# Rate of reaction

The rate of a reaction is a measure of how quickly a is used up, or a is formed.

## Collision theory

For a chemical reaction to happen:

• reactant 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. This is different for different reasons.

1. Two pairs of particles move towards each other

## Measuring rates of reaction

There are different ways to determine the rate of a reaction. The method chosen usually depends on the reactants and products involved, and how easy it is to measure changes in them.

In addition, how long a reaction is observed for depends on the rate of reaction. Reactions can vary from being almost instantaneous to taking years to complete. In the lab, reactions are usually followed over a few seconds or minutes.

Question

Rusting is a slow reaction. Give four examples of a very fast reaction.

Combustion, explosions, neutralisation reactions and precipitation reactions are very fast reactions.

### Measuring mass

The change in of a reactant or product can be followed during a reaction. This method is useful when carbon dioxide is a product which leaves the reaction container. It is not suitable for hydrogen and other gases with a small , Mr. The units for rate are usually g/s or g/m.

### Measuring volume

The change in of a reactant or product can be followed during a reaction. This method is used when a gas leaves the reaction container. The volume of a gas is measured using a gas syringe, or an upside down or measuring cylinder. The units for rate are usually cm3/s or cm3/min.

Two ways to measure the volume of a gas produced in a reaction

## Graphs

The rate of reaction can be analysed by plotting a graph of mass or volume of product formed against time. The graph shows this for two reactions.

The of the line is equal to the rate of reaction:

• the steeper the line, the greater the rate of reaction
• fast reactions finish sooner (when the line becomes horizontal) than slow reactions