Thermodynamic Decomposition Voltage

In any electrochemical system, there are two electrodes, an anode and a cathode. During electrolysis, electrons are consumed at the cathode while they are generated at the anode. The reactions occurring at the individual electrodes are usually called the component electrochemical reactions, the combination being termed a redox, or oxidation-reduction, reaction system. Let us now examine the component electrochemical reactions involved in the overall reaction. 

This redox reaction comprises two reactions involving the exchange of electrons  

A general rule that is particularly useful in computing the thermodynamic decomposition voltage, E ev, of an electrolytic cell is  

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Cell Volta.ge a.ndIts Components. The minimum voltage required for electrolysis to begin for a given set of cell conditions, such as an operational temperature of 95°C, is the sum of the cathodic and anodic reversible potentials and is known as the thermodynamic decomposition voltage, is related to the standard free energy change, AG°C, for the overall chemical reaction,... 

Table 7. Thermodynamic Decomposition Voltage of Chlor—Alkali Cells at 25°C...

Overall Energy Consumption. The voltage efficiency is the ratio of the thermodynamic decomposition voltage to the actual cell voltage ( cell) eiieigV efficiency (nener )> which is a product of the current and voltage efficiencies, can be expressed for a diaphragm cell as... 

These electrolyzers are operated at about 80°C, as opposed to the 95°C common in the industry. This lower temperature allows the use of vinyl ester resins and other plastics for cell construction. Moreover, the Dow diaphragm cell is optimized for low current density ( 0.5 kA/m2 ). It consumes less electrical energy per unit of production than others in the industry. The cell voltage at low current density is only 300—400 mV above the thermodynamic decomposition voltage. Dow does not license its diaphragm cell technology and operating data for these cells are not available in the open literature. 

The values of x for Q2, NaOH, and KOH are given in Table 4.4.1. rev in Eq. (139) refers to the thermodynamic decomposition voltage to produce the given product, which can be calculated from the free energy change involved in the overall reaction describing... 

The free energy change, AG, is related to the thermodynamic decomposition voltage, rev> as ... 

I. Thermodynamic Decomposition Voltage for Membrane and Diaphragm Cells. The primary anodic and cathodic reactions associated with the reaction scheme (140) are ... 

The thermodynamic decomposition voltage of a membrane cell can be calculated in a similar manner as that shown for the diaphragm cells. Typical operating conditions in a membrane cell are... 

Hence, the thermodynamic decomposition voltage of a mercury cell operating at SO C with 0.2% Na amalgam is equal to 3.058 V as shown below. 

The thermodynamic decomposition voltages calculated above are the equilibrium cell voltages when there is no applied current. When current passes, the system departs from equilibrium conditions to drive the reactions. The cell voltage will then he higher than the thermodynamic decomposition voltage because of the overvoltages associated with the anodic and cathodic reactions and the ohmic drops. 

The thermodynamic decomposition voltage is the minimum voltage required for a given electrochemical process to proceed in a given direction. It is based on the standard free energy change of the overall reaction as ... 

Commercial chlor-alkali cells operate above the thermodynamic decomposition voltage in order to allow reaction (52) to proceed in the forward direction and generate the desired products. The excess voltage, constituting the overvoltages and the ohmic drops, leads to the generation of heat. 

Molten MgCl2 can be prepared by the caibochlorination of MgC03 ores or Mg(OH>2 at 700-800°C. The thermodynamic decomposition voltage for the reaction... 

Plates of inert metals such as platinum and graphite serve in solid state electrochemical cells to supply electrons to the sample, but block the passage of material and ions for voltages below the thermodynamic decomposition voltage of the SE. The ionic current is blocked since the metals do not contain the required material that can provide the ions. In addition, the metal blocks are applied in a way that blocks the exchange of material with, e.g., the surrounding atmosphere, as well. Not only inert metals can serve for that purpose, but also any electron conductor, whether metal or semiconductor, that stays chemically intact and is impermeable to the material flow either in the form of ions or neutral species. 

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