Who this is for: Cambridge IGCSE Coordinated Science (0654) students who want energy changes in chemical reactions — exothermic and endothermic processes, bond breaking and making — to become a reliable source of marks instead of vague “energy is released” answers.
What query it owns: how to understand and revise energy changes in chemical reactions in Cambridge IGCSE Coordinated Science.
Why this is safe: this page owns the energy-changes-in-chemical-reactions revision-guide angle, while Tutopiya’s Energy Changes In Chemical Reactions subtopic page owns the learning resource and the free Energy Changes In Chemical Reactions quiz owns the practice.

Chemical reactions involve energy transfers. Cambridge IGCSE Coordinated Science (0654) expects you to distinguish exothermic and endothermic reactions, explain energy changes in terms of bond breaking and making, and interpret simple energy profile diagrams. This guide links each concept to what examiners reward, so you can answer explanation and classification questions with confidence.

Key takeaways

• Bond breaking requires energy (endothermic); bond making releases energy (exothermic).
• Exothermic reactions release energy to surroundings — temperature rises (e.g. combustion).
• Endothermic reactions absorb energy from surroundings — temperature falls (e.g. thermal decomposition).
• Overall energy change = energy to break bonds − energy released making bonds.
• Activation energy is the minimum energy needed for a reaction to start.

What are energy changes in chemical reactions in Cambridge IGCSE Coordinated Science?

Every reaction involves breaking existing bonds and forming new ones. Breaking bonds absorbs energy; making bonds releases it. If more energy is released than absorbed, the reaction is exothermic; if more is absorbed, it is endothermic. Activation energy is the energy barrier that must be overcome before bonds start to break.

You can read the full explanation, diagrams and notes on Tutopiya’s Energy Changes In Chemical Reactions subtopic page before you attempt questions.

Exothermic vs endothermic — comparison table

Feature	Exothermic	Endothermic
Energy flow	Released to surroundings	Absorbed from surroundings
Temperature change	Surroundings get warmer	Surroundings get cooler
Energy profile	Products lower than reactants	Products higher than reactants
Examples	Combustion, neutralisation, respiration	Thermal decomposition, photosynthesis

Bond energy — the calculation logic

Step	Process	Energy
1	Break bonds in reactants	Energy absorbed (+)
2	Make bonds in products	Energy released (−)
3	Overall change	Bonds broken − bonds made

If bonds made release more energy than bonds broken absorb → exothermic (negative ΔH).

Energy changes in past-paper wording

Command word	What the question wants	Typical stem
State	Classify reaction type	”State whether this reaction is exothermic or endothermic.”
Explain	Link to bond breaking/making	”Explain why combustion is exothermic.”
Describe	Energy profile features	”Describe the energy profile of an exothermic reaction.”
Calculate	Simple bond energy sum	”Calculate the energy change using bond energies.”

Worked exam-style stems

1. “Explain, in terms of bonds, why a neutralisation reaction is exothermic.” Bonds in reactants are broken (energy absorbed) and new bonds in products are made (energy released); more energy is released than absorbed. Reward: bond breaking + bond making + net release.
2. “State what is meant by activation energy.” The minimum energy needed for a reaction to occur / for bonds to start breaking. Reward: minimum energy + reaction starts.
3. “A reaction causes the temperature of the surroundings to decrease. Is it exothermic or endothermic?” Endothermic — energy is absorbed from the surroundings. Reward: correct classification with energy absorbed.

Test yourself with the Energy Changes In Chemical Reactions quiz once you can classify reactions and explain bond energy logic.

How energy changes connect to the syllabus

Energy changes link to rate of reaction (activation energy) and electricity and chemistry (cell potentials). Exothermic combustion connects to fuels and air pollution topics. The Cambridge IGCSE Coordinated Science resource hub links every Energy Changes subtopic.

Common mistakes students make

• Saying exothermic reactions have no activation energy (all reactions need some).
• Confusing energy of reactants/products with activation energy on diagrams.
• Stating endothermic reactions release energy (they absorb it).
• Forgetting bond breaking is endothermic and bond making is exothermic.
• Using temperature change of the reaction mixture without linking to surroundings.

When you need more support

If energy change explanation questions keep costing marks, work through the Energy Changes In Chemical Reactions quiz, then get focused help from a Cambridge IGCSE Coordinated Science tutor.

Frequently asked questions

Is energy changes hard in Coordinated Science? The bond-breaking and bond-making framework makes classification and explanation straightforward with practice.

What is an exothermic reaction? A reaction that releases energy to the surroundings, often shown by a temperature rise.

What is activation energy? The minimum energy required for a reaction to start.

How do I revise energy changes effectively? Learn exothermic vs endothermic features, practise bond energy calculations, sketch energy profiles, then take the quiz.

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