PHYSICS. ElectroChemical Effects. Rishi Gopie

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Milton Stevenson
5 years ago
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1 ElectroChemical Effects Rishi Gopie

There are two types of cells: 1) A primary cell, which cannot be recharged.

2 ELECTRO CHEMICAL EFFECTS Cells A cell is a device that converts chemical energy to electrical energy by producing electrons during chemical reactions. It gives a steady d.c. output. There are two types of cells: 1) A primary cell, which cannot be recharged. Since the chemical reactions that occur within it to produce electrons are irreversible. One example of a primary cell is the dry cell. 2

3 The chemicals contained within the cell are kept as moist pastes to allow electrical contact between materials within the cell. The e.m.f (i.e. maximum voltage) of a cell depends on i) The materials used (especially the materials used for the electrodes) ii) The temperature a. A larger cell will produce the same e.m.f as a smaller cell once the same materials are used in both. However, since a larger cell contains more material it will have a greater energy content and will therefore last longer under the same conditions of use. In addition, a large cell generally has a lower internal resistance than a smaller cell of the same e.m.f and will therefore be capable of supplying a larger current. A dry cell suffers from 2 limitations: Local action due to impurities in the zinc cathode. This causes the cell to constantly produce current on the zinc cathode, which means that active material will be used up even when the cell itself is not in use. This defect causes the cell to have a limited shelf life. To minimize local action the zinc has to be amalgamated i.e. covered with a protective layer of some other material in which it dissolves in such a way that the zinc is exposed to the electrolyte but the impurities are not. Usually a suitable material for such amalgamation is mercury. Polarization- due to the formation of hydrogen bubbles on the anode, which causes a) the production of a back emf (i.e. an emf on the opposite direction to the normal forward, or output emf of the cell), and b) an increase in the internal resistance of the cell. Both of these effects lead to a drop in the output emf, and current of the cell. To reduce polarization a depolarizer must be employed and this is usually a chemical agent (such as an oxidizing agent- examples of which include MnO2 and K2Cr2O ; 3

4 The depolarizing action of the depolarizer in the cell is slow and the cell will polarize (and therefore give reduced emf. / current output) if it is used to supply a current for a prolonged period of time (so it will appear to have run down ). However it will recover if allowed time (during which it is not used) for the depolarizer to act. Because of local action a dry cell should not be left for prolonged periods inside equipment. The material will leak out the cell eventually and may cause corrosion of the equipment. Local action also limits it shelf life. 2) A secondary cell or accumulator can be recharged since the chemical reactions that occur within it to produce electrons are reversible. Examples of secondary cells are the lead- acid cell, the nickel- cadium cell and the nickel- iron cell. Consider the lead- acid cell; 4

5 Consider some points concerning the care and maintenance of a lead- acid cell or accumulator a) The cell should not be regularly treated as this may cause i) active material on the plates to become dislodged and fall ii) spillage of the electrolyte (i.e. acid solution) to occur iii) the plates to touch. b) The level of the electrolyte should be kept above the plates by topping up with distilled water c) The cell should not be over discharged by connecting the terminals directly to each other (i.e. sharing) as this will cause a large current to flow which will produce a lot of heat that may lead to damage of the plates (e.g. buckling/bending) d) The cell should not be left for a prolonged period in the discharged state as sulphating occurs. (This is the formation of hard white deposits of lead sulphate, PbSO4, on both electrodes.) e) The cell should be charged at a specific rate- normally a small continuous d.c. for a specific period of time f) No naked flames or sparks should be in the vicinity when the cell is being charged especially when the charging process is complete and gassing occurs. Gassing indicates that charging is complete and means that the electrolyte (i.e. acid solution) is being electrolyzed (i.e. decomposed) chemically by charging current in such a way that hydrogen gas is liberated at the cathode and oxygen gas is liberated at the anode. This combination of gas is explosive in the presence of heat which can be provided by a flame or a spark. g) Discharging: 1) decreases the emf of the cell 2) decreases the specific gravity (i.e. relative density) of the electrolyte 3) changes both electrons to PbSO4. Charging reverses the chemical reactions involved in discharging and so 1) charging increases the emf of the cell 2) increases the specific gravity of the electrolyte 3) Changes PbSO4 to PbO2 on the anode and changes PbSO4 to Pb on the cathode. 5

Electrode half-equation. H 2O(l)

Electrode half-equation. H 2O(l) Q1.This table shows some standard electrode potential data. Electrode half-equation E ϴ / V Au + (aq) + e Au(s) +1.68 O 2(g) + 2H + (aq) + 2e H 2O(l) +1.23 Ag + (aq) + e Ag(s) +0.80 Fe 3+ (aq) + e Fe 2+