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 vs 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
- the rate of reaction increases
Note that the mean energy of the particles does not change. However, since the frequency of collisions increases, the frequency of successful collisions also increases.
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.
The gradient of the line is equal to the rate of reaction. Faster reaction with the powder:
- gives a steeper line
- finishes sooner
Make sure you answer questions 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.