Fuels and Combustion Revision Notes | A Level AQA Chemistry

1. Why Alkanes Make Such Effective Fuels

Every time a gas hob ignites or a car engine fires, an alkane is burning. Alkanes — saturated hydrocarbons with the general formula $C_nH_{2n+2}$ — release large amounts of energy when they react with oxygen, which is precisely what makes them so valuable as fuels.

The energy released comes from the formation of strong $C=O$ and $O-H$ bonds in the products, which more than compensates for the energy needed to break the $C-H$ and $C-C$ bonds in the alkane and the $O=O$ bonds in oxygen. The net result is a highly exothermic reaction.

Different alkanes are used in different contexts: methane ($CH_4$) is the main component of natural gas used for domestic heating; propane ($C_3H_8$) and butane ($C_4H_{10}$) are liquefied petroleum gases (LPG) used in portable stoves and heating; octane ($C_8H_{18}$) is a key component of petrol. The longer the carbon chain, the greater the energy released per mole of fuel — though the energy released per gram varies less dramatically across the series.

2. Complete Combustion — The Ideal Case

When an alkane burns in an excess of oxygen, complete combustion occurs. Every carbon atom is fully oxidised to carbon dioxide ($CO_2$) and every hydrogen atom ends up in water ($H_2O$). The flame burns with a clean, blue colour — a visual indicator that combustion is complete.

Worked example: Balancing a Complete Combustion Equation

Write a balanced equation, with state symbols, for the complete combustion of butane, $C_4H_{10}$.

1. $C_4H_{10}(g) + O_2(g) \rightarrow CO_2(g) + H_2O(l)$ (not yet balanced — coefficients to be determined) — Identify the reactants (alkane + oxygen) and the products of complete combustion (carbon dioxide + water). This skeleton equation shows the correct species but is not yet balanced; coefficients are added in the following steps.
2. $C_4H_{10}(g) + O_2(g) \rightarrow 4CO_2(g) + H_2O(l)$ (carbon balanced; hydrogen and oxygen not yet balanced) — There are 4 carbon atoms in butane, so 4 molecules of $CO_2$ are needed on the right-hand side. Hydrogen and oxygen will be balanced in subsequent steps.
3. $C_4H_{10}(g) + O_2(g) \rightarrow 4CO_2(g) + 5H_2O(l)$ (carbon and hydrogen balanced; oxygen not yet balanced) — There are 10 hydrogen atoms in butane. Each water molecule contains 2 H atoms, so $10 \div 2 = 5$ water molecules are needed. Oxygen will be balanced in the next step.
4. Right-hand side: $(4 \times 2) + (5 \times 1) = 8 + 5 = 13$ oxygen atoms — Each $CO_2$ contributes 2 oxygen atoms; each $H_2O$ contributes 1 oxygen atom. Total = 13 oxygen atoms needed on the left.
5. $C_4H_{10}(g) + \frac{13}{2}O_2(g) \rightarrow 4CO_2(g) + 5H_2O(l)$\nMultiply through by 2:\n$2C_4H_{10}(g) + 13O_2(g) \rightarrow 8CO_2(g) + 10H_2O(l)$ — 13 oxygen atoms requires $\frac{13}{2}$ molecules of $O_2$. Multiplying every coefficient by 2 clears the fraction and gives whole-number coefficients throughout. [M1] for correct balancing method, [A1] for correct final equation.

Answer: $2C_4H_{10}(g) + 13O_2(g) \rightarrow 8CO_2(g) + 10H_2O(l)$

3. Incomplete Combustion — When Oxygen is Limited

In practice, the oxygen supply to a burner or engine is rarely perfect. When oxygen is limited, carbon atoms cannot all be fully oxidised to $CO_2$. Instead, some carbon is only partially oxidised to carbon monoxide ($CO$), and some may not be oxidised at all, depositing as carbon particles — commonly called soot or particulates.

The flame colour shifts from blue to yellow or orange, and a sooty deposit may form on surfaces above the flame. These are reliable visual indicators of incomplete combustion in an exam question asking you to describe what is observed.

Carbon monoxide is particularly dangerous because it binds to haemoglobin in red blood cells with a much higher affinity than oxygen does, reducing the blood's ability to carry oxygen to tissues. This is why faulty gas appliances in poorly ventilated rooms are a serious safety hazard.

Fine carbon particulates (soot) from diesel engines and other sources of incomplete combustion are classed as $PM_{2.5}$ particles — particles smaller than 2.5 micrometres — and are linked to respiratory disease and cardiovascular problems. Examiners expect you to name both $CO$ and carbon particulates as products of incomplete combustion, not just one of them.

4. Sulphur Impurities and Air Pollution

Crude oil and coal are not pure hydrocarbons — they contain sulphur-bearing compounds as natural impurities. When these fuels are burned, the sulphur is oxidised in exactly the same way as carbon: it reacts with oxygen to form sulphur dioxide ($SO_2$), an acidic gas released into the atmosphere.

Once in the atmosphere, $SO_2$ dissolves in water droplets in clouds to form sulphurous acid ($H_2SO_3$), and can be further oxidised to sulphuric acid ($H_2SO_4$). This acidic precipitation — acid rain — damages ecosystems, corrodes metal structures and stonework, and acidifies lakes and soils, harming aquatic life.

At ground level, $SO_2$ is a respiratory irritant, worsening conditions such as asthma and bronchitis. Historically, industrial regions with heavy coal use suffered severe smog events — the London Great Smog of 1952 is a well-documented example — which prompted legislation to reduce sulphur emissions from power stations and vehicles.

Modern approaches to reducing $SO_2$ emissions include: desulphurisation of fuels before combustion (removing sulphur compounds from crude oil fractions during refining); flue-gas desulphurisation at power stations (reacting $SO_2$ in waste gases with calcium oxide or calcium carbonate to form calcium sulphate); and catalytic converters in vehicles, which oxidise $CO$ and unburnt hydrocarbons but do not directly remove $SO_2$. For AQA, you are expected to know that burning sulphur-containing fuels produces $SO_2$ and that this causes air pollution — the broader chemistry of acid rain formation is useful context but the core exam demand is the source and consequence of $SO_2$.

5. Worked Example — Combustion Equations and Environmental Context

Exam questions on this topic often combine equation writing with environmental explanation. The worked example below models how to handle a multi-part question that moves from calculation to evaluation — a common AQA pattern.

Worked example: Combustion of a Sulphur-Containing Fuel

A sample of coal contains carbon and sulphur as its combustible components. (a) Write a balanced equation for the complete combustion of carbon. (b) Write a balanced equation for the oxidation of sulphur during combustion. (c) Explain how burning this coal contributes to air pollution.

1. $C(s) + O_2(g) \rightarrow CO_2(g)$ — Carbon is fully oxidised to carbon dioxide. This equation is already balanced: 1 C and 2 O on each side. [B1]
2. $S(s) + O_2(g) \rightarrow SO_2(g)$ — Sulphur reacts with oxygen in a 1:1 molar ratio to form sulphur dioxide. The equation is balanced: 1 S on each side and 2 O on each side. [B1]
3. Pollutants: $CO_2$ (greenhouse gas) and $SO_2$ (acid rain precursor). If combustion is incomplete, $CO$ and carbon particulates are also produced. — Name the specific gases — examiners award marks for naming $SO_2$ explicitly and linking it to a consequence.
4. $SO_2(g) + H_2O(l) \rightarrow H_2SO_3(aq)$ (sulphurous acid, contributing to acid rain) — State the consequence: $SO_2$ dissolves in atmospheric moisture to form acid rain, which damages ecosystems, corrodes buildings, and causes respiratory irritation. [M1] for naming $SO_2$, [A1] for linking to acid rain or respiratory harm.

Answer: (a) $C(s) + O_2(g) \rightarrow CO_2(g)$; (b) $S(s) + O_2(g) \rightarrow SO_2(g)$; (c) $SO_2$ dissolves in atmospheric water to form acid rain, damaging ecosystems and causing respiratory problems. Incomplete combustion also produces toxic $CO$ and carbon particulates.

6. Exam Technique and Common Mistakes

Questions on fuels and combustion appear regularly in AQA papers, often as 2–6 mark questions combining equation writing, description, and explanation. The marks are straightforward to collect if you know the exact language the mark scheme expects.

To consolidate the full picture: alkanes are valuable fuels because their combustion is highly exothermic, but the environmental consequences depend critically on two factors — whether combustion is complete or incomplete, and whether the fuel contains sulphur impurities. Complete combustion of a pure alkane gives $CO_2$ and $H_2O$; incomplete combustion adds $CO$ and carbon particulates; and sulphur impurities add $SO_2$ to the mix, with its associated acid rain and respiratory health consequences. These three spec requirements are closely linked and examiners frequently test them together in a single question.

Worked example: From complete to incomplete combustion

Relate oxygen supply to the main combustion products of a fuel.

1. Complete combustion gives carbon dioxide and water. — Sufficient oxygen allows full oxidation of carbon and hydrogen.
2. Incomplete combustion can give carbon monoxide and carbon (soot), with some carbon dioxide still possible. — Oxygen limitation prevents all carbon atoms reaching the +4 oxidation state.
3. Sulphur impurities produce sulphur dioxide during combustion. — This pollutant is not a carbon-containing product but is important environmentally.