PHYSICAL CHEMISTRY IN BRIEF

CHAP. 9: CHEMICAL KINETICS [CONTENTS] 293

9.4 Simultaneous chemical reactions

We speak about simultaneous reactions when there are several reactions in progress in a system
and these reactions have at least one substance in common. In this section we will focus on
cases with only two reactions taking place in a system. More complex simultaneous reactions
will be discussed in section9.5. For the sake of simplicity we will assume that the partial orders
of the reactions equal their stoichiometric coefficients, i.e. that the reactions are elementary [see
9.5.1].
S Symbols: We will use the symbol

A →k B

to indicate that the reaction rate constant isk.

Example

The following reactions proceed in a system at the same time:

A+B →k^1 2C and R →k^2 S. (9.89)

Are they simultaneous reactions?

Solution

The answer is that they are not. The reactions do not have any reactant in common. Hence

they may be studied separately.

9.4.1 Types of simultaneous reactions

We distinguish the following types of simultaneous reactions:

a) Parallelorsidereactions. The initial substances react in competing reactions to form

various products. The simplest case is the decomposition of one reactant into two prod-

ucts

A → R 1

A → R 2

i.e. A

↗R^1

↘R 2

. (9.90)