Reaction Kinetics(11.1)

Objective

1. define rate laws,order of reaction and rate constant
2. write the rate law/rate equation
3. calculate the order with respect to a certain reactant and the overall order of reaction from the experimental data
4. calculate the value and determine the units of rate constants,k.

The Rate Law

• The rate law for a chemical reaction relate the rate of reaction to the concentrations of reactants.
• At constant T&P,rate of chemical reaction directly proportional to [reactants]

Rate directly proportional to [reactants]

Consider this equation:

                                  aA + bB to cC                                                k = rate constant 

                                  Rate directly proportional to [reactants]          m = reaction order with respect to A

                                  Rate = k[A]m[B]n                                          n = reaction order with respect to B

                                                                                                   m+n = overall reaction order

• The exponents in a rate law/ reaction order must be determined by experiment.They are not derived from the stoichiometric coefficients in an overall chemical equation.They are generally small,positive integers (0,1,2......)
• The rate constant,k and its value depends on specific reaction,T& the presence of catalyst.The larger k,the faster the reaction proceeds.Units of k depends on the values of m,n.....

Example 1

determining reaction order from Rate Laws

2NO(g) + O2(g) TO 2NO2(g)                 (rate = k[NO2]2[O2])

Solution:

The reaction order with respect to NO =2

The reaction order with respect to O2 = 1

The overall reaction order =3

Example 2

S2O8 + 3I to 2SO4 + I3

The above reaction is first order with respect to iodide and to thiosulphate ion

a)Write the rate equation for the reaction

b)What is the unit of rate constant,k?

Solution:

a) Rate = k [S2O8]-1[I]-1

b) Rate = k[S2O8]1[I]1

               k = rate/([S2O8]1[I]1 = Ms-1/M2 = M-1S-1

Determine rate law:Method of Initial Rates

 ine rate law: Method Of Initial Rates

• The method of initial rates involves a series of expt in which the initial [ ] of some reactants are held constant and others are varied in order to determine the rate law for that reaction.

• The effects of doubling one initial concentration:

·         For zero-order reactions, no effect on rate.

·         For first-order reactions, the rate doubles.

·         For second-order reactions, the rate quadruples.

·         For third-order reactions, the rate increases

             eightfold.

Method Of Initial Rates

Initial Rates of the Reaction:

2NO(g) + Cl2(g)  to 2NOCl(g)

Exp.	Initial [NO]	Initial [Cl2]	Initial rate,Ms-1
1	0.0125	0.0255	2.27 x 10-5
2	0.0125	0.0510	4.55 x 10-5
3	0.0250	0.0255	9.08 x 10-5

d

2NO(g) + Cl2(g) To 2NOCl(g)

The rate law in general is; Rate = k [NO]m [Cl2]n,Compare 2 experiments,of [one reactant] varies and the [other reactant] constant.

Rate exp2/Rate exp1 = k[NO]exp2(m) [Cl2]exp2(N) / k[NO]exp1(m) [Cl2]exp1(n)

4.55x10-5/2.27x10-5  = k(0.0125)m (0.0510)n / k(0.0125)m (0.0255)n

                                2 = 2 power of n

Thus,n,order of rxn wrt Cl2 = 1

Rate exp3 / Rate exp1 = k[NO]exp3(m) [Cl2]exp3(n) / k[NO]exp1(m) [Cl2]exp1(n)

9.08 x10-5/2.27 x10-5 = k(0.0250)m (0.0255)n / k(0.0125)m (0.0255)n

                                  4 = 2 power of m

Thus,m, order of rxn wrt NO = 2

Thus,rate law is Rate = k [NO]2 [Cl2]

Overall order of reaction  = m+n = 2+1=3