RATE LAW

Objectives:

5. calculate the value and determine the units of rate constantsAt the end of the lesson the students should be able to:1. define rate law and write the rate equation2. define the order of reaction and the rate constant3. calculate the order with respect to a certain reactant from experimental data4. determine the overall order of a reaction from experimental data

The Rate LawThe rate law expresses the relationship of the rate of a reaction to the rate constant and the concentrations of the reactants raised to some powers.Rate = k [A]x[B]y reaction order is x with respect to A reaction order is y with respect to BOverall reaction order is (x + y) The exponents x, y, can be integers, fractions or decimal or negative values. k is called rate constantRate [reactant]

Rate LawThe values of x and y can only be determined experimentally.Reaction order is usually defined in terms of reactant concentrations.The order of a reactant is not related to the stoichiometric coefficients of the reactants in the balanced chemical equation.

The units of rate constant, ki) The reaction is zero order Rate = k [A]0 Rate = k unit k =unit rate= mol L-1 s-1 or M s-1

ii) First orderRate = k [A]1iii) Second orderRate = k [A]2

Example :The above reaction is first order with respect to iodide ions and to thiosulphate ions.a) Write the rate of equation for the reaction.b) What is the unit of rate constant, k?Solution :a) Rate = k [S2O82-]1[I-]1b) Rate = k [S2O82-]1[I-]1Unit k

The order of reaction For reaction

Rate = k [A]xi) If x = 0Rate is not dependent on [A]Rate = k [A]0Rate = kTherefore this reaction is zero order with respect to A

ii) If x= 1Rate = k [A]1Assume [A]i = 1.0MRate = k (1.0M)If the [A] is doubled from 1.0M to 2.0M,henceRate = 2k[A]Doubling the [A] will double the rate of reaction.Therefore this reaction is first order with respect to A

iii) If x = 2Rate = k[A]2Assume [A]i = 1.0 MRate = k (1.0 M)2If the [A] is doubled from 1.0 M to 2.0 M,henceRate = 4k[A]Doubling [A], the rate will increase by a factor of 4.Therefore the reaction is second order with respect to A

Example overall reaction order = 3The reaction order respect to NO : 2The reaction order respect to O2 : 1

Solution: The reaction order with respect to CH3CHO : 3/2 The reaction order (overall) : 3/2Solution:The reaction of order with respect to H2O2 : 1 The reaction of order with respect to I- : 1 and zero order in H+, while overall order is 2.

Determination of the orders of reaction rate;

rate = k [O2]m[NO]nCompare 2 experiments in which the concentration of one reactant varies and the concentration of the other reactant(s) remains constant. ;Do a similar calculation for the other reactant(s).Solution:The reaction is first order with respect to O2

To find the order with respect to NO, we compare experiment 3 and 1, in which [O2] is held constant and [NO] is doubled:The reaction is second order with respect to NO

The results of the kinetic studies are given below.Exercise:ClO2(aq) + 2OH- (aq) productsExplain what is meant by the order of reaction.Reffering to the data determine (i) rate law /rate equation (ii) rate constant, k (iii) the reaction rate if the concentration of both ClO2 and OH- = 0.05 M

exp[ClO2]M[OH-]MInitial rate, Ms-110.04210.01858.21 1 0-320.05220.01851.26 1 0-230.04210.02851.26 1 0-2

rate = k [ClO2]2[OH-]

k = 250 M-2s-1 rate = 3.12 10-2 M/s

Use the following data to determine the individual and overall reaction orders:

ANS

m = 2n = 0 k = 0.5