Predicting Reactions (AQA A Level Chemistry)
Revision Note
Predicting Reactions
Direction of electron flow
The direction of electron flow can be determined by comparing the Eꝋ values of two half-cells in an electrochemical cell
Cl2 (g) + 2e- ⇌ 2Cl- (aq) Eꝋ = +1.36 V
Cu2+ (aq) + 2e- ⇌ Cu (s) Eꝋ = +0.34 V
The Cl2 more readily accept electrons from the Cu2+/Cu half-cell
This is the positive pole
Cl2 gets more readily reduced
The Cu2+ more readily loses electrons to the Cl2/Cl- half-cell
This is the negative pole
Cu2+ gets more readily oxidised
The electrons flow from the Cu2+/Cu half-cell to the Cl2/Cl- half-cell
The flow of electrons is from the negative pole to the positive pole
The electrons flow through the wires from the negative pole to the positive pole
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 Tips and Tricks
Remember that the electrons only move through the wires in the external circuit and not through the electrolyte solution.
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