Andrew

Entropy, given the symbol S, is a state function which is a measure of the disorder of a system. i.e. the larger the value of S of a system, the more disordered it is. Entropy is given a thermodynamic definition in terms of the change in entropy dS that occurs as a result of some process. The definition … Read more

There are several equivalent alternative ways of representing the Second Law of Thermodynamics. One is in terms of entropy, S, a state function which is a measure of the molecular disorder of a system. The law states that in the course of a spontaneous change, the entropy of an isolated system must increase. Note that since the universe is itself an isolated system, we … Read more

Certain processes occur spontaneously – for example cooling of a hot object to the temperature of the surroundings, and expansion of a gas to fill the volume available to it. Though these processes can be made to go the other way (heating an object up, confining a gas to a smaller volume), they do not occur … Read more

The half-reaction in a standard hydrogen electrode is as follows: H+(aq)  +  e–  →  ½ H2(g)                                 ν  =  1 and the reaction quotient, Q , is given by ( fH2 is the fugacity of hydrogen gas; fugacity has units of pressure and is the gas-phase analogue of activity – see here. For the standard hydrogen electrode,  fH2  =  Pº by definition). Note also, for the SHE,  Eº  =  0. Substituting these results … Read more

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. For a general salt of low solubility, represented by MX, the solubility may be discussed in terms of the solubility equilibrium: MX(s) ↔ M+(aq)  +  X–(aq)                Ks  =  a(M+) a(X–) … Read more

Standard cell potentials find many uses as sources of thermodynamic information of interest. We have already encountered the relation ΔGºr =  – νFEº. From this, and the thermodynamic relation (δG/δT)P = – S , we obtain: we can now use the fundamental relation ΔGºr = ΔHºr  –  T ΔSºr to write Standard potentials may also be used to construct … Read more

It is not possible to measure the electrode potential of a single electrode in isolation. This idea can be explained very simply: If we consider that the reaction at an electrode is actually a half reaction, one half of a redox process, it is obvious that to maintain the overall number of electrons, there must … Read more

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: where ΔGºr is the Standard Gibbs Energy of Reaction, and Q is the reaction quotient, dependant upon the activities of the reactants and products, and thus also dependant upon the composition … Read more

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. As long as the reaction has not reached equilibrium, the electrons being driven through the external circuit may be used to … Read more

Before a discussion of the cell reaction, it is necessary to introduce the notation used to represent electrochemical cells as cell diagrams. Phase boundaries are represented by a vertical line, |   (i.e. such a line is placed between species in different phases, such as a solid electrode and aqueous ions). Different species in the same phase (as … Read more