The key idea
Exothermic reactions release energy to the surroundings; the temperature rises.Endothermic reactions absorb energy; the temperature falls.Breaking bonds requires energy; making bonds releases energy.
energy change = energy in (breaking bonds) − energy out (making bonds)
Use the labels to explain the scientific relationship shown.
The bit that matters
Keep the idea tight, then use the worked example to practise the exact exam wording.
Exothermic and endothermic
In an exothermic reaction, energy is transferred from the reaction to the surroundings, so the temperature rises.Examples include combustion, neutralisation and respiration.In an endothermic reaction, energy is absorbed from the surroundings, so the temperature falls.Examples include thermal decomposition, dissolving ammonium chloride and photosynthesis.
Bond breaking and bond making
Breaking chemical bonds always requires energy (endothermic).Making chemical bonds always releases energy (exothermic). In any reaction, both bond breaking and bond making occur.If more energy is released making bonds than is absorbed breaking them, the overall reaction is exothermic.If more energy is needed to break bonds than is released making them, the overall reaction is endothermic.
Energy profile diagrams
An energy profile diagram shows the energy of reactants and products on the y-axis and progress of reaction on the x-axis.Exothermic reactions show products at a lower energy level than reactants; endothermic reactions show products at a higher level.The activation energy is the minimum energy required to start the reaction, shown as the peak of the energy barrier.
Bond energy calculations
Bond energies are given in kJ/mol for each type of bond.To calculate the energy change: add up the bond energies of all bonds broken (energy in), add up the bond energies of all bonds made (energy out), then calculate ΔH = energy in − energy out.A negative ΔH means exothermic; a positive ΔH means endothermic.
Definitions to learn
Exothermic reaction
A reaction that transfers energy to the surroundings, causing the temperature of the surroundings to rise.
Endothermic reaction
A reaction that absorbs energy from the surroundings, causing the temperature of the surroundings to fall.
Activation energy
The minimum energy required for a reaction to start; shown as the peak in an energy profile diagram.
Bond energy
The energy required to break one mole of a specific type of covalent bond in gaseous molecules.
Enthalpy change (ΔH)
The overall energy change of a reaction; negative = exothermic, positive = endothermic.
Using bond energies, calculate the energy change for H₂ + Cl₂ → 2HCl. (H–H = 436 kJ/mol, Cl–Cl = 242 kJ/mol, H–Cl = 432 kJ/mol)
Energy in (breaking bonds): H–H = 436; Cl–Cl = 242. Total = 678 kJ.
Energy out (making bonds): 2 × H–Cl = 2 × 432 = 864 kJ.
Energy change = energy in − energy out = 678 − 864 = −186 kJ.
Negative value means exothermic.
Energy change = −186 kJ/mol (exothermic).
Always set out bond energy calculations in two lines: energy IN (bonds broken) and energy OUT (bonds made).Subtract: negative = exothermic, positive = endothermic.
Do not confuse bond breaking (energy in, endothermic) with bond making (energy out, exothermic).Breaking bonds always requires energy.
How to score full marks
- 1Breaking bonds = energy IN (endothermic); making bonds = energy OUT (exothermic) — these two facts are tested directly.
- 2For bond energy calculations, work in two clear rows: 'bonds broken' and 'bonds made', then subtract.
- 3On an energy profile diagram, exothermic shows products LOWER than reactants — a downward overall change.
Try these yourself
Open each answer only after you have explained the full chemical process.
1State what is meant by an exothermic reaction.[2 marks]
- 1.Describe energy transfer direction.
2Give one example of an exothermic reaction and one endothermic reaction.[2 marks]
- 1.Recall a reaction that heats and one that cools.
3State whether breaking or making bonds requires energy.[1 mark]
- 1.Recall which process is endothermic.
4Explain why a reaction is exothermic if more energy is released making bonds than is needed to break bonds.[3 marks]
- 1.Link net energy change to temperature.
5Sketch an energy profile diagram for an exothermic reaction, labelling reactants, products and activation energy.[4 marks]
- 1.Show products lower than reactants, with an energy hump.
6Calculate the energy change for the reaction N₂ + 3H₂ → 2NH₃. Bond energies: N≡N = 945 kJ/mol, H–H = 436 kJ/mol, N–H = 391 kJ/mol.[4 marks]
- 1.Find energy in (breaking bonds) and energy out (making bonds), then subtract.
7Describe how a student could determine whether dissolving potassium nitrate in water is exothermic or endothermic.[4 marks]
- 1.Describe the method and expected observation.
8Explain why hand warmers use exothermic reactions and cold packs use endothermic reactions.[2 marks]
- 1.Link direction of energy flow to temperature change.