The vocabulary of this chapter tells one story: a reaction either releases heat to or absorbs heat from the surroundings, the heat change ΔH records how much and in which direction, and every heat of reaction is measured per mole with Q = mcθ.
The terms in Thermochemistry are best learned as one connected story rather than a separate list, because almost every definition flows from a single question: when a reaction happens, does energy leave the chemicals or enter them, and how much? Once you see that link, the chapter becomes a short chain of ideas, release or absorb, sign of ΔH, diagram, and calculation, and that chain is where the marks are, because a definition here is usually worth the exact wording itself.
Release or absorb: the first pair. Everything starts with the split between an exothermic reaction, which releases heat to the surroundings so the temperature rises, and an endothermic reaction, which absorbs heat from the surroundings so the temperature falls. This is the pair students most often confuse, and the fix is to tie each word to what a thermometer would show. The reacting chemicals are called the system, and everything around them that can gain or lose that heat, usually the water or solution and the container, is the surroundings. You never measure the energy inside the bonds directly; you read the temperature of the surroundings and infer the direction of energy flow from the way it moves.
The heat change and its sign. The quantity that records all of this is the heat change, or enthalpy change, given the symbol ΔH and measured in kJ mol⁻¹. Its sign carries the meaning: ΔH is negative for an exothermic reaction and positive for an endothermic one. Behind the sign lies the balance of bond breaking, which absorbs energy, against bond forming, which releases it; a reaction is exothermic when forming the new bonds releases more than breaking the old ones absorbs, and endothermic when it releases less. The sign convention, negative for a temperature rise, positive for a fall, is the single rule that ties the direction of the temperature change to the sign of ΔH, and getting it the wrong way round is one of the most common errors in the chapter.
Seeing it on a diagram. The energy level diagram turns these words into a picture: two horizontal lines for the reactant and product levels, with an arrow between them marking ΔH. Products drawn lower than reactants mean an exothermic reaction and a negative ΔH; products drawn higher mean an endothermic reaction and a positive ΔH. Students sometimes confuse this with the energy profile diagram from Rate of Reaction, which also shows an activation-energy hump; the energy level diagram of this chapter needs only the two levels and their difference, and confusing ΔH with the activation energy, the overall change with the barrier, is a mistake worth guarding against.
The four heats of reaction. The general idea of a heat of reaction, a heat change quoted per mole of a named substance, with a sign, splits into four named cases that share one method. The heat of neutralisation is defined per mole of water formed; the heat of displacement per mole of metal displaced in a redox change; the heat of precipitation per mole of precipitate formed from ions; and the heat of combustion per mole of fuel burnt in excess oxygen. The pairs students confuse here are displacement with precipitation, a redox change versus a change with no change in oxidation number, and defining neutralisation per mole of acid rather than per mole of water. Keeping the phrase “per mole of…” attached to the correct substance in each definition is what earns these marks.
The calculation that ties it together. All four heats are measured by calorimetry and computed with the heat equation, Q = mcθ, where m is the mass of the surroundings, c is the specific heat capacity taken from the value the question provides, and θ is the temperature change. Because a solution’s density is taken as 1 g cm⁻³, a volume in cm³ becomes the mass in grams. The final step is the per mole basis: divide Q by the moles of the named substance and attach the sign. A recurring theme is why an experimental heat of combustion falls below the accepted value, heat lost to the surroundings and incomplete combustion rather than complete combustion, which links the numbers back to the practical work.
These terms build on the mole concept, redox and acids from earlier chapters and reward precise, DSKP-aligned wording. Our teachers work through this vocabulary in its connected order in online one-to-one lessons, from RM50 an hour, so the exact phrasing SPM Chemistry rewards, the per-mole basis, the correct sign, the named surroundings, becomes second nature under exam pressure.
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