Page 173 - Instant notes

Empirical approaches to kinetics 159

Rate law

The rate law is the empirical relationship which describes the observed rate of reaction in
terms of the concentrations of species in the overall reaction, including possibly the
concentration of the products.
It is often observed that the rate of reaction is proportional to the product of the
individual concentrations of the reactants raised to a simple power, for example,
. Rate laws are empirical observations and do not necessarily fit the
simple stoichiometry of the balanced chemical equation for the observed reaction but
may be the consequence of a more complex underlying molecular reaction mechanism.
For example, the apparently straightforward chemical reaction:
H 2(g)+Br 2(g)→2HBr(g)

has the experimentally determined rate law (see Topic F6):

Rate constants

Rate constants, k, are the constants of proportionality which appear in the empirical rate
law linking rate of reaction and concentration of species. The dimensions of the units of k
are dependent on the formulation of the individual rate law but can always be derived by
dimensional analysis of the rate law. Thus a reaction which is second order overall must
−1
−1
have a rate constant with dimensions of concentration .time in order to provide the
right hand side of the rate law with dimensions equal to the dimensions of
−1
concentration.time for rate of reaction. The exact units of k depend on the units of
−3
concentration and time used, but mol dm and s, respectively, are common.
A rate constant for a particular reaction has a fixed value at a particular temperature,
although it usually varies with temperature and the temperature dependence is often
conveniently described by an Arrhenius equation (see Topic F3). Rate constants of
elementary reactions do not vary with pressure so the observation of a pressure
dependence in the rate of reaction indicates a more complex multistep reaction
mechanism (see Topics F4, F5 and F6).

Order of reaction

If the rate law for a reaction can be written in the form, …then the
reaction is classified as α-order in A, β-order in B,…and as (α+β+…)-order overall.
Where the exponent, or sum of exponents, equals one the reaction is said to be first
order with respect to that species, or first order overall, respectively. Where the