Science

Using carbon fuels

Fuels react with oxygen to release energy. Complete combustion happens in a plentiful supply of air and incomplete combustion occurs when the supply of air is limited.

Complete combustion releases more energy than incomplete combustion. Incomplete combustion also creates carbon monoxide, and more soot. Several factors must be considered when choosing the best fuel for a particular purpose.

Factors influencing the use of a fuel

The fossil fuels include coal, oil and natural gas. Various factors need to be considered when deciding how to use a fossil fuel. These include:

  • the energy value of the fuel in kJ/g of fuel
  • the availability of the fuel
  • how the fuel can be stored
  • the cost of the fuel
  • the toxicity of the fuel - whether it is poisonous
  • any pollution caused when the fuel is used, such as acid rain
  • how easy it is to use the fuel.

You are expected to be able to list factors such as the ones above.

Factories can cause air pollution

In general, solids such as coal are easier to store than liquids and gases but they are often more difficult to light. Liquids and gases ignite more easily. They also flow, which means they can be transported through pipelines.

Carbon dioxide is a greenhouse gas that contributes to global warming. The table shows some approximate energy values of the different fossil fuels, and the typical mass of carbon dioxide they produce when they burn.

Energy values of fuel

fuelenergy content (kJ/g)mg of carbon dioxide produced for each kJ
natural gas5253
petrol4371
coal2493

Coal releases the least amount of energy per gram of fuel. It also produces the most carbon dioxide for a given amount of energy released when it burns.

You should be able to interpret data to choose the best fuel for a particular purpose.

Combustion

Fuels are substances that react with oxygen to release useful energy. Most of the energy is released as heat, but light energy is also released.

About 21 per cent of the air is oxygen. When a fuel burns in plenty of air, it receives enough oxygen for complete combustion.

Complete combustion

Complete combustion needs a plentiful supply of air so that the elements in the fuel react fully with oxygen.

Fuels such as natural gas and petrol contain hydrocarbons. These are compounds of hydrogen and carbon only. When they burn completely:

  • the carbon oxidises to carbon dioxide
  • the hydrogen oxidises to water (remember that water, H2O, is an oxide of hydrogen).

In general, for complete combustion:

hydrocarbon + oxygen    →    carbon dioxide + water

Here are the equations for the complete combustion of propane, used in bottled gas:

propane + oxygen → carbon dioxide + water

C3H8 + 5O2 → 3CO2 + 4H2O

Incomplete combustion

Incomplete combustion occurs when the supply of air or oxygen is poor. Water is still produced, but carbon monoxide and carbon are produced instead of carbon dioxide.

In general for incomplete combustion:

hydrocarbon + oxygen    →    carbon monoxide + carbon + water

The carbon is released as soot. Carbon monoxide is a poisonous gas, which is one reason why complete combustion is preferred to incomplete combustion. Gas fires and boilers must be serviced regularly to ensure they do not produce carbon monoxide.

Here are the equations for the incomplete combustion of propane, where carbon is produced rather than carbon monoxide:

propane + oxygen → carbon + water

C3H8 + 2O2 → 3C + 4H2O

The Bunsen burner

The Bunsen burner is commonly used in school laboratories to heat chemicals. Its fuel is natural gas, which is almost pure methane, CH4. Methane is a hydrocarbon. So the Bunsen burner has an air hole that allows complete or incomplete combustion.

Air hole open

When the air hole is open, air is drawn into the chimney, where it mixes with the natural gas. This ensures complete combustion:

methane + oxygen    →    carbon dioxide + water

A very hot, blue flame is produced.

bunden burner with a flame - unburnt gas and air are at the centre of the flame

The hottest part of the flame is at the tip of the dark blue cone

Air hole closed

When the air hole is closed the natural gas can only mix with air at the mouth of the chimney. Incomplete combustion occurs as a result:

methane + oxygen    →    carbon monoxide + carbon + water

bunsen with a yellow flame

The yellow flame is often called the safety flame

A yellow flame is produced, which transfers less heat energy than the blue flame. The yellow flame is brighter than the blue flame because the specks of carbon glow when heated.

Combustion - Higher tier

You should be able to write balanced symbol equations for the complete combustion - or incomplete combustion - of a simple hydrocarbon fuel, if you are given its molecular formula.

The formula of oxygen gas is O2.

The table summarises the formulas of the possible combustion products from hydrocarbon fuels.

Formulas of the possible combustion products from hydrocarbon fuels

nameformulaformed in complete combustionformed in incomplete combustion
carbon dioxideCO2yes 
carbon monoxideCO yes
sootC yes
waterH2Oyesyes

Here are some examples of how to balance symbol equations for combustion. Remember that you should have the same number of atoms of each element on each side of the arrow.

Example 1

Complete combustion of methane, CH4

Write formulae for each substance

CH4 + O2    →    CO2 + H2O

Balance the number of H atoms

CH4 + O2    →    CO2 + 2H2O

Balance the number of O atoms

CH4 + 2O2    →    CO2 + 2H2O

Example 2

Incomplete combustion of ethane, C2H6

Write formulae for each substance

C2H6 + O2    →    CO + C + H2O

Balance the number of H atoms

C2H6 + O2    →    CO + C + 3H2O

Balance the number of O atoms

C2H6 + 2O2    →    CO + C + 3H2O

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