Revise Nitrogen and sulfur for Cambridge 9701 (12.1)

Nitrogen gas ($N_2$) is remarkably unreactive under normal conditions. This stability is due to the strong triple covalent bond between the two nitrogen atoms. A significant amount of energy, the bond enthalpy of +945 kJ mol⁻¹, is required to break this bond and allow nitrogen to react. This is why, despite its abundance, nitrogen is often a limiting nutrient for plant growth.

To overcome this inertia for industrial purposes, such as producing ammonia for fertilisers, we use the Haber process. This process combines nitrogen from the air with hydrogen (usually from natural gas) under specific conditions to produce ammonia. It is a reversible reaction, so conditions are chosen to maximise the yield and rate of reaction in a cost-effective way.

$N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g) \quad \Delta H = -92 \text{ kJ mol}^{-1}$

While the N≡N bond is strong, it can be broken by the extreme conditions found in lightning strikes or the internal combustion engine of a car. At these high temperatures and pressures, atmospheric nitrogen and oxygen react to form nitrogen monoxide (NO), a colourless gas.

$N_2(g) + O_2(g) \rightarrow 2NO(g)$

Once in the atmosphere, nitrogen monoxide is readily oxidised to form nitrogen dioxide ($NO_2$), a brown, toxic gas. $NO_2$ is a major component of photochemical smog and dissolves in atmospheric water to form a mixture of nitric acid ($HNO_3$) and nitrous acid ($HNO_2$), contributing to acid rain.

$2NO_2(g) + H_2O(l) \rightarrow HNO_3(aq) + HNO_2(aq)$

To mitigate this, modern cars are fitted with catalytic converters. These devices use a catalyst (a mixture of platinum, rhodium, and palladium) to convert harmful nitrogen oxides back into harmless nitrogen gas, while also oxidising carbon monoxide to carbon dioxide.

The primary source of atmospheric sulfur dioxide ($SO_2$) is the combustion of fossil fuels, particularly coal, which contain sulfur as an impurity. When the fuel is burned, the sulfur reacts with oxygen to form $SO_2$ gas.

In the atmosphere, $SO_2$ can be further oxidised to sulfur trioxide ($SO_3$). This reaction is slow on its own but is catalysed by pollutants like $NO_2$ or fine dust particles. $SO_3$ is highly reactive and dissolves readily in water droplets to form sulfuric acid ($H_2SO_4$), a major component of acid rain.

$2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g)$ $SO_3(g) + H_2O(l) \rightarrow H_2SO_4(aq)$

The industrial production of sulfuric acid, a vital chemical, utilises this chemistry in the Contact process. The key step is the catalytic oxidation of $SO_2$ to $SO_3$ using a vanadium(V) oxide ($V_2O_5$) catalyst at about 450 °C and near atmospheric pressure.