Concentration Dependence of the Equilibrium Cell Voltage

Equation (5.9) is the general Nemst equation giving the concentration dependence of the equilibrium cell voltage. It will be used in Section 5.4 to derive the equilibrium electrode potential for metal/metal-ion and redox electrodes. 

Electrochemical cells may consist of two electrodes of the same type, but with different concentrations of the electroactive species in the electrolyte. Such cells are known as concentration cells. For example, two platinum electrodes operate in two H+/H2 solutions of different activity, separated by a membrane. The equilibrium cell voltage is defined by Equation (21a). As the standard potential is the same for both electrode reactions, the measurable cell voltage will depend only on the activity ratios, Equation (21b). If in this system both electrolytes were in equilibrium with the same EE pressure, the measured E would respond linearly to the pH difference between the two electrolytes, Equation (21c) (i.e. a pH electrode). 

Equation (11) shows that the equilibrium cell voltage depends only on the acid concentration. It is independent of the present amount of lead, lead dioxide or lead sulfate, as long as all three substances are available in the electrode. (They are in... 

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. 

CONCENTRATION DEPENDENCE OF EQUILIBRIUM CELL VOLTAGE THE GENERAL NERNST EQUATION... 

A Gf for an anodic reaction is decreased and A Gt increased by applying a positive voltage with respect to the equilibrium condition (Fig. 7.2b). Accordingto this model, the potential dependence of the interfacial current is caused by the potential dependence of the rate constants. As will be shown later, in this aspect metal electrodes behave completely different from semiconductor electrodes. It also becomes clear from Figs. 7.1 and 7.2 that a variation of the concentration ratio leads to the same effect as that caused by an application of a voltage to the cell. This is reasonable because an increase of tVej speeds up the reaction rate, i.e. the barrier height must be smaller as shown in Fig. 7.1a. 

Introducing Nemst s law for the equilibrium case, the simation when no current (and hence power) is delivered by the cell, the equilibrium ceU voltage under nonstandard conditions for a H2/O2 ceU in dependence of the respective reactant/ product concentration (partial pressures) can be expressed as ... 

This reasoning is incorrect. The Nernst equation describes the dependence of OCV on the oxygen concentration only at equilibrium. This equation is not applicable in the situation when the cell generates current. Thus, Eq. (3.12) does not represent any real voltage loss in a cell. A true voltage loss in non-equilibrium conditions can be calculated from the kinetic equations as discussed above, this leads to the transport loss in the form Vconc = In Note that in Eq. (3.12) the factor is BT/ nF), while in the... 

If the electrolyte has large ionic disorder and since the blocked ionic current i,o is zero. Equations (9.5) and (9.6) indicate that no gradient can exist in the electrostatic potential within the sample. Then the steady-state transport of electrons and holes occurs only due to diffusion nnder the influence of gradients in their concentrations. These gradients must be uniform if the diffusion coefficient does not depend markedly on the concentration. From Equations (9.2) and (9.3) and the ionization equilibrium, the cell voltage determines the ratio of the activities of the electronic species at both sides of the electrolyte ... 

The position of equilibrium of a reaction maybe affected by changes in the concentration of reagents, temperature and pressure of gases. The voltage of an electrochemical cell will also depend on these factors, so we should use standard conditions when comparing electrode potentials. These are ... 

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