PHYSICAL CHEMISTRY IN BRIEF

CHAP. 9: CHEMICAL KINETICS [CONTENTS] 267

Determination of the rate constants, the order of reaction and the partial orders of reaction
from experimental kinetic data is discussed in section (9.3). In elementary reactions [see (9.5)],
the partial orders of reactions equal their stoichiometric coefficients.
Symbols: When there is no risk of confusion, we will use simplekinstead of the more correct
ki.

9.1.4 Reaction half-life

The half-life of a reaction is the time in which the concentration of a chosen reactant falls to
one half. It is denotedτ 1 / 2 orτ 0. 5.

Note:When there is only one reactant in the reaction, or when the initial concentrations

of reactants are in a stoichiometric ratio, the half-life of reaction can be determined un-

ambiguously. When there are more reactants and their initial concentrations are not in a

stoichiometric ratio, the half-life is usually defined with respect to the key component (a

compounds which would first disappear is usually chosen as the key component).

Example

The initial concentration of substance A reacting according to the equation

A→R

was 2 mol dm−^3. After 20 minutes it fell to 1 mol dm−^3. Calculate the half-life of the reaction.

Solution

From the definition of the reaction half-life it follows thatτ 1 / 2 = 20 min.

9.1.5 Material balance

When the concentrations of more than one substance act in a kinetic equation, we cannot
solve this equation unambiguously because we have only one equation and several dependent
variables. In order to obtain a differential equation for one dependent variable, we carry out
material balance.