IB Chemistry - Kinetics

IB Chemistry home > Syllabus 2016 > Kinetics > The rate expression

Syllabus ref: 16.1

The aim of kinetics studies is to find mathematical relationships between the rate of a reaction and the concentrations of the reactants and to use this relationship to provide evidence for the way in which the reactants interact to form the products.

This mathematical relationship is called the rate expression.

Nature of science:

Principle of Occam's razor-newer theories need to remain as simple as possible while maximizing explanatory power. The low probability of three molecule collisions means stepwise reaction mechanisms are more likely.


Essential idea: Rate expressions can only be determined empirically and these limit possible reaction mechanisms. In particular cases, such as a linear chain of elementary reactions, no equilibria and only one significant activation barrier, the rate equation is equivalent to the slowest step of the reaction.

Reactions may occur by more than one step and the slowest step determines the rate of reaction (rate determining step/RDS).

The molecularity of an elementary step is the number of reactant particles taking part in that step.

The order of a reaction can be either integer or fractional in nature. The order of a reaction can describe, with respect to a reactant, the number of particles taking part in the rate-determining step.

Rate equations can only be determined experimentally.

The value of the rate constant (k) is affected by temperature and its units are determined from the overall order of the reaction.

Catalysts alter a reaction mechanism, introducing a step with lower activation energy.

Applications and skills

Deduction of the rate expression for an equation from experimental data and solving problems involving the rate expression.

Sketching, identifying, and analysing graphical representations for zero, first and second order reactions.

Evaluation of proposed reaction mechanisms to be consistent with kinetic and stoichiometric data.

In Chapter 6.3