Collision theory and activation energy — AQA GCSE Chemistry

Collision theory is the model used to explain why reaction rates change when conditions change. It states that particles must collide with sufficient energy for a reaction to occur. Factors such as concentration, pressure, surface area, temperature and the presence of a catalyst affect how often particles collide or how many collisions have enough energy. For example, increasing the concentration of hydrochloric acid increases the number of acid particles in a given volume, so collisions with magnesium atoms become more frequent and the rate increases. Raising temperature increases both the frequency of collisions and the proportion of collisions with energy greater than or equal to the activation energy, so the rate increases more sharply. A catalyst provides an alternative pathway with a lower activation energy.

This statement gives the central condition of collision theory: a reaction happens only when particles collide and the collision has at least the activation energy. A collision with less energy does not lead to reaction because the particles simply bounce apart. The minimum energy needed is called the activation energy. For example, in the reaction between hydrogen and oxygen, most collisions at room temperature do not have enough energy, so the reaction is very slow; a spark provides enough energy for some collisions to succeed, and the reaction then proceeds rapidly. This idea explains why temperature has such a strong effect: raising temperature increases the proportion of particles with energy greater than or equal to the activation energy, so more collisions are successful.

Particles in a gas or solution move randomly and collide constantly, but most collisions do not produce a reaction. For a reaction to occur, the colliding particles must have at least a certain minimum energy, called the activation energy. This energy is needed to break the bonds in the reactants and begin forming new bonds. If the particles collide with less than the activation energy, they simply bounce apart unchanged. A useful model is a ball needing a push to roll over a hill: the height of the hill represents the activation energy. Reactions with low activation energy, such as neutralisation between H⁺ and OH⁻ ions, happen rapidly at room temperature, while reactions with high activation energy, such as burning methane, need a flame to start.

The rate of a reaction depends on how often reactant particles collide successfully. Increasing concentration means more reactant particles in the same volume of solution, so collisions become more frequent. Increasing the pressure of reacting gases squeezes the same number of gas particles into a smaller volume, again increasing collision frequency. Increasing the surface area of a solid, for example by using powder instead of a large lump, exposes more particles to the other reactant, so more collisions can occur. In each case, the energy of individual collisions is not necessarily changed, but the number of collisions per second rises, so the rate increases. A practical example is marble chips reacting with hydrochloric acid: powdered calcium carbonate reacts faster than large chips of the same mass.

Temperature affects reaction rate in two ways. First, particles move faster at higher temperature, so they collide more often. Second, and often more importantly, a greater proportion of particles have energy equal to or greater than the activation energy, so a larger fraction of collisions is successful. Both effects increase the rate of reaction. For example, magnesium ribbon reacts slowly with cold dilute hydrochloric acid but rapidly with warm acid of the same concentration. When explaining temperature effects, it is not enough to say only that particles move faster; the key point is that more particles have enough energy to overcome the activation energy. This distinguishes temperature from concentration, pressure and surface area, which mainly change collision frequency.

This skill requires you to use collision theory to predict whether a reaction will speed up or slow down when concentration, pressure or temperature changes, and then to explain why using particles and collisions. For concentration, more particles in a given volume means more frequent collisions. For pressure in gases, a smaller volume means gas particles collide more often. For temperature, particles move faster and a greater proportion have energy at or above the activation energy, so collisions are both more frequent and more energetic. A strong answer names the change, states the effect on collision frequency or energy, and links this to the rate. For example, increasing the pressure of hydrogen and nitrogen in the Haber process increases the rate because gas particles collide more frequently.

Reactions involving a solid occur when reactant particles collide at its exposed surface. Breaking a solid into smaller pieces raises its surface area to volume ratio, so more particles are exposed and collisions between reactant particles become more frequent, increasing the rate. For a fixed mass of solid, halving the average piece size roughly doubles the total surface area, so the rate increases in proportion to the surface area available. A single large lump has a small surface area to volume ratio and reacts slowly; the same mass as powder has a much larger ratio and reacts quickly. The particles themselves are unchanged, so the same products form and the same total amount of gas or precipitate is produced; only the time taken changes.