F1.3 Reaction Rate Constants

Estimation of Reaction Rate Constant and Order of Reaction

An order of the chemical reaction has to be established from experimental data. If the reaction is elementary in nature, then the rate of reaction is proportional to the product of concentration of those species raised to the stoichiometric coefficients. If the order of the reaction is known, then the reaction rate constant, k, can be obtained by the rearrangement of rate expression. If a reaction proceeds as follows:

Taking a logarithm of both sides, we obtain

If the function can be integrated then, the whole expression can be rearranged to get the best estimate of the k value. Concentration of a component may be expressed in terms of partial pressures.

Half-life: A term half-life is defined is a time required to reduce concentration of a component to one-half its original concentration. If a reaction is first order then

However, for any nth order reaction conducted under constant volume conditions,

A plot of ln(t_1/2) versus ln(C_Ao) yields a straight line with slope (- n + 1). If the reaction is conducted at constant pressure, then half-life is given as