Chemistry • Year 12 • Module 7 • Lesson 11

Combustion of Alcohols & Comparison with Fossil Fuels

Lock in the core vocabulary, the three-step calorimetry calculation, combustion equations, and the systematic sources of discrepancy in spirit burner experiments.

Build · Band 3–4 · Recall & Vocab

1. Term–definition match

Write the correct term from the list below next to each definition. 8 marks (1 per row)

Term bank: complete combustion • molar enthalpy of combustion (ΔH_c) • energy density • bomb calorimeter • carbon neutrality • spirit burner calorimeter • systematic error • incomplete combustion

#	Definition	Term
1.1	A reaction of a fuel with excess oxygen in which the only products are CO₂(g) and H₂O(g).
1.2	The heat released when one mole of a substance burns completely in excess oxygen under standard conditions.
1.3	The energy released per gram of fuel (kJ/g); calculated as \|ΔH_c\| ÷ molar mass.
1.4	A closed, oxygen-pressurised vessel that measures combustion heat with minimal heat loss; gives results close to theoretical ΔH_c.
1.5	A fuel property where the CO₂ released on combustion equals the CO₂ absorbed from the atmosphere during biomass growth.
1.6	A simple open-flame apparatus used in school labs; a spirit burner heats water in a copper can using q = mcΔT.
1.7	An error that consistently shifts all results in one direction; it does not average out with repeat trials.
1.8	Combustion in limited oxygen producing CO and/or soot (C) rather than fully converting to CO₂.

Stuck? Revisit the Key Terms panel and Card 03 of the lesson.

2. True or false — with correction

Circle T or F. If the statement is false, write the corrected version on the line. 8 marks (1 T/F + 1 correction where needed)

2.1 In the formula q = mcΔT, m refers to the mass of alcohol burned in the spirit burner. T / F

2.2 The experimental ΔH_c from a spirit burner is always lower in magnitude than the theoretical value. T / F

2.3 Longer-chain alcohols have a smaller molar enthalpy of combustion because their molecules are larger and harder to burn. T / F

2.4 Both ethanol and octane (petrol) produce only CO₂ and H₂O in complete combustion. T / F

Stuck? Revisit lesson Cards 02 and 04.

3. Cloze — fill the blanks

Complete the paragraph using the word bank. Each word is used once. 8 marks (1 per blank)

Word bank: exothermic • kJ/mol • negative • joules • water • moles • theoretical • systematic

In a spirit burner experiment, heat released by combustion is absorbed by the (3.1) __________ in the calorimeter. Using q = mcΔT gives the heat in (3.2) __________. Dividing q (converted to kJ) by the number of (3.3) __________ of alcohol burned gives ΔH_c in (3.4) __________. Because combustion is (3.5) __________, the value of ΔH_c is always (3.6) __________. The experimental result is consistently lower than the (3.7) __________ value because heat is lost to the surroundings — this is a (3.8) __________ error, not a random one.

Stuck? Revisit the Formula Panel and Card 02 in the lesson.

4. Balance the combustion equations

Write balanced equations for complete combustion of each alcohol in excess oxygen. Include state symbols (l) and (g). 6 marks (2 each)

4.1 Methanol (CH₃OH)

CH₃OH(l) + _____ O₂(g) → _____ CO₂(g) + _____ H₂O(g)

4.2 Ethanol (C₂H₅OH)

C₂H₅OH(l) + _____ O₂(g) → _____ CO₂(g) + _____ H₂O(g)

4.3 Propan-1-ol (C₃H₇OH)

C₃H₇OH(l) + _____ O₂(g) → _____ CO₂(g) + _____ H₂O(g)

Stuck? Count C atoms first (gives CO₂ coefficient), then H atoms (gives H₂O coefficient), then balance O last.

5. Sources of discrepancy — match the chain

Link each source of error to its effect on q or n, then to its effect on |ΔH_c|. Write A, B, or C in each blank. 6 marks

Sources:

Heat loss to surroundings (calorimeter walls, air, bench)
Alcohol evaporating from wick without combustion
Incomplete combustion producing CO and soot

Effect on measurement	Measurement affected (q or n)	Source letter (A, B, or C)
Less heat captured by water; measured heat lower than actual heat released	q too low
Mass decrease recorded but no combustion heat produced; n inflated relative to heat released	n too high
Fuel oxidised to CO and C not CO₂; chemical energy retained in products not released as heat	q too low

5.4 All three sources make the calculated |ΔH_c| __________ (higher / lower) than the theoretical value. This is because they are __________ (systematic / random) errors. 2 marks

Stuck? Revisit the error table in Card 03 of the lesson.

Answers — Do not peek before attempting

Q1 — Term–definition match

1.1 complete combustion • 1.2 molar enthalpy of combustion (ΔH_c) • 1.3 energy density • 1.4 bomb calorimeter • 1.5 carbon neutrality • 1.6 spirit burner calorimeter • 1.7 systematic error • 1.8 incomplete combustion

Q2 — True/False with correction

2.1 False. Correction: m in q = mcΔT is the mass of water in the calorimeter, not the mass of alcohol burned. The water temperature rise is what is measured; the alcohol mass (fuel burned) is used in Step 2: n = Δm/M.

2.2 True. The experimental ΔH_c is always lower in magnitude because of heat loss to surroundings, incomplete combustion, and alcohol evaporation — all systematic errors.

2.3 False. Correction: longer-chain alcohols have a larger magnitude ΔH_c. Each additional CH₂ unit adds C–H and C–C bonds whose combustion releases approximately 650 kJ/mol of extra energy. The claim confuses chain length with combustion difficulty.

2.4 True. Complete combustion of both ethanol and octane produces only CO₂(g) and H₂O(g). The sustainability difference lies in the carbon cycle, not combustion products.

Q3 — Cloze

3.1 water • 3.2 joules • 3.3 moles • 3.4 kJ/mol • 3.5 exothermic • 3.6 negative • 3.7 theoretical • 3.8 systematic

Q4 — Combustion equations

4.1 CH₃OH(l) + 3/2 O₂(g) → CO₂(g) + 2H₂O(g) or multiplied through: 2CH₃OH(l) + 3O₂(g) → 2CO₂(g) + 4H₂O(g). Accept fractional coefficients.

4.2 C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(g)

4.3 C₃H₇OH(l) + 9/2 O₂(g) → 3CO₂(g) + 4H₂O(g) or: 2C₃H₇OH(l) + 9O₂(g) → 6CO₂(g) + 8H₂O(g).

Marking notes: 1 mark for correct products (CO₂ + H₂O only), 1 mark for balanced coefficients including O₂. State symbols optional at this tier but award both marks if correct.

Q5 — Sources of discrepancy

Row 1 (q too low, heat loss): Source A

Row 2 (n too high, evaporation): Source B

Row 3 (q too low, incomplete combustion): Source C

5.4 All three make |ΔH_c| lower than theoretical. They are systematic errors (always in the same direction; do not average out with repeat trials).