Stepwise Chemical Reaction

a chemical reaction proceeding in distinct steps. In contrast to stepwise reactions, the reactants in single-step reactions appear in only one form. In the case of reversible single-step reactions, however, the reaction can proceed in two directions, which leads many authors to include such reactions in the stepwise category.

A stepwise reaction can be considered as the sum obtained by adding the separate steps. Sometimes, all the substances formed in the course of a stepwise reaction are products in the sense that they may be obtained in amounts comparable to those of the initial reactants. Reactions of this type include, in particular, consecutive and parallel reactions. However, most reactions are of the type illustrated by the homogeneous decomposition of nitrous oxide, N₂O. This reaction occurs in two steps:

(1) N₂O → N₂ + O 2

The total reaction is obtained by summation and characterizes the reaction’s final result. This result is established through chemical analysis or some other relatively inexact method, for example, through the change in pressure if the reaction is conducted in a closed system. In order to obtain a final equation through the addition of the reactions of steps (1) and (2), the equations of these steps must first be multiplied by the number, known as the stoichiometric number, on the right. Substances formed and consumed in various steps and not appearing in the final equation are called intermediates. Atomic oxygen is an intermediate in the decomposition of nitrous oxide.

The most important types of stepwise reactions are catalytic and chain reactions. A common feature of these reactions is the cyclical formation and consumption of intermediates; the principal difference derives from the nature of the intermediates. In catalysis, the intermediates are stable chemical species, which, in the absence of a reaction, are capable of existing by themselves indefinitely. The intermediates in chain reactions-atoms, free radicals, or excited molecules—are capable of existing for a short time only.

When a stepwise reaction occurs in an open system with a continuous supply of reactants at a constant rate and a continuous removal of reaction products, then the reaction may proceed in the steady state. Each intermediate in’ this case is formed and consumed at the same rate such that the amount of the intermediate in the system does not change with time. A reaction in a closed system may proceed in a quasi-steady state if the average lifetime of the intermediates is short in comparison with the time required for a significant change in the composition of the reaction mixture. In such a case, the reaction at each moment is practically indistinguishable from a steady-rate reaction.

Branched chain reactions, the theory of which was developed by N. N. Semenov, are an important instance of non-steady-state reactions.

The rates of the individual steps of stepwise reactions are determined through the law of mass action, which is also used for obtaining equations describing the course of stepwise reactions. This problem is significantly simplified if the reaction is a steady-state or quasi-steady-state reaction.