Collision theory and activation energy: use simple ideas about proportionality when using collision theory to explain the effect of a factor on the rate of a reaction.

The idea

Collision theory says that particles must collide with at least the activation energy for a reaction to occur. The rate depends on the frequency of these successful collisions. Simple proportionality means that if a factor increases the frequency of successful collisions by a given factor, the rate increases by roughly the same factor, provided other conditions stay constant. For example, doubling the concentration of a reactant in solution roughly doubles the number of particles in a given volume, so successful collisions per second and the rate roughly double. Similarly, increasing the surface area to volume ratio of a solid by breaking it into smaller pieces increases the exposed area and so increases the rate in proportion, until other factors limit it. Proportionality is approximate because factors interact and measurements have uncertainty.

What earns the marks

• Particles must collide with energy greater than or equal to the activation energy for a reaction to occur.
• Rate depends on the frequency of successful collisions, so a factor that increases this frequency increases the rate.
• If a factor increases the frequency of successful collisions by a given factor, the rate increases by approximately the same factor when other variables are controlled.
• Doubling the concentration of a reactant in solution roughly doubles the number of particles per unit volume, so the rate roughly doubles.
• Increasing the surface area to volume ratio of a solid exposes more particles to collision, so the rate increases in proportion to the exposed area.
• Proportionality is a simple model and may not hold exactly because of experimental uncertainty, limiting factors or changes in temperature.

Common mistakes and how to put them right

• Treating proportionality as exact.

  Put it right: Doubling concentration may not exactly double the rate because of measurement uncertainty and other limiting factors.

• Confusing rate with total amount.

  Put it right: A proportional change in rate does not change the total amount of product formed from a fixed amount of reactant.

• Saying a factor changes the activation energy.

  Put it right: Concentration, pressure and surface area change collision frequency, not the activation energy, while temperature changes both collision frequency and the proportion of particles with enough energy.

Exam tips

• State the factor changed, the factor kept constant and the measured rate, then use the phrase roughly proportional or in proportion.
• Use a numerical example, such as doubling concentration and predicting the rate roughly doubles, to show proportional reasoning.
• Explain the link in order: factor changes particle proximity or exposed area, so successful collision frequency changes, so rate changes.