What Are Energy Changes in Chemical Reactions?

In many chemical reactions, energy is either released or absorbed. These energy changes are an important part of understanding how chemical reactions occur.

In O Level Chemistry, reactions are commonly classified into two types based on energy changes:

• Exothermic reactions
• Endothermic reactions

These reactions involve the transfer of thermal energy (heat) between the reaction and its surroundings.

What Is an Exothermic Reaction?

An exothermic reaction is a chemical reaction that releases heat energy to the surroundings.

Because energy is released, the temperature of the surroundings increases.

This means the surroundings become warmer during the reaction.

Examples of Exothermic Reactions

Common examples include:

• Combustion reactions (burning fuels)
• Respiration in living organisms
• Neutralisation reactions between acids and bases

Example reaction:

Burning methane:

CH₄ + 2O₂ → CO₂ + 2H₂O + energy

This reaction releases a large amount of heat.

What Is an Endothermic Reaction?

An endothermic reaction is a chemical reaction that absorbs heat energy from the surroundings.

Because energy is absorbed, the temperature of the surroundings decreases.

This means the surroundings become cooler during the reaction.

Examples of Endothermic Reactions

Some examples include:

• Photosynthesis in plants
• Thermal decomposition reactions
• Dissolving ammonium nitrate in water

Example reaction:

Thermal decomposition of calcium carbonate:

CaCO₃ → CaO + CO₂

This reaction requires heat energy to occur.

Difference Between Exothermic and Endothermic Reactions

FeatureExothermic ReactionEndothermic ReactionEnergy changeReleases heatAbsorbs heatTemperature changeSurroundings get warmerSurroundings get coolerEnergy flowReaction → surroundingsSurroundings → reaction

Understanding this difference is important for analysing chemical reactions.

Enthalpy Change (ΔH)

The enthalpy change (ΔH) describes the energy change during a chemical reaction.

• For exothermic reactions, ΔH is negative
• For endothermic reactions, ΔH is positive

This indicates whether energy is released or absorbed during the reaction.

Everyday Examples of Energy Changes

Energy changes in reactions can be seen in many real-life situations.

Examples include:

• Hand warmers used in winter (exothermic reactions)
• Instant cold packs used for injuries (endothermic reactions)
• Burning fuels for heating and electricity

These reactions are used in everyday technology and industry.

Why Energy Changes Are Important

Understanding energy changes helps chemists:

• Predict whether reactions will release or absorb heat
• Design industrial chemical processes
• Study fuel efficiency and combustion reactions
• Understand biological energy processes

Energy changes are essential in both chemistry and everyday life.

Exam Tip (5070)

Students are commonly asked to:

• Define exothermic reactions
• Define endothermic reactions
• Identify reactions based on temperature changes

Example exam question:

Explain why the temperature increases during an exothermic reaction.

Answer:

The temperature increases because heat energy is released from the reaction to the surroundings.

Practice Question

During a chemical reaction, the temperature of the surroundings decreases.

What type of reaction is this?

Answer

This is an endothermic reaction because the reaction absorbs heat from the surroundings.

Study O Level Chemistry with IVY Online

Preparing for Cambridge O Level Chemistry (5070) becomes easier with clear explanations and exam-focused practice.

With the IVY Online App, students can access:

✔ Complete syllabus coverage
✔ Interactive concept explanations
✔ Topic-wise past paper questions
✔ Exam-focused revision

📱 Download the IVY Online App and start preparing smarter for Chemistry exams.