6A.1 Oxidation reactions in the air

The book says the atmosphere is a medium for "the oxidation of certain gases". The main examples are:

1. Lightning and nitrogen. The temperature in a lightning channel is about 30 000 °C, enough to break the N≡N bond: N₂ + O₂ → 2NO (nitric oxide) 2NO + O₂ → 2NO₂ (nitrogen dioxide, brown gas) 3NO₂ + H₂O → 2HNO₃ + NO (nitric acid, carried to the soil by rain as nitrate fertiliser) (Atom check for the last equation: N 3 = 2 + 1; O 6 + 1 = 6 + 1; H 2 = 2.) This is the natural counterpart of the acid-rain reactions in Module 12. Industrial combustion produces the same oxides at high temperature.
2. The hydroxyl radical (OH) — the "detergent" of the atmosphere. Ozone absorbing UV can produce an excited oxygen atom, which reacts with water vapour to make OH radicals: O* + H₂O → 2OH. OH oxidises many pollutants and greenhouse gases (CO, CH₄, SO₂, NO₂) and so cleans the air, for example CO + OH → CO₂ + H, and NO₂ + OH → HNO₃. The lifetime of methane in the air (about 12 years) is controlled by this reaction.
3. Sulfur oxidation. SO₂ is oxidised to SO₃ and then dissolves to make sulfuric acid (Module 12), or forms fine sulfate particles that reflect sunlight and scatter light (sulfur from volcanoes cools the climate for a short time).
4. Combustion (burning) of fuels is oxidation: complete combustion gives CO₂ and water; incomplete gives CO and soot (Module 12).