Energy Changes

Every reaction involves an energy change, almost always as heat. An exothermic reaction releases energy to the surroundings, so the temperature rises — combustion, respiration, neutralisation, and most reactions of metals. An endothermic reaction takes in energy, so the surroundings cool — thermal decomposition, photosynthesis, and dissolving some salts.

A quick lab test is just a thermometer: temperature up means exothermic, temperature down means endothermic.

An energy profile plots the energy of the reactants and products. In an exothermic reaction the products sit lower than the reactants — energy was released — so the overall energy change, ΔH, is negative. In an endothermic reaction the products sit higher and ΔH is positive.

On both profiles there is a hump between reactants and products: the activation energy, the barrier every reaction must climb before it can proceed. A catalyst lowers that hump without changing the reactants or products.

The energy change is bookkeeping on bonds. Breaking bonds takes energy in (endothermic); making bonds gives energy out (exothermic). Every reaction does both — old bonds break, new ones form — and the net result is the difference.

If making the new bonds releases more than breaking the old ones absorbed, the reaction is exothermic overall. If breaking costs more than making returns, it is endothermic.

You cannot see energy, but you can measure the temperature change it causes. In calorimetry a reaction (say burning a fuel, or a neutralisation) heats a known mass of water, and the heat transferred is found from q = mcΔT. Dividing by the moles reacted gives the energy change per mole.

Real measurements always fall short of data-book values because heat escapes to the surroundings and combustion is often incomplete — a good reason to insulate the apparatus and shield it from draughts.

Chemists name specific energy changes: the enthalpy of combustion (burning one mole in oxygen), of neutralisation (acid + alkali making one mole of water) and of formation (making one mole of a compound from its elements). All are quoted per mole under standard conditions.

Hess's law says the total enthalpy change of a reaction is the same whatever route is taken, because energy is conserved. That lets you calculate an enthalpy change you cannot measure directly by adding up the steps of an alternative route through an energy cycle.