VIEWS: 7 PAGES: 14 POSTED ON: 11/4/2011
Rate equation and order of reaction 7S Chan Po Hang ( 3 ) Ng Lai Man ( 24 ) 2001/1 1 Rate equation For a general reaction: 2A + B + 3C D The rate law for the above reaction can be expressed as Rate of reaction = k Ax By Cz where k = rate constant for the reaction at temperature [A], [B] and [C] = concentrations x, y and z = order of reaction 2 Rate constant The rate constant k, is temperature dependent. At a fixed temperature,it is a constant for a reaction. k The unit of k depends on the overall order of the reaction. rate of reaction E.g. k A B C x y z unit of k = M 1-(x+y+z) s-1 3 Order of Reaction The overall order of reaction is the sum of powers of the concentration of individual reactants. The order of reaction can only be determined experimentally. (a) For a zeroth order reaction, the rates of reaction are independent of the concentration of reaction. 4 (b) For a first order reaction, the rate is directly proportional to the concentration of the reactant. (c) For a second order reaction, the rate of the reaction is directly proportional to the square of the concentration of the reagent. 5 For a simple reaction: A products Order of reaction Rate law Unit of k d (a) Zeroth order A Mol dm-3 s-1 dt d (b) First order A k A s-1 dt d (c) Second order A k A2 Mol-1 dm3 s-1 dt d (d) N th order A k An Mol-2 dm6 s-1 dt 6 Simple rate equations determined from experimental results (1) Initial rate method Example: In the kinetic study of the reaction, CO(g) + NO2(g) CO2(g) + NO(g) Experiment Initial concentration Initial concentration Initial of CO of NO2 rate M/s 1 0.1 M 0.1 M 0.015 2 0.2 M 0.1 M 0.030 3 0.1 M 0.2 M 0.030 4 0.4 M 0.1 M 0.060 7 Establish the rate equation for the reaction. Solution 0.015 = k (0.1)m (0.1)n ----------(1) 0.030 = k (0.2)m (0.1)n ----------(2) 0.030 = k (0.1)m (0.2)n ----------(3) : 0.5 0.5 (1) (1) : 0.5 (0.5) n m (2) (2) m=1 n=1 0.060=k(0.4)(0.1) k=1.5 Rate =k[CO][NO2] 8 (2) Rate-concentration graph Example: Tabulated below are the initial rate of decomposition of hydrogen peroxide at different concentrations. Plot a graph of initial rate against concentration. 9 Solution: 10 (3) Log(rate)-log(concentration) graph log(rate)= n log[A]+log k Slope = order 11 12 (4) Integrated Rate equation Order Equation Zeroth [A]=-k0 + [A]0 A0 k1t First In A Second K2t + 1/[A]0 13 14
"Rate equation and order of reaction"