# Rates, concentration and pressure

## Collision theory

For a chemical reaction to happen:

• must collide with each other
• the particles must have enough energy for them to react

A collision that produces a reaction is called a . The is the minimum amount of needed for a collision to be successful. It is different for different reactions.

Colliding particles

1. Two pairs of particles move towards each other

The greater the of successful collisions, the greater the rate of reaction. If the of a reacting or the of a reacting gas is increased:

• the reactant particles become more crowded
• 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.

### 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.

The curved red and green lines are the reaction data. The straight dashed lines at the start of the two reactions are called . They follow the curve at the beginning of the reaction. The steeper the tangent, the faster the reaction.

Tangents can also be drawn at other points on the graph. As the reaction carries on, the steepness of the tangent to the curve decreases (eg the blue dashed line) so the reaction is slowing down.

## Calculating the rate of reaction from the gradient of a tangent - Higher

To calculate the rate at the start of a reaction, you need to find the mathematical gradient (steepness) of the gradient. Use this equation:

For example, using the green tangent below:

The green tangent shows lower pressure

Rate of reaction = 12 cm3/s