Modern inorganic chemistry

ACIDS AND BASES: OXIDATION AND REDUCTION 97

Thus the equation for the reaction is:

Cr 2 Or(aq) + 6Fe2+(aq) + 14H 3 O+ -> 2Cr3+(aq) +

STANDARD REDOX POTENTIALS

21HO

When the reaction between zinc and copper(II) sulphate was carried

out in the form of an electrochemical cell (p. 94), a potential differ-

ence between the copper and zinc electrodes was noted. This

potential resulted from the differing tendencies of the two metals to

form ions. An equilibrium is established when any metal is placed

in a solution of its ions.

The enthalpy changes A/f involved in this equilibrium are (a) the

heat of atomisation of the metal, (b) the ionisation energy of the

metal and (c) the hydration enthalpy of the metal ion (Chapter 3).

For copper and zinc, these quantities have the values (kJ moP J):

Cu

Zn

H eat of

atomisation

339

126

Sum of 1st and 2nd

ionisation energies

2703

2640

Hydration

enthalpy

-2100

-2046

AH

+ 942

+ 720

For the equilibrium M(s) ^ M2+(aq) + 2e , it might then be
(correctly) assumed that the equilibrium for copper is further to the
left than for zinc, i.e. copper has less tendency to form ions in
solution than has zinc. The position of equilibrium (which depends
also on temperature and concentration) is related to the relative
reducing powers of the metals when two different metals in solutions
of their ions are connected (as shown in Figure 4.1 for the copper-
zinc cell) ; a potential difference is noted because of the differing
equilibrium positions.
Since it is not possible to measure a single electrode potential, one
electrode system must be taken as a standard and all others measured
relative to it. By international agreement the hydrogen electrode has
been chosen as the reference:

This electrode, shown diagrammatically in Figure 4.4, is assigned
zero potential when hydrogen gas at one atmosphere bubbles over
platinised platinum in a solution of hydrogen ions of concentration
1 mol P * (strictly, at unit activity).