Combustion of alkanes and pollutionAQA A-Level Chemistry: Revision notes

Section 1

Alkanes as fuels

Alkanes are used as fuels because they burn in oxygen and release a lot of energy. Combustion of alkanes (and other organic compounds) can be complete or incomplete.

Complete combustion happens with plenty of oxygen. Carbon is oxidised to carbon dioxide and hydrogen to water:

$C_3H_8 + 5O_2 \to 3CO_2 + 4H_2O$

Incomplete combustion happens when oxygen is limited. The products include carbon monoxide and/or carbon (soot), as well as water:

$2CH_4 + 3O_2 \to 2CO + 4H_2O$

$CH_4 + O_2 \to C + 2H_2O$

Key termscomplete combustionincomplete combustion

Common mistake

Balance the oxygen last, and check the oxygen atoms on both sides. Half-integer coefficients such as 12½O₂ are allowed when balancing for one mole of fuel.

Section 2

Pollutants from the internal combustion engine

The internal combustion engine produces four main pollutants:

Carbon monoxide (CO) from incomplete combustion. It is toxic.
Carbon (soot) particles from incomplete combustion.
Unburned hydrocarbons from fuel that leaves the engine without burning.
Nitrogen oxides (NOx), formed because nitrogen and oxygen from the air react at the very high temperatures and sparks inside the engine.

Nitrogen oxides contribute to acid rain and photochemical smog.

Key termsNOxcarbon monoxideunburned hydrocarbons

Common mistake

NOx is not formed from the fuel. It forms from the nitrogen and oxygen of the air at high temperature, so say that it is the air, not the petrol, that supplies the nitrogen.

Section 3

Catalytic converters

The gaseous pollutants can be removed using catalytic converters. A converter contains a catalyst (platinum, palladium or rhodium) spread over a ceramic honeycomb, which gives a large surface area.

On the surface, the pollutants react to form less harmful gases:

$2CO + 2NO \to 2CO_2 + N_2$

Unburned hydrocarbons are oxidised to carbon dioxide and water:

$C_8H_{18} + 12\tfrac{1}{2}O_2 \to 8CO_2 + 9H_2O$

The converter converts CO, NOx and unburned hydrocarbons into CO₂, N₂ and H₂O.

Key termscatalytic converter

Exam tip

When writing the converter equation, check it balances for both nitrogen and oxygen. 2CO + 2NO → 2CO₂ + N₂ is the one that is usually asked.

Section 4

Sulfur dioxide and its removal

Fuels such as fuel oil and coal contain sulfur as an impurity. When they burn, the sulfur forms sulfur dioxide:

$S + O_2 \to SO_2$

Sulfur dioxide dissolves in rainwater to form acid rain, which damages trees, aquatic life and limestone buildings, and is a respiratory irritant.

It can be removed from flue gases using calcium oxide or calcium carbonate (often as a wet slurry). Sulfur dioxide is an acidic gas and calcium oxide and calcium carbonate are bases, so they react (neutralisation):

$CaO + SO_2 \to CaSO_3$

$CaCO_3 + SO_2 \to CaSO_3 + CO_2$

Key termssulfur dioxideacid rainflue gas desulfurisation

Exam tip

To explain why CaO removes SO₂, give the idea in one line: an acidic gas reacts with a base. Do not forget to include the equation if asked.

Must know

Complete combustion: CO₂ and H₂O. Incomplete combustion: CO and/or C, plus H₂O.
Engine pollutants: NOx, CO, carbon, unburned hydrocarbons.
Catalytic converters change CO, NOx and hydrocarbons into CO₂, N₂ and H₂O.
Sulfur in fuels gives SO₂ and acid rain.
CaO and CaCO₃ remove SO₂ from flue gases because they are bases that neutralise the acidic gas.

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Combustion of alkanes and pollution

A petrol engine burns octane, C₈H₁₈, in a limited supply of air. A mechanic notices that the exhaust gas contains carbon monoxide and black particles.
Explain why the exhaust gas contains carbon monoxide and carbon when the supply of air is limited.2 marks
A modern petrol car has a catalytic converter containing platinum and rhodium on a ceramic honeycomb. The exhaust gases leaving the engine contain nitrogen oxides (NOx), carbon monoxide and unburned hydrocarbons.
Hexane, C₆H₁₄, is one of the unburned hydrocarbons in the exhaust gases. Write an equation for its complete oxidation in the catalytic converter.2 marks
An oil-fired power station burns a fuel oil that contains sulfur impurities. Before the flue gases are released, they are passed through a wet mixture of calcium carbonate in water.
Explain why sulfur dioxide is formed when the fuel oil is burned, and describe one environmental problem that it causes.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).