§-Syllabus dot point

SAChemistrySyllabus dot point

How is the energy content of a fuel measured and compared?

Define and calculate the enthalpy of combustion of fuels, and compare fuels by their energy content per gram and per mole.

Defining and calculating enthalpy of combustion, comparing fuels by energy per gram and per mole, complete versus incomplete combustion, biofuels, and fully worked SACE-style combustion-calorimetry calculations.

Generated by Claude Opus 4.89 min answerUpdated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

Have a quick question? Jump to the Q&A page

Jump to a section

What this dot point is asking
Lead worked calculation
Defining enthalpy of combustion
Comparing fuels: per mole versus per gram
Complete versus incomplete combustion
Renewable and alternative fuels
Why it matters for managing resources

What this dot point is asking

SACE expects you to define and calculate enthalpy of combustion, compare fuels per mole and per gram, and discuss complete versus incomplete combustion and renewable fuels.

Lead worked calculation

Enthalpy of combustion from calorimetry

Burning $0.860\ \text{g}$ of propan-1-ol heats $200.0\ \text{g}$ of water by $30.5\ ^\circ\text{C}$ . Find the experimental molar enthalpy of combustion. ( $c = 4.18\ \text{J g}^{-1}\ ^\circ\text{C}^{-1}$ ; $M(\text{C}_3\text{H}_7\text{OH}) = 60.1\ \text{g mol}^{-1}$ .)

Heat gained by the water

q = mc\Delta T = 200.0 \times 4.18 \times 30.5 = 2.550 \times 10^{4}\ \text{J} = 25.50\ \text{kJ}

Moles of fuel burned

n = \frac{m}{M} = \frac{0.860}{60.1} = 1.431 \times 10^{-2}\ \text{mol}

Molar enthalpy of combustion

\Delta H_c = -\frac{q}{n} = -\frac{25.50}{1.431 \times 10^{-2}} = -1.78 \times 10^{3}\ \text{kJ mol}^{-1}

The negative sign shows combustion is exothermic. The experimental value is lower in magnitude than the data-book figure because heat is lost to the surroundings and the apparatus.

Defining enthalpy of combustion

Comparing fuels: per mole versus per gram

A fair comparison depends on the basis chosen.

For transport and portable use, energy per unit mass and per unit volume both matter, which is why hydrogen (very high energy per gram) still poses storage challenges (low energy per litre as a gas).

Complete versus incomplete combustion

Complete combustion (plenty of oxygen) gives carbon dioxide and water and releases the maximum energy: $\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}$ .
Incomplete combustion (limited oxygen) produces carbon monoxide and/or soot (carbon) and releases less energy: $2\text{CH}_4 + 3\text{O}_2 \rightarrow 2\text{CO} + 4\text{H}_2\text{O}$ . The toxic $\text{CO}$ and particulates are also pollutants.

Incomplete combustion both wastes energy and creates hazards, so well-designed burners ensure a good air supply.

Renewable and alternative fuels

Biofuels such as bioethanol (from fermenting sugars) and biodiesel (esters from plant oils) are renewable because the $\text{CO}_2$ released on burning was recently absorbed from the atmosphere by the plant, giving them a smaller net carbon footprint than fossil fuels. Hydrogen burns to give only water ( $2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}$ ) with no $\text{CO}_2$ , but its overall sustainability depends on how it is produced. Evaluating a fuel means weighing energy content against emissions, renewability and practicality.

Why it matters for managing resources

Quantifying and comparing the energy content of fuels guides the choice of energy sources, balancing energy density against cost, supply and greenhouse emissions. This connects combustion chemistry directly to the resource and sustainability decisions at the heart of Topic 4.

Exam-style practice questions

Practice questions written in the style of SACE Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

SACE 20225 marksIn a calorimetry experiment, burning

1.15\ \text{g}

of ethanol raised the temperature of

250.0\ \text{g}

of water by

24.5\ ^\circ\text{C}

. (a) Calculate the experimental molar enthalpy of combustion of ethanol. (b) The data-book value is

-1367\ \text{kJ mol}^{-1}

; calculate the percentage of the energy transferred to the water. (

c = 4.18\ \text{J g}^{-1}\ ^\circ\text{C}^{-1}

;

M(\text{C}_2\text{H}_5\text{OH}) = 46.07\ \text{g mol}^{-1}

Show worked answer →

(a) $q = mc\Delta T = 250.0 \times 4.18 \times 24.5 = 2.560 \times 10^{4}\ \text{J} = 25.60\ \text{kJ}$ . $n(\text{ethanol}) = \dfrac{1.15}{46.07} = 2.496 \times 10^{-2}\ \text{mol}$ . $\Delta H_c = -\dfrac{25.60}{2.496 \times 10^{-2}} = -1.03 \times 10^{3}\ \text{kJ mol}^{-1}$ . (3 marks)

(b) Percentage transferred $= \dfrac{1026}{1367} \times 100 = 75.1\%$ . (2 marks)

SACE 20194 marksCompare methane (

\text{CH}_4

) and octane (

\text{C}_8\text{H}_{18}

) as fuels by calculating the energy released per gram, given molar enthalpies of combustion

\Delta H_c(\text{CH}_4) = -890\ \text{kJ mol}^{-1}

and

\Delta H_c(\text{C}_8\text{H}_{18}) = -5470\ \text{kJ mol}^{-1}

. (

M(\text{CH}_4) = 16.0

M(\text{C}_8\text{H}_{18}) = 114.0\ \text{g mol}^{-1}

.) State which releases more energy per gram.

Show worked answer →

Methane: energy per gram $= \dfrac{890}{16.0} = 55.6\ \text{kJ g}^{-1}$ . (2 marks)

Octane: energy per gram $= \dfrac{5470}{114.0} = 48.0\ \text{kJ g}^{-1}$ . (1 mark)

Methane releases more energy per gram ( $55.6$ versus $48.0\ \text{kJ g}^{-1}$ ), because it has the higher hydrogen-to-carbon ratio. (1 mark)