Heat change (ΔH)

The heat change, or enthalpy change, given the symbol ΔH, is the heat energy taken in or given out during a chemical reaction, measured in kJ mol⁻¹. Malay: perubahan haba · Chinese: 热变化.

Every reaction either releases heat to the surroundings or absorbs heat from them, and ΔH records both how much and in which direction. The sign carries the meaning: ΔH is negative for an exothermic reaction, because the products hold less energy than the reactants and the difference is given out; ΔH is positive for an endothermic reaction, because the products hold more energy and that extra must be taken in. The size of ΔH tells you how much energy is involved per mole of the named substance, which is why every value is quoted per mole with its sign.

Example. When one mole of water is formed in a neutralisation, heat is released, so the heat change is written with a minus sign, for instance ΔH = −x kJ mol⁻¹. The minus sign is not optional: it is the part of the answer that says the reaction is exothermic and that the temperature of the mixture rose. For the thermal decomposition of a carbonate the value would instead be written +x kJ mol⁻¹, marking an endothermic change.

Confusion to avoid. Do not confuse heat change with activation energy. ΔH is the overall difference in energy between reactants and products; activation energy is the barrier that must be climbed first. A reaction can have a large activation energy yet still be strongly exothermic overall.

Heat change is the single quantity every part of this chapter is built to find, from reading an energy level diagram to dividing the result of Q = mcθ by the number of moles, so SPM Chemistry expects both the value and the correct sign in a complete answer.