Instant Notes: Analytical Chemistry

The free energy Gdepends on the activity (or approximately the concentra-

tion) of the reactants and products. For a general reaction:

Ox1 +Red2 =Red1 +Ox2

DG=DGn+RTln [(a(Ox2).a(Red1))/(a(Ox1).a(Red2))]

where the superscript, n, means that it applies to the standard state. For gases,

this is 1 atmosphere pressure; for liquids and solids it is less than 1 atmosphere

pressure for the pure material; and for solutions, it is an activity of 1.

The electromotive force measures the free energy change under reversible

conditions when n Faradays of electrical charge occur in the cell reaction and is

related to it by the equation:

DG=-nFE

For example, in the Daniell cell reaction, 2 moles of electrons are transferred,

and the cell emf is about 1.1 V. Therefore:

DG =- 2 ¥ 96485 ¥ 1.1=-212.3 kJ mol-^1

Combining the above equations, for the Daniell cell:

E =En+(RT/ 2 F) ln [(a(Zn^2 +).a(Cu))/(a(Zn).a(Cu^2 +))]

This is often referred to as the Nernst equation for the cell. (Note: the value of

Enmeasures the cell emf and relates to the free energy change under standard

state conditions, while the value of E relates to the free energy change under

other, specified conditions. It is important to recognize that these equations

apply at any temperature, not just a standard temperature.)

In order to establish any scale it is necessary to have a reference point. Since

measurement of emf is made using a cell, one electrode must be taken as a

reference electrode. The standard hydrogen electrode (SHE) has been chosen as

the primary reference.

A hydrogen electrode may be constructed using a platinum metal plate as

contact, and bubbling hydrogen gas through so that it makes contact with both

solution and platinum. When the gas is at 1 atmosphere pressure and the solu-

tion has an activity of hydrogen ions of 1, this is a standard hydrogen electrode

(SHE).

By convention, the standard hydrogen electrode is assigned an electrode

potential of 0.000 V and all other electrode potentials are compared to it.

A cell can be constructed with a copper electrode in copper sulfate and a

standard hydrogen electrode, as shown in Figure 2.

Electrode
potentials

C2 – Electrochemical reactions 63

H 2 supply

Pt electrode

Copper electrode

Sintered disk

Solution with Copper sulfate solution

H+ (a=1)

Fig. 2. Copper-hydrogen electrochemical cell.