Rates and surface area to volume ratio
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 rate 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 rate 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 rate of collisions increases (they happen more often), the rate 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 horizontal line shows that no more product is being made - the reaction has finished
- the mass of product formed is not affected by the rate of reaction
The gradient of the line is equal to the rate of reaction. The faster reaction with the powder:
- gives a steeper line
- finishes sooner
Always refer to 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.