A-Level Chemistry: Energetics & Thermodynamics Revision

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In an exothermic reaction, what happens energetically with the bonds?

Energetics & Thermodynamics is a core part of A-Level Chemistry. Revise the key concepts and common mistakes below, then lock them in with the free games.

Key concepts

Enthalpy change ΔHHeat energy change at constant pressure; negative for exothermic, positive for endothermic.

Standard enthalpy of formationΔH_f°, enthalpy change when 1 mole of compound forms from its elements in standard states.

Standard enthalpy of combustionΔH_c°, enthalpy change when 1 mole of substance is completely burned in oxygen under standard conditions.

Bond enthalpyAverage energy required to break 1 mole of a particular covalent bond in the gas phase.

CalorimetryExperimental measurement of heat change using q = mcΔT, often with a polystyrene cup or bomb calorimeter.

Standard conditions100 kPa pressure, a stated temperature (usually 298 K) and 1 mol dm⁻³ for solutions.

Enthalpy ΔHHeat change at constant pressure (kJ/mol).

ExothermicReleases heat; ΔH negative.

EndothermicAbsorbs heat; ΔH positive.

Hess's lawΔH independent of route taken.

Standard state298 K, 100 kPa, pure substance.

ΔHfEnthalpy of formation from elements.

Gibbs energy ΔGΔH - TΔS; <0 means feasible.

FeasibilityThermodynamic ability for reaction to occur.

What your exam board covers

Where Energetics & Thermodynamics sits in each exam board's specification. Tap a board to see what it lists.

AQAEnergeticsSpec 7405

Part of: Physical chemistry

Source: AQA A-level Chemistry (7405) specification — Subject content, accessed 23 June 2026.

AQAThermodynamics (A-level only)Spec 7405

Part of: Physical chemistry

Source: AQA A-level Chemistry (7405) specification — Subject content, accessed 23 June 2026.

EdexcelTopic 8: Energetics ISpec 9CH0

Part of: Energetics, kinetics and equilibria

Source: Pearson Edexcel A-Level Chemistry (9CH0) specification, accessed 23 June 2026.

Learn Energetics & Thermodynamics step by step

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Other boards:CCEA Eduqas/WJEC OCR

Common mistakes to avoid

Questions where students often pick the tempting wrong answer — make sure you know the right one:

In an exothermic reaction, what happens energetically with the bonds?✗ Bond breaking releases energy and bond making absorbs energy. ✓ Energy released forming new bonds exceeds energy absorbed breaking existing bonds.

Why does Hess's Law work?✗ Hess's Law works because energy always cancels out around any cycle. ✓ Enthalpy is a state function — change depends only on initial and final states, not the route taken.

Practise Energetics & Thermodynamics — free games

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Energetics & Thermodynamics

Key concepts

What your exam board covers

Common mistakes to avoid

Practise Energetics & Thermodynamics — free games

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