Energy changes in reactions
Chemistry · GCSE · The School
Reaction profiles
A REACTION PROFILE plots energy against the progress of a reaction. Reactants sit on the left, products on the right, and the hump between them is the ACTIVATION ENERGY — the minimum energy needed to start. If the products sit LOWER than the reactants the reaction is exothermic; higher, and it is endothermic. The shape carries the whole story.
Activation energy and catalysts
Even an exothermic reaction needs a push to begin: petrol does not burn until a spark supplies the activation energy. A CATALYST provides a different route with a LOWER activation energy, so more molecules have enough to react. It is not used up and does not change how much energy is released overall — only how easily the reaction starts.
Bond energies
Breaking bonds takes energy in; making bonds gives it out. Overall energy change = energy to break bonds − energy released making them. A negative answer means more was released than absorbed, so the reaction is EXOTHERMIC. Getting the sign right is where most marks are lost, so state which way each term runs before you subtract.
H₂ + Cl₂ → 2HCl. BREAKING costs energy: H–H is 436 kJ/mol and Cl–Cl is 242, total 678 kJ in. MAKING releases it: two H–Cl bonds at 431 each, 862 kJ out. Overall change = 678 − 862 = −184 kJ/mol. Negative means more was released than absorbed, so the reaction is EXOTHERMIC. The sign is the answer, and the 678 that went in first is the activation energy hump on the profile.
Everyday exothermic and endothermic reactions
A self-heating hand warmer uses an EXOTHERMIC reaction — often iron slowly oxidising, or a supersaturated solution crystallising — releasing energy as heat. An instant cold pack uses an ENDOTHERMIC change: ammonium nitrate dissolving in water absorbs energy from its surroundings, so the pack feels cold to the touch. The required practical for this topic measures a temperature change directly: record the starting temperature, add the reactants, stir, and record the highest or lowest temperature reached, repeating to check the result is reliable. A bigger temperature change means a bigger energy change per gram of the reacting mixture, not automatically a bigger overall amount of energy, unless the mass is stated too.
Drawing a catalyst onto a reaction profile
A catalysed reaction is drawn as a second curve on the same profile: it starts and finishes at the same energy as the uncatalysed reaction — the overall energy change does not change — but its hump is LOWER, showing the reduced activation energy. Reactants and products sit at the same two heights either way, because a catalyst changes the path taken, not the destination. Exams frequently ask for this as a sketch: two humps of different heights between the same start and end points, with the lower one labelled catalysed. Getting the start and end heights wrong, rather than the hump, is the more common mistake.