Thermodynamic Feasibility
Feasibility
- The Eꝋ values of a species indicate how easily they can get oxidised or reduced
- The more positive the value, the easier it is to reduce the species on the left of the half-equation
- The reaction will tend to proceed in the forward direction
- The less positive the value, the easier it is to oxidise the species on the right of the half-equation
- The reaction will tend to proceed in the backward direction
- A reaction is feasible (likely to occur) when the Ecellꝋ is positive
- For example, two half-cells in the following electrochemical cell are:
Cl2 (g) + 2e- ⇌ 2Cl- (aq) Eꝋ = +1.36 V
Cu2+ (aq) + 2e- ⇌ Cu (s) Eꝋ = +0.34 V
- Cl2 molecules are reduced as they have a more positive Eꝋ value
- The chemical reaction that occurs in this half cell is:
Cl2 (g) + 2e- → 2Cl- (aq)
- Cu2+ ions are oxidised as they have a less positive Eꝋ value
- The chemical reaction that occurs in this half cell is:
Cu (s) → Cu2+ (aq) + 2e-
- The overall equation of the electrochemical cell is (after cancelling out the electrons):
Cu (s) + Cl2 (g) → 2Cl- (aq) + Cu2+ (aq)
OR
Cu (s) + Cl2 (g) → CuCl2 (s)
- The forward reaction is feasible (spontaneous) as it has a positive Eꝋ value of +1.02 V ((+1.36) - (+0.34))
- The backward reaction is not feasible (not spontaneous) as it has a negative Eꝋ value of -1.02 ((+0.34) - (+1.36))
A reaction is feasible when the standard cell potential Eꝋ is positive
Examiner Tip
You may have to apply your understanding (from the above worked example) to questions with more than 2 equations
- The process is still the same in terms of identifying the most positive / least negative value as the reduction reactions
- This reaction will also contain the oxidising agent on the left hand side