9.Could you explain electrical connection of electrodes. And advantages of them.
16.Explain common types of electrode process (process of between electrodes and electrolyte).
Electrical connections to cells. When a cell or cell stack contains more than two electrodes, there are two ways of making the electrical connection: (1) the cell may be monopolar (Fig. 1 (a)); or (2) bipolar (Fig. 2 (b)). Monopolar requires: Low – voltage ;High – current supply. Bipolar requires: Low – current ;High – voltage supply.
Electrode potentials If you put strips of two different metals into an appropriate electrolyte solution you are likely to find that a potential difference appears between the two strips. The system is a galvanic cell, commonly known as a battery. The potential difference is produced by an EMF associated with the chemical reactions between both the metals and the electrolyte. The interface between each piece of metal and the electrolyte solution forms an electrode. Electrodes are always used in pairs. It is impossible to make voltage measurements or pass current into an electrolyte without using two electrodes. A simple example is a piece of copper (Cu) and a piece of zinc (Zn) both partly immersed in the same dilute solution of copper sulfate (CuSO4). The copper becomes about 1 volt positive with respect to the zinc. The potential change occurs in two steps at the electrode-electrolyte interfaces, not in the bulk solution. (See figure 5.2.) Both metals are negative relative to the electrolyte, but the potential of the zinc is more negative than that of the copper. So the copper is positive with respect to the zinc.It is impossible to measure these absolute electrode potentials directly because you can't make contact with the electrolyte without using another electrode. If you introduce a third electrode the problem is still there because you don't know its electrode potential. However the cell potential, which is the difference between the two electrode potentials, can be measured directly. Metal - electrolyte electrode.The chemistry, electrode potential, and other characteristics of an electrode depend not only on thesolid electrode material, but also on the solution in which it is placed. Strictly the electrode is notjust the metal, but the combination of the metal and the solution. Thus for example, when copper is placed in a copper sulfate solution we refer to a copper - copper sulfate electrode.
10.Could you explain Hall –Heroult process for aluminum extraction.
11.1Tell about Aluminum extraction process. And their parameters.
Aluminum extraction. Aluminium is normally produced from the ore, bauxite, which is a hydrated aluminium oxide containing silica and other metal oxides, particularly iron. It is converted to a pure alumina using the equilibrium:
Al2O3*3H2O + 2NaAlO2 + 4H2O
The ore is first treated with sodium hydroxide under pressure. The aluminium largely dissolves as the aluminate, the iron oxide is insoluble and the silica also remains in the form of a sodium aluminium silicate, which leads to a loss of aluminium. Hence, the best bauxites are those those low in silica. After filtration, the hydrated aluminium oxide is reprecipitated by seeding and the sodium hydroxide solution may be reused. The alumina is washed and then heated at 12000C to remove water. The final step in the production of aluminium metal has to be electrolytic since the reduction of alumina with carbon is only possible at very high temperatures and the reverse reaction occurs on cooling.
Moreover, because of the chemistry of aluminium, the electrolysis medium cannot be water; in fact, almost all commercial production of aluminium during the last 90 years has used an electrolysis in molten cryolite, Na3AlF6.
The process is based on the observation made concurrently and independently in 1886 by Hall in the USA and Heroult in France that, whereas alumina melts at 20200C to give a non-conducting liquid, it will dissolve to the extent of 15 wt% in molten cryolite at 10300C to give a conducting medium. Two years later, the second important step towards a viable process of aluminium extraction was made by Bayer who developed a method for leaching sodium aluminate to produce alumina. The high solubility of alumina in molten Na3AlF6 results from the near equality of size of fluorine and oxygen atoms in the aluminium complexes in the melt and, hence, facile formation of oxyfluoride ions on addition of the oxide to the Na3AlF6 . Indeed, the aluminium is probably present as a mixture of several related species, although the exact chemistry of the system is not known and it is therefore difficult to write complete electrode reactions. Clearly, however, the cathode reaction is the reduction of an aluminium (III) species to the metal which is molten at the electrolysis temperature; the cathode is effectively a pool of liquid aluminium. Ideally, the anode reaction would be the oxidation of oxide ion to oxygen but it is difficult to find an anode material which is inert under the electrolysis conditions. Hence, the electrolysis has always been run with consumable carbon anodes so that the overall cell reaction is:
2Al2O3 + 3C → 4Al + 3CO2
The carbon anode is consumed in a stoichiometric amount but the free energy for this reaction is 340 kJ mol-1 Al(at 10000C) compared with 640 kJ mol-1 for the reaction:
2Al2O3 → 2Al + 3O2
i.e. the overall cell reaction if oxygen was evolved at the anode. In consequence, the sacrifice of the carbon anode leads to a much reduced cell voltage and energy consumption. Thus, the reversible cell voltage is -1.18 V compared with -2.21 V if the cell reaction was.
