Nitrogen and sulfur

Nitrogen gas makes up about 78% of the Earth's atmosphere. It exists as diatomic molecules, N₂. The reason nitrogen is so unreactive, or inert, lies in its bonding. Each nitrogen molecule contains a triple covalent bond (N≡N) between the two nitrogen atoms. This triple bond is extremely strong, with a very high bond enthalpy (the energy needed to break the bond) of 945 kJ/mol. Because so much energy is required to break this bond, nitrogen only reacts under extreme conditions of high temperature and/or pressure. This high energy requirement is known as a high activation energy. Examples of these conditions include lightning strikes in the atmosphere and the industrial Haber process.

Ammonia (NH₃) is a vital nitrogen compound. It is a colourless gas with a characteristic sharp, pungent smell. Ammonia acts as a weak base. According to the Brønsted-Lowry theory, a base is a proton (H⁺) acceptor. The nitrogen atom in an ammonia molecule has a lone pair of electrons, which it can donate to form a dative covalent bond with an H⁺ ion from an acid or even a water molecule. This forms the ammonium ion, NH₄⁺. When ammonia dissolves in water, it forms a weakly alkaline solution: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻. The presence of ammonium ions in a substance can be tested for. When an ammonium salt (e.g., ammonium chloride, NH₄Cl) is warmed with a strong base like sodium hydroxide, it displaces ammonia gas. This is an acid-base displacement reaction. The ammonia gas produced is the only common alkaline gas and can be identified as it turns damp red litmus paper blue.

The vast majority of ammonia produced industrially via the Haber process is used to manufacture nitrogen-containing fertilisers. Plants need nitrogen to synthesise proteins and DNA for growth. While nitrogen is abundant in the air, plants cannot use it directly. They absorb it from the soil as soluble ions, primarily nitrate ions (NO₃⁻) and ammonium ions (NH₄⁺). When crops are harvested, this nitrogen is removed from the soil and must be replenished. Fertilisers like ammonium nitrate (NH₄NO₃) and ammonium sulfate ((NH₄)₂SO₄) provide a soluble source of nitrogen. However, the overuse of these fertilisers has a major environmental consequence: eutrophication. Being highly soluble, excess nitrates can be washed (leached) from the soil by rain into rivers and lakes. This surge in nutrients causes a rapid, excessive growth of algae, known as an 'algal bloom'. This bloom blocks sunlight from reaching plants deeper in the water, causing them to die. When the large quantity of algae also dies, it is decomposed by aerobic bacteria. These bacteria multiply rapidly and consume large amounts of dissolved oxygen from the water in the process. The resulting deoxygenation of the water kills fish and other aquatic animals.

Oxides of nitrogen, collectively known as NOx (mainly NO and NO₂), are significant air pollutants. They have both natural and man-made sources. The main natural source is lightning, where the immense energy causes atmospheric nitrogen and oxygen to react. The main man-made source is the internal combustion engine in vehicles. The high temperature and pressure inside an engine provides the activation energy for N₂ and O₂ from the air to react, forming NO. Once in the atmosphere, NO is readily oxidised to nitrogen dioxide (NO₂). This NO₂ is a pollutant in two key ways. Firstly, it dissolves in water droplets to form nitric acid (HNO₃), a component of acid rain. Secondly, it acts as a catalyst, speeding up the oxidation of another pollutant, sulfur dioxide (SO₂), into sulfur trioxide (SO₃), which makes acid rain worse. To combat this, modern cars are fitted with catalytic converters in their exhaust systems. These devices contain catalysts like platinum and rhodium on a ceramic honeycomb structure. As the hot exhaust gases pass through, the catalyst helps to convert toxic pollutants into harmless substances. Specifically, nitrogen oxides are reduced back to harmless nitrogen gas: 2CO + 2NO → 2CO₂ + N₂.

Sulfur dioxide (SO₂) is the main pollutant responsible for acid rain. Its primary man-made source is the combustion of fossil fuels (especially coal and heavy oil) in power stations and factories. These fuels naturally contain sulfur as an impurity. During combustion, the sulfur is oxidised: S + O₂ → SO₂. Once in the atmosphere, sulfur dioxide reacts further. It can be oxidised to sulfur trioxide (SO₃), a reaction catalysed by NOx. Sulfur trioxide is highly soluble and reacts instantly with water droplets in clouds to form strong sulfuric acid (H₂SO₄). This falls to the ground as acid rain. Acid rain is any form of precipitation with a pH lower than about 5.6. It causes widespread environmental damage. It reacts with and corrodes buildings and statues made of carbonate rock (limestone, marble). It lowers the pH of lakes and rivers, killing fish and other aquatic life that cannot tolerate the increased acidity. It also damages forests by leaching essential mineral ions like magnesium (needed for chlorophyll) from the soil and by damaging the waxy cuticle on leaves.