Rates of reaction

Different chemical reactions occur at different rates or speeds. Indicators of reactions are colour change, precipitate formation, release of gas, and/or a detectable energy change. 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.

At low temperature the particles move slowly and seldom collide. At high temperature the particles move quickly and collide more often.Particles at low temperatures move more slowly than those at high temperatures

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.

Graph of total volume of product against time from start of reaction. The reaction is plotted twice, once at a high temperature and once at a low temperature reaction. Both lines level off at the same volume of product produced, but it takes less time for the high temperature reaction to achieve it.Reactions at higher temperatures have faster rates than reactions at lower temperatures

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.

At a low concentration there are few particles present and hence few collisions. At a high concentration there are many particles and an increased likelihood of collisions.

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.

Rates of Reaction