Page 169 - Instant notes

F1
EMPIRICAL APPROACHES TO KINETICS

Key Notes
Experimental methods in kinetics measure change in the
composition of a reaction mixture with time, either continuously
as the reaction progresses, or at fixed intervals after the reactants
have come together. The techniques applied vary depending on
the timescale of the reaction and the chemical species under
study. Additional kinetic information is obtained by varying
experimental parameters such as the initial concentration of
reactant(s) or the temperature of the mixture.
The instantaneous rate of reaction for a species is the rate of
change of concentration with time of that species at a particular
instant during the reaction. Units of reaction rate always have
−1
dimensions of concentration time .
The rate law is the empirically determined mathematical
relationship describing the observed rate of reaction in terms of
the concentrations of the species involved in the reaction. Rate
laws do not necessarily fit the simple stoichiometry of a balanced
chemical reaction but may be the consequence of a more complex
underlying mechanism to the observed reaction.
Rate constants are the constants of proportionality within the
empirical rate law linking rate of reaction and concentration of
species involved in the reaction. The units of rate constants are
particular to the rate law and can be derived by dimensional
analysis. Rate constants usually vary with temperature.

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. The
exponents do not have to be integers, and for complex rate laws,
the order may not be a definable quantity.
The molecularity of a reaction is the number of molecules which
come together to react and is independent of the order of a
reaction. In a unimolecular reaction a single molecule breaks
apart or rearranges its constituent atoms. A bimolecular reaction
involves two atoms or molecules.
Related topics Rate law determination (F2) Rate laws in action (F5)
Energetics and mechanisms (F3) The kinetics of real systems
(F6)
Formulation of rate laws (F4)