Chemical industries: Class 10 Chemistry

Key points

A chemical industry converts raw materials into useful products by chemical processes. Raw materials come from air, water, minerals, plants, animals, natural gas and petroleum. Chemical industries make fertilizers, medicines, soaps, plastics, paints, fuels and building materials. They give employment, save foreign exchange and support agriculture.

Many industrial processes use the principle of Le Chatelier to get a good yield. If conditions of a system at equilibrium are changed, the system moves to oppose the change. Higher pressure favours the side with fewer gas molecules, and removing the product pushes the reaction forward. Temperature is chosen as a balance between rate and yield, and catalysts are used to get a faster rate.

Haber process: ammonia is made from nitrogen (from air) and hydrogen (from natural gas and steam). N₂ + 3H₂ ⇌ 2NH₃, ΔH = −92 kJ per mole of N₂ reacting. Conditions: about 450 °C, 200 atm pressure, and iron as catalyst. High pressure favours ammonia as the product has fewer moles of gas (2 against 4). A low temperature favours yield since the reaction is exothermic, but it is too slow, so 450 °C is a compromise. Ammonia is cooled to liquid and removed, and the unused gases are recycled.

Uses of ammonia: it is the base of nitrogen fertilizers (urea, ammonium nitrate and ammonium sulphate), nitric acid, explosives, and cleaning agents. Urea is made from ammonia and carbon dioxide at high pressure and temperature: 2NH₃ + CO₂ → NH₂COONH₄, then NH₂COONH₄ → CO(NH₂)₂ + H₂O. Urea has about 46% nitrogen, the highest of the common solid fertilizers. Pakistan makes urea from natural gas.

Contact process: sulphuric acid, called the king of chemicals, is manufactured in three stages. First, sulphur or iron pyrites is burned in air: S + O₂ → SO₂. Second, SO₂ is purified and passed with air over vanadium(V) oxide, V₂O₅, catalyst at about 450 °C and 1 to 2 atm: 2SO₂ + O₂ ⇌ 2SO₃ (exothermic). Third, SO₃ is absorbed in 98% sulphuric acid to form oleum, H₂S₂O₇, which is diluted with water to give H₂SO₄. SO₃ is not added to water directly since it gives a mist of acid.

Uses of sulphuric acid: making fertilizers such as superphosphate and ammonium sulphate, lead-acid batteries, detergents, paints, dyes, drugs and refining of petroleum.

Solvay process: sodium carbonate (soda ash) is made from brine, ammonia and limestone. Ammonia and carbon dioxide are passed into saturated brine and sodium hydrogencarbonate precipitates: NaCl + NH₃ + CO₂ + H₂O → NaHCO₃ + NH₄Cl. It is heated: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. The CO₂ is recycled. Ammonia is recovered from NH₄Cl with lime from the heating of limestone. Soda ash is used for glass, soap, paper and water softening.

Chlor-alkali industry: electrolysis of concentrated brine (NaCl solution) gives chlorine, hydrogen and sodium hydroxide. At the anode: 2Cl⁻ → Cl₂ + 2e⁻. At the cathode: 2H₂O + 2e⁻ → H₂ + 2OH⁻. Na⁺ and OH⁻ stay in solution as NaOH. A membrane keeps chlorine away from NaOH. Chlorine is used in water treatment and bleach, hydrogen in making ammonia and margarine, and NaOH in soap, paper and textile industries.

Cement: the raw materials are limestone (calcium carbonate) and clay (silica, alumina and iron oxide). They are ground, mixed and heated in a rotary kiln to about 1400 to 1450 °C to give clinker. The clinker is cooled and ground with a small amount of gypsum (about 3 to 5%) to slow the setting. When mixed with water, cement sets and hardens by hydration. Pakistan has many cement plants, as limestone is abundant in the Salt Range and elsewhere.

Petroleum is a mixture of hydrocarbons. Fractional distillation separates crude oil by boiling point. Fractions from top to bottom: refinery gas (LPG), petrol (gasoline), naphtha, kerosene, diesel, lubricating oil, fuel oil and bitumen. Lighter fractions have lower boiling points, shorter chains and are collected higher in the column. Heavier fractions are cracked into petrol and alkenes, which are used to make plastics.

Other industries: soap is made by boiling fat or oil with NaOH (saponification), and glycerol is a by-product. Glass uses sand, soda ash and limestone melted together. Sugar is made from sugar cane and beet. Paper comes from wood pulp, bagasse and straw. Textile, leather and fertilizer industries are important to the economy of Pakistan.

Industry also creates pollution. Care is needed in the safe disposal of waste, treatment of effluents, control of gas emission from chimneys and conserving energy. Good plant design recycles unused materials and uses the heat from exothermic steps to save fuel.

Study tip: For each major process learn the raw materials, the main equation, the catalyst, temperature and pressure, and one use of the product. Draw a simple flow diagram. For yield questions, balance the equation first and compare the actual with the theoretical amount.

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Common mistakes

• Writing wrong conditions for the Haber and contact processes, such as the wrong catalyst. Iron is for Haber, V₂O₅ is for contact.
• Saying high temperature increases the yield of an exothermic reaction. It increases only the rate.
• Adding SO₃ directly to water in the contact process.
• Forgetting to say that unused gases are recycled in industrial processes.
• Mixing up the anode and cathode products in the electrolysis of brine.

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