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Combustion

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Chemistry

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.

Combustion is an exothermic reaction that involves burning a fuel such as coal, gas or petrol, usually in oxygen.

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.

An exothermic reaction is a reaction that releases lots of energy to its surroundings. In the case of combustion, this energy takes the form of heat and light.

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.

When working with a Bunsen burner in the lab, you’ll know that closing the air hole results in the safety flame. This flame is orange-yellow. However, opening the air hole produces a blue flame. The safety flame is a lot easier to see than the blue flame, but is also a lot less hot as oxygen is limited. This means it uses an incomplete combustion reaction. Using this flame a lot causes the bottom of any beakers held above the Bunsen burner to go black and sooty. In contrast, the blue flame uses complete combustion. It releases a lot more energy when burning, resulting in a cleaner, hotter, more dangerous flame.

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.

A greenhouse gas is a gas that traps radiation from the sun reflected from the Earth, instead of letting it pass through the atmosphere and back into outer space.

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.

Combustion Diagram of the greenhouse effect StudySmarterA 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.

Carbon-neutral substances are substances that produce no net overall carbon dioxide emissions in their lifetime. All the carbon dioxide they release is counterbalanced by carbon dioxide taken in at different points in their lives.

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.

Carbon capture is still a relatively small industry but is growing rapidly as techniques improve. For example, the Swiss company Climeworks directly captures carbon from the air using special filters and either stores it deep within the ground, or recycles it into fuels.

Combustion - Key takeaways

  • Combustion is an exothermic reaction that releases large amounts of energy in the form of heat and light.
  • Combustion can be complete or incomplete.
  • The combustion of fuels such as coal, gas, and petrol has several negative environmental effects.
  • Methods to reduce the environmental impact of combustion include flue gas desulfurisation, catalytic converters, and switching to carbon-neutral fuels.

Combustion

 Combustion is an exothermic reaction that involves burning a fuel, usually in oxygen. It releases lots of energy in the form of heat and light.

Incomplete combustion is combustion that occurs in insufficient oxygen.

The products of complete combustion are carbon dioxide and water.

Final Combustion Quiz

Question

What is combustion?

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Answer

Combustion is an exothermic reaction that involves burning a fuel, usually in oxygen. It releases lots of energy in the form of heat and light.


Show question

Question

Compare complete and incomplete combustion.

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Answer

  • Both produce water and carbon dioxide.
  • Both are exothermic reactions.
  • Complete combustion takes place in excess oxygen.
  • Incomplete combustion takes place in limited oxygen.
  • Complete combustion releases more energy than incomplete combustion.

Show question

Question

What are the products of complete combustion?


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Answer

Water

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Question

Give the two carbon-based products that can be produced in incomplete combustion.


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Answer

Carbon particles

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Question

Why does using the safety flame of a Bunsen burner cause sooty deposits?


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Answer

Using the safety flame encourages incomplete combustion. One of the products is carbon, which accumulates as soot.


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Question

Write an equation to show the complete combustion of ethane.

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Answer


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Question

Write an equation to show the incomplete combustion of ethane to produce carbon monoxide and water only.


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Answer


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Question

Which of the following two alkanes releases more energy when burnt? Their enthalpies of combustion are given:


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Answer

Butane.


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Question

Name four pollutants produced in combustion.


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Answer

  • Sulfur dioxide.
  • Nitrous oxides.
  • Carbon.
  • Carbon monoxide.
  • Carbon dioxide.

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Question

Hydrocarbon fuels can contain sulfur impurities. What are the negative effects caused by these impurities?


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Answer

Sulfur burns to form sulfur dioxide, which can cause acid rain and breathing difficulties.


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Question

What are catalytic converters made of?


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Answer

Platinum and rhodium.

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Question

Write an equation to show the reaction of a nitrous oxide, , with an unburnt hydrocarbon , in a catalytic converter to form carbon dioxide, water, and nitrogen.


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Answer


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Question

Discuss the pros and cons of biofuels.

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Answer

  • Biofuels are cheap, easy to grow and are carbon-neutral.
  • However, they are relatively inefficient to burn.
  • They take up valuable growing space that could be used for food crops. 
  • They may also require processing before use, which uses energy, and can be big and bulky to transport.

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