Nitrogen and sulfur — common mistakes

A common exam question involves the test for $SO_2$. Remember the colour change and the species involved. Orange dichromate(VI) ion, $Cr_2O_7^{2-}$, is reduced by $SO_2$ to the green chromium(III) ion, $Cr^{3+}$. You must state 'acidified' potassium dichromate(VI) as the reaction requires $H^+$ ions.

The forward reaction to make ammonia is exothermic, so a low temperature would give the highest yield according to Le Chatelier's principle. However, at low temperatures, the rate of reaction is extremely slow. A moderate temperature of around 450 °C is a compromise that provides a reasonably fast rate of reaction while still achieving an economically viable yield.

Sulfur dioxide is formed primarily from an impurity (sulfur) already present in the fuel (e.g., coal) being burned. In contrast, nitrogen oxides are typically formed when gases that are naturally present in the air (N₂ and O₂) react together under the high-temperature conditions of combustion, such as in a car engine.

Nitrogen-based fertilisers, produced via the Haber process, are crucial for modern agriculture. They dramatically increase crop yields, allowing us to feed a large global population. While their overuse leads to environmental problems like eutrophication, banning them would lead to widespread food shortages. The challenge is to manage their use more efficiently through practices like precision farming to minimise environmental run-off.

The reaction between sulfur trioxide ($SO_3$) and water is extremely exothermic and produces a fine mist of sulfuric acid that is difficult to collect and hazardous. Instead, $SO_3$ is dissolved in concentrated sulfuric acid to form oleum ($H_2S_2O_7$). The oleum is then safely diluted with a controlled amount of water to produce more concentrated sulfuric acid. This is a safer and more efficient industrial method.