Concentration and reaction rate

For a reaction involving substances in solution, the concentration of a reactant tells us how many reactant particles are present in a given volume. Changing the concentration changes how quickly the reaction happens.

Reactant particles must collide for a reaction to occur. When the concentration is higher, there are more reactant particles in the same volume, so collisions happen more often. More frequent collisions lead to more successful collisions each second, increasing the rate of reaction.

When the concentration is lower, there are fewer reactant particles in the same volume. Collisions happen less often, so there are fewer successful collisions each second and the reaction is slower.

Pressure and reaction rate

In a reaction involving gases, particles must collide for a reaction to occur. Changing the pressure changes how close together the gas particles are, and therefore how often they collide.

Increasing the pressure squeezes the gas into a smaller volume. The particles are closer together, so they collide more frequently. This increases the frequency of successful collisions, making the reaction faster.

Decreasing the pressure allows the gas to occupy a larger volume. The particles are farther apart, so collisions happen less often. Successful collisions are therefore less frequent, and the reaction becomes slower.

Surface area and reaction rate

A solid reactant can react only at its surface, where particles of another reactant can reach it. Particles inside the solid are not exposed to the other reactant.

Breaking the solid into smaller pieces exposes surfaces that were previously inside it. For the same amount of solid, smaller pieces have a greater total surface area than a large lump. A powder exposes even more surface area because it consists of many small particles.

With more surface exposed, particles of the other reactant can collide with the solid at more places. More collisions can therefore occur each second, increasing the reaction rate. A large lump has less exposed surface area, so fewer collisions occur each second and the reaction is slower.

Temperature and reaction rate

Changing the temperature changes how quickly a chemical reaction happens. For particles to react, they must collide with enough energy. A collision that leads to a reaction is called a successful collision.

When the temperature increases, particles gain kinetic energy and move faster. They collide more often, and their collisions are more energetic. A greater proportion of collisions therefore have enough energy to be successful. More successful collisions occur each second, so the reaction rate increases.

When the temperature decreases, particles have less kinetic energy and move more slowly. Collisions are less frequent and less energetic, so fewer collisions have enough energy to be successful. Fewer successful collisions occur each second, and the reaction rate decreases.

Catalysts and reaction rate

A catalyst increases the rate of a chemical reaction without being used up. It provides an alternative reaction pathway that requires less energy to start than the pathway without a catalyst. This minimum energy requirement is called the activation energy.

Reactant particles must collide with enough energy for a reaction to occur. Because the catalysed pathway has a lower activation energy, a greater proportion of collisions have enough energy to be successful. More successful collisions occur in a given time, so the reaction is faster.

Without the catalyst, the reaction follows a pathway with a higher activation energy and is slower under the same conditions.