Exothermic and Endothermic Reactions

This guide covers Topic 5, Chemical energetics — subtopic 5.1 Exothermic and endothermic reactions — for Cambridge IGCSE Chemistry 0620 and Cambridge O Level Chemistry 5070, 2026–2028 series.

Where this fits in 0620/5070

This is a short topic by subtopic count, but it introduces vocabulary and diagrams — enthalpy change, activation energy, reaction pathway diagrams — that reappear constantly: in rate of reaction (Topic 6.2), and much more quantitatively at AS Level in Chemical Energetics: Hess’s Law and Enthalpy Cycles, which assumes you’re already comfortable with everything below.

Syllabus coverage

CAMBRIDGE IGCSE CHEMISTRY 0620

Core

• Stating that an exothermic reaction transfers thermal energy to the surroundings, increasing their temperature (5.1)
• Stating that an endothermic reaction takes in thermal energy from the surroundings, decreasing their temperature (5.1)
• Interpreting reaction pathway diagrams showing exothermic and endothermic reactions (5.1)

Supplement / Extended

• Naming the transfer of thermal energy during a reaction the enthalpy change, ΔH — negative for exothermic reactions, positive for endothermic reactions (5.1)
• Defining activation energy, Ea, as the minimum energy colliding particles must have to react (5.1)
• Drawing and labelling full reaction pathway diagrams, including reactants, products, ΔH and Ea (5.1)
• Stating that bond breaking is endothermic and bond making is exothermic, and explaining ΔH in these terms (5.1)
• Calculating the enthalpy change of a reaction using bond energies (5.1)

CAMBRIDGE O LEVEL CHEMISTRY 5070

5070 has no Core/Extended split — all of the above, including the Supplement-only bond-energy calculation, is required for every O Level candidate.

Exothermic and endothermic reactions

An exothermic reaction releases thermal energy to the surroundings — the surroundings get hotter. Combustion, neutralisation and most oxidation reactions are exothermic.

An endothermic reaction takes in thermal energy from the surroundings — the surroundings get colder. Thermal decomposition and photosynthesis are endothermic.

A simple experimental test: mix the reactants in a test tube (or with water, for a dissolving process) and measure temperature with a thermometer — a rise means exothermic, a fall means endothermic.

Enthalpy change, ΔH

The enthalpy change, ΔH, is the name given to the thermal energy transferred during a reaction, at constant pressure.

exothermic reaction:  ΔH is negative (energy leaves the system)
endothermic reaction: ΔH is positive (energy enters the system)

Reaction pathway diagrams

A reaction pathway diagram plots energy (vertical axis) against the progress of the reaction (horizontal axis), and shows four things at once:

• The energy level of the reactants.
• The energy level of the products.
• ΔH — the vertical gap between reactants and products. Products below reactants means exothermic (ΔH negative); products above reactants means endothermic (ΔH positive).
• Activation energy, Ea — the “hump” the reaction must climb over before it can proceed, measured from the reactants’ energy level to the peak of the curve. Every reaction has an activation energy, whether it’s exothermic or endothermic — it’s the energy needed to start breaking bonds, not a measure of the overall energy change.

Exothermic: reactants start high, climb over the activation-energy hump, then drop to a lower-energy product level — net energy released.

Endothermic: reactants start low, climb over the activation-energy hump, then settle at a higher-energy product level — net energy absorbed.

Why: bond breaking and bond making

Every reaction involves breaking bonds in the reactants and making new bonds in the products.

breaking bonds:  requires energy input   → endothermic step
making bonds:    releases energy         → exothermic step

The overall ΔH is the balance between these two steps:

ΔH = energy to break bonds in reactants − energy released making bonds in products

If more energy is released making bonds than was needed to break them, the overall reaction is exothermic (ΔH negative). If breaking the reactants’ bonds costs more energy than making the products’ bonds releases, the reaction is endothermic (ΔH positive).

Calculating enthalpy change from bond energies

Worked example. Use the bond energies below to calculate the enthalpy change for the reaction H₂ + Cl₂ → 2HCl.

Bond energies (kJ/mol): H–H = 436,  Cl–Cl = 242,  H–Cl = 431

Step 1 — energy to break bonds in reactants:
  H–H: 436
  Cl–Cl: 242
  total = 436 + 242 = 678 kJ/mol

Step 2 — energy released making bonds in products:
  2 × H–Cl: 2 × 431 = 862 kJ/mol

Step 3 — overall enthalpy change:
  ΔH = energy to break bonds − energy released making bonds
     = 678 − 862 = −184 kJ/mol

ΔH is negative, so this reaction is exothermic — consistent with more energy being released forming the new H–Cl bonds than was needed to break the original H–H and Cl–Cl bonds.

Common mistakes

• Confusing which direction is which. Exothermic = energy exits (to the surroundings, so the surroundings warm up); endothermic = energy enters the reacting system (from the surroundings, so they cool down). The easiest anchor: exothermic reactions feel hot to the touch.
• Treating activation energy as if it were ΔH. Ea is always positive (it’s a “hill to climb”) and says nothing about whether the reaction is overall exothermic or endothermic — that’s what ΔH describes.
• Getting the bond-energy calculation backwards. It’s bonds broken minus bonds made — reversing the subtraction flips the sign and turns an exothermic answer into an endothermic one.
• Assuming breaking bonds releases energy. It’s the opposite: breaking any bond always requires an energy input; only bond making releases energy.

Quick revision checklist

• Exothermic vs endothermic: direction of thermal energy transfer, and the effect on surroundings’ temperature
• Reading and interpreting a reaction pathway diagram
• (0620 Extended, 5070 required) ΔH sign convention, activation energy Ea, and drawing a fully-labelled reaction pathway diagram
• (0620 Extended, 5070 required) explaining ΔH in terms of bond breaking (endothermic) and bond making (exothermic)
• (0620 Extended, 5070 required) calculating ΔH from given bond energies

Related resources

• Rates of Reaction and Reversible Reactions — activation energy applied to reaction rate and collision theory
• Chemical Energetics: Hess’s Law and Enthalpy Cycles — the AS & A Level continuation
• Cambridge IGCSE Chemistry hub · Cambridge O Level Chemistry hub

Written against Cambridge IGCSE Chemistry 0620 and Cambridge O Level Chemistry 5070, 2026–2028 series. Always check the current syllabus for your examination year.