Free energy cell voltage

When the e.m.f. of a cell is measured by balancing it against an external voltage, so that no current flows, the maximum e.m.f. is obtained since the cell is at equilibrium. The maximum work obtainable from the cell is then nFE J, where n is the number of electrons transferred, F is the Faraday unit and E is the maximum cell e.m.f. We saw in Chapter 3 that the maximum amount of work obtainable from a reaction is given by the free energy change, i.e. - AG. Hence... 

This reaction has a positive free energy of 422.2 kj (100.9 kcal) at 25°C and hence energy has to be suppHed in the form of d-c electricity to drive the reaction in a net forward direction. The amount of electrical energy required for the reaction depends on electrolytic cell parameters such as current density, voltage, anode and cathode material, and the cell design. 

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,... 

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. 

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. 

To obtain a quantitative relation between cell voltage and concentration, it is convenient to start with the general expression for the free energy change discussed in Chapter 17 ... 

Using the reaction free energy AG, the cell voltage Aelectrons exchanged during an electrode reaction must be determined from the cell reaction. For the Daniell element (see example), two moles of electrons are released or received, respectively ... 

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 relation between reaction free energy, temperature, cell voltage, and reversible heat in a galvanic cell is reflected by the Gibbs-Helmholtz equation [Eq. (31)]. 

The consumed electrical energy is 17 7 (U, cell voltage [V], 7, cell current [A]). The part U I — AH, that exceeds the enthalpy of the cell reaction AH = AG + T AS (AG, reaction free enthalpy (Gibbs energy)... 

The maximum electrical energy available from a fuel cell is determined by the Gibbs free energy difference across the electrolyte membrane, AG. This determines the equilibrium voltage of the cell, E, through the Nemst equation, which is nothing more than a restatement in electrical units of how AG (= 2FE) changes with pressure. 

Equation 1. The free-energy change (AG) as a function of cell voltage E)... 

The thermodynamic criterion for spontaneity (feasibility) of a chemical and electrochemical reaction is that the change in free energy, AG have a negative value. Free-energy change in an oxidation-reduction reaction can be calculated from knowledge of the cell voltage ... 

Determine the spontaneity (feasibility) of the following reactions by determining the standard cell voltage and the standard free-energy change AG for the following cell reactions ... 

A. Gibbs free energy—standard cell voltage... 

If the reactants and products were in their standard states of unit activity, then of course the voltage (E°) is the "standard cell potential," and the change in free energy is the change in "standard free energy" (AG°) ... 

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