Energy Changes
Exothermic and endothermic reactions, bond energies, and fuel cells.
In this topic
Exothermic and Endothermic Reactions
Chemical reactions involve energy changes.
Exothermic reactions RELEASE energy to the surroundings — temperature increases. Examples: combustion, neutralisation, oxidation, hand warmers.
Endothermic reactions ABSORB energy from the surroundings — temperature decreases. Examples: thermal decomposition, photosynthesis, citric acid + sodium bicarbonate, cold packs.
Energy is conserved. If a reaction releases energy, the products have less energy than the reactants. On an energy profile diagram, exothermic goes DOWN, endothermic goes UP.
Key Points
- Exo = exit = energy leaves the reaction
- Endo = enter = energy enters the reaction
- Activation energy is the minimum energy needed to start a reaction
- Catalysts lower the activation energy
Example Questions
2A student adds citric acid to sodium bicarbonate solution and the temperature drops. Is this exothermic or endothermic? Explain.
Endothermic. The temperature drops because the reaction is absorbing energy from the surroundings. Energy is transferred from the solution into the reaction, causing the surroundings to cool down.
[2 marks]
3Use bond energies to calculate the overall energy change for: H₂ + Cl₂ → 2HCl. Bond energies: H-H = 436 kJ/mol, Cl-Cl = 242 kJ/mol, H-Cl = 431 kJ/mol.
Energy to break bonds: 436 + 242 = 678 kJ. Energy released forming bonds: 2 × 431 = 862 kJ. Overall: 678 - 862 = -184 kJ/mol. Exothermic (negative value).
[3 marks]