Acid Deposition
Chemistry: IB Diploma Higher Level, topic 6.4.1
Acid Deposition
Whilst rainwater is naturally acidic, sulphur and nitrogen oxides dissolve in rainwater to further lower the pH and produce acid rain.
Acidity of natural rainwater
- Rainwater naturally has a pH of around 5.6 due to the presence of dissolved CO2 forming carbonic acid:
- CO2(g) + H2O(l) ⇌ H2CO3(aq)
- H2CO3(aq) ⇌ HCO3-(aq) + H+(aq)
Acid pollution
- Human activity, such as the burning of fossil fuels, releases non-metal oxide gases such as sulphur oxides and nitrogen oxides into the atmosphere.
- These are acidic gases and dissolve in rainwater to lower the pH below 5.6 and produce acid rain.
- Wet acid deposition (acid rain) is when the pH of rainwater drops below 5.6.
- The acidic pollutants can also become incorporated into dust or smoke and fall to the ground as dry deposition.
Nitrogen oxides
- Nitrogen oxides are produced by the combination of nitrogen and oxygen under the high temperatures and pressures in a combustion engine.
- In this process, nitrogen first combines with oxygen to form nitrogen monoxide:
- N2(g) + O2(g) → 2NO(g)
- Reactive nitrogen monoxide is then further oxidised to form nitrogen dioxide:
- 2NO(g) + O2(g) → 2NO2(g)
Formation of nitric and nitrous acid
- Nitrogen dioxide gas reacts with rainwater in the presence of more oxygen to produce nitric acid:
- 4NO2(g) + 2H2O(l) + O2(g) → 4HNO3(aq)
- Alternatively, nitrogen dioxide can react directly with water to produce a mixture of nitric acid and nitrous acid:
- 2NO2(g) + H2O(l) → HNO3(aq) + HNO2(aq)
Sulphur oxides
- Sulphur oxides are produced when fossil fuels containing sulfur impurities (mainly coal or oil) are burned.
- The sulphur impurities combust in the oxygen to form mainly sulphur dioxide:
- S(g) + O2(g) → SO2(g)
- SO2 can also further react with O2 to form SO3
- 2SO2(g) + O2(g) → 2SO3(g)
Formation of sulphuric acid
- SO2 reacts with water to produce sulphuric (IV) acid:
- SO2(g) + H2O(l) → H2SO3(aq)
- SO3 reacts with water to produce sulphuric (VI) acid:
- SO3(g) + H2O(l) → H2SO4(aq)
Effects of Acid Deposition
Acid deposition can have detrimental effects on human health and ecosystems.
Human health
- Fine sulphate and nitrate particles are produced from acid rain.
- These particles when inhaled cause irritation in the respiratory tract.
- This irritation can increase trigger respiratory diseases such as asthma, bronchitis or emphysema.
Soil
- Acid rain lowers the pH of the soil and some organisms cannot tolerate this low pH so their population decreases.
- The increased acidity of the soil increases the solubility of important nutrients such as Ca2+ and K+.
- These ions are leached from the soil and become less available for the plants.
Water
- The ecological effects of acid rain are evident in aquatic ecosystems, such as rivers, lakes and marshes.
- Many organisms cannot tolerate decreased pH and many fish eggs cannot hatch below pH 5.
- Low pH levels cause leaching of toxic aluminum ions, which run into streams and lakes.
Materials - calcium carbonate
- Many buildings and statues are made of limestone, which consists of mainly calcium carbonate (CaCO3).
- Calcium carbonate reacts with the acids in acid rain to produce soluble salts:
- CaCO3(s) + 2HNO3(aq) → Ca(NO3)2(aq) + H2O(l) + CO2(g)
- CaCO3(s) + H2SO4(aq) → CaSO4(aq) + H2O(l) + CO2(g)
- Over the years this causes damage to the structures as the limestone is worn away by the reactions with acids.
Materials - metal structures
- Acid rain causes metal structures to corrode as the metals become oxidised on reacting with acids in the rainwater:
- Fe(s) + H2SO4(aq) → FeSO4(aq) + H2(g)
Test yourself on Acid Deposition
Which of the following gives the correct equation for the formation of sulphuric (IV) acid from sulphur dioxide?
Why is rainwater naturally acidic, with a pH of 5.6?
Which of the following can NOT be caused by acid rain?