Hydrocarbons as fuels

Hydrocarbon fuels are useful because chemical energy stored in their bonds can ultimately be released as thermal energy during combustion. Petrol, for example, contains a mixture of hydrocarbons that burn inside an internal combustion engine, providing the energy needed to move a vehicle.

The chemistry becomes more interesting when we ask what actually leaves the exhaust pipe. Combustion inside an engine is not perfectly controlled: oxygen may be limited, temperatures can become extremely high, and some fuel may escape without burning completely. Understanding these conditions explains both the pollutants produced and the chemistry used to reduce their environmental impact.

Why are hydrocarbons useful as fuels?

Hydrocarbons contain only carbon and hydrogen. When they react with oxygen, new bonds are formed in carbon dioxide and water, and the overall combustion reaction is strongly exothermic. The released energy can be transferred to the surroundings as heat and then converted into useful forms of energy.

In a car engine, for example, combustion produces hot gases that expand rapidly. The expanding gases push the pistons, allowing chemical energy stored in the fuel ultimately to be converted into mechanical work.

Petrol is a mixture, not a single compound

It is useful to remember that petrol is not one pure hydrocarbon. It is a mixture containing several hydrocarbons and other components. Examination questions may therefore use a particular hydrocarbon, such as octane, as a simplified model for petrol combustion.

The general equation for complete combustion

For a hydrocarbon with formula CₓHᵧ, complete combustion can be represented as:

The important idea is simpler than the general equation may first appear: every carbon atom ends up in CO₂, while every hydrogen atom ends up in H₂O. Oxygen is then balanced last.

For example, complete combustion of octane can be written as:

2C₈H₁₈ + 25O₂ → 16CO₂ + 18H₂O

Carbon and hydrogen are fully oxidised because sufficient oxygen is available.

Complete and incomplete combustion

The products obtained from burning a hydrocarbon depend strongly on the oxygen supply. With sufficient oxygen, complete combustion occurs. When oxygen is limited, however, the hydrocarbon may undergo incomplete combustion.

Formation of carbon monoxide

When there is not enough oxygen to oxidise carbon completely to CO₂, carbon monoxide can form instead.

Carbon monoxide is particularly dangerous because it is colourless and odourless, so its presence may not be obvious.

Formation of carbon particles

If the oxygen supply is even more restricted, some carbon may remain in a very low oxidation state and appear as fine carbon particles or soot. In practice, incomplete combustion can produce a mixture of products rather than one single incomplete-combustion product.

This is why it is better to reason from the oxygen supply than to memorise that incomplete combustion always gives exactly one product.

Pollutants from internal combustion engines

An internal combustion engine is not simply a container in which a hydrocarbon reacts perfectly with oxygen. Fuel and air are mixed rapidly, combustion takes place in a very short time, and extremely high temperatures are reached. These conditions lead to several important pollutants.

Carbon monoxide, CO

Carbon monoxide is produced by incomplete combustion of the hydrocarbon fuel. Its formation is therefore associated with regions of the engine where insufficient oxygen is available for complete oxidation to CO₂.

Oxides of nitrogen, NOₓ

Students sometimes assume that the nitrogen in nitrogen oxides comes from the hydrocarbon fuel. For the reactions considered here, the important source is actually the air entering the engine.

Nitrogen molecules are normally relatively unreactive because the N≡N bond is very strong. Inside an operating engine, however, temperatures become sufficiently high for some N₂ and O₂ molecules to react.

After leaving the hottest part of the engine, some NO can be further oxidised by oxygen:

Nitrogen oxides contribute to air pollution. They irritate the respiratory system and can take part in atmospheric reactions that contribute to acid rain and photochemical smog.

Unburnt hydrocarbons

Not every hydrocarbon molecule entering an engine necessarily burns. Some fuel may leave the engine unchanged or only partly reacted and enter the atmosphere through the exhaust.

Unburnt hydrocarbons can participate, together with nitrogen oxides, in sunlight-driven atmospheric reactions that contribute to the formation of photochemical smog.

How does a catalytic converter reduce exhaust pollution?

Modern vehicles use catalytic converters to change harmful exhaust components into less harmful substances before the gases are released into the atmosphere.

A typical catalytic converter contains a ceramic honeycomb coated with catalytic metals such as platinum, palladium and rhodium. The honeycomb structure provides a large surface area over which the exhaust gases can contact the catalyst.

Removing carbon monoxide and nitrogen monoxide

One particularly important reaction allows CO and NO to react with each other. Carbon monoxide is oxidised to carbon dioxide while nitrogen monoxide is reduced to nitrogen.

This is a redox reaction. Carbon in CO increases in oxidation state from +2 to +4, while nitrogen in NO decreases from +2 to 0.

Oxidising carbon monoxide

Carbon monoxide can also be oxidised directly by oxygen:

Oxidising unburnt hydrocarbons

Unburnt hydrocarbons passing through the converter can be oxidised towards carbon dioxide and water. For a general hydrocarbon:

The converter therefore tackles several harmful components of the exhaust at the same time.

Why use a honeycomb structure?

The exhaust gases must come into contact with the catalyst before the reactions can occur rapidly. A honeycomb structure exposes a very large catalytic surface while still allowing exhaust gases to flow through the converter.

A larger catalyst surface area means more gas molecules can reach catalytic sites at the same time, increasing the overall rate of reaction.

If an examination question asks why a catalytic converter has a honeycomb structure, a useful answer is:

“The honeycomb provides a large surface area of catalyst for contact with the exhaust gases, increasing the rate of the catalytic reactions.”

Catalytic converters do not solve the carbon dioxide problem

Catalytic converters greatly reduce emissions of CO, NOₓ and unburnt hydrocarbons, but they do not remove carbon dioxide from the exhaust. In fact, oxidation of CO and unburnt hydrocarbons produces CO₂.

This distinction is important. A catalytic converter addresses several forms of harmful local air pollution, whereas CO₂ is associated primarily with the enhanced greenhouse effect and climate change.

The greenhouse effect and the enhanced greenhouse effect

The natural greenhouse effect is essential for maintaining temperatures on Earth that allow liquid water and life to exist. The problem is not the existence of the greenhouse effect itself, but an increase in its strength as concentrations of greenhouse gases rise.

How does the greenhouse effect work?

Radiation arriving from the Sun is mainly at relatively short wavelengths. Much of this radiation passes through the atmosphere and is absorbed by Earth's surface.

The warmed surface then emits energy as longer-wavelength infrared radiation. Greenhouse-gas molecules can absorb some of this infrared radiation because it can cause changes in their molecular vibrations.

The absorbed energy is subsequently redistributed and infrared radiation is emitted in different directions. Some is directed back towards Earth's surface. Increasing greenhouse-gas concentrations therefore reduces the rate at which energy escapes directly to space, causing the surface and lower atmosphere to warm until a new energy balance is established.

Carbon dioxide and methane

Carbon dioxide is formed whenever hydrocarbon fuels undergo complete combustion. Large-scale combustion of fossil fuels therefore transfers carbon that had been stored underground into atmospheric CO₂.

Methane, CH₄, is itself a hydrocarbon and an important greenhouse gas. It can enter the atmosphere from sources such as leakage during natural-gas production and transport, as well as biological processes.

Molecule for molecule, CH₄ can have a stronger warming influence than CO₂, although the overall climatic importance of a greenhouse gas also depends on factors such as its atmospheric concentration and lifetime.

Putting the whole story together

The chemistry of hydrocarbon fuels can be understood as a chain of cause and effect. We begin with an exothermic combustion reaction that makes the fuel useful. We then consider what happens when real combustion conditions depart from the ideal situation.

Limited oxygen can produce CO and soot. High temperatures allow atmospheric N₂ and O₂ to form NOₓ. Some fuel may remain unburnt. A catalytic converter then uses redox chemistry to remove much of the CO, NOₓ and unburnt hydrocarbon before the exhaust is released.

Yet even when combustion is complete and the catalytic converter works effectively, CO₂ is still produced. This is why reducing toxic exhaust pollutants and reducing greenhouse-gas emissions are related but distinct environmental goals.

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