# Rates of reaction

## Home learning focus

Learn about the rate of chemical reactions, and how different factors affect it.

This lesson includes:

• one video
• two activities

# Learn

In this video, Fran Scott explains the different ways you can change the rate of a chemical reaction.

## Rate of reaction

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

### Collision theory

For a chemical reaction to happen:

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

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

The rate of reaction can be calculated in two ways:

mean rate of reaction = quantity of reactant used ÷ time taken

or

mean rate of reaction = quantity of product formed ÷ time taken

The quantities of the reactants/products can be calculated by observing changes in either mass or volume.

Therefore, the units for rate of reaction can either be g s⁻¹ (grams per second) or cm³ s⁻¹ (cubed centimetres per second).

### Mass/volume vs time graphs

Mass/volume vs time graphs can be used to plot changes in either mass or volume over time. The gradient of the line is equal to the rate of reaction:

• the steeper the line, the greater the rate of reaction
• the reaction has ended when the line becomes horizontal
• the sooner the graph becomes horizontal, the quicker the reaction

## Changing reaction rates

### Increasing the concentration or pressure

If the concentration of, or pressure on, reactants is increased, then the number of successful collisions will increase, and the rate of reaction will increase.

This is because:

• the reactant particles become more crowded
• the frequency of collisions between reactant particles increases

### Increasing the surface area

For a given mass of a solid, larger lumps have a smaller surface area to volume ratio 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

If the surface area to volume ratio of a reacting solid is increased, the rate of reaction increases. This is because:

• more reactant particles are exposed at the surface
• the frequency of collisions between reactant particles increases

### Increasing the temperature

If the temperature of the reaction mixture is increased, the rate of reaction increases. Because:

• reactant particles move more quickly
• the energy of the particles increases
• the frequency of successful collisions between reactant particles increases
• the proportion of collisions which are successful increases

### Catalysts

A catalyst is a substance that:

• speeds up the rate of a reaction
• does not alter the products of the reaction
• is not chemically changed or used up at the end of the reaction

Catalysts do not appear in the overall chemical equation for a reaction.

Only a very small mass of catalyst is needed to increase the rate of a reaction. However, not all reactions have suitable catalysts.

Catalysts in biological reactions are called enzymes.

## How catalysts work

A catalyst provides an alternative reaction pathway that has a lower activation energy than the uncatalysed reaction. This does not change the frequency of collisions. However, it does increase the frequency of successful collisions because more particles have energy greater than the activation energy, therefore there are more successful collisions.

The effect of a catalyst on the activation energy is shown on a chart called a reaction profile. This shows how the energy of the reactants and products change during a reaction.

# Practise

## Activity 1

Correct or incorrect?

Test your knowledge of rates of reaction by deciding whether the statements on this downloadable worksheet from teachit science are correct or incorrect.

You can print off the worksheet or write your answers on a piece of paper.

## Activity 2

Reaction factors

Test your knowledge of rates of reaction by saying whether each factor will make a difference to the rate of reaction and why, with this downloadable worksheet from teachit science.