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.








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