Atmospheric pollutants from fuels — AQA GCSE Chemistry
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Atmospheric pollutants from fuels explained
Atmospheric pollutants from fuels covers 5 specification points, including The combustion of fuels is a major source of… and Most fuels, including coal, contain carbon….
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The combustion of fuels is a major source of atmospheric pollutants.
Combustion means burning a fuel in oxygen, releasing energy. When the fuel is a hydrocarbon, complete combustion gives carbon dioxide and water vapour. In a limited supply of oxygen, incomplete combustion also produces carbon monoxide and solid carbon particles. Fuels may contain sulfur, so burning them releases sulfur dioxide, and the high temperature inside an engine or furnace lets nitrogen and oxygen from the air combine to form oxides of nitrogen. These products enter the atmosphere and cause problems such as acid rain, respiratory harm and global climate change. For example, burning petrol in a car engine releases carbon dioxide, water vapour, carbon monoxide, nitrogen oxides and particulates. This makes fuel combustion a major source of atmospheric pollutants.
Most fuels, including coal, contain carbon and/or hydrogen and may also contain some sulfur.
Fuels are substances that release energy when burned. Most common fuels are based on carbon and hydrogen. Coal is mostly carbon but also contains hydrocarbons and mineral matter. Petrol, diesel and natural gas are hydrocarbons, meaning their molecules contain only carbon and hydrogen. Many fuels also contain sulfur as an impurity. For example, coal and crude-oil fractions can contain sulfur compounds. When the fuel burns, carbon is oxidised to carbon dioxide, hydrogen is oxidised to water vapour, and sulfur is oxidised to sulfur dioxide. Knowing the elements present in a fuel lets you predict which oxides will form. This is why the composition of a fuel matters when assessing its pollution.
The gases released into the atmosphere when a fuel is burned may include carbon dioxide, water vapour, carbon monoxide, sulfur dioxide and oxides of nitrogen.
Burning a fuel releases a mixture of gases into the atmosphere. Carbon dioxide and water vapour form when carbon and hydrogen in the fuel are oxidised completely. If oxygen is limited, incomplete combustion also produces carbon monoxide, a toxic gas. Sulfur impurities in the fuel burn to sulfur dioxide. At the high temperatures inside engines and furnaces, nitrogen and oxygen from the air react to form oxides of nitrogen. For example, a car engine burning petrol can release carbon dioxide, water vapour, carbon monoxide, sulfur dioxide and nitrogen oxides in its exhaust. The word may is important because the exact mixture depends on the fuel, the oxygen supply and the temperature.
Solid particles and unburned hydrocarbons may also be released that form particulates in the atmosphere.
Combustion does not always convert every fuel molecule into simple gases. When oxygen is limited, some carbon is not fully oxidised and is released as solid particles, often called soot. Some fuel molecules may escape without burning at all, and these unburned hydrocarbons are also released. In the atmosphere, solid particles and unburned hydrocarbons can join together to form particulates, which are tiny solid or liquid particles suspended in the air. For example, diesel engines can emit black soot and unburned fuel droplets. Particulates can reduce air quality, cause breathing problems and contribute to smog. They also affect climate by absorbing or scattering sunlight.
Students should be able to: describe how carbon monoxide, soot (carbon particles), sulfur dioxide and oxides of nitrogen are produced by burning fuels predict the products of combustion of a fuel given appropriate information about the composition of the fuel and the conditions in which it is used.
Burning fuels releases pollutants depending on the fuel's composition and the conditions. Incomplete combustion, caused by limited oxygen, produces carbon monoxide and soot. Carbon monoxide forms when carbon in the fuel is only partly oxidised; soot forms as unburnt carbon particles. Sulfur dioxide forms when sulfur impurities in the fuel burn and combine with oxygen. Oxides of nitrogen form when the high temperature inside an engine or furnace allows nitrogen and oxygen from the air to react. To predict products, identify the elements present in the fuel and the oxygen supply: complete combustion of a hydrocarbon gives carbon dioxide and water, while limited oxygen adds carbon monoxide and soot. Sulfur impurities add sulfur dioxide, and high temperatures add oxides of nitrogen.
Your focus
- Describe combustion as burning a fuel in oxygen and identify the products of complete and incomplete combustion.
- Explain how sulfur impurities and high-temperature reactions with air produce sulfur dioxide and oxides of nitrogen.
- Relate the products of fuel combustion to named atmospheric pollution problems.
Show all 15 objectives
- State that most fuels contain carbon and/or hydrogen and may contain sulfur.
- Identify coal as a carbon-rich fuel that also contains hydrocarbons and sulfur impurities.
- Predict which oxide products can form from the elements present in a fuel.
- Name the gases that may be released when a fuel burns.
- Explain how each named gas is formed during combustion.
- Describe how fuel composition, oxygen supply and temperature affect the gases released.
- Describe how solid particles and unburned hydrocarbons are released during combustion.
- Explain how these substances form particulates in the atmosphere.
- Relate particulates to effects on human health and the environment.
- Describe how carbon monoxide, soot, sulfur dioxide and oxides of nitrogen are produced when fuels burn.
- Predict the products of combustion when given the fuel composition and the conditions of use.
- Explain the link between limited oxygen supply and the formation of carbon monoxide and soot.
Atmospheric pollutants from fuels exam tips
Marking Points
- Combustion is the reaction of a fuel with oxygen, releasing energy as heat and light.
- Complete combustion of a hydrocarbon produces carbon dioxide and water vapour only.
- Incomplete combustion, caused by insufficient oxygen, produces carbon monoxide and solid carbon particles as well as carbon dioxide and water.
- Sulfur impurities in fuels burn to form sulfur dioxide.
- High temperatures in engines and furnaces allow nitrogen and oxygen from the air to react, forming oxides of nitrogen.
- These released substances are atmospheric pollutants linked to acid rain, breathing problems and climate change.
- Most fuels contain carbon and/or hydrogen, so burning them can produce carbon dioxide and water vapour.
- Coal is a carbon-rich fuel that also contains hydrocarbons and other material.
- Sulfur is present in many fuels as an impurity, not as the main fuel component.
- Burning sulfur in a fuel produces sulfur dioxide.
- The elements present in a fuel determine which oxide pollutants can form when it burns.
- Carbon dioxide is released when carbon in a fuel is oxidised.
- Water vapour is released when hydrogen in a fuel is oxidised.
- Carbon monoxide is released when a fuel burns in a limited supply of oxygen.
- Sulfur dioxide is released when sulfur impurities in a fuel burn.
- Oxides of nitrogen are released when nitrogen and oxygen from the air react at high temperatures.
- The gases released depend on the fuel composition, oxygen supply and combustion temperature.
- Solid particles, often carbon soot, form when carbon in a fuel is not fully burned.
- Unburned hydrocarbons are fuel molecules that escape combustion unchanged.
- Solid particles and unburned hydrocarbons can form particulates in the atmosphere.
- Particulates are tiny solid or liquid particles suspended in the air.
- Particulates can harm human health, reduce air quality and contribute to smog.
- Incomplete combustion increases the release of solid particles and unburned hydrocarbons.
- Incomplete combustion occurs when oxygen supply is limited, producing carbon monoxide and soot instead of only carbon dioxide and water.
- Carbon monoxide is produced by partial oxidation of carbon in the fuel; soot is unburnt carbon released as particles.
- Sulfur dioxide is produced when sulfur impurities in a fuel burn and react with oxygen.
- Oxides of nitrogen are produced when nitrogen and oxygen from the air react at the high temperatures found in engines and furnaces.
- Predicting products requires identifying the elements in the fuel and the conditions, especially whether oxygen is plentiful or limited and whether temperatures are high.
- Complete combustion of a hydrocarbon fuel produces carbon dioxide and water; additional pollutants depend on impurities and conditions.
Examiner Tips
- 💡Link each pollutant to its origin: carbon dioxide and water from complete combustion, carbon monoxide and carbon from incomplete combustion, sulfur dioxide from sulfur impurities, and nitrogen oxides from hot air.
- 💡Use the phrase limited supply of oxygen when explaining incomplete combustion, because it shows the cause rather than just naming the products.
- 💡When asked why combustion is a major source of pollutants, refer to the huge quantities of fuel burned in vehicles, power stations and homes, not just to one example.
- 💡Use the wording carbon and/or hydrogen when describing fuel composition, because it covers fuels that contain both elements as well as those containing only one.
- 💡If a question gives the formula of a fuel, identify the carbon, hydrogen and any sulfur atoms before predicting the combustion products.
- 💡Distinguish clearly between an element present in a fuel and the oxide pollutant formed from it, for example sulfur in coal and sulfur dioxide in the exhaust gases.
- 💡For each gas, give both its name and how it is formed, because marks often depend on linking the gas to its source.
- 💡Use the word may when listing exhaust gases, since the mixture varies with fuel and conditions.
- 💡If asked to name pollutant gases from burning a named fuel, check the fuel composition first so you include sulfur dioxide only when sulfur is present.
- 💡Use the term unburned hydrocarbons for fuel molecules that escape combustion, and particulates for the tiny particles they help to form.
- 💡Link particulates to health and environmental effects such as breathing problems and smog, because questions often ask for consequences as well as formation.
- 💡When explaining soot formation, mention a limited supply of oxygen, since that is the condition that prevents complete oxidation of carbon.
- 💡Link each pollutant to its source: carbon monoxide and soot to incomplete combustion, sulfur dioxide to sulfur impurities, oxides of nitrogen to high-temperature reaction of air.
- 💡When predicting products, first list the elements in the fuel, then decide whether oxygen is plentiful or limited and whether conditions are hot enough for nitrogen oxides.
- 💡Use the phrase 'limited oxygen supply' rather than vague wording such as 'not enough air' to show precise understanding.
- 💡Check that every product you name can be formed from the elements present; do not invent products containing elements absent from the fuel and air.
Common Mistakes
- Thinking that all combustion is complete and produces only carbon dioxide and water. Correction: state that a limited oxygen supply causes incomplete combustion, which also forms carbon monoxide and carbon particles.
- Believing that oxides of nitrogen come from impurities in the fuel. Correction: they form when nitrogen and oxygen in the air react at high temperatures.
- Writing that combustion destroys matter or makes pollutants disappear. Correction: atoms are conserved, so fuel and oxygen atoms rearrange into the product gases and particles.
- Saying that coal contains only carbon. Correction: coal is mainly carbon but also contains hydrocarbons and sulfur impurities.
- Confusing the element sulfur with the product sulfur dioxide. Correction: sulfur is the impurity in the fuel; sulfur dioxide is the oxide formed when it burns.
- Assuming that all fuels contain sulfur. Correction: sulfur is a common impurity in some fuels, but its amount varies and some fuels contain very little.
- Listing carbon monoxide as a product of complete combustion. Correction: carbon monoxide forms during incomplete combustion when oxygen is limited.
- Saying that oxides of nitrogen come from nitrogen in the fuel. Correction: they form from nitrogen and oxygen in the air at high temperatures.
- Treating water vapour as a pollutant gas in the same way as carbon monoxide or sulfur dioxide. Correction: water vapour is a normal combustion product, while the other named gases cause specific pollution problems.
- Calling particulates a gas. Correction: particulates are tiny solid or liquid particles suspended in the atmosphere.
- Thinking that unburned hydrocarbons are the same as carbon dioxide. Correction: unburned hydrocarbons are fuel molecules that did not react, while carbon dioxide is a fully oxidised product.
- Assuming that all carbon in a fuel becomes carbon dioxide. Correction: with limited oxygen, some carbon forms solid particles instead.
- Thinking carbon monoxide is produced only from carbon dioxide; the correction is that it forms when carbon is partly oxidised because oxygen is limited.
- Assuming sulfur dioxide comes from atmospheric nitrogen; the correction is that it comes from sulfur impurities in the fuel.
- Believing oxides of nitrogen form from fuel nitrogen only; the correction is that they mainly form when nitrogen and oxygen in the air react at high temperature.
- Writing that soot is carbon dioxide; the correction is that soot is unburnt carbon particles.