Dependence of Rates on Concentration

CHEMICAL KINETICS

Chemical Kinetics

Kinetics is the study of how fast chemical reactions occur.

Four important factors affect rates of reactions:

1.reactantconcentration

Temperature

Catalyst

4.surfacearea

Reaction Rates

Speed of a reaction is measured by the change in concentration with time.

Suppose A reacts to form B.

For the reaction A B there are two ways of measuring rate:

1. the speed at which the products appear (i.e. change in concentration of B per unit time), or

2. the speed at which the reactants disappear (i.e. the change in concentration of A per unit time).

Average rate = change in concentration of B or A = D[B] = -D[A]

time required for this change D t D t

Unit of average rate is mol/L*s or [M/s].

The rate at any instant (instantaneous rate) is the slope of the tangent to the curve (r =-d[A]/dt) Instantaneous rate is different from average rate.

We usually call the instantaneous rate the rate.

Reaction Rates and Stoichiometry

In general for

aA + bB cC + dD

rate = - 1d[A] = - 1d[B] = 1d[C] = 1d[D]

a dt b d t c d t d d t

Dependence of Rates on Concentration

In general, rates increase as concentrations increase.

For the reaction:

NH₄⁺(aq) + NO₂^-(aq) N₂(g) + 2H₂O(l)

We note:

- as [NH₄⁺] doubles with [NO₂^-] constant, the rate doubles,

- as [NO₂^-] doubles with [NH₄⁺] constant, the rate doubles.

We conclude rate a [NH₄⁺] [NO₂^-]

Rate law:

Rate = k*[NH₄⁺]*[NO₂^-]

The constant k is therate constant

Note that the rate constant k does not depend on concentration.

For a general reaction with rate law:

Rate = k*[reactant 1]^m*[reactant 2]ⁿ

We say the reaction is m-th order in reactant 1 and n-th order in reactant 2.

The overall order of reaction is m+n.

A reaction can be zeroth order if m, n are equal to zero.

· Note the values of the exponents (orders) have to be determined experimentally. They are not simply related to stoichiometry.