Who this is for: Cambridge IGCSE Chemistry (0620) students who want chemical energetics — exothermic and endothermic reactions, energy level diagrams and bond energy calculations — to become reliable marks instead of confusion about which direction energy flows.
What query it owns: how to understand and revise chemical energetics in Cambridge IGCSE Chemistry.
Why this is safe: this page owns the chemical energetics revision-guide angle, while Tutopiya’s Chemical Energetics subtopic page owns the learning resource and the free Chemical Energetics quiz owns the practice.

Chemical energetics deals with energy changes during reactions. In Cambridge IGCSE Chemistry (0620), examiners test whether you can classify reactions as exothermic or endothermic, draw energy level diagrams, and calculate enthalpy changes using bond energies. This guide covers the syllabus definitions, the bond-breaking and bond-making rules that prevent sign errors, and the question types that appear every year.

Key takeaways

• Exothermic reactions release energy — products have less energy than reactants (ΔH negative).
• Endothermic reactions absorb energy — products have more energy than reactants (ΔH positive).
• Bond breaking requires energy (endothermic step); bond making releases energy (exothermic step).
• ΔH = energy to break bonds − energy released making bonds (or bonds broken − bonds formed).
• Energy level diagrams show reactants and products on a vertical energy axis with ΔH arrow.

What is chemical energetics in Cambridge IGCSE Chemistry?

Chemical energetics is the study of energy changes that accompany chemical reactions. Exothermic reactions transfer thermal energy to the surroundings (temperature rises). Endothermic reactions take in thermal energy from the surroundings (temperature falls). Bond energy calculations use average bond energies to estimate the overall enthalpy change of a reaction.

You can read the full explanation, diagrams and worked examples on Tutopiya’s Chemical Energetics subtopic page before you attempt questions.

The core ideas you must master

Idea	What it means	How the exam uses it
Exothermic	Energy released; surroundings warm up	”State whether the reaction is exothermic.”
Endothermic	Energy absorbed; surroundings cool down	”Explain why the temperature decreases.”
Energy level diagram	Reactants/products on energy axis	”Draw an energy level diagram for…”
Bond energy calculation	ΔH = bonds broken − bonds formed	”Calculate the enthalpy change using bond energies.”
Activation energy	Minimum energy to start reaction	”Label the activation energy on the diagram.”

How to calculate enthalpy change from bond energies — step by step

1. Write the balanced equation and identify all bonds broken and formed.
2. Sum bond energies broken (reactants side) — this is energy absorbed.
3. Sum bond energies formed (products side) — this is energy released.
4. Calculate ΔH = bonds broken − bonds formed.
5. Interpret the sign: negative ΔH = exothermic; positive ΔH = endothermic.
6. State answer with units (kJ/mol) if required.

Once you have worked through a few, test yourself with the free Chemical Energetics quiz — it tells you fast whether the calculations have actually stuck.

Exothermic vs endothermic: which idea does the question want?

Feature	Exothermic	Endothermic
Energy flow	Released to surroundings	Absorbed from surroundings
Temperature change	Surroundings get warmer	Surroundings get cooler
Energy level diagram	Products lower than reactants	Products higher than reactants
ΔH sign	Negative	Positive
Examples	Combustion, neutralisation	Thermal decomposition of CaCO₃

Chemical energetics in past-paper wording: command words that matter

Command word / phrase	What the question wants	Typical energetics stem
State	Exothermic or endothermic	”State whether the reaction is exothermic or endothermic.”
Explain	Link energy change to temperature	”Explain why the temperature rises during the reaction.”
Draw	Energy level diagram with labels	”Draw an energy level diagram for the reaction.”
Calculate	Bond energy enthalpy change	”Calculate the enthalpy change using the bond energies given.”
Label	Parts of a diagram	”Label the activation energy and the enthalpy change.”

Worked exam-style stems (how to answer the wording)

1. “The reaction H₂ + Cl₂ → 2HCl releases energy. State whether the reaction is exothermic or endothermic and explain what happens to the temperature of the surroundings.” Exothermic. Energy is transferred to the surroundings, so the temperature increases. Mark-scheme reward: correct classification + temperature linked to energy release.
2. “Calculate the enthalpy change for H₂ + Cl₂ → 2HCl using bond energies: H–H 436, Cl–Cl 242, H–Cl 431 kJ/mol.” Bonds broken: 436 + 242 = 678. Bonds formed: 2 × 431 = 862. ΔH = 678 − 862 = −184 kJ/mol (exothermic). Reward: correct bond count + subtraction order.
3. “Draw an energy level diagram for an endothermic reaction, labelling reactants, products, activation energy and ΔH.” Products higher than reactants; activation energy hump above reactants; ΔH arrow from reactants to products pointing upward. Reward: correct relative levels + labels.

When you can recognise the wording instantly, work the full set on the Chemical Energetics quiz and revisit topical stems via the Cambridge IGCSE Chemistry resource hub to lock the method in.

How chemical energetics connects to the rest of the syllabus

Chemical energetics links to reaction types across the course — combustion is exothermic, thermal decomposition is endothermic. It feeds into rates of reaction (activation energy) and equilibria (energy changes in forward and reverse reactions). When you are ready to mix topics, the Cambridge IGCSE Chemistry resource hub lets you move straight from a weak subtopic into the next.

Common mistakes students make

• Reversing the bond energy formula: ΔH = bonds broken − bonds formed, not the other way round.
• Drawing products higher for exothermic reactions on energy level diagrams.
• Confusing activation energy with enthalpy change — they are different arrows on the diagram.
• Forgetting to multiply bond energy by the number of bonds (e.g. 2 H–Cl bonds in 2HCl).
• Saying endothermic reactions “produce cold” — they absorb energy from surroundings.

When you need more support

If energetics calculations keep tripping you up — especially bond energy signs and diagrams — work through the Chemical Energetics quiz to pinpoint the exact gap, then get focused help from a Cambridge IGCSE Chemistry tutor to fix it quickly.

Frequently asked questions

Is chemical energetics hard in Cambridge IGCSE Chemistry? No — the rules are fixed. Marks are lost when students reverse the bond energy subtraction or draw energy level diagrams with products at the wrong height.

What is the bond energy formula? ΔH = sum of bond energies broken − sum of bond energies formed. Negative ΔH means exothermic.

How do I tell exothermic from endothermic on a diagram? Exothermic: products lower than reactants. Endothermic: products higher than reactants.

How do I revise chemical energetics effectively? Learn the definitions, practise bond energy calculations with sign checks, draw one diagram per type, then take the Chemical Energetics quiz.

Ready to master Cambridge IGCSE Chemistry energetics?

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