AGNI COLLEGE OF TECHNOLOGY, THALAMBUR SUBJECT NAME: ENGINEERING CHEMISTRY-I YEAR/SEMESTER: I / 01
SUBJECT CODE: CY6151 DEPARTMENT: Common to all branches
IMPORTANT PART A & B QUESTION AND ANSWER PART A 1. Define degree of polymerisation. (June 2016, Jan 2014) The number of repeating units (monomeric units) in a polymer chain is known as degree of polymerisation. Molecular weight of the polymeric network Degree of polymerization (Dp) = Molecular weight of the monomeric unit 2. What is functionality of polymers? (June 2014) Functionality: The number of bonding sites or reactive sites or functional groups present in a monomer. Example: CH2 = CH2 (Ethylene) Functionality is 2 (Breaking of a double bond provides two bonding sites) 3. What are co-polymers? (Dec 2014) Co-polymerisation is a joint polymerisation in which two or more different types of monomeric units combine to form high molecular weight co-polymers. Example: Butadiene and styrene copolymerize to give SBR rubber.
4. How are polymers classified? (Any method) (Dec 2015)
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5. Define poly dispersity index. (May 2015) The ratio of the weight–average molecular weight (Mw) to number–average molecular weight (Mn) is known as polydispersity index (PDI) (or) distribution ratio.
6. State Clausius and Kelvin statement of second law of thermodynamics. (or) State the second law of thermodynamics. (Jan 2016, Dec 2015, Dec 2014) Clausius statement: It is impossible to construct a machine that can transfer heat from a cold body to a hot body, unless some external work is done on the machine. Kelvin or Kelvin-Planck statement: It is impossible to take heat from a hot body and convert it completely into work by a cyclic process without transferring a part of heat to a cold body. 7. What happens to entropy of the following? (Dec 2015, May 2015) (a) Ice is converted into water at room temperature – Entropy increases (b) Solid iodine is sublimed to its vapour – Entropy increases (c) Gaseous nitrogen is converted to liquid nitrogen – Entropy decreases 8. Write down the criteria of spontaneity. (June 2016)
9. Define entropy. (June 2014, Jan 2014) 10. Definition: Entropy is measure of degree of disorder or randomness of the system. It can also be considered as a measure of unavailable form of energy. ∆S = qrev / T 11. Problems based on physical transformation: (a) Calculate the entropy change involved in converting one mole of water at 373K to its vapour at the same temperature. Molar heat of vapourisation of water is 40.66 kJK-1mole-1. (May 2015) Given: T = 373K, ∆Hvap = 40.66 kJK-1mole-1, ∆Svap = ? Solution: ∆Svap = ∆Hvap / T = 40.66 / 373 = 0.109008 kJK-1mole-1 = 109.008 JK-1mole-1 (b) Calculate the entropy change when 10g of ice is converted into liquid water at 0OC. Latent heat of fusion of ice is 80 cal/g. (Jan 2016) 2
Given: T = 0OC = 273 K, L = 80 cal/g, ∆Sfusion = ? Solution: ∆Sfusion = ∆Hfusion / T = L / T =80 /273 = 0.293 cal/g / K When 1g of ice is converted into liquid, the entropy change is 0.293 cal/g / K Therefore, when 10g of ice is converted into liquid, the entropy change is 0.293 x 10 = 2.93cal/g / K 12. State Beer-Lambert law. (June 2016, Jan 2016) “When a beam of monochromatic radiation is passed through a solution of an absorbing substance, the rate of decrease of intensity of radiation ‘dI’ with thickness of the absorbing solution ‘dx’ is proportional to the intensity of incident radiation ‘I ’ as well as the concentration of the solution ‘C’. 13. State and explain (a) Grothus-Draper law (Dec 2015) (b) Stark-Einstein law of photochemistry. (May 2015) Grotthus -Draper law states that only the light, which is absorbed by a substance, can bring about a photochemical change. Stark-Einstein law of photochemical equivalence states that, in a primary photo chemical process (first step) each molecule is activated by the absorption of one quantum of radiation (one photon). 14. What is meant by absorption of radiation and wave number? (Jan 2014, June2014) Uptake of radiation by a solid body, liquid or gas is known as absorption of radiation. The absorbed energy may be transferred or re-emitted. In an electromagnetic wave, wave number is the reciprocal of wavelength (i.e) the number of waves in unit distance. υ = 1 / λ 15. What is fluorescence and phosphorescence? (June 2016, June 2014) Fluorescence: It is the process of emission of radiation due to the transition from singlet excited state (S1) to the ground state (S0) (i.e) S1 →S0. This transition is fast and occurs in 10 - 8 seconds. Fluorescence stops as soon as the incident radiation is cut off. Phosphorescence: It is the process of emission of radiation due to the transition from triplet excited state (T1) to the ground state (S0) (i.e) T1 →S0. This transition is slow and forbidden transition. 16. Mention the possible electronic transitions that can occur in organic molecules (or ethylene molecule). (Dec 2015, May 2015)
17. State reduced (or) condensed phase rule. (Jan 2016, May 2015, Jan 2014) In solid-liquid equilibrium of an alloy system, the pressure variable is kept constant during the study of the system because of the absence of gaseous phase. So, small changes of pressures have very little effect on the systems. Such systems are called condensed systems. For such condensed system, 3
the phase rule equation is F’ = C – P + 1. This equation is known as reduced or condensed phase rule. 18. Define degree of freedom and component with a suitable example. (June 2016, Jan 2016, Dec 2015) The number of components of a system is the minimum number of chemical constituents required to express the composition of all the phases present in the system. Example: Consider a water system, Ice (s) ↔ Water (l) ↔ Water vapour (g) The chemical composition of all the three phases is H2O, but is in different physical form. Hence the number of component is one. The number of degrees of freedom of a system is the minimum number of independent variable factors such as temperature, pressure and concentration (composition) required to describe the system completely. Example: Consider a water system, Ice (s) ↔ Water (l) ↔ Water vapour (g) F = C – P + 2 = 1 – 3 + 2 = 0 (non-variant) 19. Mention any two significance of alloy making. (Dec 2014) Significance of alloy making: (i)
To increase the hardness of the metal
(ii)
To lower the melting points of the metal
(iii)
To resist the corrosion of the metal.
(iv)
To modify chemical activity of the metal.
20. What are the basic difference between brass and bronze? (June 2016) Brass is an alloy of copper (60 - 90%) and zinc (40 – 10%) Bronze is an alloy of copper (80 – 95%) and tin (20 – 5%) 21. What is meant by nanochemistry? (Dec 2014) Nano chemistry is the branch of nano science, which deals with the chemical applications of nanomaterials. It also includes the study of synthesis and characterization of nanomaterials. 22. Differentiate the terms nano rods and nano wire? (June 2016, Jan 2016, May 2015, Dec 2014, June 2014) S.No.
Nano rod
Nano wire
1.
Size Range:
0.1-10 nm
Size Range: 10-100nm
2.
Aspect Ratio less than 20
Aspect Ratio greater than 20
3.
Thick
Thin
4.
Hard
Greater Flexibility
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23. What are CNT? Mention two applications of CNTs. (May 2015, Jan 2014) Carbon nanotubes are cylindrical carbon molecules (graphite sheet is folded into a tube) with a nanostructure having a length-to-diameter ratio greater than 1,000,000. Application of CNTs: (i)
It is used in battery technology and in industries as catalyst.
(ii)
CNTs are used effectively inside the body for drug delivery.
24. What are nanomaterials? List out any four applications of nanomaterials. (June 2016, Dec 2015) Nanomaterials are the materials having components with size less than 100 nm atleast in one dimension. Application of nanomaterials: Nano drugs - Nano materials are used as nano drugs for the cancer and TB therapy, Laboratories on a chip - Nano technology is used in the production of laboratories on a chip. 25. Distinguish between bulk materials and nano materials. (June 2014) S.No. 1. 2. 3. 4.
Nano particles Size is less than 100 nm. Collection of few molecules. Surface area is more. Strength and hardness are more.
Bulk particles Size is larger in micron size. Collection of infinite number of molecules. Surface area is less. Strength and hardness are less. PART B
1. (a) Bring out the differences between thermoplastics and thermosetting resins. ( Dec 2015, May 2015, June 2014, Jan 2014) S.No. 1. 2. 3. 4. 5. 6. 7. 8. 9.
Thermoplastic resins They are obtained by addition polymerisation. They have either linear or branched structure. All polymeric chains are held together by weak Vander Waal’s forces of attraction.
Thermosetting resins They are obtained by condensation polymerisation. They possess three dimensional cross linked structures. All polymeric chains are held together by strong covalent bonds.
They are weak, soft and less brittle.
They are strong, hard and more brittle.
They can be softened on heating and hardened on cooling. They can be remoulded. They have low molecular weights. They are soluble in organic solvents. Examples: Polyethylene, PVC
They get hardened on heating and they cannot be softened on reheating. They cannot be remoulded. They have high molecular weights. They are insoluble in organic solvents. Examples: Bakelite, Epoxy resin 5
(b) Describe the free radical mechanism of addition polymerisation. (June 2016, Jan 2016, Dec 2015, May 2015, June 2014) Free radical mechanism of addition polymerization: Three major steps are: Chain Initiation Chain Propagation Chain Termination STEP 1: Chain Initiation: This step involves two reactions. Production of free radicals by homolytic cleavage or dissociation of an initiator (I) (or catalyst) to yield a pair of free radicals (RO)
Some examples of thermal initiators: Thermal initiator is a substance used to produce free radicals by homolytic dissociation at high temperatures.
Addition of this free radical to the first monomer to produce chain initiating species.
STEP 2: Chain Propagation It consists of the growth of chain initiating species by successive addition of large number of monomers.
The growing chain of the polymer is termed as living polymer. STEP 3: Chain Termination Termination of the living polymer may occur either by coupling reaction or by disproportionation.
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Combination (or) Coupling : Coupling of free radical of one chain end to another free radical forming a macro molecule.
Disproportionation: Transfer of a hydrogen atom of one radical centre to another radical centre, forming one saturated and another unsaturated macro molecule. These are termed as dead polymers.
2. (a) Write down the preparation, properties and uses of nylon 6,6 and epoxy resin. (June 2016, Jan 2016, Dec 2014, June 2014,Jan 2014) NYLON – 6,6 : PREPARATION: It is obtained by condensation polymerisation of hexamethylene diamine with adipic acid in the ratio 1:1.
PROPERTIES: •
They are translucent, whitest and horny polymers.
•
Its melting point is 264OC.
•
They possess high melting point and low co-efficient of friction.
•
They possess high thermal stability and good abrasion resistance.
•
They are insoluble in common organic solvents but soluble in phenol and formic acid.
USES: •
Nylon -6,6 is used for textile fibres, which is used in making socks, dresses, carpets, etc.
•
Mainly used for moulding purposes for gears, bearing, electrical mountings, etc.
•
They are used for making filaments for ropes, bristles for tooth-brushes, etc. 7
EPOXY RESIN: PREPARATION: It is prepared by condensing epichlorohydrin with bisphenol.
PROPERTIES: Due to the presence of stable ether linkage, epoxy resin possesses high chemical-resistance to water, acids, alkalis, various solvents and other chemicals. They are flexible, tough and possess very good heat resisting property. Because of the polar nature of the molecules, they possess excellent adhesion quality. USES: Epoxy resins are used as surface coatings, adhesives like araldite, glass-fibre-reinforced plastics. These are applied over cotton, rayon and bleached fabrics to impart crease-resistance and shrinkage control. These are also used as laminating materials in electrical equipment. Moulds made from epoxy resins are employed for the production of components for aircrafts and automobiles.
(b) Discuss in detail about emulsion and solution polymerisation techniques. (Jan 2016, May 2015, Dec 2014, June 2014) EMULSION POLYMERISATION:
Water insoluble monomer is dispersed in large amount of water to form a uniform emulsion. This emulsion is stabilized by adding surfactants like soap. Then initiator is added. The whole content is taken in a flask and heated at a constant temperature with vigorous agitation in a thermostat with nitrogen atmosphere. After 4 to 6 hours, the pure polymer can be isolated from the emulsion by addition of de-emulsifier like 3% Al2(SO4)3. Examples: PVAc, PVC are prepared by this method.
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Advantages •
Heat transfer is very efficient and hence viscosity built up is low.
•
Rate of polymerisation is high.
•
The continuous supply of monomers to the growing chain results in getting polymers of very high molecular weight.
Disadvantages •
Polymer needs purification.
•
It requires rapid agitation.
•
It is very difficult to remove entrapped emulsifier and de-emulsifier.
Applications •
The latex obtained can be used for making water-soluble emulsion paints, adhesives, etc.
SOLUTION POLYMERISATION:
The monomer is dissolved in a suitable inert solvent and then the initiator and chain transfer agent is added to form a homogeneous mixture. The whole mixture is kept under constant agitation. After required time, the polymer produces is precipitated by pouring it in a suitable non-solvent. The inert solvent medium helps to control viscosity increase and promotes a proper heat transfer. Examples: Polyacrylic acid, polyisobutylene and polyacrylonitrile can be prepared by this method. Advantages •
Viscosity built up is negligible.
•
Agitation and heat control are easy.
Disadvantages •
It is difficult to get very high molecular weight polymer as the solvent molecules may act as chain terminator.
•
The polymer formed must be isolated from the solution either by evaporation of the solvent or by precipitation in a non-solvent.
Applications •
It can be used as adhesives and coatings.
3. (a) Derive an expression for entropy change in the isothermal expansion of an ideal gas. (or) Derive an expression for entropy change of an ideal gas at constant pressure. (June 2016, Dec 2015)
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(b) Derive Gibbs-Helmholtz equations and give its two important applications. (June 2016, Jan 2016, May 2015, Dec 2014, June 2014, Jan 2014)
GIBBS HELMHOLTZ EQUATION (OR) RELATION BETWEEN ∆G AND ∆H
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4. (a) Derive Maxwell’s relations from the fundamental definitions of thermodynamic properties. (June 2016, Jan 2016, Dec 2015, May 2015, June 2014)
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(b) Derive Clausius – Clapeyron equation. (June 2016, Jan 2016, Dec 2014)
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5. (a) Derive the relation, ∆GO = - RT ln Keq. (or) Derive Vant Hoff’s isotherm equation. (or) What is meant by Vant Hoff’s reaction isotherm? Derive the expression for a reaction isotherm of the general reaction: aA + bB → cC + dD (May 2015, Dec 2014, Jan 2014) VAN’T HOFF ISOTHERM: Van’t Hoff isotherm gives a quantitative relationship between the free energy change and the equilibrium constant. It can be derived as follows: Consider the general reaction , aA + bB → cC + dD 19
We know that
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(b) Derive an expression for Vant-Hoff’s isochore. (Jan 2016)
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6. (a) Explain the principle, instrumentation and working of a UV-Visible spectrophotometer. (May 2015, Dec 2014, June 2014, Jan 2014) INSTRUMENTATION OF UV-VISIBLE SPECTROPHOTOMETER I. COMPONENTS The various components of a visible UV spectrometer are as follows: 1.
Radiation Source: In visible – UV spectrometers, the most commonly used radiation sources are
hydrogen (or) deuterium lamps. Requirements of a radiation source (a) It must be stable and supply continuous radiation. (b) It must be of sufficient intensity. 2.
Monochromators: The monochromator is used to disperse the radiation according to the
wavelength. The essential elements of a monochromator are an entrance slit, a dispersing element and an exit slit. The dispersing element may be a prism or grating (or) a filter. 3.
Cells (Sample Cell and Reference Cell: The cells, containing samples or reference for analysis,
should ulfill the following conditions. (i) They must be uniform in construction. (ii)The material of construction should be inert to solvents. (iii) They must transmit the light of the wavelength used. 4.
Detectors: There are three common types of detectors used in visible UV spectrophotometers. They
are Barrier layer cell, Photomultiplier tube and Photocell. The detector converts the radiation, falling on which, into current. The current is directly proportional to the concentration of the solution. 5.
Recording System: The signal from the detector is finally received by the recording system. The
recording is done by recorder pen. II. WORKING OF VISIBLE AND UV SPECTROPHOTOMETER The radiation from the source is allowed to pass through the monochromator unit. The monochromator allows a narrow range of wavelength to pass through an exit slit. The beam of radiation coming out of the monochromator is split into two equal beams. One-half of the beam (the sample beam) is directed to pass through a transparent cell containing a solution of the compound to be analysed. Another half (the reference beam) is directed to pass through an identical cell that contains only the solvent. The instrument is designed in such a way that it can compare the intensities of the two beams. If the compound absorbs light at a particular wavelength, then intensity of the sample beam (I) will be less than that of the reference beam (Io). The instrument gives output graph, which is a plot of wavelength vs absorbance of the light. This graph is known as an absorption spectrum. 24
Block diagram of UV-visible spectrophotometer
(b) Explain the principle and instrumentation of IR spectroscopy with a neat block diagram. (June 2016, Jan 2016, Dec 2015, Jan 2014) INFRARED SPECTROSCOPY PRINCIPLE: IR spectra is produced by the absorption of energy by a molecule in the infrared region and the transitions occur between vibrational levels. IR spectroscopy is also known as Vibrational Spectroscopy. Range of Infrared Radiation: The range in the electromagnetic spectrum extending from 12,500 to 50 cm-1 (0.8 to 200 µ) is commonly referred to as infrared. This region is further divided into three sub regions. Sources of IR: Electrically heated rod of rare-earth oxides Range of IR radiation
(i)
Near infrared: The region is from 12500 to 4000 cm-1
(ii)
Infrared (or) ordinay IR:The region is from 4000 to 667 cm-1
(iii)
Far infrared: The region is from 67 to 50 cm-1
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INSTRUMENTATION I. Components 1. Radiation source - The main sources of IR radiation are Nichrome wire and Nernst glower, which is a filament containing oxides of Zr, Th, Ce held together with a binder. When they are heated electrically at 1200 to 2000OC, they glow and produce IR radiation. 2. Monochromator It allows the light of the required wave length to pass through, but absorbs the light of other wavelength 3. Sample Cell The cell, holding the test sample, must be transparent to IR radiation. 4. Detector IR detectors generally convert thermal radiant energy into electrical energy. There are so many detectors, of which the followings are important. (i) Photoconductivity cell. (ii) Thermocouple. (iii) Pyroelectric detectors. 5. Recorder The recorder records the signal coming out from the detector. II. Working of IR Spectrophotometer The radiation emitted by the source is split into two identical beams having equal intensity. One of the beams passes through the sample and the other through the reference sample.
Block diagram of double beam IR spectrophotometer When the sample cell contains the sample, the half- beam travelling through it becomes less intense. When the two half beams (one coming from the reference and the other from the sample) recombine, they produce an oscillating signal, which is measured by the detector. The signal from the detector is passed to the recording unit and recorded. 26
7. (a) Discuss Fluorescence (Jan 2016, May 2015, June 2014, Jan 2014) Phosphorescence (Jan 2016, May 2015, Jan 2014) Chemiluminescence (Dec 2015, Dec 2014) and Photosentisation (June 2016, Jan 2016, Dec 2015, Dec 2014, June 2014) in detail.
(1) FLUORESCENCE: When a beam of light is allowed to fall on a substance, it gets excited and emits radiation within a short time (about 10-8 sec). Emission stops as soon as the incident radiation is cut off. This process is called fluorescence. The substance which shows fluorescence is called fluorescent substance. Examples: Fluorite (naturally occurring CaF2), petroleum, organic dyes like eosin, fluorescein, ultramarine and vapours of Na, Hg and I2. TYPES OF FLUORESCENCE: (i)
Resonance fluorescence If the excited atom emits the radiation of the same frequency, the process is known as resonance
fluorescence. Example: When mercury vapour at low pressure is exposed to radiation of wavelength 253.7 nm, it gets excited. Subsequently, when it returns to its ground state, it emits radiation of the same frequency, which it absorbed (ii)
Sensitized Fluorescence If the molecule is excited, due to the transfer of part of excitation energy from the foreign substance, it
emits the radiation of lower frequency, the process is known as sensitized fluorescence. Example: Vapours of mercury, sodium, etc. MECHANISM OF FLUORESCENCE: Molecules have even number of electrons in the ground state (S0) and are paired. When it absorbs light, one of the paired electrons moves to the higher energy states (excited states) (S1, S2, S3, etc.). From the excited state the molecules returns to the ground state by the following process. (a) From the excited state, the molecules return to the first excited state (S1) through Internal Conversion (IC). S3 → S1
S2 → S1
(b) From the S1 state the molecules returns to the ground state (S0) by emitting radiation, called fluorescence. S1 → S0
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QUENCHING OF FLUORESCENCE: The fluorescence may be quenched (stopped), when the excited molecule collides with a normal molecule before it fluoresces. During quenching, the energy of the excited molecule gets transferred to the molecule with which it collides. Quenching occurs in two ways: (a) Internal quenching Quenching may also occur when the molecule changes from the singlet excited state to the triplet excited state. This phenomenon is called internal quenching. (b) External quenching Quenching may also occur from the addition of an external substance, which absorbs energy from the excited molecule. This phenomenon is called external quenching.
(2)PHOSPHORESCENCE When a beam of light is allowed to fall on a substance, it gets excited and emits radiation, for some time. Emission continuous for some time even after the incident radiation is cut off. This process is called phosphorescence (or) delayed fluorescence. The substance, which shows phosphorescence, is called phosphorescent substance. Examples: ZnS, alkaline earth sulphides and sulphates of Ca, Ba &Sr. MECHANISM OF PHOSPHORESCENCE: When a molecule absorbs light, one of its paired electrons move from the ground state (S0) to the higher energy states (excited states) (S1, S2, S3, etc.) From the excited state the molecules return to the ground stae by the following process. (a) The molecule crosses from the singlet excited state to the corresponding triplet excited states through Inter System Crossing (ISC). 28
(b) From the triplet excited state, the molecule returns to the first triplet excited state through Internal Conversion (IC).
(c) From the T1 state, the molecule returns to the ground state (S0) by emitting radiation called phosphorescence.
(3) PHOTOSENSITIZATION:
In some photochemical reactions, the reactant molecules do not absorb
radiation and no chemical reaction occurs. But, if a suitable foreign substance (sensitizer), which absorbs radiation, is added to the reactant, the reaction takes place. The foreign substance gets excited during absorption of radiation and transfers its energy to the reactants and initiates the reaction. 1. Photosensitization The foreign substance, which absorbs the radiation and transfers the absorbed energy to the reactants, is called a photosensitizer. This process is called photosensitized reaction (or) photosensitization. Examples: (a) Atomic photosensitizers: Mercury, cadmium, zinc (b) Molecular photosensitizers: Benzophenone, sulphur dioxide 2. Quenching - When the foreign substance in its excited state collides with another substance it gets converted into some other product due to the transfer of its energy to the colliding substance. This process is known as quenching. MECHANISM OF PHOTOSENSITIZATION The mechanism of photosensitization and quenching can be explained by considering a general Donor (D) – Acceptor (A) system. In a donor-acceptor system, only the donor D (ie., the sensitizer) absorbs the incident photon. When the donor absorbs the photon, it gets excited from ground state (S0) to singlet state (S1); Then the donor, via 29
inter system crossing (ISC), gives the triplet excited state (T1 or 3D). The triplet state of the donor is higher than the triplet state of the acceptor (A).This triplet excited state of the donor then collides with the acceptor produces the triplet excited state of the acceptor (3A) and returns to the ground state (S0). If the triplet excited state of the acceptor (3A) gives the desired products, the mechanism is called photosensitization However, if the products are resulted directly from the excited state of the donor (3D), then A is called the quencher and the process is called quenching.
Mechanism of Photosensitization The sequence of photosensitization and quenching may be represented as follows:
It is necessary that the energy of the triplet excited state of the donor (sensitizer) must be higher than the triplet excited state of the acceptor (reactant).So, that the energy available is enough to excite the reactant molecule to its excited state. The dotted line indicates the transfer of energy from the sensitizer to reactant.
(4) CHEMILUMINESCENCE If light is emitted at ordinary temperature, as a result of chemical reactions, the phenomenon is known as chemiluminescence. Thus, it is the reverse of a photochemical reaction. As the emission occurs at ordinary temperature, the emitted radiation is also known as “cold light“.
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Explanation: In a chemiluminescent reaction, the energy released during the chemical reaction makes the product molecule electronically excited. The excited molecule then emits radiation, as it returns to the ground state. Examples of chemiluminescence: 1. Glow of phosphorus and its oxide, in which the oxide in its excited electronic state emits light. 2. Oxidation of 5-aminophthalic hydrazides or cyclic hydrazides by H2O2, emits bright green light. 3. Bioluminescence: Emission of “cold light” by fire flies
(glow-worm) due to the aerial oxidation of
luciferon (a protein) in the presence of enzyme (luciferase). 4. Grignard reagent produces greenish blue light, on oxidation by air. 5. Oxidation of decaying wood containing certain bacteria. Mechanism of Chemiluminescence - It can be explained by considering anion-cation reactions. Example
Illustration
Mechanism of chemiluminescence in anion-cation reaction The aromatic anion ((Ar-) contains two paired electrons in the bonding molecular orbital (BMO) and one unpaired electron in the antibonding molecular orbital (ABMO). The ABMO of the aromatic cation (Ar+) + is empty. When the electron is transferred from the ABMO of the anion ((Ar-) to the ABMO of the cation ((Ar+) + ), the singlet excited state1Ar* is formed. The excited state can be deactivated by the emission of photon hv.
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(b)What is quantum efficiency? How is it determined? (June 2016, Dec 2015, May 2015, Dec 2014) Quantum efficiency (Ф) is defined as, “the number of molecules of reactants reacted or products formed per quantum of light yield (or) absorbed”, ie.,
DETERMINATION OF QUANTUM YEILD We know that, the quantum yield Ф of a photochemical reaction is expressed as
Thus we can calculate the quantum yield from the determination of the followings. (i)
Determination of the number of molecules reacted in a given time.
(ii)
Determination of the amount of photons absorbed in the same time.
Experimental determination of number of molecules reacted The number of molecules reacted in a given time can be determined by the usual analytical techniques, used in chemical kinetics. Measurement of Rate of Reaction The rate of reaction is measured by the usual methods. Small quantities of the samples are pipetted out from the reaction mixture from time to time and the concentration of the reactants are continuously measured by the usual volumetric methods (or) the change in some physical property such as refractive index (or) absorption (or) optical rotation. From the data, the amount and hence number of molecules can be calculated. Experimental Determination of amount of Photons Absorbed A photochemical reaction occurs by the absorption of photons of the light by the reactant molecules. Therefore, it is essential to determine the intensity of the light absorbed. An experimental set up for the study of photochemical reaction is illustrated in the following diagram.
Apparatus used for measurement of light intensity
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Radiation emitted from a source of light (L) (Sunlight, tungsten filament, mercury vapor lamp) is passed through the lens, which produces parallel beams. The parallel beams are then passed through a filter (or) monochromatic ‘B’, which yields a beam of the desired (one) wavelength only. The light from the monochromatic (monochromatic light) is allowed to enter into the reaction cell ‘C’ immersed in a thermostat, containing the reaction mixture. The part of the light that is not absorbed fall on a detector X’, which measures the intensity of radiation. Among so many detectors, the most frequently employed is the chemical actinometer. The Chemical Actinometer: It is a device used to measure the amount of radiation absorbed by the system in a photochemical reaction. Using chemical actinometer, the rate of a chemical reaction can be measured easily. Uranyl Oxalate actinometer - It is a commonly used chemical actinometer. It consists of 0.05 M oxalic acid and 0.01 M uranyl sulphate in water. When it is exposed to radiation, oxalic acid undergoes decomposition to give CO2, CO and H2O.
The residual concentration of oxalic acid can be found out by titrating with standard KMnO4. The amount of oxalic acid consumed (decomposed) is a measure of the intensity of radiation.
8. (a) Draw the phase diagram of lead-silver system. Explain the salient features and its application. (June 2016, Jan 2016, Dec 2015, May 2015, June 2014)
LEAD SILVER SYSTEM (SIMPLE EUTECTIC BINARY ALLOY SYSTEM) This system has two components and four phases. The phases are (i) solid silver (ii) solid lead (iii) solution of molten Ag & Pb and (iv) vapour. The boiling points of Ag & Pb being considerably high, the vapour phase is practically absent. Thus Pb/Ag is condensed system with three phases. In such a case, 33
pressure can have no effect on the system. Therefore we need only two variables, namely temperature (T) and composition (C). PHASE DIAGRAM OF LEAD-SILVER SYSTEM
SALIENT FEATURES OF THE PHASE DIAGRAM: •
Two lines (or) curves AC and BC
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Eutectic point ‘C’
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Four areas : Area above ACB, below AC, below BC and below DCE
(i) Curve AC: •
‘A’ represents the melting point of pure Ag (961OC).
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The curve AC is the freezing point curve of Ag.
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Addition of Pb lowers the melting point of Ag along the curve AC.
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Along AC, solid Ag and liquid melt (solution of Ag & Pb) are in equilibrium.
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Applying reduced phase rule, F’ = C - P + 1 = 2 – 2 + 1 = 1. Hence the system is univariant along the curve AC.
(ii) Curve BC: •
‘B’ represents the melting point of pure lead (327OC).
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The curve BC is the freezing point curve of Pb.
•
Addition of Ag lowers the melting point of Pb along the curve BC.
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Along BC, the solid Pb and liquid melt (solution of Ag & Pb) are in equilibrium.
•
Applying reduced phase rule, F’ = C - P + 1 = 2 – 2 + 1 = 1. Hence the system is univariant along the curve BC.
(iii) Eutectic Point ‘C’: •
The curve AC and BC intersect at ‘C’ which is called eutectic point. 34
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Below the point ‘C’ both Pb & Ag exist in the solid state.
•
At this point three phases (solid Ag, solid Pb and their liquid melt) are in equilibrium. Solid Pb + Solid Ag
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Liquid melt
Applying reduced phase rule, F’ = C – P + 1 = 2 – 3 + 1 = 0. Hence the system is nonvariant at point ‘C’.
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Eutectic point is the lowest possible temperature (303oC) in the system that corresponds to fixed composition (97.4% Pb & 2.6 % Ag), below which a liquid phase cannot exist and above which the solid phases disappear.
•
The temperature and composition corresponding to the eutectic point ‘C’ are called eutectic temperature and eutectic composition.
•
A liquid mixture of two components – Ag & Pb, which has the lowest freezing point compared to all other liquid mixtures, is called eutectic mixture.
(iv) Areas: •
The area above ACB represents a single phase (solution of molten Pb & Ag).
•
Applying reduced phase rule, F’ = C – P + 1 = 2 – 1 + 1 = 2. The system is bivariant.
•
Area below AC represent the phases solid Ag + liquid melt, area below BC represents the phases solid Pb + liquid melt and area below ‘DCE’ represents solid Pb + solid Ag.
•
All the three areas have two phases and hence the system is univariant. (F’ = C – P + 1 = 2 – 2 + 1 = 1)
APPLICATION OF LEAD-SILVER SYSTEM Pattinson’s Process for the Desilverisation of Argentiferous Lead: The process of recovery of silver from argentiferous lead is called as desilverisation.
Argentiferous
lead contain very small amount of silver (less than 0.1%). Desilverisation of lead is based on the formation of eutectic mixture. Argentiferous lead is heated well above the melting point of pure lead (327oC) so that the system consists only of the liquid phase represented by the point ‘a’ in the phase diagram. When the liquid melt is allowed to cool gradually, the temperature of the melt falls along the dashed line ‘ab’. As soon as the temperature corresponding to ‘b’ is reached, solid lead begins to separate and the solution would contain relatively larger amount of silver. On further cooling, more and more of lead is separated along BO. At ‘O’, a eutectic mixture consisting of 2.6% Ag and 97.4% Pb is obtained. The eutectic alloy is then treated for the recovery of silver profitably. The process of raising the relative proportion of Ag in the alloy is known as Pattinson’s process.
35
(c) Draw the phase diagram of one component water system and explain in detail. (or) Apply phase rule to water system and explain the characteristics. (June 2016, Jan 2016, Dec 2015, May 2015, Dec 2014, Jan 2014)
APPLICATION OF PHASE RULE TO ONE COMPONENT WATER SYSTEM Water exists in three possible phases namely: solid, liquid and vapour. Hence, there can be three forms of equilibria. Solid
Liquid
Liquid
Vapour
Solid
Vapour
Each of the above equilibrium involves two phases.
SALIENT FEATURES OF THE PHASE DIAGRAM: (a) Curves or lines OA, OB, OC and OA’ (b) Areas AOB,BOC and AOC (c) Triple point ‘O’ (i) CURVE OA: •
The curve OA is called vapourisation curve.
•
It represents the equilibrium between water and water vapour. At any point on the curve the following equilibrium will exist. Water
Water vapour 36
•
The degree of freedom along the line OA is one, as predicted by the phase rule F = C − P + 2; F = 1 − 2 + 2; F = 1 (univariant)
•
This equilibrium (i.e. line OA) will extend upto the critical temperature (374°C). Beyond the critical temperature the equilibrium will disappear and only water vapour will exist.
(ii)CURVE OB: •
The curve OB is called sublimation curve of ice.
•
It represents the equilibrium between ice and water vapour. At any point on the curve the following equilibrium will exist. Ice
•
Vapour
The degree of freedom along the line OB is one, as predicted by the phase rule F = C − P + 2; F = 1 − 2 + 2; F = 1 (univariant)
•
This equilibrium (line OB) will extend upto the absolute zero (− 273°C), where no vapour can be present and only ice will exist.
(iii) CURVE OC: •
The curve OC is called melting point curve of ice.
•
It represents the equilibrium between ice and water. At any point on the curve the following equilibrium will exist. Water
Ice •
The curve OC is slightly inclined towards pressure axis. This shows that melting point of ice decreases with increase of pressure.
•
The degree of freedom along the line OC is one, as predicted by the phase rule F = C − P + 2; F = 1 − 2 + 2; F = 1 (univariant)
(iv) TRIPLE POINT ‘O’: •
The three curves OA, OB and OC meet at a point ‘O’, where three phases namely solid, liquid and vapour are simultaneously at equilibrium.
•
This point is called triple point, at this point the following equilibrium will exist: Ice(s)
•
Water(l)
Vapour(g)
The degree of freedom of the system is zero, as predicted by the phase rule F = C − P + 2; F = 1 − 3 + 2; F = 0 (nonvariant)
•
Temperature and pressure corresponding to the triple point ‘O’ are 0.0075°C and 4.58 mm.
(v) CURVE OA’ (METASTABLE EQUILIBRIUM): •
The curve OA′ is called vapour pressure curve of the super-cool water or metastable equilibrium where the following equilibrium will exist. 37
Super - cool water •
Vapour
Sometimes water can be cooled below 0°C without the formation of ice, this water is called supercooled water. Super cooled water is unstable and it can be converted into solid by “seeding” or by slight disturbance.
(vi) AREAS: •
Area AOC, BOC, AOB represents water, ice and vapour respectively. In order to define the system at any point in the areas, it is essential to specify both temperature and pressure.
•
The degree of freedom of the system is two, as predicted by the phase rule F = C − P + 2; F = 1 − 1 + 2; F = 2 (Bivariant)
9. (a) Write a note on heat treatment of steel. (June 2016, Dec 2015, Dec 2014, June 2014, Jan 2014)
HEAT TREATMENT OF STEEL: Heat treatment is defined as, “the process of heating and cooling of solid steel article under carefully controlled conditions”. During heat treatment certain physical properties are altered without altering its chemical composition. OBJECTIVES OF HEAT TREATMENT: Heat treatment causes (i) Improvement in magnetic and electrical properties. (ii) Refinement of grain structure. (iii)Removal of the imprisoned trapped gases. (iv) Removal of internal stresses. (v) Improves fatique and corrosion resistance. TYPES OF HEAT TREATMENT OF STEEL: 1. Annealing
4. Normalizing
2. Hardening
5. Carburizing
3. Tempering
6. Nitriding
(1) ANNEALING Annealing means softening. This is done by heating the metal to high temperature, followed by slow cooling in a furnace Purpose of annealing: (i) It increases the machinability. (ii) It also removes the imprisoned gases.
38
Types of Annealing: (i) Low temperature annealing (or) process annealing. (ii) High temperature annealing (or) full annealing. (i)Low temperature annealing (or) process annealing: It involves in heating steel to a temperature below the lower critical temperature followed by slow cooling. Purpose: (i) It improves machinability by relieving the internal stresses or internal strains. (ii) It increases ductility and shock - resistance. (iii)It reduces hardness. (ii) High temperature annealing (or) full - annealing It involves in heating steel to a temperature about 30 to 50° C above the higher critical temperature and holding it at that temperature for sufficient time to allow the internal changes to take place and then cooled to room temperature. The approximate annealing temperatures of various grades of carbon steel are: 1. mild steel = 840 − 870° C 2. medium-carbon steel = 780 − 840° C 3. high-carbon steel = 760 − 780° C Purpose: (i) It increases the ductility and machinability. (ii) It makes the steel softer, together with an appreciable increase in its toughness. (2) HARDENING (OR) QUENCHING It is the process of heating steel beyond the critical temperature and then suddenly cooling it either in oil or brine-water or some other fluid. Hardening increases the hardness of steel. The faster the rate of cooling harder will be the steel produced. Medium and high-carbon steels can be hardened, but low-carbon steels cannot be hardened. Purpose: (i) It increases its resistance to wear, ability to cut other metals and strength, but steel becomes extra brittle. (ii) It increases abrasion-resistance, so that it can be used for making cutting tools. (3) TEMPERING It is the process of heating the already hardened steel to a temperature lower than its own hardening temperature and then cooling it slowly. In tempering, the temperature to which hardened steel is reheated is of great significance and controls the development of the final properties. Thus, (i)
for retaining strength and hardness, reheating temperature should not exceed 400°C.
(ii)
for developing better ductility and toughness, reheating temperature should be within 39
400 − 600° C. Purpose: (i) It removes any stress and strains that might have developed during quenching. (ii) It reduces the brittleness and also some hardness but toughness and ductility are simultaneously increased. (iii)Cutting-tools like blades, cutters, tool-bites always require tempering. (4) NORMALISING It is the process of heating steel to a definite temperature (above its higher critical temperature) and allowing it to cool gradually in air. A normalised steel will not be as soft as an annealed job of the same material. Also normalising takes much lesser time than annealing process. Purpose: (i) It recovers the homogeneity of the steel structure. (ii) It refines grains. It removes the internal stresses. (iii) It increases the toughness. (iv) Normalised steel is suitable for the use in engineering works. (5) CARBURIZING The mild steel article is taken in a cast iron box containing small pieces of charcoal (carbon material). It is then heated to about 900 to 950°C and allowed to keep it as such for sufficient time, so that the carbon is absorbed to required depth. The article is then allowed to cool slowly within the iron box itself. The outer skin of the article is onverted into high-carbon steel containing about 0.8 to 1.2% carbon. Purpose: To produce hard-wearing surface on steel article. (6) NITRIDING Nitriding is the process of heating the metal alloy in presence of ammonia at a temperature to about 550°C. The nitrogen (obtained by the dissociation of ammonia) combines with the surface of the alloy to form hard nitride. Purpose: To get super-hard surface.
(c) Give the composition and applications of (i) Brass (ii) Bronze (Any two non-ferrous alloys) (Dec 2014, Jan 2014) (iii) Nichrome (iv) Stainless steel (Any two ferrous alloys) (June 2014) (May 2015)
40
IMPORTANT NON – FERROUS ALLOYS (i) BRASS (COPPER ALLOY): Brass is an important copper alloy, containing mainly copper and zinc. They possess, (i) greater strength, durability and machinability than copper, (ii) lower melting points than Cu and Zn, (i) good corrosion resistance and water resistance property. IMPORTANT BRASSES THEIR COMPOSITION, PROPERTIES AND USES
(ii) BRONZE (COPPER ALLOY): Bronze is also a copper alloy containing copper and tin. They possess, (i) lower melting point than steel and are more readily produced from their constituent metals. (ii) better heat and electrical conducting property than most of the steels, (ii) non-oxidizing, corrosion resistance and water resistance property. 41
IMPORTANT BRONZE THEIR COMPOSITION, PROPERTIES AND USES
42
IMPORTANT FERROUS ALLOYS: (iii)NICHROME - It is an alloy of nickel and chromium. Its composition is:
USES: (i) It is widely used for making resistance coils, heating elements in stoves. (ii) It is also used in electric irons and other household electrical appliances. 43
(iv) STAINLESS STEELS (OR) CORROSION RESISTANT STEELS These are alloy steels containing chromium together with other elements such as nickel, molybdenum, etc. Chromium is effective if its content is 16% or more. The carbon content in stainless steel ranges from 0.3 to 1.5%. TYPES OF STAINLESS STEEL: (a) HEAT TREATABLE STAINLESS STEEL: COMPOSITION: These steels mainly contain upto 1.2% of carbon and less than 12-16% of chromium. PROPERTIES: Heat - treatable stainless steels are magnetic, tough and can be worked in cold condition. USES: (i) They can be used upto 800°C. (ii) They are very good resistant towards weather and water. (iii)They are used in making surgical instruments, scissors, blades, etc., (b) NON - HEAT TREATABLE STAINLESS STEEL: These steels possess less strength at high temperature. They are more resistant to corrosion. USES: (i) It is used in making chemical equipments and automobile parts. (ii) It is used in making household utensils, sinks, dental and surgical instruments.
10. (a) Explain the functions and effects of alloying elements. (or) What is the purpose of alloy making? Illustrate with suitable examples. (June 2016, Jan 2016, Dec 2015, May 2015)
IMPORTANCE (OR) NEED (OR) PURPOSE OF MAKING ALLOYS Generally pure metals possess some useful properties such as high melting point, high densities, malleability, ductility, good thermal and electrical conductivity. The properties of a given metal can be improved by alloying it with some other metal or non-metal. (1) To increase the hardness of the metal Generally pure metals are soft, but their alloys are hard. Examples: (i) Gold and silver are soft metals; they are alloyed with copper to make them hard. (ii) Addition of 0.5% arsenic makes lead so hard and used for making bullets. (iii) Addition of 0.15-2% carbon to iron for getting steel, hardness is improved to it. (2) To lower the melting points of the metal 44
Alloying makes the metal easily fusible. Example: Wood’s metal (an alloy of lead, bismuth, tin and cadmium) melts at 60.5° C, which is far below the melting points of any of these constituent metals. (3) To resist the corrosion of the metal •
Metals, in pure form, are quite reactive and easily corroded by surrounding, thereby their life is reduced. If a metal is alloyed, it resist corrosion.
Example: Pure iron gets rusted, but when it is alloyed with carbon or chromium (stainless steel), resists corrosion. (4) To modify chemical activity of the metal Chemical activity of the metal can be increased or decreased by alloying. Example: Sodium amalgam is less active than sodium, but aluminium amalgam is more active than aluminium. (5) To modify the colour of the metal The dull coloured metals are improved by alloying with metals. Example: Brass, an alloy of copper (red) and zinc (silver-white), is white in colour. (6) To Get Good Casting of Metal Some metals expand on solidification but are soft and brittle. The addition of other metals produce alloys which are hard, fusible and expand on solidification and thus give good casting. Example: An alloy of lead with 5% tin and 2% antimony is used for casting printing type, due to its good casting property.
(b) Draw the phase diagram of zinc-magnesium system and explain in detail. (Jan 2016, Dec 2014, Jan 2014)
ZINC-MAGNESIUM SYSTEM The zinc-magnesium system is an example of two component system which forms a compound with congruent melting point. The phase diagram of Zn-Mg binary alloy system appears to be made up of two eutectic phase diagrams place side-by-side. •
Left side consists of Zn and MgZn2 system.
•
Right side consists of MgZn2 and Mg system.
45
PHASE DIAGRAM OF ZINC-MAGNESIUM SYSTEM Curve AE1: •
The curve AE1 is freezing point curve of Zn.
•
Point ‘A’ is the melting point of pure Zn (420oC).
•
The curve AE1 shows the melting point depression of Zn by the successive addition of Mg.
•
Along this curve AE1, solid Zn and the melt are in equilibrium. Solid Zn
Melt
(ii) Curve CE2: •
The curve CE2 is freezing point curve of Mg.
•
Point ‘C’ is the melting point of pure Mg (650oC).
•
The curve CE2 shows the melting point depression of Mg by the successive addition of Zn.
•
Along this curve CE2, solid Mg and the melt are in equilibrium.
Solid Mg
Melt
(iii) Curve E1BE2: •
Addition of zinc to magnesium or magnesium to zinc leads to the formation of a compound MgZn2 at the points E1 and E2.
•
Hence, the curve E1BE2 is the freezing point curve of the compound MgZn2 and ‘B’ is its melting point.
•
Along the curve, solid MgZn2 and the liquid melt are in equilibrium.
Solid MgZn2 •
Melt
Along the curves the system is univariant (F’ = C – P + 1 = 2 – 2 + 1 = 1).
46
(iv) Maximum Point ‘B’: •
The maximum point ‘B’ on the curve E1BE2 is the melting point of the pure compound MgZn2.
•
The temperature at the point is 590oC. Here the compound has the same composition both in liquid and solid states. So, MgZn2 is said to possesses congruent melting point.
•
The composition of MgZn2 is 33.3% Mg and Zn is 66.7% (i.e., the ratio of MgZn2 and Zn is 1:2).
•
At the point ‘B’, the two component system becomes one component system because both the liquid and solid phases can be represented by the component MgZn2. Hence at the point ‘B’, the system is non-variant (F’ = C – P + 1 = 1 – 2 + 1 = 0).
(v) Points E1 and E2 (Eutectic point): •
There are two eutectic points in the phase diagram.
•
At the eutectic point E1 (solid Zn, solid MgZn2 and their liquid melt) and at E2 (solid Mg, solid MgZn2 and their liquid melt) three phases exist in equilibrium.
•
The temperature corresponding to the points E1 and E2 are 380oC & 347oC.
•
The degree of freedom at these points is non-variant (F’ = C – P + 1 = 2 – 3 + 1 = 0).
(vi) Areas: (a) Below the line AE1: The area below the line AE1 consists of solid Zn and the solution. (b) Below the line CE2: The area below the line CE2 consists of solid Mg and the solution. (c) Below the line E1BE2: The area below the line E1BE2 consists of solid MgZn2 and the solution. All these areas have two phases each and hence the system is univariant. (F’ = C – P + 1 = 2 – 2 + 1 = 1) (d) Above the line AE1BE2C: •
The area above the line AE1BE2C consists of only liquid phase.
•
The single phase system at any point in this area is bivariant. (F’ = C – P + 1 = 2 – 1+ 1 = 2).
(e) Below the point E1 and E2: •
The area below the point E1 and E2 consists of solid Zn + solid MgZn2 and solid MgZn2 + solid Mg respectively.
•
Two phases are present below the point E1 & E2 and hence the system is univariant (F’ = C – P + 1 = 2 – 2 + 1 = 1).
11. (a) (i) Write briefly about the properties (size dependent properties) (June 2016, June 2014) (or) What are the properties that change from its bulk form to nano size form? Explain each with example. (May 2015) (ii)Applications of nano materials in various fields. (Jan 2016, Dec 2014, June 2014) 47
(i) SIZE DEPENDENT PROPERTIES OF NANO-MATERIALS 1. Melting Points Nano-materials have a significantly lower melting point and appreciable reduced lattice constants. This is due to huge fraction of surface atoms in the total amount of atoms. 2. Optical Properties Reduction of material dimensions has pronounced effects on the optical properties. Optical properties of nano-materials are different from bulk forms. The change in optical properties is caused by two factors. (i) The quantum confinement of electrons within the nano-particles increases the energy level spacing. Example: The optical absorption peak of a semiconductor nano-particles shifts to a short wavelength, due to an increased band gap. (ii) Surface plasma resonance, which is due to smaller size of nano-particles than the wavelength of incident radiation. 3.Magnetic properties Magnetic properties of nano materials are different from that of bulk materials. Ferro-magnetic behaviour of bulk materials disappear, when the particle size is reduced and transfers to superparamagnetics. This is due to the huge surface area. 4. Mechanical Properties The nano-materials have less defects compared to bulk materials, which increases the mechanical strength. (i) Mechanical properties of polymeric materials can be increased by the addition of nano-fillers. (ii)As nano-materials are stronger, harder and more wear resistant and corrosion resistant, they are used in spark plugs. 5. Electrical Properties (i) Electrical conductivity decreases with a reduced dimension due to increased surface scattering. However, it can be increased, due to better ordering in micro-structure. Example: Polymeric fibres (ii)
Nanocrystalline materials are used as very good separator plates in batteries, because they can hold
more energy than the bulk materials. Example: Nickel-metal hydride batteries made of nanocrystalline nickel and metal hydride, require far less frequent recharging and last much longer.
48
6. Chemical Properties Any heat treatment increases the diffusion of impurities, structural defects and dislocations and can be easily push them to the nearby surface. Increased perfection will have increased chemical properties. 7. Thermal conductivity Thermal conductivity of the nanomaterial are lower than the bulk materials because of the energy gap between valence band and conduction band is high.
(ii) APPLICATION OF NANO MATERIALS (Any four applications in each field) I. MEDICINE 1. Nano drugs Nano materials are used as nano drugs for the cancer and TB therapy, 2. Laboratories on a chip Nano technology is used in the production of laboratories on a chip. 3. Nano-medibots Nano particles function as nano-medibots that release anti-cancer drug and treat cancer. 4. Gold-coated nanoshells It converts light into heat, enabling the destruction of tumours. 5. Gold nano particles as sensors Gold nano particles undergo colour change during the transition of nano particles. 6. Protein analysis Protein analysis can also be done using nanomaterials. 7. Gold nanoshells for blood immuno assay Gold nano shells are used for blood immuno assay. 8. Gold nano shells in imaging Optical properties of the gold nano shells are utilized for both imaging and therapy. 9. Targeted drug delivery using gold nano particles It involves slow and selective release of drugs to the targeted organs. 10. Repairing work Nano technology is used to partially repair neurological damage. II. INDUSTRIES 1. As Catalyst It depends on the surface area of the material. As nano-particles have an appreciable fraction of their atom at the surface, its catalytic activity is good. Examples Bulk gold is chemically inert, whereas gold nano-particles have excellent catalytic property. 49
2. In water purification Nano-filtration makes use of nano-porous membranes having pores smaller than 10 nm. Dissolved solids and colour producing organic compounds can be filtered very easily from water. Magnetic nano-particles are effective in removing heavy metal contamination from waste water. 3. In fabric industry The production of smart-clothing is possible by putting a nano-coating on the fabric. (i) Embedding of nano-particles on fabric makes them stain repellent. (ii) Socks with embedded silver nano-particles fills all the bacteria and makes it odour free. 4. In Automobiles (i) Incorporation of small amount of nano - particles in car bumpers can make them stronger than steel. (ii)Specially designed nano-particles are used as fuel additive to lower consumption in vehicles. 5. In food industry The inclusion of nano-particles in food contact materials can be used to generate novel type of packing materials and containers. 6. In energy sector In solar power, nano-technology reduces the cost of photovoltaic cells by 10 to 100 times. III. ELECTRONICS Quantum wires are found to have high electrical conductivity. The integrated memory circuits have been found to be effective devices. A transistor, called NOMFET, (Nano particle organic memory field effect transistor) is created by combining gold nano particles with organic molecules. Nano wires are used to build transistors without p - n junctions. Nano radios are the other important devices, using carbon nanotubes. MOSFET (Metal Oxide Semi conductor Field Effect Transistor), performs both as switches and as amplifiers. IV. BIO-MATERIALS (Biology) Nano materials are used as bone cement and bone plates in hospitals. It is also used as a material for joint replacements. Nano technology is being used to develop miniature video camera attached to a blind person’s glasses. Nano materials are also used in the manufacture of some components like heart valves and contact lenses. Nano materials are also used in dental implants and breast implants. CNTs are used as light weight shielding materials for protecting electronic equipment against electromagnetic radiation. 50
(b) Write short notes on: Nano cluster, Nano wires, (June 2016, Dec 2015, Jan 2014) nano rods (Dec 2015) and CNTs. (Dec 2015, Dec 2014) NANO CLUSTER: Nano clusters constitute an intermediate state of matter between molecules and bulk materials. These are fine aggregates of atoms or molecules. They are bound by forces, which may be metallic, covalent, ionic, hydrogen bond or vander waals force in character. The size of nanocluster ranges from sub-nanometer to 10 nm in diameter. It has been found that clusters of certain critical size (clusters with a certain number of atoms in the group) are more stable than others. Nanoclusters consisting of upto a couple of hundred atoms, but larger aggregates containing 103 or more atoms are called Nanoparticles. Magic number: It is the number of atoms in the clusters of criticle sizes with higher stability. Different types of clusters can be distinguished by the nature of the force between the atoms. Clusters containing transition metal atoms have unique chemical, electronic and magnetic properties, which vary with the number of constituent atoms, the type of element and the charge on the cluster. Production of Nano Cluster
Production of nano clusters from atoms or molecules or from bulk materials Clusters can be produced from atomic or molecular constituents or from the bulk materials as shown in the figure. Atomic clusters or molecular clusters are formed by nucleation of atoms or molecules respectively. Clusters of the same type may be obtained by top down process also. Sources of Clusters There are many kinds of cluster sources. Two of them are 1. Supersonic nozzle source
2. Gas-aggregation source
1. Supersonic Nozzle Source Here metal is vapourized in an oven and the vapour is mixed with an inert carrier gas (seeded) at a pressure of several atmosphere at a temperature of 75 - 1500 K. The metal/carrier gas mixture is then allowed through a nozzle in to high vacuum, which creates supersonic beam. Seeding produces large clusters while in the absence of a carrier gas smaller clusters are formed. 51
2. Gas-aggregation source The source utilizes the property of aggregation of atoms in an inert media. The vapours generated by any method are introduced in to a cold inert gas at a higher pressure. The species at high temperature are thermalized. The gas phase is super saturated with the species and they aggregate. These sources produce continuous beams of clusters of low-to-medium boiling metals.
NANO-WIRES Nano-wire is a material having an aspect ratio ie. length to width ratio greater than 20. 1. Nano-wires of metals
:
Au, Ni, Pt.
2. Nano-wires of semiconductors
:
InP, Si, GaN
3. Nano-wires of Insulators
:
SiO2, TiO2
4. Molecular nanowires
:
DNA
Properties of Nano-wires: 1. Nano-wires are one-dimensional material. 2. Conductivity of a nano-wire is less than that of the corresponding bulk materials. 3. It exhibits distinct optical, chemical, thermal and electrical properties due to this large surface area. 4. Silicon nano-wires show strong photoluminescence characteristics. Synthesis of Nano-wires: Nanowires can be synthesised by any one of the following methods. 1. Template-assisted synthesis Template assisted synthesis of nanowires is simple way to fabricate nanostructures. These templates contain very small cylindrical pores or voids within the host material and the empty spaces are filled with the chosen material to form nanowires. Examples for templates Alumina (Al2O3), nano-channel glass, mica films, ion track-edged polymers. 2. VLS method It involves the absorption of the source material from the gas phase into a liquid droplet of catalyst. Upon supersaturation of the liquid alloy, a nucleation event generates a solid precipitate of the source material. This seed serves as a preferred site for further deposition of material at the interface of the liquid droplet, promoting the elongation of the seed into a nanowire. Applications of Nano-wires: 1. Nanowires are used for enhancing mechanical properties of composites. 2. It is also used to prepare active electronic components such as p-n junction and logic gates. 52
3. Semiconductor nanowire crossings are expected to play a important role in future of digital computing. 4. Nanowires find applications in high-density data storage either as magnetic read heads or as patterned storage media.
NANO-RODS Nano-rod is a material having an aspect ratio in the range 1 to 20 with short dimension of the material being 10-100 nm. Characteristics of Nano-rods 1. Nano-rods are one-dimensional materials. 2. It also exhibits optical and electrical properties Synthesis Nano-rods are produced by direct chemical synthesis. A combination of ligands acts as shape control agents and bond to different facets of the nano-rods with different strength. Applications It finds applications in display technologies and micromechanical switches.
CARBON NANO TUBES (CNTs) CNT is a tubular form of carbon with 1-3mm diameter and a length of few nm to micron. Carbon nanotubes are allotropes of carbon with a nanostructure having a length-to-diameter ratio greater than 1,000,000. CNT are tubular forms of carbon. When graphite sheets are rolled into a cylinder, their edges join to each other form CNTs nanotubes i.e., carbon nanotubes are extended tubes of rolled graphite sheets. Nanotubes naturally align themselves into “ropes” and held together by Vander waals forces. But each carbon atoms in the carbon nanotubes are linked by the covalent bond. STRUCTURE (OR) TYPES OF CARBON NANOTUBES Carbon nanotubes are lattice of carbon atoms, in which each carbon is covalently bonded to three other carbon atoms. Depending upon the way in which graphite sheets are rolled, two types of CNTs are formed. 1. Single - walled nanotubes (SWNTs). 2. Multi – Walled nanotubes (MWNTs) 1. Single - walled nanotubes (SWNTs) SWNTs consist of one tube of graphite. It is one-atom thick having a diameter of 2 nm and a length of 100 µm. SWNTs are very important, because they exhibit important electrical properties. It is an excellent conductor. Three kinds of nanotubes are resulted, based on the orientation of the hexagon lattice. 53
(a) Arm-chair structures: The lines of hexagons are parallel to the axis of the nanotube. (b) Zig-zag structures:The lines of carbon bonds are down the centre. (c) Chiral nanotubes: It exhibits twist or spiral around the nanotubes. It has been confirmed that arm-chair carbon nanotubes are metallic while zig-zag and chiral nanotubes are semiconducting. 2. Multi - walled nanotubes (MWNTs) MWNTs (nested nanotubes) consist of multiple layers of graphite rolled in on themselves to form a tube shape. It exhibits both metallic and semiconducting properties. It is used for storing fuels such as hydrogen and methane. SYNTHESIS OF CARBON NANOTUBES Carbon nanotubes can be synthesized by any one of the following methods. 1. Pyrolysis of hydrocarbons. 2. Laser evaporation. 3. Carbon arc method. 4. Chemical vapour deposition. 1. Pyrolysis Carbon nanotubes are synthesized by the pyrolysis of hydrocarbons such as acetylene at about 7000C in the presence of Fe-silica or Fe-graphite catalyst under inert conditions. 2. Laser evaporation It involves vapourization of graphite target, containing small amount of cobalt and nickel, by exposing it to an intense pulsed laser beam at higher temperature (12000C) in a quartz tube reactor. An inert gas such as argon is simultaneously allowed to pass into the reactor to sweep the evaporated carbon atoms from the furnace to the colder copper collector, on which they condense as carbon nanotubes. 3. Carbon arc method It is carried out by applying direct current (60 - 100 A and 20 - 25 V) arc between graphite electrodes of 10 - 20 µm diameter. 4. Chemical vapour deposition It involves decomposition of vapour of hydrocarbons such as methane, acetylene, ethylene, etc., at high temperatures (11000C) in presence of metal nanoparticle catalysts like nickel, cobalt, iron supported on MgO or Al2O3. Carbon atoms produced by the decomposition condense on a cooler surface of the catalyst. Properties of CNTs 1. CNTs are very strong, withstand extreme strain in tension and posses elastic flexibility. 2. The atoms in a nano-tube are continuously vibrating back and forth. 3. It is highly conducting and behaves like metallic or semiconducting materials. 4. It has very high thermal conductivity and kinetic properties. 54
Uses of CNTs 1. It is used in battery technology and in industries as catalyst. 2. It is also used as light weight shielding materials for protecting electronic equipment. 3. CNTs are used effectively inside the body for drug delivery. 4. It is used in composites, ICs.
12. (a) Discuss various types of synthesis involved in the preparation of nano material (Ant two). (June 2014, Jan 2014) (i) Laser ablation (June 2016, Jan 2016, May 2015, Dec 2014) (ii) Thermolysis (June 2016) (iii) CVD (June 2016, Dec 2015, May 2015, Dec 2014, Jan 2014) (iv) Hydrothermal (Jan 2016) (v) Electrodeposition (Jan 2016) (vi) Precipitation (Dec 2015) (vii) Solvothermal (May 2015)
SYNTHESIS OF NANO MATERIALS (i) SYNTHESIS OF CNT BY LASER ABLATION
Laser ablation chamber The laser ablation apparatus consists of: a furnace, a quartz tube with a window, a graphite target doped with small amount of catalytic metals like cobalt and nickel, flow systems for argon gas to maintain constant pressure and flow rates and a water cooled copper collector placed somewhat outside the furnace. In this technique, a high power pulsed laser beam of Nd:YAG (Neodymium-doped-yttrium aluminium garnet) is introduced through the window of quartz tube. It is used to vaporize carbon from a graphite target, located at the centre of the furnace maintained at a high temperature of 1200oC. The laser vapourises graphite and produces carbon molecules and atoms. Inert gas such as argon is simultaneously passed into the reactor to carry the vapourised carbon atoms from the high temperature zone to the colder copper collector. The ablated species condense on the walls of the flow tube placed opposite to the target and the carbon nanotubes will self-assemble from the carbon vapours. 55
Advantage: •
It is easy to operate.
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Eco-friendly method because no solvent is used.
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The product obtained is stable.
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This process is economical.
Disadvantage: The method is more expensive than the CVD method. Uses: •
Nanotubes having a diameter of 10 to 20 nm and 100µm can be produced.
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Other materials like silicon, carbon can also be converted into nanoparticles.
(ii) SYNTHESIS OF NANO CLUSTERS BY THERMOLYSIS METHOD This method operates at high temperatures and is based on thermal decomposition of metal precursors together with a stabilizing agent to yield nano materials.
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In thermolysis, an inorganic material is vapourised inside a vacuum chamber into which an inert gas (Ar or He) is periodically admitted.
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The source of vapour can be an evaporation boat with a sputtering target or a laser ablation target. Once the atoms boil off, they lose their energy quickly by colliding with inert gas molecules.
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Now the vapour cools rapidly and supersaturates to form nanoclusters with size range 2-100nm that collect on a finger cooled by nitrogen.
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The nanocluster that forms on the cold finger are harvested by the scraper and collected.
Synthesis of lithium metal nanoclusters: •
Lithium metal nanoclusters can be made by decomposing lithium azide, LiN3.
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The material is placed in an evacuated quartz tube and heated to 400oC.
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At about 370oC the LiN3 decomposes, releasing N2 gas, which is observed by an increase in the pressure on the vacuum gauge. 56
2Li3N •
6Li + N2
In a few minutes the pressure drops back to its original low value, indicating that all the N2 has been removed.
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The remaining lithium atoms coalesce to form small colloidal metal particles.
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Passivation can be achieved by introducing an appropriate gas.
Disadvantages: •
Results in limited controllability of particle morphology and size. Hence magnetic properties will vary and not be consistent.
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High operating temperature and the need of expensive precursors.
(iii) SYNTHESIS OF NANO MATERIALS BY CHEMICAL VAPOUR DEPOSITION (CVD) CVD is a process in which the reactants are transported towards the substrates, where they undergo certain chemical reactions to form products, which diffuse into the substrate, nucleate and grow as films, coatings, wires or tubes. THERMAL CVD REACTOR
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It consists of a quartz tube enclosed in a tubular furnace.
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The process uses methane, ethylene or acetylene gas as the carbon source and substrate like silica, mica, quartz or alumina coated with Fe, Co or Ni nanoparticles as the catalyst.
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The hydrocarbon gas flows through the quartz tube being in a furnace at a high temperature (~720oC).
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The dissociation of the gas occurs at the hot catalyst surface. The precipitation of carbon from the saturated metal particle leads to formation of tubular carbon structures in the form of nanotubes or nanofibers.
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The diameter of the growing tube is determined by the size of nanoparticles, which work as catalyst.
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This method produces CNTs with open ends and allows continuous fabrication.
Advantages: •
Low power input, lower temperature range and relatively high purity. 57
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Nano tubes grow directly on a desired substrate; in other methods they have to be collected.
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It is economical.
(iv) SYNTHESIS OF NANO PARTICLES BY HYDROTHERMAL METHOD This is a wet chemical procedure for manufacturing nano particles. Hydrothermal synthesis also called thermal hydrolysis or hydrothermal hydrolysis. This method refers to processing aqueous solution of metal salts on autoclaving of precursor material at elevated temperatures (typically 100 -300oC) and pressure above 1 atm. It involves crystallization of substances from high temperature aqueous solutions at high vapour pressure. Hydrothermal synthesis is usually performed below the super critical temperature of water (3740C).
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In this process, water is used as the reaction medium. Water is mixed with metal precursor and the solution mixture is placed in an autoclave and maintained at relatively high temperatures and pressures to carry out the growth of nanoparticles.
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At these super critical conditions:
Solubility of most ionic species increases. Viscosity of the water decreases and exhibits greater mobility. •
Size and morphological control is achieved by controlling the conditions of time and temperature.
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The reactant conditions of precursor material and pH dictate the phase purity of nanoparticles.
Advantages: •
Easy to control the size and shape distribution.
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Most materials can be made soluble in proper solvent by heating and pressuring the system close to its critical point.
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Time and energy required is less.
Disadvantages: •
Expensive autoclaves are required.
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Safety issues during the reaction.
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Could not monitor and observe the reaction. 58
(v) SYNTHESIS OF NANO WIRES BY ELECTRO DEPOSITION METHOD •
It is an electrochemical method in which ions from an aqueous electrolyte solution are deposited at the surface of the cathode with the help of applied voltage.
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Template assisted electro-deposition is an important technique for synthesizing metallic nano materials with controlled shape and size.
• • •
The method consists of an electrochemical cell.
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An active template act as a cathode of the cell. The substrate on which electro deposition of the nano structure takes place. It can be made of either non-metallic or metallic materials
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This is brought in contact with aqueous solution containing metal cations to be deposited.
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The anode is placed in the deposition solution parallel to the cathode.
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When electric potential is applied, cations diffuse into the pores and reduce at the cathode, resulting in the growth of nanowires inside the pores of the template.
Electrodeposition of gold on silver: •
Anode: Gold sheet
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Cathode: Silver plate
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Electrolyte: AuCl3
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Alumina template is kept over the cathode
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When current is applied through the electrodes, Au+ ions diffuse into the pores of the template and get reduced at the cathode.
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Resulting in the growth of nanowires or nanorods inside the pores of the templates.
Advantages: •
It is a simple and inexpensive method.
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Complex shaped objects can be coated. 59
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Can control the size and shape of the deposit by adjusting deposition conditions (current density and electrolyte composition)
Uses: •
The film or wire obtained is uniform.
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Metals nanowires including Ni, Co, Cu and Au can be fabricated by this method.
(vi) SYNTHESIS OF NANO PARTICLES BY CHEMICAL PRECIPITATION METHOD It is one of the oldest and simple techniques. In this technique, the metal precursors are dissolved in a suitable solvent and on adding a precipitating agent, insoluble product of nano particles are formed. Metallic, metal oxide and non-oxide nano particles can be produced by this method. EXAMPLES: (i) Precipitation of BaSO4 Nano-particles 10 gm of sodium hexameta-phosphate (stabilizing agent) was dissolved in 80 ml of distilled water in 250 ml beaker with constant stirring. Then 10 ml of 1M sodium sulphate solution was added followed by 10 ml of 1M Ba(NO3)2 solution. The resulting solution was stirred for 1 hr. Precipitation occurs slowly. The resulting precipitate was then centrifuged, washed with distilled water and vacuum dried.
In the absence of stabilizing agent, bulk BaSO4 is obtained. (ii) Precipitation by reduction Reduction of metal salt to the corresponding metal atoms. These atoms act as nucleation centres leading to formation of atomic clusters. These clusters are surrounded by stabilizing molecule that prevents the atoms agglomerating. Example: Nanoparticles of molybdenum can be produced from MoCl2 in toluene solution using NaBH(C2H5)3 as a reducing agent at room temperature.
(vii)
SOLVOTHERMAL SYNTHESIS
Solvothermal synthesis involves the use of solvent under high temperature (between 1000C to 10000C) and moderate to high pressure (1 atm to 10,000 atm) that facilitate the interaction of precursors during synthesis.
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Method A solvent like ethanol, methanol, 2-propanol is mixed with certain metal precursors and the solution mixture is placed in an autoclave kept at relatively high temperature and pressure in an oven to carry out the crystal growth. The pressure generated in the vessel, due to the solvent vapour, elevates the boiling point of the solvent.
Solvothermal synthesis Examples for solvent: Ethanol, methanol, toluene, cyclohexane, etc. Solvothermal synthesis of zinc oxide Zinc acetate dihydrate is dissolved in 2-propanol at 500C. Subsequently, the solution is cooled to 00C and NaOH is added to precipitate ZnO. The solution is then heated to 650C to allow ZnO growth for some period of time before a capping agent (1-dodecanethiol) is injected into the suspension to arrest the growth. The rod shaped ZnO nano-crystal is obtained. Uses 1. Many geometries including thin film, bulk powder and single crystals can be prepared. 2. Thermodynamically stable novel materials can also be prepared easily.
(b) Compare the properties of molecules, nano particles and bulk materials. (May 2015, Jan 2014)
DISTINCTION BETWEEN NANO PARTICLES, MOLECULES AND BULK MATERIALS: The size of nano particles are less than 100 nm in diameter, molecules are in the range of picometers, but bulk materials are larger in micron size. Molecule is a collection of atoms, Nanoparticles a collection of few molecules ie less than hundred nanometre but bulk materials contains thousands of molecules. Surface area of nano particles is more than the bulk materials. Hardness of nano materials is 5 times more than the bulk materials. Strength of nano materials is 3 - 10 times higher than the bulk materials. Nano particles possess size dependent properties, but bulk materials possess constant physical properties. 61
Corrosion resistance is more than the bulk materials; hence localised corrosion in nano materials is stopped. Behavior of bulk materials can be changed, but cannot enter inside the nano particles. Nano particles, due to its size, possess unexpected optical (visible) properties. EXAMPLES: I.
Gold nano particles appear deep red to black colour in solution compared to yellow colour with Gold.
II. III.
ZnO nano particles possess superior UV blocking property compared to bulk material. Absorption of solar radiation in photovoltaic cell containing nano particles is higher than the film (bulk material). Nano particles possesses lower melting point than the bulk materials.
EXAMPLES: Gold nanoparticles melt at lower temperature (3000C) for 2.5 nm but Gold slab melts at 1064OC.
Comparison of atom/molecule, nano particles/cluster, bulk materials
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