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Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph.011-47623456 Chapter Contents Chemical kinetics deals with the rate of the reactions and factors influencing the rate of a reaction. Gibb's free energy concerns with the spontaniety of the reaction whereas the kinetics will deal with the speed of the reaction. The rate of reaction are affected by light and also by electric and magnetic fields. RATE OF CHEMICAL REACTION C1 C2 t1 t2 t slope dt ]R[d rinst –= – = time Concentration [R]0 time t [P] (a) (b) (t – t ) 2 1 t ]R[ rav =  –  = C1 C2 (t – t ) 2 1 t ]P[ =   = [Reactant] [Product] dt ]P[d = slope – (C – C ) 2 1 C – C 2 1 Concentration rinst rav t1 t2 t Rate of reaction is defined as the speed with which the reactants are converted into products at a given time. It may be expressed as the change in concentration of any one reactant or product per unit time. A  B Rate = Decrease in conc. of A time taken = Increase in conc. of B time taken Rate = –Δ A  Δt = + Δ B  Δ t Negative sign with the reactant concentration is to nullify the minus sign of change of concentration of the reactants and to make the rate a positive term. H2 + I2  2HI Rate = Δ H 2  Δt  = Δ I 2  Δt  = + 1 Δ HI   2 Δt Rate of Chemical Reaction Rate Law or Rate Equation of a Reaction Integrated Rate Equations Methods to Determine the Order of Reaction Factors Affecting Rate of a Chemical Reaction Effect of Temperature Collision Theory of Reaction Rates Chapter 12 Chemical Kinetics
158 Chemical Kinetics NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph.011-47623456 For a general chemical transformation, nA + mB  pC + qD The rates of disappearance of reactants and rates of appearance of products are equalized by dividing them with the coefficients in the balanced chemical equation. Thus, 1 [A] 1 [B] 1 [C] 1 [D] n t mt pt qt          Average and Instantaneous rate of reaction : By dividing the total change in concentration of reactant or product by a time interval, we actually get the average rate of the reaction. The instantaneous rate is obtained when a very small change in concentration (dC) is divided by very small time interval (dt). It is assumed that in dt time, the rate of the reaction remains constant. Thus, instantaneous rate, dC dt  Let us consider the change, 2SO2 (g) + O2 (g)  2SO3 (g) the rate of the reaction can be represented by 1 1 d[SO ] d[O ] d[SO ] 2 dt dt 2 dt    2 2 3 The theoretical rate of a reaction is given by law of mass action. Thus, for the general chemical change mA + nB  Products Theoretical Rate dx m n k[A] [B] dt   The observed rate of reaction is given by the kinetic study of the reaction. Rate can be measured by plotting a curve between concentration of reactant and time and drawing a tangent at a specified time. Rate at time t = – dx dt = slope of the tangent dx dt time conc Example 1 : For a reaction, 2A + B 3C, the rate of appearance of C at time t is 1.2 × 10–4 mol L–1 s–1. Find the rate of reaction. Solution : d[C] 4 1.2 10 dt − = × Rate of reaction = 1 d[C] 3 dt = 1 4 1.2 10 3 − × × = 4 × 10–5 mol L–1 s –1.
NEET Chemical Kinetics 159 Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph.011-47623456 Example 2 : The decomposition of NH3 on platinum surface is a zero order reaction. What would be the rate of production of N2 and H2 if k = 2.5 × 10–4 mol L–1 s –1? Solution : For zero order reaction Rate of reaction = Rate constant (k) 2NH3 N + 3H 2 2 Rate of reaction = d[N ] 2 dt Hence, rate of production of N2 = 2.5 × 10–4 mol L–1 s –1 Again, Rate of reaction = 1 d[H ] 2 3 dt d[H ] 2 dt = 3 × Rate of reaction = 3 × 2.5 × 10–4 = 7.5 × 10–4 mol L–1 s –1 Law of Mass Action : (Guldberg and Waage Law): The rate at which a substance reacts is proportional to its active mass (molar concentration) and the rate of a reaction is directly proportional to the product of molar concentrations raised to the powers equal to the number of participating molecules A  B. For the reaction, 2A + 3B  products, theoritically. Rate  [A]2 [B]3 or Rate = k[A]2 [B]3 , where k is called specific reaction rate or velocity constant. It is defined as the rate of the reaction at a given temperature when the molar concentration of each reactant at a given temperature is unity. RATE LAW OR RATE EQUATION OF A REACTION The mathematical expression which gives a relation between rate of the reaction and concentrations of reactants is known as rate law or rate equation. The rate law is established by the kinetic study of a reaction experimentally. Let us consider a general reaction mA + nB  pC + qD dx [A] [B] dt   a b or dx k [A] [B] dt   a b The reaction is said to be of 'a' order with respect to reactant 'A' and of 'b' order with respect to reactant 'B'. The overall order of the reaction is (a+b). Thus rate law is given by experimental observation. Sometimes for elementary or simple reactions rate law can be written according to law of mass action. Molecularity of a Reaction It is defined as the number of reactant species which collide simultaneously to form the products. It is always a positive integer like 1, 2, 3, ..... Higher molecularity (> 3) are rare because chances of colliding more than three molecules at a time are very less. Molecularity is defined only for elementary (one step) reactions. There is no meaning of molecularity for complex (reactions occurring in more than one steps) reactions.
160 Chemical Kinetics NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph.011-47623456 Order of Reaction : The order of a reaction with respect to a reactant is the power of the concentration term of that reactant to which the rate of the reaction is directly proportional. The overall order of a reaction is the sum of the powers or exponents to which the concentration terms are raised in the rate law expression. Let us consider a general reaction aA + bB  pC + qD If the rate law for this reaction comes out to be dx k [A] [B] dt   m n , m is the order of reaction with respect to A and n is order of the reaction with respect to B. then, the overall order of the reaction is m + n. Order of a reaction is always experimentally determined quantity. Order of a reaction cannot be written from the balanced chemical equation. Order of a reaction may be zero, whole number, fractional or even negative. Molecularity and order of a reaction may be the same for bimolecular or termolecular reactions.. Order is defined for elementry as well as complex reactions. Example 3 : The form of the rate law for a reaction is expressed as : Rate = k[Cl2] [NO]2 . Find out the order of reaction with respect to Cl2 and with respect to NO and also the overall order of reaction. Solution : Order with respect to Cl2 = 1 Order with respect to NO = 2 Overall order = 3 EXERCISE 1. For the reaction, 2N2O5  4NO2 + O2 Select the correct statement (1) Rate of formation of O2 is same as rate of formation of NO2 (2) Rate of disappearance of N2O5 is two times the rate of formation of NO2 (3) Rate of formation of O2 is 0.5 times rate of disappearance of N2O5 (4) Rate of formation of NO2 is equal to rate of disappearance of N2O5 2. dx dt = K [A]0.5 [B]0.5 [c]0.5. What will be the order of the reaction? (1) 1 (2) 1.5 (3) Zero (4) 2