Industrial and environmental chemistry: Class 12 Chemistry

Key points

Industrial chemistry converts raw materials into useful products on a large scale. An industrial process must give a good yield at a low cost, use easily available raw materials, and cause little pollution. Conditions such as temperature, pressure and catalysts are chosen to balance speed of reaction against yield and cost. Le Chatelier's principle guides these choices.

Ammonia is made by the Haber process: N₂ + 3H₂ ⇌ 2NH₃, ΔH = −92 kJ/mol. Nitrogen comes from air and hydrogen from natural gas. The reaction is exothermic and the number of gas moles falls from four to two. High pressure (about 200 atm) favours ammonia. A low temperature favours the yield but is too slow, so a compromise of about 450 °C is used with an iron catalyst. Ammonia is removed by cooling and liquefying, and unreacted gases are recycled.

Sulfuric acid by the contact process:

• Sulfur or pyrite is burned in air to make sulfur dioxide: S + O₂ → SO₂.
• SO₂ is oxidised to SO₃ over a vanadium(V) oxide (V₂O₅) catalyst at about 450 °C and 1 to 2 atm: 2SO₂ + O₂ ⇌ 2SO₃, ΔH = −197 kJ.
• SO₃ is absorbed in 98% sulfuric acid to form oleum, H₂S₂O₇, because direct addition to water gives a fine acid mist. SO₃ + H₂SO₄ → H₂S₂O₇.
• Oleum is diluted with water to the required strength: H₂S₂O₇ + H₂O → 2H₂SO₄.

Sulfuric acid is the most widely made chemical. It is used for fertilizers (superphosphate and ammonium sulfate), detergents, paints, batteries, dyes, explosives and refining of petroleum.

Sodium carbonate by the Solvay process:

• Ammonia and carbon dioxide are passed into saturated brine: NaCl + NH₃ + CO₂ + H₂O → NaHCO₃ + NH₄Cl.
• Sodium hydrogencarbonate is less soluble and separates. It is heated: 2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O.
• CO₂ from this step and from heating limestone (CaCO₃ → CaO + CO₂) is reused. Ammonia is recovered from NH₄Cl with Ca(OH)₂, leaving CaCl₂ as the main by-product.
• Overall: CaCO₃ + 2NaCl → Na₂CO₃ + CaCl₂. Sodium carbonate is used in glass, soap, paper and water softening.

Urea, NH₂CONH₂, is made from ammonia and carbon dioxide at about 180 °C and 150 atm: 2NH₃ + CO₂ → NH₂COONH₄ (ammonium carbamate), which loses water to give urea: NH₂COONH₄ → NH₂CONH₂ + H₂O. It has the highest nitrogen content (46.7%) among common solid fertilizers and is also used in plastics and animal feed. Pakistan produces large amounts of urea from natural gas.

Fertilizers supply the nutrients nitrogen (N), phosphorus (P) and potassium (K). Examples are urea, ammonium nitrate, ammonium sulfate, DAP (diammonium phosphate), superphosphate and potassium chloride. Over-use causes eutrophication, in which nitrates and phosphates washed into water bodies cause algae to grow, use up dissolved oxygen and kill fish.

Cement is made by heating limestone (calcium carbonate) and clay to about 1450 °C in a rotary kiln to form clinker, which is cooled and ground with 3 to 5% gypsum. The gypsum slows the setting so that the cement does not harden too quickly. Main compounds are calcium silicates and aluminates. Cement making releases CO₂, and Pakistan has many cement plants in Punjab and Khyber Pakhtunkhwa.

Air pollution:

• Carbon monoxide from incomplete combustion binds haemoglobin and reduces oxygen transport.
• Oxides of sulfur and nitrogen dissolve in rain to give acid rain (pH below 5.6), which damages buildings, soil, crops and lakes.
• Carbon dioxide and methane are greenhouse gases that trap heat and cause global warming.
• CFCs destroy ozone, which shields the earth from UV rays.
• Particulate matter and smog harm the lungs. Control methods include catalytic converters, desulfurisation of fuels, filters, scrubbers and cleaner energy.

Water pollution comes from sewage, industrial effluents, fertilizers, pesticides and oil. Indicators are low dissolved oxygen, high BOD (biochemical oxygen demand) and the presence of toxic metals such as lead and mercury. Treatment includes screening, sedimentation, biological treatment and chlorination. Hardness is caused by calcium and magnesium ions, and is removed by boiling (temporary hardness), washing soda or ion exchange.

Soil pollution arises from excess agrochemicals, solid waste and industrial waste. Solid waste is managed by reducing, reusing, recycling, composting and controlled landfill. Safe disposal of plastic, batteries and chemical waste protects health, and sustainable practice keeps natural resources for the future.

Study tip: For each manufacturing process learn the raw materials, the equation, the catalyst, the conditions and the reason for them, then the main uses. Questions often ask why a temperature or pressure was chosen, which is a Le Chatelier answer. For pollution topics use a cause, effect and control layout in your answer.

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Common mistakes

• Saying that the Haber process uses a low temperature to favour the yield only. Rate is also needed, so a compromise is used.
• Naming the catalyst of the contact process as Fe or Ni. It is vanadium(V) oxide.
• Writing that NaHCO₃ is made soluble in the Solvay process. It precipitates because it is less soluble.
• Confusing the greenhouse effect with ozone depletion. They have different gases and different causes.
• Giving the pH of acid rain as below 7. Normal rain is already slightly acidic, and acid rain is below 5.6.

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