Standard cell voltage

One of the most important characteristics of a cell is its voltage, which is a measure of reaction spontaneity. Cell voltages depend on the nature of the half-reactions occurring at the electrodes (Section 18.2) and on the concentrations of species involved (Section 18.4). From the voltage measured at standard concentrations, it is possible to calculate the standard free energy change and the equilibrium constant (Section 18.3) of the reaction involved. 

The standard voltage for a given cell is that measured when the current flow is essentially zero, all ions and molecules in solution are at a concentration of 1M, and all gases are at a pressure of 1 atm. To illustrate, consider the Zn-H+ cell. Let us suppose that the half-cells are set up in such a way that the concentrations of Zn2+ and H+ are both 1 M and the pressure of H2(g) is 1 atm. Under these conditions, the cell voltage at very low current flow is +0.762 V. This quantity is referred to as the standard voltage and is given die symbol fi°. 

Standard half-cell voltages are ordinarily obtained from a list of standard potentials such as those in Table 18.1 (page 487). The potentials listed are the standard voltages for reduction half-reactions, that is,... 

As pointed out previously, the value of the standard cell voltage, E°, is a measure of the spontaneity of a cell reaction. In Chapter 17, we showed that the standard free energy change, AG°, is a general criterion for reaction spontaneity. As you might suppose, these two quantities have a simple relation to one another and to the equilibrium constant, K, for the cell reaction. 

In this equation, E is the cell voltage, E° is the standard voltage, n is the number of moles of electrons exchanged in the reaction, and Q is the reaction quotient. Notice that—... 

Nemst equation An equation relating cell voltage E to the standard voltage ° and the concentrations of reactants and products E = E° — (0.0257/ )(lnQ), 493,508-509q application, 494-495 ion concentrations, 494-495 Nernst, Walther, 493... 

Since concentration variations have measurable effects on the cell voltage, a measured voltage cannot be interpreted unless the cell concentrations are specified. Because of this, chemists introduce the idea of standard-state. The standard state for gases is taken as a pressure of one atmosphere at 25°C the standard state for ions is taken as a concentration of 1 M and the standard state of pure substances is taken as the pure substances themselves as they exist at 25°C. The half-cell potential associated with a halfreaction taking place between substances in their standard states is called ° (the superscript zero means standard state). We can rewrite equation (37) to include the specifications of the standard states ... 

A half-cdl consisting of a palladium rod dipping into a 1 M Pd(NOj)2 solution is connected with a standard hydrogen half-cell. The cell voltage is 0.99 volt and the platinum dectrode in the hydrogen half-cell is the anode. Determine E° for the reaction... 

Cell type Cell reaction Standard potential (V) Terminal Capacity voltage, (Ah kg 1) Actio (V) Specific energy (Wh kg 1 )... 

As a result of the combination of Eqs. (20) and (21), the reaction free energy, AG, and the equilibrium cell voltage, A< 00, under standard conditions are related to the sum of the chemical potentials //,. of the substances involved ... 

The increase in the standard cell voltage is 0.09 V at the higher pressure. 

The pressure dependency of the cell voltage (and correspondingly the electrode potential) can also be derived using standard thermodynamic equations... 

B We first need to determine the standard cell voltage and the cell reaction. 

We determine the standard cell voltage of each chemical reaction. Those voltages that are negative are those of chemical reactions that require electrolysis. 

Since the final cell voltage is negative, the disproportionation reaction will not occur spontaneously under standard conditions. Alternatively, we can calculate Keq by using ln/Cq = -AG°/RT and AG° = -nFE°. This method gives a Keq = 8.6 x 10 17. Clearly, the reaction will not go to completion. 

The cell voltages in KU test cells on GDE with silver catalyst are of the order of 1.9-2.0 V under standard conditions. Assuming similarly good k-factor values of the FFE with GDE and of FFE without GDE, the energy consumption will be as shown in Fig. 16.15. 

We can use this table of standard reduction potentials to write the overall cell reaction and to calculate the standard cell potential, the potential (voltage) associated with the cell at standard conditions. There are a couple of things to remember when using these standard reduction potentials to generate the cell reaction and cell potential ... 

Figure 14.10 gives a schematic presentation of the current-voltage characteristics of hydrogen and oxygen on platinum. The theoretical open circuit cell voltage at 25 °C and standard conditions is 1.229 V, for both the fuel cell and electrolyzer. 

Electrochemical experiments fall into two broad categories. Some experiments are concerned with standard cell voltages, while other experiments use the Nernst equation directly or indirectly. Experiment 21 in the Experimental chapter uses these concepts. 

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