Topic M: Industrial Chemistry (Elective)

Industrial Chemistry applies the principles of rate and equilibrium to large-scale manufacture, where yield, rate, cost and safety must all be balanced. This elective examines the Haber and Contact processes in detail, introduces green chemistry as a guide to sustainable production, and surveys the electrolytic industries that supply reactive metals and key chemicals.

Rate and equilibrium in industry

Industrial chemists choose conditions by combining what they know about rate and equilibrium with economic reality. A high yield at equilibrium is no use if the reaction is too slow, and the fastest conditions may give a poor yield, so a compromise is usually required. Catalysts are central because they raise the rate without affecting the position of equilibrium, allowing lower temperatures that favour exothermic yields while keeping production fast. Other practical measures include removing the product to pull the equilibrium forward, and recycling unreacted starting materials to improve overall efficiency. Energy costs, equipment durability and safety also shape the final choice. Understanding these trade-offs explains why real processes rarely run at the conditions that give the maximum theoretical yield.

The Haber process

The Haber process makes ammonia from nitrogen and hydrogen, N2 + 3 H2 <=> 2 NH3, a reversible exothermic reaction with fewer moles of gas on the product side. From Le Chatelier's principle, a high yield needs high pressure and low temperature, but a low temperature makes the rate too slow. The chosen conditions are a compromise: a pressure of about 200 atmospheres, a temperature of around 450 degrees Celsius, and an iron catalyst to speed the attainment of equilibrium. Ammonia is removed by cooling and liquefying it, and unreacted nitrogen and hydrogen are recycled. Ammonia is vital for fertilisers, which support world food production, illustrating how equilibrium principles are applied to meet a major human need.

The Contact process

The Contact process manufactures sulfuric acid, with its key reversible step being the oxidation of sulfur dioxide to sulfur trioxide, 2 SO2 + O2 <=> 2 SO3, which is exothermic and has fewer moles of gas on the product side. The conditions chosen are a moderate temperature of about 450 degrees Celsius, a pressure only slightly above atmospheric (because the yield is already high so high pressure is not worth the cost), and a vanadium(V) oxide catalyst. The sulfur trioxide is absorbed into concentrated sulfuric acid to form oleum, which is then diluted, rather than added directly to water, which would create a dangerous mist. Sulfuric acid is one of the most important industrial chemicals, used in fertilisers, detergents and many manufacturing processes.

Principles of green chemistry

Green chemistry aims to make industrial processes more sustainable by reducing waste, energy use and hazards. Key principles include maximising atom economy so that as much of the reactant mass as possible ends up in the desired product, using catalysts to lower energy demands, preferring renewable feedstocks, designing safer solvents and reaction conditions, and minimising hazardous by-products. Atom economy is calculated as the molar mass of the desired product divided by the total molar mass of all products, times 100, and complements percentage yield by measuring efficiency rather than just quantity. Adopting green chemistry reduces environmental impact and cost, and is increasingly important as industry responds to concerns about pollution, resource depletion and climate change.

Electrolytic industries: aluminium

Aluminium is extracted by electrolysis because it is too reactive to be reduced by carbon. Its ore, bauxite, is purified to aluminium oxide, which is dissolved in molten cryolite to lower the melting point and save energy. In the cell, aluminium ions are reduced to molten aluminium at the carbon cathode, while oxide ions are oxidised at the carbon anodes to give oxygen, which burns the anodes away so they must be replaced regularly. The process consumes large amounts of electricity, which is why aluminium smelters are often sited near cheap power and why recycling aluminium, which uses far less energy, is strongly encouraged. This industry shows electrolysis applied on a vast scale.

Electrolytic industries: the chlor-alkali process

The electrolysis of concentrated sodium chloride solution (brine) is the basis of the chlor-alkali industry, producing three valuable products. At the anode, chloride ions are oxidised to chlorine gas, used to make bleach, disinfectants and PVC. At the cathode, water is reduced to hydrogen gas, used as a fuel and in making ammonia and margarine. The solution left behind is sodium hydroxide, a strong alkali used in soaps, paper and many chemical processes. Because chlorine and sodium hydroxide must be kept apart to avoid reacting, the cell is designed with a membrane that separates the products while allowing ions through. This single process supplies several key industrial chemicals from cheap, abundant brine.