Section 1 of 5Combustion Basics
Combustion is the reaction of a fuel with oxygen, and the type of combustion depends on how much oxygen is available.
Combustion
- Fuels react with oxygen in an exothermic reaction that releases energy.
Elements in fuels
- Most fuels contain carbon and hydrogen, and some contain sulfur as an impurity.
Complete combustion
- Plenty of oxygen converts the carbon into carbon dioxide and the hydrogen into water.
Incomplete combustion
- A limited supply of oxygen produces solid carbon particles, also called soot or particulates.
Section 2 of 5Products and Pollutants
The products of combustion can be traced back to the elements that the fuel contains.
Products and Pollutants
Carbon dioxide
- Formed from the carbon in the fuel during complete combustion;
- it is a greenhouse gas that contributes to climate change.
Particulates
- Formed by incomplete combustion; they cause global dimming and form soot that darkens buildings and statues.
Sulfur dioxide
- Formed from sulfur impurities; it dissolves in rainwater and causes acid rain.
Oxides of nitrogen
- Formed when nitrogen and oxygen react at the high temperatures inside engines, and they also contribute to acid rain.
| Product | Source | Main environmental effect |
|---|---|---|
| Carbon dioxide | Carbon in the fuel | Greenhouse gas and climate change |
| Particulates | Carbon in the fuel, incomplete combustion | Global dimming and soot |
| Sulfur dioxide | Sulfur impurities | Acid rain |
| Oxides of nitrogen | Air at high temperature in engines | Acid rain |
Section 3 of 5How Acid Rain Forms
Acid rain is caused by gases released when fuels burn.
1
Sulfur dioxide released from burning fuels mixes with clouds and dissolves in rainwater.
2
Because sulfur dioxide is a non-metal oxide, it forms an acidic solution when it dissolves in water.
3
The acid rain corrodes metal structures and damages buildings and statues made of carbonate rocks such as limestone.
4
Acid rain also damages aquatic organisms and crops, and it can irritate the lungs, throat and eyes.
Section 4 of 5Worked Example
Question Write a balanced equation for the complete combustion of methane.
- Write the reactants and productsCH₄ + O₂ → CO₂ + H₂O.
- Balance the carbonthere is one carbon atom on each side.
- Balance the hydrogenfour hydrogen atoms on the left need two H₂O molecules on the right.
- Balance the oxygenthe right side then has 2 oxygen atoms in CO₂ and 2 in 2H₂O, so 2O₂ is needed on the left.
Answer CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l)
Section 5 of 5Exam Checklist & Tips
10.2.1 – Key Points for the Exam
- Combustion A reaction in which a fuel reacts with oxygen and releases energy.
- Complete combustion Combustion with enough oxygen, producing carbon dioxide and water.
- Incomplete combustion Combustion with limited oxygen, producing carbon particles.
- Particulates Solid particles, including soot, produced by incomplete combustion.
- Acid rain Rain made acidic by sulfur dioxide and oxides of nitrogen dissolving in it.
- Global dimming The reduction of sunlight reaching the Earth, caused partly by particulates.
Common Mistakes
- Stating that acid rain is caused mainly by carbon dioxide;
- it comes from sulfur dioxide and oxides of nitrogen.
- Forgetting that the products reveal the elements in a fuel: CO₂ shows carbon, H₂O shows hydrogen and SO₂ shows sulfur.
- Saying that incomplete combustion produces only carbon dioxide; it produces particulates instead.
- Leaving out the balancing of oxygen in an equation, which makes the equation incorrect.