The commonest Thermochemistry mistakes are getting the sign of ΔH wrong, using the wrong mass in Q = mcθ, forgetting to divide the heat by the number of moles, dropping units, and drawing energy level diagrams the wrong way round. Each mistake below is shown with why it loses marks and the correct version.
Thermochemistry is one of the most predictable chapters in the SPM Chemistry papers, because almost every question follows the same calculation route. That also means the same mistakes recur year after year. Below are the errors our teachers see most often, grouped by content standard. For each one we show what students write, why it loses marks, and the correct version, so you can check your own working against it.
11.1 Exothermic and endothermic reactions
Mistake 1, Getting the sign of ΔH wrong. Students write a positive ΔH for a reaction whose temperature rises. A temperature rise means heat is released, so the reaction is exothermic and ΔH must be negative. A temperature fall means heat is absorbed, so the reaction is endothermic and ΔH is positive. Pair the direction of the temperature change with the sign every single time.
Mistake 2, Leaving out the sign completely. An answer of “56 kJ mol⁻¹” is not the same as “−56 kJ mol⁻¹”. The sign is part of the value in thermochemistry, and an unsigned heat of reaction usually loses the mark. Always attach + or − to the final figure.
Mistake 3, Explaining the sign with the wrong energy idea. Students say a reaction is exothermic “because bonds are broken”. In fact breaking bonds absorbs energy and forming bonds releases energy. A reaction is exothermic when more energy is released forming new bonds than is absorbed breaking old bonds. State the comparison, not just one half of it.
Correct habit. Temperature rise → heat released → exothermic → ΔH negative. Temperature fall → heat absorbed → endothermic → ΔH positive. Memorise this chain and read it in either direction.
11.2 Energy level diagrams
Mistake 4, Drawing the diagram the wrong way round. For an exothermic reaction the products must sit lower than the reactants; for an endothermic reaction the products sit higher. Drawing an exothermic reaction with the products higher contradicts your own words and loses the diagram mark even when the explanation is correct.
Mistake 5, Leaving the diagram unlabelled. A diagram with no axis label, no “reactants” and “products”, and no ΔH arrow cannot score. Label the vertical axis Energy, mark the reactant and product levels, and draw the arrow for ΔH from the reactant level to the product level.
Mistake 6, Confusing activation energy with ΔH. The activation energy is the height of the “hump” from the reactant level to the peak, the minimum energy needed to start the reaction. ΔH is the difference between the reactant and product levels. They are two different quantities on the same diagram; do not label the hump as ΔH.
11.3 Heat of neutralisation
Mistake 7, Defining it without “one mole of water”. The heat of neutralisation is the heat released when one mole of water is formed from the neutralisation of an acid by an alkali. Answers that drop the phrase “one mole of water” are incomplete. Every heat of reaction is quoted per mole of a stated substance, so name that substance.
Mistake 8, Using only the acid volume as the mass. In Q = mcθ the mass is the mass of the whole mixture, acid plus alkali. A student who mixes 50 cm³ of acid with 50 cm³ of alkali must use 100 g, not 50 g. Using half the mass halves the answer.
Mistake 9, Not explaining the strong-versus-weak difference. When asked why a weak acid gives a lower heat of neutralisation than a strong acid, it is not enough to say “it is weaker”. The reason is that some heat is absorbed to complete the ionisation of the weak acid or weak alkali, so less heat is released overall. Give the reason, not just the observation.
11.4 Heat of displacement and precipitation
Mistake 10, Using the mass of the metal or precipitate in Q = mcθ. This is one of the costliest slips in the chapter. In displacement and precipitation you still use the mass of the solution, not the mass of the zinc added or the precipitate formed. The solution is what changes temperature, so it is the mass that goes into Q = mcθ.
Mistake 11, Dividing by the wrong number of moles. The heat of displacement is per one mole of metal displaced; the heat of precipitation is per one mole of precipitate formed. Work out the limiting quantity from the moles of the reacting ions, then divide by that. Dividing by the moles of the substance in excess gives a wrong, usually too-small, answer.
11.5 Heat of combustion
Mistake 12, Forgetting “complete combustion” and “one mole”. The heat of combustion is the heat released when one mole of a substance is completely burnt in excess oxygen. Omitting “completely” or “in excess oxygen”, or forgetting the per-mole basis, makes the definition incomplete.
Mistake 13, Using the mass of fuel instead of the mass of water in Q = mcθ. In the combustion experiment, Q = mcθ uses the mass of water being heated, because it is the water whose temperature is measured. The mass of fuel burnt is used separately, to find the number of moles of fuel (mass ÷ molar mass) that you divide by in the second step.
Mistake 14, Not giving reasons for a low experimental value. When the experimental heat of combustion is lower than the accepted value, you must explain why: heat is lost to the surroundings and to the copper can, combustion may be incomplete, and some heat escapes with the hot gases. A bare statement that “heat was lost” is weaker than naming where it went.
Method mistakes that cross every standard
Mistake 15, Forgetting to convert joules to kilojoules. Q = mcθ gives an answer in joules. Heat of reaction is quoted in kJ mol⁻¹, so divide by 1000 before or after dividing by moles. Leaving the answer a thousand times too large is a very common slip.
Mistake 16, Dropping units at the final line. A number with no unit cannot score full marks in a calculation. The heat change comes out in kJ, and the heat of reaction in kJ mol⁻¹. Write the unit on every line where a quantity appears, and especially on the final answer.
Mistake 17, Computing θ carelessly. The temperature change θ is the difference between the initial and highest (exothermic) or lowest (endothermic) temperature. Read the thermometer values carefully and subtract the right way round so θ is a positive magnitude, then decide the sign of ΔH separately from the direction of the change.
Check before you finish. Did you use the mass of the solution or water (not the solid)? Did you divide by the correct number of moles? Is the answer in kJ mol⁻¹ with a sign? Three quick checks that recover most of the marks lost in this chapter. A one-to-one teacher can mark a few of your calculations line by line so these habits become automatic well before the exam.
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