Collision theory and activation energy: Increasing the concentration of reactants in solution, the pressure of reacting gases, and the surface area of solid reactants increases the frequency of collisions and so increases the rate of reaction.

The idea

Reaction rate depends on how often reactant particles collide with at least the activation energy. In solution, raising concentration packs more reactant particles into the same volume, so collisions become more frequent. For gases, raising pressure squeezes the same particles into a smaller volume, again increasing collision frequency. For solids, breaking a lump into powder exposes more surface, so more particles can collide at once. In each case the particles themselves are not made more energetic; only the number of collisions per second rises, so the rate increases.

For example: Marble chips react faster with 2 mol/dm³ hydrochloric acid than with 1 mol/dm³ acid, and powdered calcium carbonate reacts faster than the same mass of large chips.

What earns the marks

• State that rate depends on the frequency of collisions between reactant particles.
• Explain that higher concentration means more reactant particles per unit volume in solution, so collisions are more frequent.
• Explain that higher gas pressure means the same particles occupy a smaller volume, increasing collision frequency.
• Explain that greater surface area of a solid exposes more particles, so more collisions occur per second.
• Link the increased collision frequency to a faster rate of reaction.
• Use a named example, such as marble chips with dilute versus concentrated hydrochloric acid, or powder versus lumps.

Common mistakes and how to put them right

• Saying that increasing concentration makes particles move faster.

  Put it right: It increases the number of particles per unit volume, so collisions are more frequent, not more energetic.

• Confusing pressure with concentration for gases.

  Put it right: Pressure changes the volume the gas occupies, which changes how often particles meet.

• Thinking surface area changes the total mass of solid.

  Put it right: It changes how much solid is exposed, so the same mass reacts faster when powdered.

Exam tips

• Name the variable you are changing and state clearly that collision frequency increases.
• For surface area questions, compare powder with lumps of the same mass rather than different masses.
• Use the phrase 'more frequent collisions' rather than 'more collisions' alone, and avoid claiming particles gain energy unless temperature changes.