MEHRAN UNIVERSITY OF ENGINEERING AND TECHNOLOGY, JAMSHORO, SINDH, PAKISTAN

Electrochemistry deals with cell potential as well as energy of chemical reactions.

The energy of a chemical system drives the charges to move, and the driving force give rise to the cell potential of a system called galvanic cell.

The energy aspect is also related to the chemical equilibrium.

All these relationships are tied together in the concept of Nearnst equation.

The Nernst equation was named after the German chemist Walther Nernst who established very useful relations between the energy or potential of a cell to the concentrations of participating ions.

This equation can be derived from the equation linking free energy changes to the reaction quotient (Qreaction)

where, for a generalized equation of the form:

The capital letters A, B, M and N in equation represent respectively the reactants and products of a given reaction while the small letters represent the coefficients required to balance the reaction.

In the case of an electrochemical reaction, substitution of the relationships:

&

into the expression of a reaction free energy and division of both sides by -nF gives the Nernst equation for an electrode reaction:

This equation can be written as:

where R is the gas constantz is the number of electrons participating in

either of the half-cell reactions (n is also used)F is the Faraday constant, 96,500 C/mol—the

magnitude of charge per mole (6.023 x 1023) of electrons.

It is interesting to note the relationship between equilibrium and the Gibb's free energy at this point. When a system is at equilibrium, ∆E = 0,

At any specific temperature, the Nernst equation derived above can be reduced into a simple form. For example, at the standard condition of 298 K (25°), the Nernst equation becomes

Please note that log is the logrithm function based 10, and ln, the natural logrithm function.

DISCUSSION

Understandably, the Zn2+ ions try to move from the concentrated half cell to a dilute solution. That driving force gives rise to 0.0592 V.

If you write the equation in the reverse direction,

Zn2+ (0.024 M) = Zn2+ (2.4 M), its voltage will be -0.0592 V. At equilibrium concentrations in the two half cells will have to be equal, in which case the voltage will be zero.

Show that the voltage of an electric cell is unaffected by multiplying the reaction equation by a positive number.

Nickel is connected to Cadmium and then immersed in a solution containing both Ni2+ and Cd2+ charge.

I. Which metals corrodes?II.Write equations to describe the reactions

which occurs at each electrode, assuming each electrode has a valency of 2.

III.Calculate the maximum possible potential of the resulting corrosion cell.