Rates of Chemical Change

The rate of a chemical reaction tells us how fast a reaction happens. Understanding what affects reaction rates helps chemists control reactions — making them faster, slower, or more efficient depending on the goal. This topic also includes reversible reactions and how conditions affect equilibrium.

Practicals on this page:
Rates of Reactions

What Is Rate of Reaction?

The rate of reaction is how quickly reactants are turned into products. It's usually measured by:

• How fast a reactant is used up
• How fast a product is formed

Measuring Rates of Reaction

There are several ways to measure the rate of a reaction:

1. Gas volume – Collect gas with a gas syringe or over water.
2. Mass loss – Measure decrease in mass using a balance.
3. Time for visual change – e.g. the disappearing cross in the sodium thiosulfate + hydrochloric acid reaction.

These methods are commonly used in required practicals.

Factors Affecting the Rate of Reaction

Several factors affect how often particles collide and how quickly a reaction happens.

Collision Theory

Reactions happen when particles collide with enough energy. This is called a successful collision. The more frequent and energetic the collisions, the faster the reaction.

Concentration

Increasing the concentration of a solution means more particles in the same space, so collisions happen more often.

Pressure (for gases)

Higher pressure squashes gas particles into a smaller space, increasing the frequency of collisions.

Surface Area

Breaking up a solid into smaller pieces gives it a larger surface area, exposing more particles to react.

Temperature

Higher temperatures give particles more kinetic energy, so they move faster and collide more often with more energy.

Catalysts

A catalyst speeds up a reaction without being used up. It lowers the activation energy, making it easier for collisions to be successful.

Rate of Reaction Graphs

You can plot reaction data on a graph with:

• Time on the x-axis
• Amount of product or reactant on the y-axis

Steeper lines = faster reaction
Flattened line = reaction has finished

Finding Rate from a Graph

To find the rate at a specific time:

• Draw a tangent to the curve at that point

• Calculate the gradient (change in y ÷ change in x)

This gives you the instantaneous rate of reaction at that moment.

Reversible Reactions

In a reversible reaction, the products can react to form the original reactants again. These reactions are shown with a  ⇌ symbol.

Example:

When a reversible reaction happens in a closed system, it can reach equilibrium.

Equilibrium and Le Chatelier’s Principle

Equilibrium is reached when the forward and backward reactions happen at the same rate — the amounts of reactants and products stay constant (though not necessarily equal).

According to Le Chatelier’s Principle, if conditions change, the equilibrium position will shift to oppose the change.

Changing Concentration

Increasing the concentration of a reactant shifts the equilibrium to make more products

Changing Temperature

Increasing temperature favours the endothermic direction

Decreasing temperature favours the exothermic direction

Changing Pressure (for gases)

Increasing pressure favours the side with fewer gas molecules

Decreasing pressure favours the side with more gas molecules

Endothermic and Exothermic Reactions

• Exothermic reactions give out energy to the surroundings (e.g. combustion, many neutralisations)
• Endothermic reactions take in energy (e.g. thermal decomposition, certain reversible reactions)

Understanding energy changes is key to predicting how the equilibrium will shift.

Also see Chemical Changes, Energy Changes, Electrolysis

PRACTICAL - Rates of Reactions

This practical explores how changing conditions affects the rate of a chemical reaction. You’ll usually investigate the effect of temperature, concentration, or surface area.

Two common methods are:

• Measuring gas produced – e.g. reaction of marble chips (calcium carbonate) with hydrochloric acid, collecting gas over time.
• Disappearing cross method – reaction between sodium thiosulfate and hydrochloric acid produces a cloudy sulfur precipitate that hides a cross on paper.

By timing how long it takes for a set change to occur, you can compare how different variables affect the reaction speed.

Links for Learning

BBC Bitesize: Rate of reaction
The Science Sauce: Rates of reaction
Study Mind: Factors affecting rate of reaction
BBC Bitesize: Indicators of a chemical reaction
BBC Bitesize: Reversible reactions
Revise Chemistry: Reversible Reactions
Study Rocket: Le Chateliers Principle