The essence of a chemical reaction is the exchange of atoms and electrons between molecular species. In most reactions, the exchange of the electrons is not manifest because it occurs in an intimate mixture of reactants and products. However, in a device known as an electrochemical cell, participants in a reaction are physically separated in a manner that renders the electron transfer process observable, measurable, and usable for work. The study of chemical reactions with emphasis on the electron-transfer process is called electrochemistry.
For example, the reaction involving metals X and Y and their oxides XO and YO can be written in the conventional form:
This overall reaction can be analyzed by breaking it up into the formation reactions of the oxides:
However, this manner of describing reaction (10.1) does not reveal the electron transfer process. Suppose that the initial state consists of equal numbers of moles of XO and Y. Assume also that AG° is negative so that the reaction tends to proceed from left to right. What occurs in an electrochemical sense is the transfer of oxygen ions (O2-) bound to X2+ to Y atoms to form YO. Simultaneously, Y must lose two electrons (thus becoming Y2+) and X2+ gains two electrons to produce X. Reaction (10.1) is an example of an oxidation-reduction reaction: Y is oxidized because its valence, or oxidation state is increased from 0 to +2; X is reduced from the +2 oxidation state to the neutral elemental form.
Electrochemistry is divided into two branches, depending on whether or not liquid water is present. If water is absent and the reaction involves solid phases, as in reaction (10.1), the branch is called solid-state electrochemistry. These electrochemical reactions usually take place at high temperatures so that the kinetics are sufficiently rapid for attainment of equilibrium in a reasonable time.
Many solutes dissolved in water do so as positive and negative ions rather than molecular entities. Reactions of these ions among themselves or with dissolved gases or solid metals immersed in the water involve transfer of electrons between species. This branch of electrochemistry is called aqueous electrochemistry.
The electrochemical cell, whether solid-state or aqueous, provides a method for using electrical measurements (EMF) for determination of free energy changes, such as AGo of reaction (10.1). Alternatively, the electron flow produced by an electrochemical cell can be harnessed to perform useful work. Such a cell is a battery or a fuel cell.
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Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.