Heat of reaction

The heat of reaction is the general term for the heat change when a reaction occurs, always quoted per mole of a named substance, in kJ mol⁻¹, and given with a sign. Malay: haba tindak balas · Chinese: 反应热.

It is the parent idea behind the four specific heats you meet in this chapter, the heats of neutralisation, displacement, precipitation and combustion. Each of those is a heat of reaction defined for a particular kind of change and a particular mole: one mole of water formed, one mole of metal displaced, one mole of precipitate formed, or one mole of fuel burnt. Because the value is always expressed per mole, you can compare reactions fairly and slot the number straight into a stoichiometry calculation.

Example. If burning a certain mass of ethanol releases a measured quantity of heat, you first find the moles of ethanol burnt, then divide the heat by those moles. The answer, in kJ mol⁻¹ with a negative sign, is the heat of reaction, here the heat of combustion of ethanol.

Confusion to avoid. Do not report a heat of reaction as a bare number of joules from Q = mcθ. That value of Q is the heat for the actual amount reacted; the heat of reaction is Q divided by the number of moles and carried with its sign. Forgetting the per-mole step, or dropping the sign, changes the meaning of the answer entirely.

Grasping the heat of reaction as one idea with a shared method, measure Q, then divide by moles, is what lets you treat all four named heats in this SPM Chemistry chapter with a single, reliable routine. Note too that the sign of a heat of reaction follows straight from whether the temperature rises or falls: an exothermic reaction takes a negative sign, an endothermic one a positive sign. Because the value is stated per mole, it can be compared between different reactions and carried into further calculations, so get into the habit of writing the unit kJ mol⁻¹ each time you report a value, so that its full meaning stays clear.