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Cambridge IGCSE Chemistry (0620) · Topic 5 — Chemical energetics
Mini-Lesson

Chemical Energetics

This mini-lesson walks you through the whole of Cambridge IGCSE Topic 5 — Chemical energetics: exothermic and endothermic reactions, reaction-pathway energy diagrams with the activation energy Ea and overall energy change ΔH, and bond-energy calculations.

Supplement note: some statements in Topic 5 are Supplement (extended) only — the ΔH sign convention and everything about bond energies. They are clearly marked SUPPLEMENT as you go.

exothermic gives out heat endothermic takes in heat 🌡️ energy moves to / from the surroundings

Work through each screen, answer the questions as you go (some wordy, some calculations) and collect ⭐ stars. Press Start when you're ready.

Exothermic & endothermic

Heat given out, or taken in

Every reaction involves an energy change with its surroundings. There are two kinds:

  • An exothermic reaction gives out heat energy to the surroundings, so the temperature of the surroundings rises.
  • An endothermic reaction takes in heat energy from the surroundings, so the temperature of the surroundings falls.
Exothermic temp ↑ heat out Endothermic temp ↓ heat in
You detect the type by the temperature change of the surroundings: warming = exothermic, cooling = endothermic.

Examples: combustion (burning fuels) and neutralisation are exothermic; thermal decomposition (e.g. heating limestone) and the reaction in a cold pack are endothermic.

Watch out: "exothermic" means energy goes out of the reaction into the surroundings — so the surroundings get hotter, not colder. Many students flip this.

Quick check

Reading the thermometer

?A student adds a powder to water in a beaker and the temperature of the mixture drops from 21 °C to 9 °C. What type of reaction is this?
Reaction pathway diagrams

Energy diagram — exothermic

A reaction pathway (energy-level) diagram plots energy up the side as the reaction goes from reactants to products. For an exothermic reaction the products sit lower than the reactants — energy was released to the surroundings.

energy progress of reaction → reactants products Eₐ (activation energy) ΔH negative
Products below reactants → energy released. Ea is the height of the "hump" the reactants must climb to react.

Activation energy Ea is the minimum energy the colliding particles need to start the reaction — it is the height from the reactant level up to the top of the hump, not the whole curve.

SUPPLEMENT For an exothermic reaction the overall energy change ΔH is negative (energy is given out). The arrow for ΔH points downward, from reactants to the lower products.

Quick check

Reading the hump

?On a reaction pathway diagram for an exothermic reaction, what does the activation energy Ea represent?
Reaction pathway diagrams

Energy diagram — endothermic

For an endothermic reaction the products end up higher than the reactants — energy was taken in from the surroundings and stored in the products.

energy progress of reaction → reactants products Eₐ (activation energy) ΔH positive
Products above reactants → energy absorbed. The same Ea hump must still be climbed first.

SUPPLEMENT For an endothermic reaction the overall energy change ΔH is positive (energy is taken in). The ΔH arrow points upward, from reactants to the higher products.

Supplement · Quick check

Sign of ΔH

?On an energy-level diagram the products are higher than the reactants. Which statement is correct?SUPPLEMENT
Supplement · Bond energies

Bonds: breaking vs making SUPPLEMENT

During a reaction, bonds in the reactants must first be broken, then new bonds in the products are made. These two steps move energy in opposite directions:

  • Bond breaking is endothermic — energy must be put in (taken from the surroundings) to pull a bond apart.
  • Bond making is exothermic — energy is given out (released to the surroundings) when a new bond forms.
breaking = endothermic energy IN to split them making = exothermic energy OUT as they join

So why is a reaction exothermic or endothermic overall? If more energy is released making bonds than was used breaking them, the reaction gives out heat (exothermic). If breaking the bonds costs more than making bonds releases, it takes heat in (endothermic).

Watch out: bond breaking takes energy in; bond making gives energy out — students often swap these.

Supplement · Calculating ΔH

ΔH from bond energies SUPPLEMENT

A bond energy is the energy needed to break (or released on making) one mole of that bond. We find the overall energy change as:

ΔH = bonds broken − bonds madeΔH = energy taken in breaking all reactant bonds − energy released making all product bonds

Worked example — the formation of hydrogen chloride:

H–H + Cl–Cl → 2 H–Cl Bonds BROKEN (in) 1 × (H–H) = 436 1 × (Cl–Cl) = 242 = 678 kJ/mol Bonds MADE (out) 2 × (H–Cl) = 2 × 431 = 862 = 862 kJ/mol
ΔH = 678 − 862 = −184 kJ/mol → negative → exothermic.
Worked example

Bonds broken = (H–H) + (Cl–Cl) = 436 + 242 = 678 kJ/mol

Bonds made = 2 × (H–Cl) = 2 × 431 = 862 kJ/mol

ΔH = 678 − 862 = −184 kJ/mol (negative → exothermic ✓)

Supplement · Calculate

Your turn — ΔH from bond energies SUPPLEMENT

1For the combustion of hydrogen: 2H₂ + O₂ → 2H₂O. Bond energies (kJ/mol): H–H = 436, O=O = 498, O–H = 464. Calculate ΔH in kJ/mol (include the sign).
kJ/mol
Hint: broken = 2×(H–H) + O=O = 872 + 498 = 1370. Made = 4×(O–H) = 4×464 = 1856. ΔH = 1370 − 1856.
Supplement · Calculate

Your turn — is it exo or endo? SUPPLEMENT

2A reaction breaks bonds worth a total of 945 kJ/mol and makes bonds worth a total of 812 kJ/mol. Calculate ΔH in kJ/mol (include the sign).
kJ/mol
Hint: ΔH = bonds broken − bonds made = 945 − 812. A positive answer means endothermic.
Sort it

Exothermic or endothermic?

Tap whether each change gives out heat (exothermic) or takes in heat (endothermic).

Sort it

Match the clue to the type

Tap a clue, then tap the box it belongs in.

🔥 Exothermic (heat out)

❄️ Endothermic (heat in)

Recap

The key points to know

Exothermic: heat given out → surroundings get hotter (e.g. combustion, neutralisation).

Endothermic: heat taken in → surroundings get colder (e.g. thermal decomposition).

Pathway diagrams: Ea = the activation-energy "hump"; products lower (exo) or higher (endo) than reactants.

ΔH sign SUPPLEMENT: negative for exothermic, positive for endothermic.

Bonds SUPPLEMENT: breaking = endothermic; making = exothermic.

Calculation SUPPLEMENT: ΔH = bonds broken − bonds made.

You've covered all of Cambridge IGCSE Chemistry Topic 5 — Chemical energetics. Press Finish to see your score.

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Mini-lesson complete!

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