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Different chemical reactions occur at different rates or speeds.

Some are very slow, like a car rusting, while others are very fast like a sudden explosion.

There are four factors that affect the rate of a chemical reaction:

• temperature
• concentration
• particle size
• use of a catalyst

Temperature

Increasing the temperature a reaction takes place at increases the rate of reaction. At higher temperatures, particles can collide more often and with more energy, which makes the reaction take place more quickly.

The graph below shows how changing the temperature affects the rate of reaction between an acid and chalk. The magenta line represents a faster reaction because it is steeper. Both reactions release a gas and both finish at the same volume.

Concentration

If you increase the concentration of a reactant, there will be more of the chemical present. More reactant particles moving together allow more collisions to happen and so the reaction rate is increased. The higher the concentration of reactants, the faster the rate of a reaction will be.

Particle size

By decreasing the particle size of a reactant, there are more surfaces that collisions can take place on. The smaller the particle sizes the faster the reaction.

This video clip has demonstrations with charcoal, gunpowder and steel wool that show the effect of particle size and concentration on reaction rate.

Use of a catalyst

A catalyst speeds up the rate of a reaction but it is not used up in the reaction. If a catalyst is present, the reacting particles can collide more successfully with less energy and so the reaction can take place at a lower temperature.

Elephant's toothpaste experiment

The elephant's toothpaste experiment shows how a catalyst can speed up the rate of a reaction.

Hydrogen peroxide breaks down slowly into water and oxygen. Adding a catalyst such as sodium iodide or potassium iodide makes this happen much more quickly, as shown in the video below.