Topic 6: Chemical kinetics - 6.1 Collision theory and rates of reaction

Nature of science: The principle of Occam's razor is used as a guide to developing a theory-although we cannot directly see reactions taking place at the molecular level, we can theorize based on the current atomic models. Collision theory is a good example of this principle. (2.7)


Essential idea: The greater the probability that molecules will collide with sufficient energy and proper orientation, the higher the rate of reaction.

Species react as a result of collisions of sufficient energy and proper orientation.

The rate of reaction is expressed as the change in concentration of a particular reactant/product per unit time.

Concentration changes in a reaction can be followed indirectly by monitoring changes in mass, volume and colour.

Activation energy (Ea) is the minimum energy that colliding molecules need in order to have successful collisions leading to a reaction.

By decreasing Ea, a catalyst increases the rate of a chemical reaction, without itself being permanently chemically changed.

Applications and skills

Description of the kinetic theory in terms of the movement of particles whose average kinetic energy is proportional to temperature in Kelvin.

Analysis of graphical and numerical data from rate experiments.

Explanation of the effects of temperature, pressure/concentration and particle size on rate of reaction.

Construction of Maxwell-Boltzmann energy distribution curves to account for the probability of successful collisions and factors affecting these, including the effect of a catalyst.

Investigation of rates of reaction experimentally and evaluation of the results.

Sketching and explanation of energy profiles with and without catalysts


Calculation of reaction rates from tangents of graphs of concentration, volume or mass vs time should be covered.

Students should be familiar with the interpretation of graphs of changes in concentration, volume or mass against time