Exothermic and endothermic reactions

Chemistry · Energy changes. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

An exothermic reaction transfers energy to the surroundings, so the temperature of the surroundings rises. Combustion, neutralisation and many oxidation reactions are exothermic. An endothermic reaction takes energy in, so the temperature falls. Thermal decomposition is endothermic.

Particles react only if they collide with enough energy. The minimum energy needed is the activation energy. On a reaction profile it is the height from the reactants up to the top of the curve.

Breaking bonds takes energy in and making bonds gives energy out. If more energy is given out making the new bonds than was taken in breaking the old ones, the reaction is exothermic overall.

Rules to remember

Exothermic: temperature rises, products are at a lower energy than the reactants, energy change is negative
Endothermic: temperature falls, products are at a higher energy than the reactants, energy change is positive
Activation energy = energy at the top of the curve − energy of the reactants
Overall energy change = energy of the products − energy of the reactants
Bond breaking is endothermic; bond making is exothermic
Energy change = energy needed to break bonds − energy released making bonds
Uses: hand warmers and self-heating cans (exothermic); sports injury cold packs (endothermic)

Common mistake

In bond energy questions students forget the numbers in front of the formulae or the number of bonds in each molecule. Count every bond: 2H₂O has four O–H bonds, not two.

Worked example

N₂ + 3H₂ → 2NH₃. Bond energies in kJ/mol: N≡N 945, H–H 436, N–H 391. What is the overall energy change?

Bonds broken: one N≡N and three H–H = 945 + (3 × 436) = 945 + 1308 = 2253 kJ.
Bonds made: two NH₃ have six N–H bonds = 6 × 391 = 2346 kJ.
Energy change = broken − made = 2253 − 2346 = −93 kJ/mol.
The sign is negative, so the reaction is exothermic.

Answer: −93 kJ/mol (exothermic)

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