PHYSICAL CHEMISTRY IN BRIEF

CHAP. 9: CHEMICAL KINETICS [CONTENTS] 315

9.6 Temperature dependence of the rate of a chemical reaction

The rate of reaction depends on temperature via the rate constant. In a simple reaction, both
the rate constant and the rate of reaction increase with temperature. The rate of parallel and
consecutive reactions also increases with temperature. In reactions proceeding by more complex
mechanisms, the rate may decrease with temperature.

Example

When does the rate of formation of a product proceeding according to the Lindemann mechanism

[see9.5.6] decrease with increasing temperature?

Solution

The rate is given by the kinetic equation (9.145)

dcB

dτ

=

k 1 k 3

k 3 +k 2 cA

c^2 A.

When the temperature is raised, all the three rate constantsk 1 ,k 2 ,k 3 will increase. However,

the overall rate of reaction will decrease if the denominator in the kinetic equation increases more

than the numerator.

9.6.1 Van’t Hoff rule

With the temperature raised by 10◦C, the rate of a chemical reaction increases 1.5- to 3-times.
This qualitative rule often allows for distinguishing chemical reactions from physical processes
and from biochemical reactions proceeding in living organisms. If the rate of reaction obeys the
van’t Hoff rule, it is highly probable that the reaction in question is chemical. If, on the contrary,
the rate of reaction increases substantially less, the studied process is most likely of a physical
nature. This may be, e.g., gas diffusion or adsorption on a solid surface. A typical physical
process is a radioactive decay where the rate does not depend on temperature at all. However,
this may be also a chemical reaction with a more complex mechanism (see the example in the
preceding section). If a reaction ceases to proceed above a certain limit when the temperature