Chapter C6: Making useful chemicals — OCR GCSE Combined Science

Chapter C6: Making useful chemicals covers the production of key industrial chemicals, including the Haber process for ammonia, the Contact process for sulfuric acid, and the electrolysis of brine. It explains the conditions, catalysts, and yields involved, alongside the importance of these chemicals in fertilisers and everyday products.

Chapter C6, 'Making useful chemicals', is a cornerstone of industrial chemistry in the OCR GCSE Combined Science course. It focuses on how raw materials are transformed into valuable products on a large scale, with an emphasis on the principles of chemical equilibrium, rates of reaction, and energy changes. You will study two major industrial processes: the Haber process for making ammonia (used in fertilisers) and the Contact process for making sulfuric acid (used in many industries). Additionally, you will explore the electrolysis of brine (sodium chloride solution) to produce chlorine, hydrogen, and sodium hydroxide.

Understanding these processes is not just about memorising conditions; it's about applying the concepts of reversible reactions and Le Chatelier's principle. For example, you need to explain why certain temperatures and pressures are chosen to maximise yield while keeping costs manageable. This topic also links to environmental concerns, such as the energy demands of these processes and the importance of sustainability. Mastering this chapter will help you appreciate how chemistry contributes to everyday life, from food production to cleaning products.

In exams, you will be expected to recall specific details, interpret graphs of yield against conditions, and evaluate the economic and environmental trade-offs. The content builds on earlier topics like atomic structure, bonding, and rates of reaction, so a solid grasp of those will be beneficial. By the end of this chapter, you should be able to describe the raw materials, stages, and conditions for each process, and explain the underlying chemistry with confidence.

• →Reversible reactions and dynamic equilibrium: In a closed system, forward and reverse reactions occur at the same rate, and the concentrations of reactants and products remain constant.
• →Le Chatelier's principle: If a system at equilibrium is subjected to a change in concentration, temperature, or pressure, the equilibrium shifts to counteract the change.
• →The Haber process: N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = -92 kJ/mol. Conditions: 450°C, 200 atm, iron catalyst. Ammonia is used to make fertilisers.
• →The Contact process: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) ΔH = -196 kJ/mol. Conditions: 450°C, 2 atm, vanadium(V) oxide catalyst. Sulfuric acid is used in many industries.
• →Electrolysis of brine: 2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + H₂(g) + Cl₂(g). Products: chlorine at anode, hydrogen at cathode, sodium hydroxide remains in solution.

• Correct identification of products from acid reactions with metals, hydroxides, and carbonates.
• Accurate description of laboratory procedures for salt preparation (filtration, evaporation, crystallisation, drying).
• Correct use of pH scale and understanding of H+ ion concentration.
• Explanation of factors affecting reaction rates (temperature, concentration, pressure, surface area) using collision theory.
• Description of catalytic action in terms of activation energy.
• Understanding of dynamic equilibrium in reversible reactions.
• Prediction of equilibrium shifts based on changes in conditions.

• 💡When asked about rate of reaction, always refer to collision frequency and energy.
• 💡Ensure balanced symbol equations include state symbols where required.
• 💡Use the term 'dynamic equilibrium' when discussing reversible reactions in closed systems.
• 💡Practice calculating pH changes based on H+ concentration shifts.
• 💡Clearly distinguish between the effect of conditions on rate versus the effect on equilibrium position.
• 💡Always quote exact conditions (temperature, pressure, catalyst) for each industrial process. Marks are often awarded for specific values, not just 'high temperature'.
• 💡When explaining why a particular condition is used, always link to both rate and yield, and mention the compromise between them. For example, 'A higher temperature would increase rate but decrease yield because the reaction is exothermic, so 450°C is a compromise.'
• 💡For electrolysis questions, write half-equations and state which electrode each product forms. Use the mnemonic 'An Ox, Red Cat' to remember oxidation at anode, reduction at cathode.

• Confusing 'strong' and 'weak' acids (degree of ionisation) with 'concentrated' and 'dilute' (amount of substance).
• Incorrectly describing the effect of catalysts as changing the yield rather than just the rate.
• Failing to mention that dynamic equilibrium only occurs in closed systems.
• Misinterpreting the relationship between H+ ion concentration and pH (factor of 10 per pH unit).
• Incomplete descriptions of collision theory (e.g., omitting 'frequency' or 'energy' of collisions).
• Misconception: Increasing pressure always increases the yield of a reaction. Correction: Only if the forward reaction produces fewer gas molecules. In the Haber process, high pressure favours ammonia, but in reactions with equal gas moles, pressure has no effect.
• Misconception: A catalyst increases the yield of a reaction. Correction: A catalyst speeds up the rate of both forward and reverse reactions equally, so it does not change the position of equilibrium or the yield, but helps reach equilibrium faster.
• Misconception: The Haber process and Contact process use the same conditions. Correction: They are different reactions with different optimised conditions. Haber: 450°C, 200 atm, iron; Contact: 450°C, 2 atm, vanadium(V) oxide.

1. 1Week 1, Day 1-2: Review reversible reactions and dynamic equilibrium. Write down definitions and draw a simple diagram of a closed system.
2. 2Week 1, Day 3-4: Study the Haber process in detail. Create a fact file with raw materials, stages, conditions, and uses of ammonia. Practice explaining why each condition is used.
3. 3Week 1, Day 5-6: Study the Contact process similarly. Compare and contrast with the Haber process.
4. 4Week 2, Day 1-2: Learn electrolysis of brine. Draw a labelled diagram of the cell and write half-equations. Practice predicting products.
5. 5Week 2, Day 3-4: Work through past exam questions on these processes. Focus on 6-mark questions that ask you to evaluate conditions.
6. 6Week 2, Day 5-6: Use active recall to test yourself on key facts. Create mind maps linking the processes to real-world applications.

• 📋Multiple-choice questions: Often ask for the correct catalyst or conditions. Tip: Memorise the exact values and catalysts.
• 📋Short-answer questions: 'State the raw materials for the Haber process.' Tip: Be precise – nitrogen from air, hydrogen from natural gas (methane).
• 📋Data analysis: You may be given a graph of yield vs temperature/pressure and asked to interpret it. Tip: Describe the trend and explain using Le Chatelier's principle.
• 📋6-mark extended response: 'Evaluate the use of high pressure in the Haber process.' Tip: Structure your answer with points for and against, and conclude with a justified decision.