Direction of electron flow

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

Cl2?(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

   

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

Feasibility

• The?E??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?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?E??value of -1.02 ((+0.34) - (+1.36))

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