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.