Author: admin
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Simplified Activity Correction
Now that we have a way of obtaining for use in Equation 2.11, we must add the activity correction to get the desired expression for U. A discussion of activities and standard states for electrolytes is provided later in Section 2.14. Activity is a dimensionless quantity that depends on the standard state for each species. As a first…
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Effect of Temperature on Standard Potential
The cell potential in the relationships shown previously was determined at 25 °C, where the standard values have been tabulated. However, it is often desirable to calculate the cell voltage at a different temperature. To do so, we need the standard potential at that temperature. This section describes how to correct the standard potential to the…
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Standard Potentials
For convenience, values of standard potentials have been determined and tabulated for a large number of electrochemical reactions, a subset of which is found in Appendix A. Although listed as half-cell reactions, the potentials represent the difference between the potential of the reaction of interest and a reference reaction. The universal reference is the standard hydrogen…
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Expression for Cell Potential
Note that Equation 2.3 relates the Gibbs energy to the cell potential, U, which is the difference between the potential of the two electrodes. From a practical standpoint, that potential will be positive if you measure it one way, and negative if you switch the wires and measure it again. The fact that the potential can be measured…
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Cell Potential
At open circuit, no current flows between the electrodes. Furthermore, for our thermodynamic analysis, each electrode half-cell reaction is at equilibrium (no net anodic or cathodic reaction occurs at either electrode). Under these conditions, it is appropriate to write the half-cell reactions as reversible reactions. Since electrons are participants in each of the two reactions, the…
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Electrochemical Reactions
An electrochemical reaction is a reaction where the transfer of electrons from a species being oxidized to a species undergoing reduction takes place through an electronic conductor. Typically, that conductor is a metal. Because the electron transfer takes place through a conductor rather than directly between the reacting species, we can separate the two electron-transfer…
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Electrochemical Systems: Example
We end this chapter with an example of an industrial electrolytic process—chlorine production by the chlor-alkali route. Our objectives in this section are twofold. First, to review principles and terminology within the context of an important electrochemical system. Second, to introduce the concept of the current–voltage relationship, also called the I–V curve. The connection between current and voltage is essential…
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Potential and OHM’S
Another important quantity in the study of electrochemical systems is the potential. We can define an electrostatic potential, ϕ, in terms of the work required to move a unit (positive) charge from infinity to a specific position in the metal or in solution. This work can also be thought of in terms of energy. The unit…
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Current Density
To this point in the chapter, we have considered the total current, I. However, as you know from your experience, some electrochemical devices are small like a hearing aid battery. Others are significantly larger, such as the battery used to start your car. You would not expect the total current from these devices to be similar.…
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Faradaic Efficiency
The electrochemical reactions that we have been discussing are called faradaic reactions since they involve electron transfer that is directly related to the consumption of reactants and the formation of products as described by Faraday’s law. To this point, we have only considered one reaction at each electrode. However, it is possible, and in fact common, for…