Factors Affecting the Rate of Reaction

The rate of reaction is how quickly reactants turn into products. In particle terms, reactions happen when particles collide successfully.

Collision theory (the simple idea)

• Particles must collide to react. More frequent collisions → faster reaction.
• Collisions must have enough energy to get over the activation energy, $E_a$. Higher kinetic energy makes more collisions successful.
• In short: $\text{rate} \propto \text{successful collisions per second}$.

What changes the rate?

1) Concentration of solutions

Higher concentration means more particles in the same volume. Collisions happen more often, so the reaction is faster. Example: dilute acid reacts more slowly with a metal than concentrated acid.

2) Pressure of gases

Increasing pressure squeezes gas particles closer together. Collisions are more frequent, so gas reactions speed up. This factor applies to gases only.

3) Surface area of solids

Smaller pieces or powders have more exposed surface. More reacting particles can collide at once. A spoon of sugar dissolves faster than a sugar cube because of larger surface area.

4) Temperature

Higher temperature gives particles more kinetic energy. They move faster (more collisions) and a larger fraction have energy $\ge E_a$ (more successful collisions). This usually has a strong effect on rate.

5) Catalysts (including enzymes)

A catalyst provides an alternative pathway with lower $E_a$. More collisions have enough energy, so the rate increases. The catalyst is unchanged at the end. Enzymes are catalysts in living things.

Common misconceptions

• “Catalysts get used up.” They do not; they are unchanged at the end.
• “Pressure changes the rate for solids and liquids.” Pressure mainly affects gases.
• “More reactant mass always means faster reaction.” If concentration and surface area are the same, extra mass does not make each second faster; it just makes the reaction last longer.
• “Heat is the same as temperature.” Raising temperature (not just adding warm surroundings) increases particle kinetic energy.