The big idea: Every reaction either gives out energy or takes in energy. We measure this as the enthalpy change, ΔH — the heat energy released or absorbed at constant pressure.
- Exothermic reaction → energy is released to the surroundings → the surroundings get hotter → ΔH is negative. - Endothermic reaction → energy is absorbed from the surroundings → the surroundings get colder → ΔH is positive.
The system is the reacting chemicals; the surroundings is everything else (the solution, the beaker, the air).
Exothermic: the products sit LOWER than the reactants, so energy is released to the surroundings — ΔH is negative.
Interactive diagram
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Define ΔH: Enthalpy change (ΔH) is the heat energy transferred between the system and the surroundings during a reaction at constant pressure (units: kJ mol⁻¹).
The sign tells you the direction: − means the system loses energy (exothermic); + means the system gains energy (endothermic).
Whether a reaction is exothermic or endothermic comes down to bonds. Breaking a bond always needs an energy input; making a bond always gives energy out.
Breaking bonds — endothermic
- Energy must be put in to pull bonded atoms apart.
- This is an energy input (positive contribution).
- Happens in the reactants as they react.
Making bonds — exothermic
- Energy is released when new bonds form.
- This is an energy output (negative contribution).
- Happens as the products form.
The overall sign: Compare the total energy in versus out:
- If making the new bonds releases more energy than breaking the old bonds absorbed → net energy out → exothermic (ΔH < 0). - If breaking the old bonds costs more than making the new bonds releases → net energy in → endothermic (ΔH > 0).
Memory hook: Breaking Bonds = Big energy in (endothermic). Making bonds gives energy out (exothermic). 'exo' = exit = energy out.
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An energy (reaction) profile plots enthalpy on the vertical axis against the reaction progress. Three features are always labelled: the reactant level, the product level, the activation energy (the hump) and the enthalpy change (the gap between the two levels).
Endothermic: the products sit HIGHER than the reactants, so energy is absorbed from the surroundings — ΔH is positive.
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
Define the two energies: - Activation energy (Eₐ) — the minimum energy reactants need to react; measured from the reactant level up to the peak. - Enthalpy change (ΔH) — the energy difference between reactants and products; measured from the reactant level to the product level.
How to tell which is which at a glance: Look at the product level relative to the reactants:
- products below reactants → energy released → exothermic → ΔH negative. - products above reactants → energy absorbed → endothermic → ΔH positive.
The Eₐ hump is always there for both — it does not decide the sign of ΔH.
How this is tested: Energetics opens with a guaranteed energy-profile task.
- Paper 1A (MCQ): interpret a temperature change, e.g. 'the temperature of the surroundings falls — what does this tell you about the system and ΔH?' - Paper 2: sketch and fully label an energy profile for a stated reaction — reactants, products, Eₐ and ΔH — and explain the sign.
The markers want every label in the right place and the product level drawn correctly above/below the reactants.
Don't lose the easy marks: On a sketch: (1) put reactants and products at the correct relative heights, (2) draw the Eₐ hump from the reactant level, (3) mark ΔH between the two levels (not from the peak). For exothermic, products go lower.
IB-style question — energy profile of an exothermic reaction (a)
When a magnesium ribbon is added to dilute hydrochloric acid the mixture becomes noticeably warmer. (a) Sketch and label the energy profile for this reaction, including the reactants, products, the activation energy Eₐ and the enthalpy change ΔH. [3]
How to score the marks
- Mark 1 — the levels. The mixture warms up, so the reaction is exothermic: draw the products lower than the reactants and label both levels.
- Mark 2 — activation energy. Draw the curve rising to a peak between the levels and label Eₐ from the reactant level up to the peak.
- Mark 3 — enthalpy change. Label ΔH as the gap between the reactant and product levels, with an arrow pointing down (ΔH negative).
Final answer
An exothermic profile: reactants high, an Eₐ hump, products lower, ΔH (negative) marked downward between the two levels — shown below.
Exothermic: the products sit LOWER than the reactants, so energy is released to the surroundings — ΔH is negative.
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
IB-style question — interpreting a temperature change (b)
(b) In a different experiment, dissolving a solid in water causes the temperature of the solution to fall. Identify the type of reaction and state the sign of ΔH, the energy change of the system, and the relative stability of the products. [2]
How to score the marks
- Mark 1 — classify. The surroundings (the solution) cool down, so energy is absorbed from the surroundings: the change is endothermic, so ΔH is positive (> 0).
- Mark 2 — system and stability. The system gains energy; the products are at a higher energy than the reactants, so the products are less stable than the reactants.
Final answer
Endothermic; ΔH > 0; the system gains energy (taken from the surroundings); the products are higher in energy and so less stable than the reactants.