For M 2+ /M and M 3+ /M 2+ systems the EV values for some metals are as follows: Cr 2+ /Cr -0.9V Cr 3 /Cr 2+ -0.4 V Mn 2+ /Mn -1.2V Mn 3+ /Mn 2+ +1.5 V Fe 2+ /Fe -0.4V Fe 3+ /Fe 2+ +0.8 V Use this data to comment upon: (i) the stability of Fe 3+ in acid solution as compared to that of Cr 3+ or Mn 3+ and (ii) the ease with which iron can be oxidized as compared to a similar process for either chromium or manganese metal.

Question

Philoid · Accepted Answer

(i) Reduction potential tells us the ease with which the Metal can get reduced, As E° for Cr³⁺/Cr²⁺ is negative (–0.4 V), this means that Cr³⁺ ions in solution cannot be reduced to Cr²⁺ ions i.e., Cr³⁺ ions are very stable. As a further comparison of E° values show that Mn³⁺ ions more readily than Fe³⁺ ions which means that Mn³⁺ is least stable.

So Stability of metal ions is as follows:

Mn³⁺3+3+.

(ii) Reduction potential tells us the ease with which the Metal can get reduced or the difficulty with which they are oxidized, As the reduction potential increases in the following order Mn²⁺/Mn < Cr²⁺/Cr< Fe²⁺/Fe

So oxidation of Fe is not as easy as Cr and Mn.

So increasing order of the metals to get oxidised is as follows:

Fe < Cr < Mn

Cr²⁺/Cr	-0.9V	Cr³/Cr²⁺	-0.4 V
Mn²⁺/Mn	-1.2V	Mn³⁺/Mn²⁺	+1.5 V
Fe²⁺/Fe	-0.4V	Fe³⁺/Fe²⁺	+0.8 V