Dichromate(VI) reduction predictions

We need to use the following standard electrode potentials:

[Cr₂O₇^2-(aq) + 14H⁺(aq)], [2Cr³⁺(aq) + 7H₂O(l)]\|Pt	+ 1.33 V
Cr³⁺(aq), Cr²⁺(aq)\|Pt	- 0.41 V
Zn²⁺(aq)\|Zn(s)	- 0.76 V

The zinc standard electrode potential is the most negative of these three. This means that it will give up electrons to the other two, and therefore, reduce them. So both dichromate(VI) to chromium(III), and chromium(III) to chromium(II) reductions should take place using zinc metal.

We can do this formally by writing down a cell diagram and working out the value of E_cell. A positive value means the forward reaction (as represented in the cell diagram) is feasible. Note that the second change has a value of + 0.35 V and, therefore, will not go to completion. A value of + 0.6 V or greater is required before the reaction is said to go to completion. However, with this value, the product concentration will be many times greater than the remaining reactant concentration.

Zn(s)| Zn²⁺(aq)::Cr³⁺(aq), Cr²⁺(aq)|Pt

E_cell = + 0.35 V

The blue, hexaquochromium(II) solution is unstable and it is readily oxidized by the air back to the green, hexaquochromium(III) solution. This change can be seen at the end of the video. However, the chromium(II) ion is stabilized by the formation of the crimson ethanoate complex and it resists oxidation by the air, and so remains unchanged.