Effect of surface area on rate

The rate of a chemical reaction can be raised by increasing the surface area of a solid reactant. This is done by cutting the substance into small pieces, or by grinding it into a powder.

To summarise:

  • large pieces = small surface area
  • small pieces (powder) = large surface area
Diagram to show the different surface areas of large particles, small particles, and very small particles.Diagram showing increase in surface area

If the surface area of a reactant is increased:

  • more particles are exposed to the other reactant
  • there is a greater chance of particles colliding, which leads to more successful collisions per second
  • the rate of reaction increases
Graph of total mass of product against time from start of reaction, for large and small surface area.

Compared to a reaction with lumps of reactant, the graph line for the same reaction but with powdered reactant:

  • has a steeper gradient at the start
  • becomes horizontal sooner

This shows that the rate of reaction is greater when the surface area is increased.

The reactions will produce the same volume if all of the reactant factors are the same, ie mass of solid, volume of acid. If one of the factors is halved, the volume produced will be half as much. If the factor is doubled, the reaction will produce double the volume.

This applies to all factors that can alter the rate.