# 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

## Lumps vs powders

The greater the 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 frequency of collisions between reactant particles increases
• the rate of reaction increases

Note that the mean of the particles does not change. However, since the frequency of collisions increases, the frequency of successful collisions also increases.

## Graphs

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

Comparing two reactions - the greater the surface area, the greater 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
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.
Question

Icing sugar is made from a fine powder, whereas granulated sugar is made from much larger crystals. State and explain which will burn more quickly in air.

Icing sugar will burn in air more quickly than granulated sugar because it has a much larger surface area to volume ratio than granulated sugar. This means that successful collisions with the oxygen molecules in the air will be more frequent.