Theoretical Decomposition Voltage

The first mathematical method is based on the equivalence of minimum energy (— W ) which has to be supplied to the electrolyzer in the form of electric current and the change in free energy AG accompanying the considered 

Since all electrolytio processes are characterized by the positive value of AG, the voltage E m n, yvill always have the negative sign which is an indication that the electrolytic process is not a spontaneous one but requires a supply of energy from an external source. 

As far as the equation (VII-2) is concerned the potential difference represents the negative value of the theoretical decomposition voltage (ER)t, necessary for the beginning of the electrolysis in a reversibly operating system. When substituting the relation E mi — — (ER)t into the equation (VII-3), we obtain 

Should, e. g. the decomposition of hydrogen iodide solution (anJ =1), proceeding according to the equation 

Theoretical decomposition voltage oan also be calculated from the reversible electrode potentials. Because the potential difference E mu, equals, according 

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Molten halides are liquid electrolytes in many instances, and their decomposition may be canned out in principle to produce the metal and halogen, usually in the gaseous state. The theoretical decomposition voltage, E°, is calculated from the Gibbs energy of formation tlrrough the equation... 

To determine the theoretical decomposition voltage of hydrochloric acid in solution with molal concentration m = l at a pressure of 1 atm. and temperature 25 °C with y i, = 0.811, the same procedure will be used ... 

When water is decomposed eleotrolytically, oxygen is liberated at the anode and hydrogen at the cathode. When the gases escape under pressure of 1 atm., the theoretical decomposition voltage at 25 °C consists of the following potentials at the electrodes ... 

Hence all aqueous solutions of acids and bases have the same theoretical decomposition voltage when during electrolysis hydrogen and oxygen are liberated. 

The values of decomposition voltage [ER)S of some compounds in solution of normal concentration measured by means of platinum electrodes are given in Table 11. For comparison, theoretical decomposition voltages (ER)t are also tabulated, calculated by the aforementioned method the difference... 

Electrolyte Eleotrolysis products Decomposition voltage (ER), Theoretical decomposition voltage (EH)t (Eh), — (EB)t... 

The minimum voltage necessary for the decomposition of water coincides with the theoretical decomposition voltage (Elt)t which can be calculated by using formula (VII-5) (see in the theoretical part) from the reversible oxidation potential at the anode (spt,o. oh-) and the reversible reduction potential at the cathode (tcoh- i n2.pt) ... 

As has been already mentioned the theoretical decomposition voltage of water equals 1.23 V while its practical value amounts to minimum 1.7 V and the actual operating voltage to 1.9 V. It would be erroneous to think that all the energy corresponding to the difference between the actual and the theoretical voltage across the electrolytic cell is converted into heat which warms up the electrolyte. 

If we first use the equation (VII-4) which expresses the relation between the change in free energy and the theoretical decomposition voltage (ER)t, we obtain ... 

The theoretical decomposition voltage of chlorides can be calculated from the value of equilibrium oxidation potential of the chlorine electrode e t. cia1 ci-in the anolyte and the reversible reduction potential of the hydrogen electrode 7Toh- Ha. pt in alkaline catholyte. If we apply the Nemst equations for the corresponding electrochemical processes [see (XI-9) and (XI-10)] we obtain the... 

The theoretical decomposition voltage of sodium chloride may be calculated as the sum of the reversible oxidation potential of the chlorine electrode and the reversible reduction potential of the amalgam electrode. 

The computed value E1 indicates the theoretical decomposition voltage of sodium chloride in a solution with unit activity of both sodium and chloride ions if a reversible insoluble anode and a reversible amalgam cathode which contains 0.206 per cent of Na are used (at a temperature of 25 °0). 

When comparing the theoretical decomposition voltage of sodium chloride on a mercury cathode and on an iron cathode [see equation (XI-18a) in the preceding chapter] it will be noted that this voltage has a value by about 1 V higher with a mercury cathode electrolyzer. 

Because of a higher theoretical decomposition voltage mercury cathode electrolyzers have also a higher theoretical energy consumption. If for the preparation of 100 kg of sodium hydroxide 67 kA-hr. are required according to Faraday s law, then the theoretical energy consumption equals... 

On adding the reduction and oxidation potential Ae obtain the A alue of the theoretical decomposition voltage at 25 °C ... 

By adding the values (XVII-16 and XVII-18), the resulting theoretical decomposition voltage can be ascertained ... 

The standard potential of reaction (6), which takes place in acid solutions, is 1.23 V vs. standard hydrogen electrode (SHE). Reaction (7), in alkaline solutions, has a potential of 0.40 V vs. SHE. Both reactions are 1.23 V positive to the reversible potential of the hydrogen evolution reaction (HER) in the respective media. Therefore, the theoretical decomposition voltages of certain electrochemical processes can be reduced by 1.23 V if an oxygen-consuming cathode is substituted for the conventional hydrogen-evolving type. Examples of these processes [5] are ... 

The cell voltage, V (volt), is expressed by Eq. 10, where EIR is the sum of the ohmic drops related to the solution, separator, and electrodes. Sr is the sum of the overvoltage and Vo is the theoretical decomposition voltage. 

Rare Earth Metal Production by Molten Salt Electrolysis, Table 3 The theoretical decomposition voltages for... 

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