Rates and surface area to volume ratio

Dividing lumps

For a given mass of a solid, large lumps have smaller surface area to volume ratios than smaller lumps or powders. If a large lump is divided or ground into a powder:

  • its total volume stays the same
  • the area of exposed surface increases
  • the surface area to volume ratio increases

Lumps versus powders

The greater the frequency of successful collisions, the greater the rate of reaction. If the surface area to volume ratio of a reacting solid is increased:

  • more reactant particles are exposed at the surface
  • the frequency of collisions between reactant particles increases
  • therefore the rate of reaction increases

Note that the mean energy of the particles does not change. However, since the frequency of collisions increases (they happen more often) the frequency of successful collisions also increases.

Graphs

The rates of two or more reactions can be compared using a graph of mass or volume of product formed against time. The graph shows this for two reactions.

Graph of total mass of product against time from start of reaction, for large and small surface area.

The gradient of the line is equal to the rate of reaction. The faster reaction with the powder:

  • gives a steeper line
  • finishes sooner
curriculum-key-fact
Make sure questions are answered in terms of surface area to volume ratio, rather than just surface area. This is because the surface area also depends on the mass of solid reactant used.