(d) Vitamin C is water soluble, so it is easily excreted from the body and does not accumulate in humans. Humans are naturally adapted to handle fairly large amounts of this vitamin. Omnivores often ingest significant amounts of it from fruits and vegetables in their diets. Vitamin E is not water soluble and cannot be excreted readily. It tends to accumulate quickly to dangerous (toxic) levels if there is too much in the diet. (Large carnivores such as polar bears and lions can have so much vitamin E stored in their livers that eating the organ can be fatal to humans.) 15. Plastic cling wrap is made with a significant amount of a softening material, called a plasticizer, added to the polyvinyl chloride polymer. This causes the film to be very soft and flexible; consequently, it moulds well to any smooth surface (including itself), and the closeness of contact combined with large surface area makes the London force quite significant—the plastic wrap is “clingy.” (It is also likely that this plastic easily acquires an electrostatic charge that helps it cling to itself and nonmetallic objects.) Some plasticizers, particularly di-(2-ethylhexyl) adipate (DEHA), have come under fire because of suspicions that they may act as endocrine disruptors, with possible long-term harmful effects on the body. These compounds can be dissolved out of the wrap if the wrapped food contains fats and cheese would be a primary example of this. It should be noted that plasticizer molecules are liquid and nonpolar, so London forces will make them soluble in nonpolar fats.

3.5 STRUCTURES AND PHYSICAL PROPERTIES OF SOLIDS Mini Investigation: Building an Ionic Crystal Model (Page 122) 1. (a) Na+ ions are nearest to each chloride ion. There are six sodium ions near each chloride ion that is inside the crystal lattice structure. 2. (b) Cl– ions are nearest to each sodium ion. There are six chloride ions near each sodium ion that is inside the crystal lattice structure. 3. (c) Ion charges are not written because in any ionic crystal, the number of positive ions is balanced by the number of negative ions in a specific ratio, so the overall charge on the crystal is always zero. 4. (d) “n” represents the total number of sodium ions and of chloride ions in the crystal lattice. 5. (e) This numbering system is not used because the sizes of crystals vary, and therefore the total number of ions in each crystal varies. We are only interested in the simplest wholenumber ratio of oppositely charged ions in any crystal. 6. (f) The subscript numbers in an ionic compound formula refer only to the ratio in which oppositely charged ions combine to form a neutral formula unit in the crystal. 7. (g) Alternating positive and negative ions maximizes attraction and minimizes repulsion. This arrangement usually forms a cubic or other three-dimensional polygon crystal structure. Practice (Pages 122–123) 1. (a) (b) (c)

70

Unit 1 Solutions Manual

Copyright © 2007 Thomson Nelson

(d)

(e)

(f) [Mg]2+ 2. (a) HCl(g) + NH3(g) o NH4Cl(s) (b)

3. Soluble ionic compounds are found in salt water and fresh water. Ionic compounds with low solubility make up most rocks and minerals. 4. Nonmetals have much higher electronegativities than metals. When a metal and a nonmetal react, the valence electrons of the metal are removed or pulled away by the nonmetal, forming positive and negative ions with stable octets. 5. The hardness of some ionic compounds (and their high melting and boiling points) suggests that ionic bonds are strong. All are solids at SATP. 6. 7. calcium oxide 8. 9. 10. Based only on differences in electronegativity, francium fluoride (FrF) would be expected to be the most strongly ionic of all binary compounds. [Note: this is a purely theoretical answer; since francium is so radioactive that no quantity great enough to be a measurable mass has ever been prepared.] 11. The chemical formula of an ionic compound indicates only the simplest ratio of ions in the ionic crystal. The chemical formula of a molecular compound indicates the actual number of atoms in a molecule of the substance. Web Activity: Canadian Achievers—Jillian Buriak (Page 123) 1. Nanotechnology is the development and use of devices that have a size of only a few nanometres. 2. In 2005, Buriak won the Royal Society of Canada Rutherford Memorial Medal in Chemistry. 3. Five recent projects of Buriak’s research group are synthesis of metal nanoparticle arrays of semiconductor surfaces, synthesis of complex nanostructured metals, patterning of organic monolayers on surfaces of semiconductors, homogeneous catalyst design and synthesis, and hybrid nanoscale inorganic-organic structures.

Copyright © 2007 Thomson Nelson

Unit 1 Solutions Manual

71

Practice (Page 128) 12. The main factors that determine the hardness of a solid are the strength and direction of bonds between its entities. 13. (a) covalent bonding covalent network crystal (b) ionic bonding ionic crystal (c) covalent bonding molecular crystal (d) covalent bonding covalent network crystal (e) metallic bonding metallic crystal (f) ionic bonding ionic crystal 14. The melting point of a solid is proportional to the attractive forces between the entities of the solid. Strong bonds, such as covalent bonds, will result in very high melting points, whereas much weaker bonds such as London forces will result in lower melting points. 15. Metals are generally malleable, ductile, and flexible because the bonding between atoms in metals is nondirectional, so changing the position of the atoms (shape of the solid) does not “break” the bonding. 16. (a) Aluminium is a light, relatively soft, flexible, silvery metal solid, with a fairly low melting point for metals. Aluminium oxide is a very hard, rigid crystalline solid with an extremely high melting point. In aluminium, the bonding is metallic, with a lower-thanaverage strength for metals. In oxide, very strong ionic bonds lock the aluminium and oxide ions into a rigid three-dimensional ionic crystal lattice. (b) Carbon dioxide is a soft molecular solid with a very low boiling point. Silicon carbide is a very hard network crystal solid with an extremely high boiling point. In carbon dioxide, the molecules are held together by relatively weak nondirectional London forces only. In silicon carbide, the silicon and carbon atoms are locked in a three-dimensional network by very strong covalent bonds. 17. (a) The blade must be oriented in the same direction as the plane of the atoms (or ions) in the crystal because the crystal can only break along these “cleavage” planes. (b) To cleave a sodium chloride crystal, the knife blade should be perpendicular to a crystal face because the crystal is a cube and planes of ions are at 90° to each other. (c) The crystal will shatter into small pieces if struck incorrectly. (d) Diamonds and other precious stones are cut into smaller gemstones by this technique before they are polished. 18. (a) high melting point, conduct electricity vanadium, V(s) (b) low melting point, soft phosphorus pentoxide, P2O5(s) (c) high melting point, soluble in water sodium bromide, NaBr(s) (d) very high melting point, nonconductor silicon dioxide, SiO2(s) 19. (a) It seems logical to assume that conductivity of electric current and of heat are related. A hypothesis would be that both electrical conductivity and heat conductivity depend on the “sea” of electrons surrounding the metal atoms. (b) Thick wires of equal diameter and length, made of several common metals, are tested for electrical conductivity with an electrical multimeter. The same wires are tested for heat conduction by heating one end and recording the time for the other end to reach a specified temperature. The manipulated variable is the type of metal; the responding variables are the electrical conductivity and time to heat; and the fixed variables are length, diameter, initial temperature, final temperature, and heat source. (A common laboratory device has spokes of different metals with a concave end to attach a blob of wax. The relative heat conductivity can be determined by noting the order or time for the wax to fall off each metal.)

72

Unit 1 Solutions Manual

Copyright © 2007 Thomson Nelson

20. Graphite may be better than oil in lubricating moving parts of a machine because it is a powdered solid—a dry lubricant that will not stick to dirt, or cake or build up on moving parts. Graphite may also be more stable at higher temperatures than an oil. 21. Flint is a covalent network solid, a substance in which silicon and oxygen atoms are combined in a 2:1 ratio and are covalently bonded to each other alternately. It is a very hard and brittle substance that will break under pressure. 22. Obtain samples of each of the compounds and determine the melting point of each. The melting point of each compound varies directly with its molar mass. The order of compounds, from lowest to highest melting point, is NaF, NaCl, NaBr, and NaI. Investigation 3.5: Classifying Unknown Solids (Pages 129, 134) Purpose The purpose of this investigation is to use empirical definitions to classify solid substances. Problem To what class of solids do the four mystery solids belong? Design The class of each solid is identified by observing the appearance, electrical conductivity, solubility in water, electrical conductivity in water, and effect of heating (relative melting point). Materials Ɣ lab apron Ɣ eye protection Ɣ conductivity meter Ɣ hot plate Ɣ four 50 mL beakers Ɣ pure water bottle Ɣ four unknown solids Procedure 1. Observe the appearance of each solid. 2. Test a crystal of each solid for electrical conductivity. 3. Place a crystal of each solid on a hotplate at a low heat setting. Observe the solids as the heat is slowly increased. 4. Test the electrical conductivity of pure water in a clean, dry beaker. 5. Place a few crystals of each solid in separate beakers and add about 10 mL of water to each. Stir to dissolve as much as possible. 6. Test the electrical conductivity of each mixture. 7. Dispose of solids in the waste basket and liquids in the sink. Evidence Properties of the Mystery Solids Solids Appearance

1 clear, colourless

Conductivity of solid Effect of heating Solubility in water Conductivity in water

Copyright © 2007 Thomson Nelson

3 silvery grey

4 clear, colourless

none no change

2 clear, colourless, some white patches none no change

good none

did not dissolve none

dissolved good

did not dissolve none

none melted quickly and turned black dissolved none

Unit 1 Solutions Manual

73

Analysis According to the evidence collected, solid 1 is network covalent, 2 is ionic, 3 is metallic, and 4 is molecular. Evaluation Most of the evidence was sufficient to classify the majority of the solids. The classification of the network covalent solid fits with the properties of network covalent solids but was done mainly by elimination once the others were classified. This classification is very uncertain and it is possible that solid 1 may be a low-solubility ionic solid. The classification of solids 2, 3, and 4 seems relatively certain. Other properties, such as hardness and melting points, would help to make the classification more certain. The purpose was achieved in this investigation. Section 3.5 Questions (Pages 129–130) 1. Ionic substances do not conduct while in solid state because all ions are locked in position. When melted, the ions are free to move and the substance conducts freely. In aqueous solution, the ions are also free to move and the solution will conduct to varying degrees, depending on concentration. 2. A substance conducts electricity when its entities have a charge and are free to move, meaning that the particles must be held by weak forces. 3. In calcium oxide, the ions have double the charge that the ions in sodium chloride have, creating a significantly greater interionic attraction. 4. In solid carbon dioxide, the bonding consists of London forces between molecules. These relatively weak forces result in very low melting and boiling points, and make the solid a soft substance. By contrast, in silicon dioxide, the bonding consists of a continuous network of covalent bonds between atoms. These very strong forces result in very high melting and boiling points, and make the solid a very hard, brittle substance. 5. Most metals have a relatively high density because their atoms are closely packed together in solid form and held together strongly by mobile valence electrons that are dispersed throughout a structure of positive ions. 6. Covalent network structures have the highest hardness and also the highest melting and boiling points, indicating that covalent bonds are the strongest bonds. Molecular structures have the lowest values for these properties, indicating that intermolecular bonding forces are the weakest. Metals and ionic compound properties fall between these first two categories, but the values for both metals and ionic compounds vary widely, depending on the specific substance. 7. Rubbing your zipper with your pencil will coat it with graphite, which will act as a lubricant. Graphite crystals form in layers one atom thick, held to each other only by very weak London forces, so they slide very easily over each other. 8. Diamond is composed of carbon atoms bonded four times each in a three-dimensional covalent network. It is a colourless solid, extremely hard, and a nonconductor. Its main use is as an abrasive; pure crystals are used as gemstones. Graphite is composed of carbon atoms bonded three times each in a two-dimensional covalent network, resulting in layers one atom thick, which are held to each other by London forces. It is a grey-black solid, very soft and slippery, and a good electrical conductor because of the mobility of the delocalized, unbonded fourth electron of each carbon atom. It has a myriad of uses in industry, most of which have to do with its high melting point and lubricant properties.

74

Unit 1 Solutions Manual

Copyright © 2007 Thomson Nelson

9. XCla must be an ionic compound. The high melting point suggests that it has very strong bonds holding the entities together, but is water soluble, so it is not likely to be a covalent network crystal. YClb must be a molecular compound. The melting and boiling points are low, indicating that London forces hold the entities together. The solubility indicates that the molecules are nonpolar. 10. Molecular research in the medical and pharmaceutical fields has produced materials such as human insulin for diabetics, and diagnostic devices such as MRI scanners. The plastics industry has produced new contact lens materials as well as many new fabrics, containers, and construction materials. The electronics industry has created new products such as rechargeable NiMH batteries, fuel cells, and LEDs for traffic lights. 11. Clay is a term that refers to a general class of minerals produced by long-term weathering of igneous rock into very tiny particles, usually less than a few micrometres in size. The predominant compound present (of a very complex mixture)—in the clays that can be heated to produce pottery—is normally kaolinite, Al2Si2O5(OH)4(s). In general, all clays may be thought of as hydrated aluminium silicates with varying amounts of other atoms, such as K, Mg, Fe, and Ca included in the crystal structure of the mineral. All clays contain at least some tiny particles of SiO2 and Al2O3. Ceramic is a term that refers to any manufactured materials that have essentially a network crystalline structure. This includes abrasives, porcelain, china, refractories (heatshielding materials), structural clay products (brick and pottery), electrical ceramics (for electronics), and glasses. Glasses account for nearly half of all ceramic production. Clay, when wet, is a soft, slippery plastic material. It is easily moulded into shapes, because the tiny particles are attracted to each other by low-strength London forces and hydrogen bonds. By heating these silicaceous minerals at very high temperatures, new bonds form to change the material into a very hard, brittle network solid. The networks are mostly covalent, but also somewhat ionic, and crystalline in character. The bonding structure is extremely complex, but may be thought of as a tetrahedral silicon–oxygen network modified with many substituted atoms and ions, all held together within an overall glass structure. All glasses have a crystal structure that is unique: the arrangement of atoms in glasses is like the arrangement in liquids because there is no regular repetitive pattern to the structure. (Students should also supply an illustration.) 12. Boron nitride, BN(s), is a network covalent compound with unique properties. It has very high melting and boiling points, conducts heat well, and is a soft, slippery solid like graphite. Unlike graphite, however, boron nitride is a nonconductor of electricity. It is used as an additive to plastics, ceramic mixes, and lubricants, where it adds lubricating and thermal transmission properties and, to ceramics, increased strength. It can be used as a dry lubricant in powdered form. The network structure is planar hexagonal sheets, much like graphite, except that the hexagon corners are alternating B and N atoms. As in graphite, the atoms within a single crystal sheet of boron nitride are strongly bonded with covalent bonds, while each sheet is attracted to the next only by London forces. Extension 13. Composites provide significant weight reduction in aircraft structure due to their high strength-to-weight ratios. Weight reductions in the order of 20% have been achieved by replacing aluminium with graphite–epoxy composites, thereby reducing fuel consumption and direct operating costs. 14. (a) Each hemoglobin molecule can transport four oxygen molecules. (b) Oxygen binds to the Fe2+ ion in the haemoglobin molecule (the “heme” group). Carbon dioxide binds at the alpha-amino group of the molecule.

Copyright © 2007 Thomson Nelson

Unit 1 Solutions Manual

75

(c) The O2–Fe bond can be readily broken, depending on whether oxygen needs to be picked up or released to tissue cells. The ability of hemoglobin to bind to oxygen depends on the partial pressure of O2 in the surrounding air or water, and on temperature, pH, and CO2 levels. The affinity of hemoglobin for oxygen decreases if temperature increases, pH decreases, or CO2 levels increase. Make a Summary (Page 136) 1. Table 1: Forces Acting Between Entities Force or bond covalent covalent network dipole–dipole hydrogen ionic London metallic

Central entity central atom nonmetal or semi-metal atom positive end of one polar molecule hydrogen atom nucleus (proton) bonded to a F, N, or O atom of one molecule cation nucleus of atom in one molecule cation of a metal

Surrounding entities surrounding atoms all surrounding nonmetal atoms negative end of another polar molecule pair of electrons on F, O, or N atom on another molecule anion electrons in surrounding molecules electrons of surrounding cations in same metal

2. Table 2: Summary of Properties of Substances Substance molecular ionic covalent network metallic

Hardness

Melting point

low high high

low high high

Solid none none none

variable

high

high

Electrical conductivity Liquid Solution none none high high none none high

—

3. 1. When atoms collide, bonding electrons may be shared if their electronegativities are equal or relatively close, or bonding electrons may be transferred if their electronegativities are significantly different. 2. The shape of a molecule affects physical properties such as melting point and boiling point. This effect is primarily due to the effect of shape on intermolecular forces. The shape of a molecule would also affect how the molecule reacts. We know that the shape of protein molecules (e.g., enzymes) directly impacts the function of these proteins. If there is any change in shape of a protein molecule, the protein molecule will no longer function properly. 3. The bonding between entities of a substance largely controls the physical and chemical properties of that substance. Different entities form different types of intramolecular and intermolecular bonds. For example, sodium and chlorine combine to form sodium chloride held together by ionic bonding. A group of sodium chloride ions will form a lattice structure that provides additional strength to the molecules of sodium chloride. This lattice structure helps explain why sodium chloride has a high melting point. 4. For C2H5OH(l) and CH3OCH3(l), the differences in physical properties can be explained by intermolecular forces. These molecules are isoelectronic (same London forces) and both are polar. However, C2H5OH(l) will have hydrogen bonding forces between its molecules, significantly affecting various physical properties. Both SiO2(s) and CO2(s) appear to be molecular, but SiO2(s) is a covalent network solid with a 3-D arrangement of covalent bonds within the sample. This covalent bonding throughout the sample greatly affects the physical properties. On the other hand,

76

Unit 1 Solutions Manual

Copyright © 2007 Thomson Nelson