HSC Exam Practice

Sample response. The molar enthalpy of combustion is the enthalpy change when one mole of a substance undergoes complete combustion in oxygen under standard conditions (25°C, 100 kPa). For combustion reactions it is always negative because energy is released to the surroundings (exothermic), making the products lower in enthalpy than the reactants.

Marking notes. 1 mark: definition includes “one mole” and “complete combustion in oxygen”; 1 mark: correct sign (−) with explanation that combustion is exothermic / energy released.

Sample response. q = heat energy absorbed by the water (J); m = mass of the water in the calorimeter (g); c = specific heat capacity of water = 4.18 J g⁻¹ K⁻¹; ΔT = temperature change of the water (K or °C, same magnitude).

Marking notes. 1 mark per correctly identified variable with unit: q in J, m in g, c in J g⁻¹ K⁻¹ or ΔT in K/°C. Accept any three correct for 3 marks.

Sample response. The bomb calorimeter gives far more accurate ΔH_c values than the spirit burner because it is a sealed, thermally insulated system. The key structural difference is that in a bomb calorimeter the fuel is ignited under high-pressure oxygen inside a sealed steel vessel, preventing any heat escaping to the surroundings; in a spirit burner the flame burns in open air, allowing a large proportion of combustion energy to heat the surrounding air before it can warm the water, making the experimental ΔH_c always less negative.

Marking notes. 1 mark: bomb calorimeter gives more accurate/accepted values; 1 mark: one valid structural difference (sealed/pressurised vs open-air, insulated vs uninsulated, constant volume vs open); 1 mark: explanation of why that structural difference improves accuracy (links to reduced heat loss).

Sample response. The experimental ΔH_c is less negative because not all the combustion energy heats the water — heat escapes to the surroundings.
Error 1: Heat loss to surroundings — convection and radiation carry energy away from the flame before it reaches the calorimeter. This reduces q, which reduces |ΔH_c|, making the result less negative than the true value.
Error 2: Incomplete combustion — insufficient O₂ supply causes some fuel carbon to oxidise only to CO or soot rather than CO₂. Less energy is released per mole (incomplete combustion of carbon releases less energy), reducing q and making ΔH_c less negative.

Marking notes. 1 mark: general reason (not all heat reaches water). 1 mark each (up to 2) for a specifically named error. 1 mark for stating the directional effect (less negative) for at least one error. Do not award marks for “human error” or “inaccurate measurement” without physical specificity.

Sample response. Before burning: record initial mass of spirit burner + fuel on a balance; record initial temperature of the water in the calorimeter. During: light the burner; after reaching the desired temperature rise, extinguish the flame and cap the burner immediately. After burning: record final temperature of water; record final mass of spirit burner + fuel. Mass of fuel consumed = initial mass − final mass.

Marking notes. 1 mark: both initial mass and temperature recorded before; 1 mark: both final mass and temperature recorded after; 1 mark: mass of fuel consumed = initial − final burner mass.

Worked solution.
ΔT = 30.5 − 20.0 = 10.5°C = 10.5 K [1]
q = mcΔT = 150.0 × 4.18 × 10.5 = 6584 J = 6.584 kJ [1]
n(methanol) = 0.62/32.04 = 0.01936 mol [1]
ΔH_c = −q/n = −6.584/0.01936 = −340 kJ mol⁻¹ [1]
(Accepted −726 kJ mol⁻¹; % error = 53%.)

(a) 2 marks. |ΔH_c| increases as carbon chain length increases: methanol (726) < ethanol (1367) < propan-1-ol (2021) < butan-1-ol (2676) kJ mol⁻¹ [1]. The approximate increment per additional –CH₂– group is: (2676 − 726)/3 = ≈ 650 kJ mol⁻¹ [1].

(b) 3 marks. Each additional –CH₂– unit adds one C atom and two H atoms to the molecule [1]. During complete combustion, each C is oxidised to CO₂ and each pair of H atoms to H₂O, both highly exothermic processes that require breaking C–H and C–C bonds and forming C=O and O–H bonds [1]. The energy released per mole of CO₂ and H₂O formed is approximately constant across the series, so each additional CH₂ contributes an approximately constant increment of ~650 kJ mol⁻¹ to ΔH_c [1].

(c) 2 marks. Extrapolated estimate: butan-1-ol (2676) + 650 ≈ −3325 kJ mol⁻¹ for pentan-1-ol (accepted −3328 kJ mol⁻¹) [1]. The extrapolation is valid within this homologous series because the increment per CH₂ group is approximately constant for lower alcohols; however, for very long chains (e.g. decan-1-ol), intermolecular forces and combustion kinetics may introduce deviations, so extrapolation is more reliable within a few steps of the known data [1].

(a) 4 marks.
m(butan-1-ol) = 162.45 − 161.67 = 0.78 g [1]
n = 0.78/74.12 = 0.01052 mol [1]
ΔT = 34.9 − 21.4 = 13.5°C; q = 200.0 × 4.18 × 13.5 = 11,286 J = 11.29 kJ [1]
ΔH_c = −11.29/0.01052 = −1073 kJ mol⁻¹ [1]

(b) 2 marks.
% error = |−1073 − (−2676)| / 2676 × 100 = 1603/2676 × 100 = 59.9% [1]
Interpretation: approximately 60% of the combustion energy was lost before heating the water, primarily to the surrounding air through the open-air flame; the spirit burner calorimeter is a poor approximation of complete energy capture [1].

(c) 2 marks. Assumption: all heat released by combustion is absorbed by the water (no heat loss to the calorimeter walls, air, or stand) [1]. If this assumption is violated — i.e. the copper calorimeter or stand absorbs some heat — the measured ΔT of the water will be lower than it should be; q will be underestimated and ΔH_c will appear less negative than the true experimental value [1].

Mark allocation (7 marks):

1 mark — Identifies that spirit burner calorimetry consistently underestimates |ΔH_c| (less negative than accepted), and that this is a systematic rather than random error.

2 marks — Names and correctly explains the directional effect of at least two distinct systematic errors (e.g. heat loss to surroundings makes q smaller; incomplete combustion reduces total energy released; evaporation from uncapped wick overestimates n; thermometer not submerged underestimates ΔT). 1 mark per error, must name mechanism AND state direction.

1 mark — Addresses reliability: multiple repeat trials would give consistent results in the same direction (consistently less negative), indicating the error is systematic and reproducible, which improves reliability even though accuracy is poor.

1 mark — Comments on fitness for purpose in a school context: the spirit burner correctly demonstrates that (i) combustion is exothermic, (ii) ΔH_c increases with chain length, and (iii) the calculation chain q → ΔH_c; these learning goals are achievable even with poor absolute accuracy.

1 mark — Identifies a specific improvement that would reduce error (e.g. insulation around calorimeter, correcting for heat capacity of calorimeter, using a more enclosed apparatus, extrapolating back the cooling curve).

1 mark — Evidence-based judgement: the spirit burner is not fit for purpose as a precise analytical instrument (errors of 40–70% are typical) but is fit for purpose as a school demonstration tool for qualitative and semi-quantitative comparison of fuel energy content.

Do NOT award marks for: “human error”, “inaccurate readings”, or errors stated without directional effect.