Factors which affect the rates of chemical reactions: Factors which affect the rates of chemical reactions include: the concentrations of reactants in solution, the pressure of reacting gases, the surface area of solid reactants, the temperature and the presence of catalysts.

The idea

Reaction rate depends on how often and how energetically particles collide. Increasing the concentration of a solution or the pressure of a gas packs particles closer, so collisions are more frequent and the rate rises. Increasing the surface area of a solid, for example using powder instead of a lump, exposes more particles to collision. Raising the temperature makes particles move faster, so collisions are more frequent and more energetic, increasing the proportion of collisions that succeed. A catalyst provides an alternative pathway with lower activation energy, so more collisions are successful without changing the reactants or products. These factors can be investigated by measuring gas volume, mass loss or the time for a colour change.

What earns the marks

• Concentration: a higher concentration of reactants in solution means more particles in a given volume, so more frequent collisions and a faster rate.
• Pressure: increasing the pressure of reacting gases brings particles closer together, increasing collision frequency and rate.
• Surface area: a solid with a larger surface area, such as a powder, allows more particles to collide at once, increasing the rate.
• Temperature: raising the temperature increases particle speed, so collisions are more frequent and more energetic, increasing the rate.
• Catalysts: a catalyst lowers the activation energy by providing an alternative reaction pathway, increasing the rate without being used up.
• Rate can be followed by measuring the volume of gas produced, the loss in mass, or the time taken for a colour change or precipitate to appear.

Common mistakes and how to put them right

• Saying a catalyst increases the rate by increasing collision frequency.

  Put it right: It lowers activation energy so more collisions are successful, while collision frequency is largely unchanged.

• Confusing surface area with concentration.

  Put it right: Surface area applies to solid reactants, while concentration applies to reactants in solution.

• Claiming temperature increases rate only because particles collide more often.

  Put it right: The main effect is that more particles have energy greater than or equal to the activation energy.

Exam tips

• Link each factor to collision frequency or collision energy, then to the rate, so the explanation shows the mechanism rather than just naming the factor.
• When describing an experiment, name the independent variable, the dependent variable and at least two control variables.
• Use comparative language such as 'more frequent collisions' or 'greater proportion of successful collisions' to make the reasoning explicit.