IB Chemistry SL

Revision Notes

8.1.13 Acid Deposition

Acid Deposition

What is acid deposition?

  • Rain is naturally acidic because of dissolved CO2 which forms carbonic acid

 H2O (l) + CO2 (g)  ⇌  H2CO3 (aq)

  • Carbonic acid is a weak acid and dissociates in the following equilbrium reaction giving a pH of 5.6

H2CO3 (aq)  ⇌  H+ (aq) +  HCO3(aq)

  • For that reason acid rain is defined as rain with a pH of below 5.6
  • Acid deposition includes all processes by which acidic components leave the atmosphere
    • This could be gases or precipitates
  • There are two types of deposition: wet acid deposition and dry acid deposition
    • Wet acid deposition refers to rain, snow, sleet, hail, fog, mist and dew
    • Dry acid deposition refers to acidic particles and gases that fall to the ground as dust and smoke
  • Acid deposition is formed when nitrogen or sulfur oxides dissolve in water to form HNO3, HNO2, H2SO4 and H2SO3


Acid deposition equations

Formation of sulfur based acids

  • Fossil fuels are often contaminated with small amounts of sulfur impurities
  • When these contaminated fossil fuels are combusted, the sulfur in the fuels get oxidised to sulfur dioxide

 S (s) + O2 (g) → SO2 (g)

  • Sulfur dioxide may be further oxidised to sulfur trioxide

2SO2 (g) + O2 (g) ⇌ 2SO3 (g)

  • The sulfur dioxide and sulfur trioxide then dissolve in rainwater droplets to form sulfurous acid and sulfuric acid

SO2(g) + H2O  (l) → H2SO3 (aq)

SO3 (g) + H2O (l) → H2SO4 (aq)

  • These acids are components of acid rain which has several damaging impacts on the environment

Formation of acid rain by nitrogen oxides

  • The temperature in an internal combustion engine can reach over 2000 °C
  • Here, nitrogen and oxygen, which at normal temperatures don’t react, combine to form nitrogen monoxide:

N2 (g)+ O2 (g) ⇌ 2NO (g)

  • Nitrogen monoxide reacts further forming nitrogen dioxide:

2NO (g) + O2 (g) ⇌ 2NO2 (g)

  • Nitrogen dioxide gas reacts with rain water to form a mixture of nitrous and nitric acids, which contribute to acid rain:

2NO2 (g) + H2O (l)  → HNO2 (aq) + HNO3 (aq)

  • Lightning strikes can also trigger the formation of nitrogen monoxide and nitrogen dioxides in air
  • Nitrogen dioxide gas reacts with rain water and more oxygen to form nitric acid

4NO2 (g) + 2H2O (l) + O2 (g)→ 4HNO3 (aq)

  • When the clouds rise, the temperature decreases, and the droplets get larger
  • When the droplet containing these acids are heavy enough, they will fall down as acid rain

Nitrogen & Sulfur Acid Rain Formation, downloadable AS & A Level Chemistry revision notes

The diagram shows the formation of acid rain by the oxidation of nitrogen dioxide


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