Reaction kinetics

Factors which affect the rate of a chemical reaction

1 Concentration of reactants

2 Pressure

3 Temperature

4 Physical state of reactants

5 catalysts

Decomposition of hydrogen peroxide

[H2O2}]

Time

Rate = gradient = conc example units are mol s-1 timeNow we find gradient at various concentrations and plot rate vs concentration

rate

concentration

a) In this case the rate concentration. When rate is directly proportional to the concentration we call this 1st order with respect to the concentration of the reactant we are monitoring

rate

concentration

b) In this case the rate is independent of the concentration. We call this zero order with respect to the reactant we are monitoring

rate

concentration

c) We call this one 2nd order with respect to the reactant we are monitoring

For a) where the rate is proportional to the conc

rate [H2O2]

or rate = k [H2O2] where k is called the rate constant

and the above equation is called the rate equation

We could write this as rate = k [H2O2]1 but the superscript is always assumed to be 1 if no other number is present.

This is the number that is referred to in the expression 1st order

Lets suppose we now have a reaction involving 2 reactants

A + B products

Rate will depend on both [A] and [B] The rate equation is

rate = k[A]x [B]y

x and y are called the order with respect to A and B

X and y may be 0,1,2 (very rarely 3)

If the order is 0 the the rate does not depend on the concentration of that reactant

e.g for the reaction between A and B previously.

If order w.r.t A = 1 and order w.r.t B = 0

The rate equation will be

Rate =k [A]1[B]0 any number to the power of 0 = 1 Rate equation becomes

Rate = k [A]

The overall order of a reaction is the sum of all the superscripts

So for the reaction with a rate equation as follows

Rate = k[A]1][B]2

The overall order is 3

It is important to remember that no information about the order or the rate of a reaction can be found from an equation. All the kinetic data must be found by experiment

Definitions

The order of reaction is the number of concentration terms in the experimentally determined rate equation.

The rate of a reaction is proportional to the concentration and the rate constant is the proportionality constant.

In order to find the order w.r.t. individual reactants a series of experiments must be performed.

e.g. for the reaction

A + B P

We need to

1) Keep A the same and change B

2) Keep B the same and change A

Initial conc A mol/l

Initial conc B mol/l

Initial rate mol/L/h

2 x 10-3 3 x 10-3 3 x 10-3

2 x 10-3 6 x 10-3 1.2 x 10-2

4 x 10-3 6 x 10-3 2.4 x 10-2

Lets look at A first. When B is kept the same and A is doubled the rate doubles.

rate [A]1

i.e. order w.r.t A = 1

When A kept the same and B doubled

The rate increases x 4 (i.e 22)

Rate [B]2

i.e. order w.r.t B = 2

Overall order = 3

our rate equation then is rate = k [A][B]2

As k is a constant it will be the same for any of the 3 experiments so we can use any experiment and put in the values of [A] [B] and initial rate to find k.

k = rate k = 3.0 x 10-3

[A][B]2 (2 x 10-3)(3.0 x 10-3)2

= 1.67 x 105

What are the units?

mol/L/h = mol-2L2 h-1

(mol/L)3

Although we write equations for reactions with only one step many reactions have more than one step

i.e. for the overall reaction

A + B C

The actual procedure could be

Step 1 A intermediate

Step 2 intermediate + B C

The first step is often the slowest step and is called the rate determining step

In any reaction the slowest step (usually the first step) is the rate determining step (r.d.s)

The order of reaction is the number of molecules or atoms taking part in the r.d.s

So in the previous example the order w.r.t. A would be 1 and the overall order would also be 1. the rate depends on the first step so [B] has no effect on the rate.

For reaction with the following rate equation

Rate = k[A][B]Order w.r.t A = 1Order w.r.t B = 1 1 molecule A and 1 molecule B take part in the r.d.s

For a reaction with the following rate equation Rate = k[A]2B]0

This would be a 2 step reactionStep 1 A + A XStep 2 X + B products