Combustion

Alkanes are great fuels. One litre of petrol contains roughly 32.6 megajoules of energy and provides 7800 calories. If you could eat petrol, this would be enough to feed an average man for over three days. When burnt, all this stored energy is released into the environment as heat. We can say that they have high negative enthalpies of combustion (see Enthalpy Changes). This is why we combust alkanes as fuels for our cars, planes, and other engines; to heat our homes, and to power our electronics.

Try placing a small piece of magnesium ribbon in a test tube filled with dilute hydrochloric acid. You should see bubbles. If you wrap your hand around the tube, it should feel slightly warmer. This is a good example of an exothermic reaction.

What are the different types of combustion?

There are two different types of combustion reactions:

• Complete combustion.
• Incomplete combustion.

They vary in their conditions and products. Let’s take a quick look.

Complete combustion

In complete combustion, a fuel is burnt in excess oxygen. Burning any hydrocarbon (such as methane or propane) in this way produces carbon dioxide and water and releases lots of heat to the environment. The equation for the complete combustion of methane is given below:

Incomplete combustion

In incomplete combustion, there isn’t enough oxygen to fully oxidise all the carbon atoms into carbon dioxide. Instead, the main product is carbon monoxide. If no oxygen is present whatsoever, pure carbon is produced in the form of soot. Incomplete combustion is much less efficient than complete combustion and releases less energy.

The above equation shows the incomplete combustion of methane. Compare it to the equation we gave earlier for the complete combustion of methane. You'll notice that complete combustion requires 2 moles of oxygen for each mole of methane, whereas incomplete combustion only requires 1.

How do you write combustion equations?

Throughout your time as a chemist, you’ll probably have been told to always write whole number equations. An exception to this rule is combustion, where reacting half a mole of oxygen molecules is perfectly acceptable. In fact, it often makes it easier to clearly see the reactants and products of the reaction.

To write balanced equations for complete combustion, start with one mole of your hydrocarbon. Let’s take propane, , as an example. Write out your basic equation, remembering that the products will be carbon dioxide and water, and leave gaps to represent the unknown number of moles of both these compounds and oxygen:

The alkane is the only source of carbon on the left-hand side of the equation. Hence we have exactly three carbons present. In order to balance the equation, there must only be three molecules of carbon dioxide produced on the right-hand side:

Likewise, we have eight hydrogen atoms present on the left-hand side of the equation. Our only product containing hydrogen is water, but each water molecule has two hydrogen atoms, so we must produce exactly 8 ÷ 2 = 4 moles of water for each mole of propane burnt:

To finish off, let’s count up the oxygen on the right-hand side. We have six from the carbon dioxide and four from the water, making ten. This means we need ten oxygen atoms on the left-hand side. As each molecule of oxygen provides two oxygen atoms, we need exactly five moles of oxygen for each mole of propane:

Writing equations for incomplete combustion follows exactly the same process, but remember that your products will be carbon monoxide and water, not carbon dioxide. Taking our example of propane again, we can work out that we produce three moles of carbon monoxide and four moles of water, but this only amounts to seven oxygen atoms. Therefore, we only need three and a half moles of oxygen on the left hand side of the equation:

In reality, reactions are never as simple as that. There will generally always be a mixture of complete and incomplete combustion reactions.

What are the environmental impacts of combustion?

As mentioned above, burning alkanes is an extremely useful reaction. We rely on hydrocarbon fuels for transport, electricity, and heating. Your car probably runs on petrol or diesel, both derived from crude oil, and your house might be warmed by a gas boiler. However, the pressure on governments, businesses, and consumers to move away from hydrocarbons and look towards renewable energy sources is steadily increasing due to their negative environmental impacts. We’ll explore some of the products of burning hydrocarbons and their effects below.

Carbon dioxide

As we learnt, complete combustion of alkanes releases carbon dioxide. Carbon dioxide is a powerful greenhouse gas.

By trapping this heat, greenhouse gases warm the planet and contribute towards global warming. As the levels of carbon dioxide particles in the atmosphere have climbed since the start of the Industrial Revolution, average global temperatures have risen rapidly - by over 1.2℃ compared to pre-industrial levels. We see increases in extreme weather, crop failure and mass extinction of major species.

A diagram of the greenhouse effect. Greenhouse gas particles trap the sun’s radiation reflected back from the Earth and reemit part of it back at the planet, instead of letting it pass through into outer space.commons.wikimedia.org

Carbon monoxide and carbon particles

Carbon monoxide is a colourless, odourless gas. It is highly toxic to humans and animals. Carbon particles from soot can cause respiratory irritation, certain cancers, and global dimming. Both carbon monoxide and carbon particles are produced in the incomplete combustion of alkanes.

Sulfur dioxide

Sulfur impurities in fuels can react to produce sulfur dioxide when burnt. This sulfur dioxide then reacts with oxygen and water in the air to form acid rain, which damages buildings and plant life. The equation for the formation of sulfur dioxide is shown below:

Nitrous oxides

Nitrous oxides are produced in combustion engines when the temperatures reach levels high enough for nitrogen and oxygen from the air to react. Nitrous oxides react to form acid rain. They can also cause breathing difficulties and photochemical smog.

Limiting the impacts of combustion

Moving completely away from burning fossil fuels isn’t an easy task, and scientists have worked hard to come up with processes to reduce the effects of alkane combustion on the environment. These include:

• Flue gas desulfurisation.
• Catalytic converters.
• Carbon-neutral fuel alternatives.

Flue gas desulfurisation

Flue gas is the gas produced when burning coal in power stations. It contains sulfur dioxide due to impurities in the fuel. The sulfur is removed by reacting the flue gas with either calcium oxide and water, or calcium carbonate and oxygen, to form gypsum. Gypsum is a saleable product and we can use it to make plasterboard.

For example:

Catalytic converters

In 1993, it became law for all new cars in the UK to have catalytic converters fitted to their exhausts. This is because internal combustion engines in cars produce most of the pollutants listed above, such as sulfur dioxide and carbon monoxide, although we now remove sulfur from petrol before it is burnt. Catalytic converters work by reducing the amounts of carbon monoxide, nitrous oxides, and unburnt hydrocarbons in the exhaust fumes. Catalytic converters have a honeycomb shape, to maximise surface area, and are made up of platinum and rhodium. These expensive metals coat the honeycomb frame in a thin layer to minimise the amount of them needed. As the exhaust fume gases pass over the catalytic converter, the catalysts speed up the reactions that produce less harmful products. For example, carbon monoxide reacts with nitrous oxides to produce nitrogen and carbon dioxide, and unburnt hydrocarbons react with nitrous oxides to produce nitrogen, carbon dioxide and water.

Carbon-neutral alternatives

As you now know, burning fossil fuels such as coal and gas causes a net increase in carbon dioxide levels in the atmosphere. This is strongly linked to global warming, so finding a solution to rising temperatures is becoming ever more pressing. However, some fuels can be carbon-neutral.

These fuels produce no overall carbon emissions and so don’t heat our planet. Examples include biofuels and synthetic fuels.

• Biofuels are made from biomass, such as sugar cane or wood. They are quick to grow and all the carbon they release upon combustion was taken in during their lifetime. However, the fertilisers, water, and land required to produce biofuels can create other problems like food security issues, monoculture, and deforestation.
• Synthetic fuels are artificial hydrocarbons. They are made from carbon dioxide captured from the air and hydrogen produced in electrolysis.