| Contents for this page | Related topics |  |
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The dry cell (Leclanché cell) The lead-acid accumulator The fuel cell Additional questions |
The petroleum industry The chloralkali and Solvay processes The polymer industry (1) The polymer industry (2) The fertiliser industry Ammonia - Nitric acid - Sulphuric acid |
 Data Glossary |
| Learning Outcomes | ||
| After studying this section, you will be familiar with certain practical examples of electrochemical cells, such as the dry cell, the car battery, and the fuel cell. | ||
| Helpful background knowledge | ||
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The Cu-Zn electrochemical cell Electrolysis | ||
The dry cell dry cell, invented in 1867 by the French engineer Georges Leclanché (1839 - 1889), is widely used as a source of electric energy in electric torches and small appliances such as transistor radios. It makes use of the two reactions, which (in a simplified form) may be described as
The manganese is supplied as manganese dioxide, and the actual cathode reaction taking place is
The H+ ions are in turn provided by ammonium ions NH4+, through the reaction
The dry cell is an example of a primary cell, as once it is discharged it cannot be recharged, and must be discarded.
A typical dry cell is shown below, with a section cut away in order to expose the interior parts.
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The top of the battery is closed with a non-conducting sealing material (A).
The cathode consists of a graphite (carbon) rod (B)(tipped with a metal contact), which serves as the positive pole of the battery. The anode is a cylindrical zinc casing (C) (the bottom of the battery is normally exposed and serves as the negative pole). The battery is filled with a mixture of manganese dioxide (MnO2) as oxidant, ammonium chloride (NH4Cl) as a source of H+ ions, and zinc chloride (ZnCl2) (D). These two salts serve as electrolytes. The above mixture is separated from the zinc walls of the battery by a porous material soaked in a solution of the two salts. The e.m.f. of the battery is about 1.5 V. |
In 1859, the French physicist Gaston Planté invented a device called the LEAD-ACID ACCUMULATOR, which, with minor design changes, is still used today as the motor car battery. It is an example of a SECONDARY ELECTROCHEMICAL CELL, since it can be recharged.
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The battery consists of 6 cells connected in series, each cell having an emf of about 2 V, giving 12 V as the overall emf of the battery, |
| The general appearance of the battery is shown on the right, with part of its casing removed to show the individual cells. |  |
When the battery is discharging, i.e., when it is supplying a current, the reactions are
At the ANODE (the so-called "negative" terminal of the battery):
At the CATHODE (the "positive" terminal of the battery):
The overall reaction (during discharge) is therefore
Note that the electrolyte is sulphuric acid (H2SO4), which is gradually used up to form the lead sulphate. Both electrodes become coated with an insoluble layer of lead sulphate, which, being an insulator, would eventually ruin the battery.
During the manufacturing process, the electrodes are filled with litharge (PbO), and immersed in the electrolyte (H2SO4, 6 mol.dm-3). When an electric current is passed through the battery, the electrode that is labelled "negative" is converted to lead, while the electrode that is labelled "positive" is converted to PbO2. There is no net change in the concentration of acid. This process is called FORMING the battery, and is in fact an electrolytic process.
When the battery is fully charged, the "negative" electrode (actually the anode" consists of metallic lead in a spongy form (with a high area per unit mass), while the "positive" electrode (the cathode) consists of lead dioxide:
The electrolyte is an aqueous solution of sulphuric acid, at a concentration of approximately 6 mol.dm-3. This is the battery acid that one can buy at service stations.
As the battery provides energy, it is discharged and this lead to the gradual formation of lead sulphate at the electrodes, and a steady decrease in the concentration of the sulphuric acid:
At the ANODE (the "negative" terminal):
At the CATHODE ("positive" terminal):
The electrical status of the battery is indicated by the concentration of the sulphuric acid, which is indicated by its specific gravity, SG. The electrolyte of a fully charged battery has an SG of 1.30. An SG of 1.10 indicates that the battery is discharged. If the battery is totally discharged, both electrodes will have been converted to lead sulphate. It will no longer be able to provide a current, and cannot be recharged. It is only good for scrap! The SG will be slightly above 1, indicating that most of the sulphuric acid will have been used up.
When the battery is being charged, illustrated in the diagram above, (using a battery charger or the motor car's alternator), an electrolytic process takes place. Electrons are supplied to the electrode that is labelled "negative" (which now becomes a cathode, as it accepts electrons from the external source, and is the site of reduction) and this converts the lead sulphate to lead. At the same time, the lead sulphate at the "positive" electrode (which now becomes the anode, as it gives up electrons to the external circuit and is the site of oxidation) is converted to lead dioxide.
Care must be taken to distinguish between which electrode is "anode" and "cathode", as this will depend on whether the battery is charging or discharging.
When discharging, the cathode will be the site of reduction (Pb4+ + 2 e- ® Pb2+) and the anode will be the site of oxidation (Pb ® Pb2+ + 2 e-).
When charging, the cathode will be the site of reduction (Pb2+ + 2 e- ® Pb), and the anode will be the site of oxidation (Pb2+ ® Pb4+ + 2 e-).
This arises because current flows in opposite directions during the charging and discharging processes.
The reactions are:
The fuel cell converts the energy of combustion directly into electrical energy. For example, one type of such cells, used in spacecraft and advanced non-nuclear submarines, makes use of the reactions
giving the overall reaction
with a theoretical emf of 1.23V. Platinum is used as a catalyst, and the only by-product, water, is utilized by the spacecraft or submarine crews. The hydrogen and oxygen are stored in liquid form in high pressure tanks.
Note that this fuel cell is not a primary cell, since neither reactants nor products are stored, or a secondary cell which can be recharged. Rather, the reactants have to be fed in continuously and the product removed.
The diagram below (left) shows the basic design of a fuel cell. In practice, it is a very complex piece of equipment, as shown in the picture below on the right.
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 Fuel cell in use in some German submarine |
