Thermochemistry rewards students who prepare, because its questions come in a handful of recognisable forms and the same two formulas keep reappearing. The examiner tests three things: can you read energy, can you calculate heat, and can you compare reactions fairly. This guide groups the chapter’s questions by type, different from a pure calculation walkthrough, so you can spot which move a question is asking for before you start writing.
Type 1: Exothermic or endothermic?
The opening type gives a reaction or an observation and asks you to classify it and justify the answer. The rule to state is simple: if the surroundings get warmer, heat is released, the change is exothermic and ΔH is negative; if they get colder, heat is absorbed, the change is endothermic and ΔH is positive.
Worked example: Ammonium chloride dissolving in water makes the beaker feel cold. Classify it. Because the temperature falls, the process absorbs heat, so it is endothermic and ΔH is positive. You can revise the definitions in our exothermic reaction glossary entry.
Type 2: Drawing and reading energy level diagrams
A very common structured question shows or asks for an energy level diagram. You must place reactants and products at the correct relative heights and draw the ΔH arrow the right way.
Worked example: For an exothermic reaction, the products sit lower than the reactants, and the ΔH arrow points downward from reactants to products; the difference in height is the heat released. For an endothermic reaction the products sit higher and the arrow points up. Label both levels and the arrow, an unlabelled diagram rarely scores full marks.
Type 3: The heat calculation
The bread-and-butter type gives temperature and volume data and asks for a heat of reaction. The method is always Q = mcθ then ΔH = Q ÷ n, with c taken as 4.2 J g⁻¹ °C⁻¹ and the sign attached at the end. Our heat of reaction per mole page shows the full working.
Worked example: 100 cm³ of solution rises by 6.5 °C when 0.05 mol of acid is neutralised. Q = 100 × 4.2 × 6.5 = 2730 J = 2.73 kJ, and ΔH = −2.73 ÷ 0.05 = −54.6 kJ mol⁻¹, negative because the temperature rose. Watch the mass, it is the mass of the solution, not of any solid added.
Type 4: Comparing heats of combustion
A favourite comparison type gives the heats of combustion of a series of alcohols and asks you to describe and explain the trend. As you go from methanol to ethanol to propanol to butanol, the number of carbon and hydrogen atoms rises, so more bonds burn and more heat is released per mole, the heat of combustion increases down the series.
Worked example: Explain why the heat of combustion of ethanol is greater than that of methanol. Ethanol has more atoms and therefore more bonds to burn; combustion of one mole releases more energy, so its heat of combustion is larger in magnitude. Data on the reaction itself is on our heat of combustion reaction page.
Type 5: Strong versus weak in neutralisation
A sharper type compares the heat of neutralisation of different acids. A strong acid with a strong alkali gives a value close to −57 kJ mol⁻¹. A weak acid, such as ethanoic acid, gives a smaller magnitude, because some energy is used to fully ionise the weak acid before neutralisation.
Worked example: Explain why the heat of neutralisation of ethanoic acid is less exothermic than that of hydrochloric acid. Ethanoic acid is only partially ionised; energy must be absorbed to ionise it completely, so the net heat released is smaller.
Type 6: The practical and its errors
Paper 2 and Paper 3 questions often build on the neutralisation or displacement experiment and ask about accuracy, why a polystyrene cup is used (it insulates, reducing heat loss), why the mixture is stirred (even temperature), and why the measured value is lower than the true value (heat lost to the surroundings and the apparatus). Walk through the method on our determining heat of neutralisation page.
Revising across the six types
The efficient route is to rehearse each move once: classify by temperature change, draw a labelled energy level diagram, run Q = mcθ then ΔH = Q ÷ n, explain a combustion trend, explain strong versus weak, and list the practical precautions. That is how our online one-to-one lessons drill this chapter, in English from RM50 an hour with a paid one-hour trial. Get the sign convention automatic and the diagram labels precise, and Thermochemistry becomes a chapter you can rely on.
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