# Rates and surface area to volume ratio

## Dividing lumps

For a given of a solid, large lumps have smaller to 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
A solid divided into smaller lumps has a higher surface area to volume ratio than the same mass of solid divided into larger lumps

## Lumps vs powders

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

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

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

## Graphs

The rates of two or more reactions can be compared using a graph of mass or volume of 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 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.