WJEC Chemistry for A2: Study and Rev Guide

Uses of redox reactions Redox reactions are very common in chemistry. You have seen many redox reactions in previous units, but they are also common in industry, where metals are extracted from their ores; in biology, respiration and photosynthesis both involve a complex series of reductions and oxidations. Redox reactions in organic synthesis You have seen the use of reduction and oxidation in your work in other units. These include: ■ Oxidation of alcohols to aldehydes, ketones or carboxylic acids using acidified potassium dichromate(VI). ■ Reduction of nitriles to amines or reduction of carboxylic acids, aldehydes and ketones to alcohols using lithium tetrahydridoaluminate(III). ■ Reduction of nitrobenzene and its derivatives to phenylamines using tin and hydrochloric acid. Fuel cells Fuel cells use electrochemical methods to get energy from fuels, typically hydrogen gas. At one platinum electrode, hydrogen is oxidised to H + ions, whilst at the other platinum electrode, oxygen gas is reduced to water, H 2 O: At the anode H 2 2 H + + 2e – At the cathode O 2 + 4 H + + 4e – 2 H 2 O The overall reaction that occurs is: 2 H 2 + O 2 2 H 2 O. This method of obtaining energy has many advantages and disadvantages. Advantages ■ Water is the only product, so no carbon dioxide (a greenhouse gas) is produced. ■ Highly efficient, as less energy is wasted as heat, so much more energy is used effectively. ■ Hydrogen gas can be produced using renewable resources by the electrolysis of water. Disadvantages ■ Hydrogen gas is highly flammable and difficult to store. ■ Hydrogen gas is usually produced from fossil fuels, which leads to a net energy loss. Pointer Remember that the unit 3 exam contains synoptic questions, so you can be expected to use redox reactions that you have seen in other units to answer these questions. Grade boost Make sure you can recall at least one advantage and at least one disadvantage of fuel cells and express these clearly. Common errors are to use vague answers such as ‘less pollution’ or ‘gases are harmless’. 12 A2 Chemistry: Study and Revision Guide