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Sakshi Bhavita Online Edition www.sakshieducation.com/bhavitha.aspx

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The students must focus on the following concepts ◆

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The chemical reactions that proceed with moderate rate can be studied by using chemical kinetics. Ionic reactions cannot be studied by Chemical Kinetics since they are very fast. Rate of reaction: Change in the concentration of reactant in unit time or change in the concentration of product in unit time. Rate of reaction can be classified

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R −R

Intermediate Chemistry

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rate of reaction. =

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dR → dt

Rate

P − P Δ[P] = 2 1= → t 2 − t1 Δt

average

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d[P] → instantaneous rate dt

This is one area where questions can be asked in the competitive exams. Therefore students are suggested to focus on this concept and solve as many as questions possible. Rate of appearance of C = 2 × disappearance of A (or) B.

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1 d[C] d[A] d[B] =− =− 2 dt dt dt Ex : N 2 + 3H2 � 2NH3 , 1 d[NH 3 ] d[H 2 ] d[N2 ] =− =− 2 dt 3dt dt H 2 + I2 � 2HI, 1 d[HI] d[H 2 ] d[I ] =− =− 2 2 dt dt dt ◆

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Law of Mass Action: According to this law, the rate of the reaction is directly proportional to the product of concentration of the reactants at that instant. Rate law: It explains the mathematical dependence of rate of reaction on the concentration terms of reactants. It is derived on the basis of experimental results: mA + nB → Product. According to law of mass action, rate ∝ [A]x [B]y rate = k[A]x [B]y, k = specific rate constant. x + y → order of the reaction, x + y may or may not be equal to m + n. Order of the reaction is always derived on the basis of experimental results. Order can be 0 or fraction or integer and can also be negative.

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rate c 1 = × 2 = c − 1 t −1 , 2 [A] t c

ER

rate c 1 = × 3 = c −2 t −1 3 [A] t c

For 0 order reaction, the units are as same as that of rate of the reaction. Zero order reaction: If the rate of the reaction is independent of the concentration of the reactant then the order of the reaction is zero w.r.t. the reactant. The problems on finding out the order of the reaction are very frequently asked in all forms of exams. So, students are advised to solve as many problems as possible in this area. Example: NCERT Problem: The following results have been obtained during the kinetic studies of the reaction: 2A + B → C + D

Ex..t [A]/ [B]/ Initial rate of mol L–1 mol L–1 formation of D/mol L–1 min–1 I 0.1 0.1 6.0×10–3 II 0.3 0.2 7.2×10–2 III 0.3 0.4 2.88×10–1 IV 0.4 0.1 2.40×10–2

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[B]2.

∴ rate = k The relation between the concen-

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[A]0 2.303 log t [A]

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Slope=k/2.303

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time

0.693 t1/2

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The relation between half-life and order of the relation is given

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order of the reaction. Therefore in case of first order reaction, half-life is the independent of the initial concentration. In case of 0 order reactions halflife is directly proportional to initial concentration. Now-a-days problems based on time taken for completion of reaction with respect to half-life period are asked in all forms of the exams. Therefore students are advised to prepare the ratios in the following way. t 75% 2.303 100 k 1 = log × t 50% k 25 2.303 log 100 50

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Intercept = lnA

For a reversible reaction, the relation between rate constants and temperature and ΔH is given by: log

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k2 ΔH  1 1  =  −  k1 2.303R  T1 T2 

Students must note that as the temperature increases rate of the reaction increases. Now-a-days questions are also asked on temperature coefficient concepts. For every 10°C rise in temperature the rate of the reaction is increased by 2 or 3 times. K t °C + 10°C = 2 or 3 K t °C

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[A]

n 0 given by [A] = 2 , where n =

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Slope = –E a/RT

1/T

The relation between initial concentration and final concentration and number of half-life is

number of half-lives. Theoretical questions are asked on molecularity of a reaction. Students must remember molec-

k2 Ea  1 1  =  −  k1 2.303R  T1 T2 

ln K

t75% = 2t50%

t 87.5% t = 3, 99.9% = 10 t 50% t 50% t 90% t = 3.33, 99.99% = 13.33 t 50% t 50% t 99% t 3 t = 2, 99.9% = , 99.9% = 3 t 90% t 99% 2 t 90%

Students must remember the following points: Normal collisions do not lead to the formation of the products. Only active collisions lead to the formation of the products. The difference between the threshold energy and energy of the normal molecules is activation energy. Rate of the reaction is inversely proportional to activation energy. The relation between rate constant k and activation energy and temperature is given by : k = A.e–Ea/RT The rate constants at two different temperatures is given by: log

1 ∝ t by 1/2 [A]n −1 , where n is the

y log2 = log 4 y 0.3010 = + log 4 y = +2 [A]1

time

As the first order reactions never go to the completion, it would be appropriate to calculate half life. k=

7.2 × 10 −2 [0.2] y sol. = 2.88 × 10 −1 [0.4] y

6 × 10 −3 (0.1) x 1 1 = ⇒ = ⇒ x= 1 2.4 × 10 −2 (0.4) x 4 4x

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time

Determine the rate law and the rate constant for the reaction.

72 1 288 = y ⇒ 2y = =4 288 2 72

k= –slope

k= –slope

ΔH EP

[R0]

ln[R0]

TE Ea

For first order reaction: k=

where c = mol / litre. For 3rd order reaction: 3A → B k=

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rate c 1 1 −1 = × = =t [A]1 t c t

For 2nd order reaction: 2A → B k=

rate. =

Rate constant for zero order reaction: [R]0 − [R] k= t

Units of rate constant For 1st order reaction: A → B, Rate = k[A]1 k=

instantaneous rate of

reaction. At time t1 the concentration of product is P1 and at t2 it is P2.

PE

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Δ[R]

2 1 Rate = t − t = Δt → average 2 1

time

time

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ularity cannot be zero or fractional and cannot exceed 3. The relation between the temperature and rate of the reaction:

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Chemical Kinetics is the one of the important topic in which the questions are very frequently asked in Competitive exams like JEE (MAINS & ADV.), EAMCET etc. Chemical Kinetics is the branch of Chemistry that explains the speed with which reactants are connected into product. Student must focus on the contents given in the syllabus for Competitive exams. Syllabus for JEE (Mains) Rate of a chemical reaction, factors affecting the rate of reactions: concentration, temperature, pressure and catalyst; elementary and complex reactions, order and molecularity of reactions, rate law, rate constant and its units, differential and integral forms of zero and first order reactions, their characteristics and half - lives, effect of temperature on rate of reactions - Arrhenius theory, activation energy and its calculation, collision theory of bimolecular gaseous reactions (no derivation). Syllabus for JEE (Adv.) Rates of chemical reactions, Order of reactions, Rate constant, First order reactions, Temperature dependence of rate constant (Arrhenius equation). In EAMCET, they are mainly concentrating on State Board Syllabus. The students must follow NCERT book thoroughly to get more marks in IPE and competitive exams. The advantage of well preparation in this chapter is very much helpful in understanding Nuclear Chemistry in better way. Because all the Nuclear radioactive disintegration reactions follow the first order kinetics.

tration and time can be represented in the following graphs:

Log([R]0/[R])

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into two types. a) Average rate of reaction b) Instantaneous rate of reaction. If the rate of reaction is calculated in large intervals then it is called average rate of reaction. If the rate of reaction is calculated in infinitesimal time intervals. At time t1 the concentration of reactant is R1. At time t2 the concentration of reactant is R2.

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Chemical Kinetics (Inter 2nd year)

Must focus on Chemical Kinetics

conc.of R

Subject Expert

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ln [R]

P.Vijay Kishore

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Sol:

For example: [JEE Mains - 2013] The rate of a reaction doubles when its temperature changes from 300K to 310K.Activation energy of such a reaction will be: (R = 8.314 JK–1 mol–1 and log 2 = 0.301) a) 48.6 kJ mol–1 b) 58.5 kJ mol–1 c) 60.5 kJ mol–1 d) 53.6 kJ mol–1 2.303log 2 =

Ea  1 1  − 8.314  310 300 

Ea = 53.6 kJ mol–1.