Topic 16: Chemical kinetics - 16.1 Rate expression and reaction mechanism

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. (2.7)


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.


Calculations will be limited to orders with whole number values.

Consider concentration-time and rate-concentration graphs.

Use potential energy level profiles to illustrate multi-step reactions; showing the higher Ea in the rate-determining step in the profile

Catalysts are involved in the rate-determining step.

Reactions where the rate-determining step is not the first step should be considered.

Any experiment which allows students to vary concentrations to see the effect upon the rate and hence determine a rate equation is appropriate