Collision theory and activation energy: Collision theory explains how various factors affect rates of reactions. According to this theory, chemical reactions can occur only when reacting particles collide with each other and with sufficient energy. The minimum amount of energy that particles must have to react is called the activation energy.

The idea

Collision theory states that particles must collide for a reaction to happen, but most collisions do not cause a reaction. A successful collision needs enough energy, called the activation energy, to break bonds in the reactants. Factors such as concentration, pressure, surface area, temperature and catalysts change the rate by altering either the frequency of collisions or the proportion of particles with energy greater than or equal to the activation energy.

For example: Increasing temperature makes particles move faster, so they collide more often and with more energy; adding a catalyst provides an alternative pathway with lower activation energy.

What earns the marks

• State that reacting particles must collide with each other for a reaction to occur.
• Explain that collisions must have at least the activation energy to be successful.
• Define activation energy as the minimum energy that particles need to react.
• Use collision theory to explain how a named factor changes the rate, for example higher concentration increases collision frequency.
• Explain how a catalyst increases rate by providing an alternative reaction pathway with lower activation energy.

Common mistakes and how to put them right

• Saying that any collision causes a reaction.

  Put it right by stating that collisions must also have sufficient energy, at least the activation energy.

• Confusing activation energy with the overall energy change of a reaction.

  Put it right by defining activation energy as the minimum energy needed to start the reaction, not the energy difference between reactants and products.

• Claiming that a catalyst increases the energy of particles.

  Put it right by explaining that a catalyst lowers the activation energy by providing an alternative pathway.

Exam tips

• Use the phrase 'collide with sufficient energy' when explaining successful collisions.
• When explaining a rate change, state whether collision frequency, collision energy, or both are affected.
• Link catalyst explanations to a lower activation energy and an alternative pathway.