WJEC GCSE Chemistry 2.6 Reversible Reactions, Industrial Processes and Important Chemicals Exam Questions

About WJEC GCSE Chemistry 2.6 Reversible Reactions, Industrial Processes and Important Chemicals

This final Unit 2 topic looks at reversible reactions and the industrial manufacture of ammonia, sulfuric acid and fertilisers. You need to describe the Haber process and explain how the conditions chosen give the most economical yield, though Le Chatelier's principle is not required. You must outline the three stages of the contact process, the uses of sulfuric acid and its action as a dehydrating agent. The topic covers making ammonium fertilisers by neutralisation, the tests for ammonia gas and ammonium ions, and the benefits and problems of nitrogenous fertilisers. It links to acids and salts in 2.2 and eutrophication in Biology.

What you need to know

• Reversible reactions Products can react to re-form the reactants, shown by ⇌. In a closed system both reactions continue at the same rate once equilibrium is reached.
• Haber process N2 + 3H2 ⇌ 2NH3. Nitrogen from air and hydrogen from natural gas react over an iron catalyst at about 450 °C and 200 atmospheres. These conditions are a compromise between yield, rate and cost; unreacted gases are recycled.
• Ammonia test Ammonia gas turns damp red litmus paper blue (and forms white fumes with hydrogen chloride). Ammonium ions are detected by warming with sodium hydroxide solution and testing the gas given off for ammonia.
• Contact process Sulfur burns to form sulfur dioxide; 2SO2 + O2 ⇌ 2SO3 over a vanadium(V) oxide catalyst at about 450 °C; sulfur trioxide is then absorbed to make sulfuric acid. Sulfuric acid is used for fertilisers, paints, dyes, fibres, plastics and detergents.
• Dehydrating agent Concentrated sulfuric acid removes water: sugar turns to a black mass of carbon, and blue hydrated copper(II) sulfate turns white.
• Fertilisers Ammonia solution is neutralised by acids: NH3 + HNO3 → NH4NO3 and 2NH3 + H2SO4 → (NH4)2SO4. Nitrogenous fertilisers increase crop growth, but if washed into rivers they cause algal blooms and loss of oxygen (eutrophication).

Common mistakes in 2.6 Reversible Reactions, Industrial Processes and Important Chemicals

• Describing the Haber process conditions as giving the highest possible yield. They are a compromise chosen for a reasonable yield at a fast enough rate and acceptable cost.
• Using blue litmus to test for ammonia. Ammonia is alkaline, so it turns damp red litmus blue.
• Forgetting the catalyst in either process. The Haber process uses iron and the contact process uses vanadium(V) oxide.
• Writing (NH4)SO4 for ammonium sulfate. Two ammonium ions balance one sulfate ion, giving (NH4)2SO4.

Try three 2.6 Reversible Reactions, Industrial Processes and Important Chemicals exam-style questions