F.6 redox equilibra

(a) (i)
Cr(s) + 2H⁺(aq) → Cr²⁺(aq) + H2(g)
Ecell = (0) - (-0.91) = +0.91 V > 0 V
The above reaction is FEASIBLE.

(a) (ii)
O2(g) + 4H⁺(aq) + 4Cr²⁺ (aq) → 2H2­O(l) + 4Cr³⁺
Ecell = (+1.23) - (-0.40) = +1.63 > 0 V
The above reaction is FEASIBLE.

(a) (iii)
Zn(s) + 2Cr³⁺(aq) → Zn²⁺(aq) + 2Cr²⁺(aq)
Ecell = (-0.40) - (-0.76) = +0.36 > 0 V
The above reaction is FEASIBLE.


= = = = =
(b)
Cr2O7²⁻(aq) + 14H⁺(aq) + 6Cl⁻(aq) → 2Cr³⁺(aq) + 7H2O(l) + 3Cl2(aq)
Ecell = (+1.33) - (+1.36) = -0.03 V < 0 V
The above reaction is NOT feasible.
Since there is no redox reaction between K2Cr2O7 and HCl, dilute HCl can be used to acidify K2Cr2O7 solution.

2MnO4⁻(aq) + 16H⁺(aq) + 10Cl⁻(aq) → 2Mn²⁺(aq) + 8H2O(l) + 5Cl2(g)
Ecell = (+1.51) - (+1.36) = +0.15 V > 0 V
The above reaction is FEASIBLE.
Since there is reaction between KMnO4­ and HCl, dilute HCl CANNOT be used to acidify KMnO solution.

2011-04-04 13:31:18 補充：
Dioxygenated dilute HCl means that the HCl is in the absence of oxygen. You only need to consider the reaction of HCl, but it is not necessary to consider the reaction of O2.

2011-04-04 13:31:42 補充：
To acidify a solution using dilute HCl means to add dilute HCl to the solution in order to make the solution acidic. The term "acidified HCl" is meaningless, because HCl is acidic and it is not necessary to add a dilute acid to make it acidic.