What is acid deposition?

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

H₂O (l) + CO_2?(g) ??? H₂CO_3?(aq)

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

H₂CO_3?(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

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) + O_2?(g) → SO_2?(g)

• Sulfur dioxide may be further oxidised to sulfur trioxide

2SO_2?(g) + O_2?(g) ? 2SO_3?(g)

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

SO₂(g) + H₂O? (l) → H₂SO_3?(aq)

SO_3?(g) + H₂O (l) → H₂SO_4?(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:

N_2?(g)+ O_2?(g) ? 2NO (g)

• Nitrogen monoxide?reacts further forming?nitrogen dioxide:

2NO (g) + O_2?(g) ? 2NO_2?(g)

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

2NO_2?(g) + H₂O (l)? → HNO_2?(aq) + HNO_3?(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

4NO_2?(g) + 2H₂O (l) + O_2?(g)→ 4HNO_3?(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

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