Oxides of Sulfur
A concise revision guide to sulfur dioxide pollution from combustion, sulfur-containing impurities in fuels, coal as a sulfur dioxide source, acid rain and environmental effects.
Where Sulfur Dioxide Comes From
Some petroleum fractions contain sulfur-based impurities. When these fuels are burned, the sulfur impurities are oxidised and form sulfur dioxide, SO2.
This means combustion can release pollutants even when the hydrocarbon fuel itself mainly contains carbon and hydrogen. The pollution comes from impurities mixed with the fuel or from sulfur-containing organic molecules present in the fuel source.
Key idea: Sulfur dioxide is not produced from pure alkanes alone. It forms when sulfur-containing substances in the fuel are burned.
Formation of Sulfur Dioxide
Sulfur present as an element reacts directly with oxygen during combustion. The balanced equation is:
S + O2 → SO2
Sulfur can also be present inside organic molecules. For example, methanethiol, CH3SH, burns to form sulfur dioxide alongside carbon dioxide and water.
CH3SH + 3O2 → SO2 + CO2 + 2H2O
Exam focus: When sulfur is present in the fuel, combustion produces sulfur dioxide. Do not confuse this with incomplete combustion, which produces carbon monoxide or soot.
Sulfur Dioxide and Acid Rain
Sulfur dioxide is an acidic gas. Once released into the atmosphere, it can dissolve in rainwater and contribute to the formation of acid rain.
Acid rain is a major environmental problem because it can damage buildings, harm aquatic life and affect soil chemistry.
| Stage | Chemistry idea | Consequence |
|---|---|---|
| Combustion | Sulfur-containing substances burn in oxygen. | Sulfur dioxide, SO2, is released. |
| Atmosphere | SO2 is acidic and dissolves in rainwater. | Acid rain can form. |
| Environment | Acidic rainwater reacts with materials and ecosystems. | Buildings, aquatic life and soils can be damaged. |
Sulfur dioxide released during combustion can dissolve in rainwater, contributing to acid rain and environmental damage.
Coal and Other Sources of Sulfur Dioxide
Coal often contains a relatively high sulfur content. When coal is burned in power stations, large quantities of sulfur oxides can be released.
This is why sulfur dioxide pollution is closely associated with the combustion of sulfur-containing fossil fuels, especially where fuel sulfur content is high and pollution control is limited.
| Fuel or source | Reason SO2 may form | Pollution issue |
|---|---|---|
| Petroleum fractions | May contain sulfur-based impurities. | SO2 is released during combustion. |
| Sulfur-containing organic molecules | The sulfur atom is oxidised during burning. | SO2 forms alongside CO2 and H2O. |
| Coal | Often has relatively high sulfur content. | Large amounts of sulfur oxides can be emitted in power stations. |
Environmental Effects to Remember
Sulfur dioxide pollution is important because it links combustion chemistry to large-scale environmental damage. In exam answers, the strongest responses connect the source, the pollutant and the effect.
- Source: sulfur-containing impurities or sulfur-containing fuels are burned.
- Pollutant: sulfur dioxide, SO2, is released into the atmosphere.
- Effect: sulfur dioxide contributes to acid rain.
- Damage: acid rain damages buildings, harms aquatic life and affects soil chemistry.
Exam focus: A precise answer should name sulfur dioxide and link it to acid rain. Avoid vague phrases such as “bad gases” or “general pollution”.
Check Your Understanding
Use these short activities to check the key ideas before moving on to oxides of nitrogen and catalytic converters.
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Some ionic radii are shown.
| Ion | Ionic radius / nm |
|---|---|
| Na+ | 0.102 |
| K+ | 0.138 |
| F− | 0.133 |
| Cl− | 0.180 |
Which compound has the strongest ionic bonding?
Explain why the metallic bonding in magnesium is much stronger than that in sodium.
Oxides of Sulfur FAQs
Common questions about sulfur dioxide pollution from combustion.
Why does combustion produce sulfur dioxide?
Combustion produces sulfur dioxide when the fuel contains sulfur-based impurities or sulfur-containing organic molecules. The sulfur reacts with oxygen to form SO2.
What is the equation for sulfur burning in oxygen?
The balanced equation is S + O2 → SO2.
How does sulfur dioxide cause acid rain?
Sulfur dioxide is an acidic gas. Once released into the atmosphere, it dissolves in rainwater and contributes to the formation of acid rain.
Why is coal an important source of sulfur dioxide pollution?
Coal often contains a relatively high sulfur content, so burning coal in power stations can release large quantities of sulfur oxides.
What damage can acid rain cause?
Acid rain can damage buildings, harm aquatic life and affect soil chemistry.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.