Syllabus Context & Application
Specification Context: Acid deposition was an explicit syllabus topic in the 2016 IB Chemistry syllabus (Topic 8.5). In the 2025 syllabus, environmental aspects appear across reactivity contexts. These notes are provided as high-value enrichment and contextual application.
Definition of Acid Deposition
Natural unpolluted rainwater has a pH of \(\approx 5.6\) due to dissolved atmospheric \(\text{CO}_2\) forming carbonic acid (\(\text{H}_2\text{CO}_3\)).
Acid rain is defined as precipitation with a \(\text{pH} < 5.6\), caused primarily by atmospheric sulfur oxides (\(\text{SO}_x\)) and nitrogen oxides (\(\text{NO}_x\)).
Sources of Acid Rain
- Sulfur dioxide (SO2) - from burning coal and oil containing sulfur impurities
- Nitrogen oxides (NOx) - formed when N2 and O2 react at the high temperatures inside vehicle engines and power stations
Key Equations
Chemical Pathways to Acid Rain
Sulfur Oxides (\(\text{SO}_2, \text{SO}_3\)):
\[\text{SO}_2(\text{g}) + \text{H}_2\text{O}(\text{l}) \rightarrow \text{H}_2\text{SO}_3(\text{aq}) \quad (\text{sulfurous acid})\] \[\text{SO}_3(\text{g}) + \text{H}_2\text{O}(\text{l}) \rightarrow \text{H}_2\text{SO}_4(\text{aq}) \quad (\text{sulfuric acid})\]Nitrogen Oxides (\(\text{NO}_2\)):
\[2\text{NO}_2(\text{g}) + \text{H}_2\text{O}(\text{l}) \rightarrow \text{HNO}_2(\text{aq}) + \text{HNO}_3(\text{aq})\]Environmental Effects
| Effect | Detail |
|---|---|
| Lakes & rivers | Acidification kills fish and aquatic organisms |
| Soil | Leaches essential minerals (Ca2+, Mg2+) and releases toxic Al3+ ions |
| Buildings | Corrodes limestone (CaCO3) and marble structures |
| Forests | Damages leaves and weakens trees |
| Metals | Accelerates corrosion of iron and steel structures |
Prevention
- Catalytic converters in vehicles reduce NOx emissions
- Flue gas desulfurisation removes SO2 from power station exhaust using CaCO3
- Liming acidic lakes with CaCO3 neutralises the acid
- Switching to renewable or low-sulfur fuels
Pre- vs Post-Combustion Mitigation
Always distinguish clearly between pre-combustion methods (e.g. hydrodesulfurisation of fuels) and post-combustion methods (e.g. flue-gas desulfurisation with \(\text{CaCO}_3\) or \(\text{CaO}\)) when answering questions on reducing \(\text{SO}_2\) emissions.
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