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Electrochemistry

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Standard Potentials

It is not possible to measure the electrode potential of a single electrode in isolation. This idea can be explained very simply.

The Nernst Equation

This equation relates the zero-current cell potential to the activities of the reactants and products in the cell reaction. The Gibbs Energy of reaction, ΔGr , can always be written in the following way.

Uses of Standard Potentials

Standard cell potentials find many uses as sources of thermodynamic information of interest.

Solubility Constants

The solubility, S, of a salt may be defined as the molality of a saturated solution of that salt, and, for a relatively insoluble salt, may be estimated from the standard potential of the appropriate cell.

Measurement of pH and pKa

The half-reaction in a standard hydrogen electrode is as follows.

The Cell Potential and Gibbs Free Energy

As the cell reaction in an electrochemical cell progresses, electrons move through a wire connecting the two electrodes until the equilibrium point of the cell reaction is reached, at which point the flow of electrons ceases.

The Cell Reaction

Before a discussion of the cell reaction, it is necessary to introduce the notation used to represent electrochemical cells as cell diagrams.

The Extended Debye-Hückel Law

When the ionic strength of a solution is too high for the Debye-Hückel limiting law to be valid (ie the molality of the solution is too high), the mean activity coefficient may be estimated from the extended Debye-Hückel law

The Debye-Hückel Limiting Law

Coulombic interactions between ions in solution are relatively strong, long-range forces compared to the other types of intermolecular force in solution. They are thus an important contributor to the non-ideality of ionic solutions, and in the Debye-Hückel theory of such solutions, they are taken to dominate the non-ideality to such an extent that all other contributions may be neglected.

Introduction to Electrochemical cells

An electrochemical cell consists of two electrodes, which are simply metallic conductors, in contact with an ionic conductor called the electrolyte. (Though the most commonly encountered electrolytes are ionic solutions, they may also be liquids, e.g. molten potassium bromide, or solids, e.g. solid silver iodide. The only criterion is that the substance must be an ionic conductor.

Half-equations

Oxidation is defined as the removal of electrons from a species, while reduction is the addition of electrons to a species. Any reaction in which there is a transfer of electrons from one species to another is thus called a redox reaction.

Electrode Reactions

It is convenient to separate an overall redox reaction into two half-reactions (see previous page) because the reduction and oxidation processes which make up the overall reaction of an electrochemical cell are separated in space.

Activity of Ionic Solutions

An ideal solution may be defined as one in which the interactions between all the species present are equal (equivalent to saying ΔH of mixing is zero).