CHAP. 9: CHEMICAL KINETICS [CONTENTS] 307
used:
- rate-determining process principle,
- Bodenstein’s steady-state principle,
- pre-equilibrium principle.
9.5.4 Rate-determining process
In simultaneously proceeding reactions, the reaction whose rate has the decisive effect on the
rate of formation of products is called the rate-determining reaction or the rate-determining
process. The rate-determining process is usually either the fastest or the slowest of the simul-
taneously proceeding reactions.
In parallel reactions, the rate-determining process is the fastest reaction while in consecutive
reactions it is the slowest reaction.
Example
Which of reactions (9.93) is the rate-determining process?Solution
Let the rate constant of the reactionA→R 1 be denotedk 1 , the rate constant of the reaction
A→R 2 be denotedk 2 , and the rate constant of the reaction R 2 →R 3 be denotedk 3 .We
assume that the values of the rate constants are different in order because otherwise the term
rate-determining process would not make sense. Several cases may occur:- k 1 k 2 : the rate-determining process is the reactionA→R 1 regardless of the value of
the constantk 3. - k 1 k 2 :
- k 2 k 3 : the rate-determining process is the reactionA→R 2 ,
- k 2 k 3 : the rate-determining process is the reactionR 2 →R 3.
9.5.5 Bodenstein’s steady-state principle
Except for a short initial and final stage of reaction, the concentrations of radicals, activated
molecules and other reactive components of elementary reactions vary relatively little with time,