E11 Acid deposition (HL)

E.11.1 Describe the mechanism of acid deposition caused by the oxides of nitrogen and oxides of sulfur.

Two primary pollutants that cause acid deposition are sulphur dioxide and nitrogen dioxide. They are converted into acids by a free radical mechanism involving hydroxyl free radicals, formed either by the reaction of water vapour with ozone or by the reaction of water vapour with oxygen free radicals formed when ozone decomposes.

H2O(g) + O3 (g) --> 2HO∙(g) + O2(g)
H2O(g) + O∙(g) --> 2HO∙(g)

Reacts with sulphur dioxide and nitrous oxides in the presence of water to give the dissolved acids.

HO∙(g) + NO2(g) --> HNO3(aq)
HO∙(g) + NO(g) --> HNO2(aq)
HO∙(g) + SO2(g) --> HOSO2∙(g)
then HOSO2∙(g) + O2(g) --> HO2∙(g) + SO3(g)
followed by SO3(g) +H2O(l) --> H2SO4(aq)

E. 11.2 Explain the role of ammonia in acid deposition.
The atmosphere contains trace amounts of ammonia and also in the soil from the action of rhizobia (bacteria) in the root nodules of leguminous plants. The ammonia in the atmosphere neutralises partially the acids to form ammonium sulphate, which is a slightly acidic as it is a product of a weak base and strong acids. As they are washed out by precipitation or sink to the ground the ammonium ion is deposited and enters the soil where acidification and nitrification an occur.

NH4+(aq) + 2O2(g) --> 2H+(aq) + NO3-(aq) + H2O(l)