OCR GCSE Chemistry B C6 Making Useful Chemicals Exam Questions

About OCR GCSE Chemistry B C6 Making Useful Chemicals

Chapter C6 brings the course together by asking how chemists make useful products efficiently. On OCR Chemistry B you study the reactions of acids, how to make a pure dry salt, pH and the difference between strong and weak acids. You then investigate how to control rates of reaction using collision theory and catalysts, and how reversible reactions and equilibrium affect yield. Separate chemistry adds industrial production, including the Haber process and fertilisers. C6 draws on moles from C5 and energy changes from C1. Examiners favour practical method questions, rate graph interpretation and yield calculations.

What you need to know

• C6.1 Acids and salts Acids react with metals to give a salt and hydrogen, with bases and alkalis to give a salt and water, and with carbonates to give a salt, water and carbon dioxide. To make a salt from an insoluble base, add excess base to warm acid, filter, then evaporate and crystallise.
• C6.1 pH and acid strength Acids produce H+ ions in solution and alkalis produce OH- ions; neutralisation is H+ + OH- to H2O. Strong acids fully ionise while weak acids, such as ethanoic acid, partly ionise. A decrease of 1 pH unit means a tenfold increase in H+ concentration.
• C6.2 Rates of reaction Rate can be followed by measuring gas volume, mass loss or time for a colour change. Higher temperature, concentration, pressure and surface area increase the rate by increasing the frequency of successful collisions. Catalysts provide a pathway with lower activation energy and enzymes are biological catalysts.
• C6.3 Yield and equilibrium Percentage yield = actual yield / theoretical yield x 100 and atom economy = Mr of desired product / total Mr of reactants x 100. In a closed system a reversible reaction reaches dynamic equilibrium, and changing temperature, pressure or concentration shifts the position of equilibrium.
• C6.4 Industrial chemistry Separate chemistry only: the Haber process makes ammonia from nitrogen and hydrogen using an iron catalyst at a compromise temperature and high pressure. Ammonia is used to make fertilisers that supply nitrogen, and industry balances rate, yield, cost, energy and environmental impact.

Common mistakes in C6 Making Useful Chemicals

• Saying a catalyst gives particles more energy. A catalyst lowers the activation energy by providing an alternative pathway.
• Confusing strong with concentrated. Strength is about how fully an acid ionises; concentration is about how much acid is in a given volume.
• Saying temperature increases rate only because collisions are more frequent. The main reason is that a greater proportion of collisions have energy equal to or above the activation energy.
• Saying reactions stop at equilibrium. Forward and reverse reactions continue at the same rate, so concentrations stay constant.
• Mixing up percentage yield and atom economy. Yield compares actual with theoretical mass; atom economy compares the useful product with all products from the equation.

Try three C6 Making Useful Chemicals exam-style questions