AQA A Level Chemistry

Revision Notes

5.4.3 Predicting Reactions

Predicting Reactions

Direction of electron flow

  • The direction of electron flow can be determined by comparing the Evalues of two half-cells in an electrochemical cell

2Cl2 (g) + 2e⇌ 2Cl (aq)        E = +1.36 V

Cu2+ (aq) + 2e⇌ Cu (s)        E = +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

Principles of Electrochemistry - Direction of Flow of Electrons, downloadable AS & A Level Chemistry revision notes

The electrons flow through the wires from the negative pole to the positive pole

Feasibility

  • The Evalues 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 Ecell is positive
  • For example, two half-cells in the following electrochemical cell are:

Cl2 (g) + 2e⇌ 2Cl (aq)        E = +1.36 V

Cu2+ (aq) + 2e⇌ Cu (s)        E = +0.34 V

  • Cl2 molecules are reduced as they have a more positive E 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 E 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 E value of +1.02 V ((+1.36) – (+0.34))
  • The backward reaction is not feasible (not spontaneous) as it has a negative Evalue of -1.02 ((+0.34) – (+1.36))

Principles of Electrochemistry - Reaction Feasibility (1), downloadable AS & A Level Chemistry revision notesPrinciples of Electrochemistry - Reaction Feasibility (2), downloadable AS & A Level Chemistry revision notes

A reaction is feasible when the standard cell potential E is positive

Exam Tip

Remember that the electrons only move through the wires in the external circuit and not through the electrolyte solution.

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