(a) How does the collision theory explain the rate of a chemical reaction? (b) State how each of the following affects the rates of chemical reactions: (i) ...

(a) The collision theory and the rate of reaction

The collision theory states that for a chemical reaction to occur, the reacting particles must collide with one another. However, not every collision leads to a reaction. Only collisions in which the particles possess energy equal to or greater than the activation energy, and in which the particles are correctly orientated, are effective and lead to product formation. The rate of a reaction therefore depends on the frequency of effective collisions: the greater the number of effective collisions per unit time, the faster the reaction.

(b) Effect of the following on rate of reaction

• (i) Surface area: increasing the surface area of a solid reactant (for example by powdering it) exposes more particles to collision. This increases the frequency of effective collisions and so increases the rate of reaction.
• (ii) Catalyst: a catalyst provides an alternative reaction pathway of lower activation energy. More colliding particles then possess the required energy, so the number of effective collisions and hence the rate increases.