Unit 18 Dry cells in daily life Teaching Notes 18.3 Different types of dry cells Zinc-carbon cell At the negative electrode (oxidation): Zn(s) Zn2＋(aq) + 2e－ At the positive electrode (reduction): 2NH4＋(aq) + 2e－ 2NH3(aq) + H2(g) Hydrogen is produced and collected on the surface of the positive electrode. Since hydrogen is a poor conductor of electricity, the accumulation of hydrogen at the positive electrode may hinder further reactions and decrease the current of the cell. Manganese(IV) oxide, an oxidizing agent, is used to remove the hydrogen. 2MnO2(s) + H2(g) Mn2O3(s) + H2O(l) The ammonia is taken up by zinc ions. Zn2＋(aq) + 2NH3(aq) + 2Cl－(aq) Zn(NH3)2Cl2(s) The overall cell reaction is: 2MnO2(s) + 2NH4Cl(aq) + Zn(s) Zn(NH3)2Cl2(s) + H2O(l) + Mn2O3(s) Alkaline manganese cell At the negative electrode (oxidation): Zn(s) + 2OH－(aq) ZnO(s) + H2O(l) + 2e- At the positive electrode (reduction): 2MnO2(s) + H2O(l) + 2e－ Mn2O3(s) + 2OH－(aq) The overall cell reaction is: Zn(s) + 2MnO2(s) ZnO(s) + Mn2O3(s) Silver oxide cell At the negative electrode (oxidation): Zn(s) + 2OH－(aq) ZnO(s) + H2O(l) + 2e－ At the positive electrode (reduction): Ag2O(s) + H2O(l) + 2e－ 2Ag(s) + 2OH－(aq) The overall cell reaction is: Zn(s) + Ag2O(s) ZnO(s) + 2Ag(s) Nickel-cadmium (Ni-Cd) rechargeable cell The recharge of secondary cells is safe only when it is carried out properly. Explosion may result if they are recharged using high currents or overcharged at a high rate. The overall cell equation for nickel-cadmium cell is: discharge Cd(s) + NiO2(s) + 2H2O(l) Ni(OH)2(s) + Cd(OH)2(s) charge Storage batteries The storage battery is based on a reversible electrochemical system. It can be recharged. Up until the later 80s, the two main technologies available on the market were lead storage batteries and nickel-cadmium storage batteries. In the early 90s, with the growth of the portable equipment market, two new technologies emerged: nickel-metal hydride storage batteries and lithium storage batteries. The lead-acid battery The lead-acid battery is commonly used in motor vehicles. The 12 volt storage battery, or accumulator, consists of six cells connected in series. Each cell has a voltage of approximately 2 V. The lead-acid battery is rechargeable. The cells are contained in a heavy-duty external casing. The electrodes in each cell consist of a bank of lead grids supporting a large surface area of the electrode material. The negative electrode grid is filled with spongy metallic lead and the positive electrode grid is filled with lead(IV) oxide. The electrolyte is sulphuric acid with a concentration of approximately 4.5 M. When discharging, the electrode reactions are as follows. Negative electrode: Pb(s) + SO42－(aq) PbSO4(s) + 2e－ Positive electrode: PbO2(s) + 4H＋(aq) + SO42－(aq) + 2e－ Overall: Pb(s) + PbO2(s) + 4H＋(aq) + 2SO42－(aq) PbSO4(s) + 2H2O(l) 2PbSO4(s) + 2H2O(l) As the battery runs down, the electrodes become coated with insoluble lead(II) sulphate and the sulphuric acid is used up. Thus the density of the electrolyte decreases. A hydrometer is commonly used to check the degree of charge of a battery by testing the electrolyte density. When the battery is recharged, the electrode reactions are reversed by connecting the terminals to another electrical source of higher voltage and reversing the direction of the electric current through the circuit. The overall equation in recharging is: 2PbSO4(s) + 2H2O(l) Pb(s) + PbO2(s) + 4H＋(aq) + 2SO42－(aq) In motor vehicles, recharging occurs during normal driving. This is achieved by an alternator. If the battery runs flat, as when the headlights are accidentally left on, an external recharger can be used. The lead-acid accumulator is regarded as a storage battery because electrical energy is stored as chemical energy in the recharging process. This stored chemical energy is released as electrical energy during discharge when the battery is used to start the vehicle, operate the lights, etc. Batteries of the future The dry cell has changed little since its development over 100 years ago. The development of more powerful batteries often relys on whether scientists can overcome major technical problems. One such problem is the use of highly reactive material in batteries. Theoretically, maximum energy output can be obtained by using a strong oxidizing agent and a strong reducing agent. However, these reactive materials may react with water, making water-based pastes unsuitable as electrolytes. Many of the new batteries currently under development use solid or molten electrolytes. Lithium storage batteries Lithium-based technology has been extensively researched, with the result that both non-rechargeable and rechargeable lithium batteries are now available. Their advantage is that, theoretically, they can store a lot of electrical charge per kilogram of battery. There are two reasons for this improvement: firstly, lithium metal has a very low density; secondly, the redox potential of lithium is much more negative than that of other materials used in battery technology, so the voltage of the cell is greater. Unfortunately, the second characteristic also creates a major snag. Lithium’s high reactivity means that it is readily oxidized, causing oxide films to form over its surface which partially insulate the metal from the electrolyte. Researchers overcome this by developing insertion electrodes which can accommodate the small lithium ion within their lattices. Two kinds of electrode are used: one made of a metal compound such as manganese(IV) oxide and the other of carbon, often graphite. When the cell discharges, lithium ions from the electrolyte insert into the manganese(IV) oxide. xLi＋（electrolyte）+ MnO2(s) + xe－ LixMnO2(s) At the other electrode a lithium ion is extracted from the carbon matrix. xLi（electrode） xLi＋（electrolyte）+ xe－ The electrolyte is polymer-based rather than an aqueous solution. Reversing the two processes recharges the cell. Such a battery can generate approximately 4 V. This type of cell is known as a lithium-ion cell. It is used in newer laptop computers and mobile phones. Non-rechargeable types of lithium cells have a voltage of 1.5 V and are available as long-life replacements. They are more environmentally-friendly because they do not contain heavy metals and are currently used where lightness is important. Hydrogen-oxygen fuel cell Fuel cells are cells in which one or both reactants are continuously supplied and so, unlike primary or secondary cells, they do not need to be discarded or recharged. In a sense, therefore, they are more like an engine than an energy storage device, since once started they keep going while the fuel supply continues. This means that they are suited to continuous and not intermittent use. The most successful fuel cell so far developed has been based on the combination of hydrogen and oxygen. The net reaction is the same as that for the combustion of hydrogen. 2H2(g) + O2(g) 2H2O(l) The figure below shows one form of hydrogen-oxygen fuel cell. Hydrogen gas is supplied to the negative electrode chamber and oxygen to the positive electrode chamber. The gases diffuse through the electrodes which are porous metals such as platinum or nickel and act as catalysts. The reactions are given by the following equations: 2H＋(aq) + 2e－ Negative electrode: H2(g) Positive electrode: O2(g) + 4H＋(aq) + 4e－ 2H2O(l) Fuel cells are part of a developing technology. Major advantages of fuel cells are their relatively small mass and high fuel efficiency. Spacecraft including the Apollo and space shuttle programs have been using fuel cells for many years as a source of electrical energy and water. In spacecraft, the hydrogen and oxygen are stored as liquids until needed. The following figure shows a diagram of a space shuttle design incorporating a hydrogen-oxygen fuel cell. The external tank holds the gases for the hydrogen-oxygen fuel cell which provides the thrust for the space shuttle to leave the Earth’s atmosphere. 18.5 Choosing a dry cell for a particular use Chemistry and Society Finding alternative fuels to power vehicles What sort of vehicles do you expect to be driving 20 years from now? Will it be just a newer, lighter version of the cars you see on the road today? How much will petrol cost? How much will it cost to fill the petrol tank of a medium-sized vehicle? Will you have trouble breathing the polluted air? It is unlikely that you will be driving a car with an engine powered by petrol. Petrol is made from petroleum and the world’s petroleum supply is rapidly decreasing. Petrol-driven vehicles cause air pollution. Finding alternative fuels to power vehicles is an obvious way of solving the problem. Electric vehicles Electric vehicles obtain their energy from a rechargeable battery pack. Most electric vehicles can travel more than 100 miles on an eight-hour charge. Although electric vehicles are more expensive than petrol-driven ones, they do not require petrol and their maintenance costs are lower. The use of electric vehicles will An electric car being recharged reduce the demand for petrol. The world’s petroleum reserves will last longer. Besides, these vehicles do not release exhaust gases which pollute the environment. There are also some drawbacks. Suppose everyone uses electric vehicles. Power stations need to supply more energy for recharging batteries. Although air pollution in cities is reduced, the air pollution around the power stations may increase (as power stations use fossil fuels to generate electricity). The limited lifetime of the batteries also raises issues about recycling or disposal. Fuel cell vehicles Another option for providing electricity to power vehicles is the use of fuel cells. One common type of fuel cell converts hydrogen gas and oxygen gas into electrical energy and water. The figure below shows the schematic diagram of a hydrogen-oxygen fuel cell. Fuel cells do not need to be recharged. A schematic diagram of a hydrogen-oxygen fuel cell Fuel cells of this type are ideal for vehicles aiming at zero emissions: water vapour is the only exhaust gas. However, in fairness, the whole energy cycle has to be considered. The source of hydrogen and oxygen may well have been electrolysis of water, using electricity that probably comes from power stations burning fossil fuels. Nevertheless, this clean technology is ideal for urban areas. Hybrid vehicles Hybrid vehicles have batteries as well as a small petrol engine. The petrol engine generates energy to recharge the batteries while the vehicle is moving. As a result, less energy is required from the batteries. Hybrid vehicles produce much less harmful emissions than petrol-driven ones. Questions 1 What design features would you expect to appear in future cars? 2 How do you think chemistry can help in the quest for better cars? Notes for questions 1 Students may suggest the following features: smaller cars; lighter cars; lean-burn engines; (The fuel-to-air ratio can affect the emissions from a car exhaust. Using catalysts can reduce emissions, but the fuel-to-air ratio has to be just right for the engine to be efficient. The best ratio is 14.7 parts fuel to 1 part of air. However, this ratio results in high emission of carbon monoxide and hydrocarbons. As a compromise, the fuel-to-air ratio is set at 18:1 or 21:1. This is known as lean burn. This ratio is too rich in oxygen and the emissions poison the catalyst, so the carbon monoxide and hydrocarbons are reduced by using catalysts.) Electric cars that have zero emission. (However the electricity that powers them may come from generating stations that generally do not have zero emission.) 2 Students may suggest the following points: lightweight alloys made from metals such as aluminium, magnesium and titanium are used in body-shell parts; using plastic for parts can lead to a reduction in mass (newer plastics can withstand higher temperatures); bumpers and headlamps can be made from polyester composites, engine parts from glass-reinforced nylon and other polymers, and body panels from polythene terephthalate composites; steel in tyres can be replaced with Kevlar, a heat-resistant polymer.