The standard electrode potential of a half-cell can be determined by connecting it to a standard hydrogen electrode
Metal / metal-ion half-cell
Example of a metal / metal ion half-cell connected to a standard hydrogen electrode
Non-metal / non-metal ion half-cell
Example of a non-metal / non-metal ion half-cell connected to a standard hydrogen electrode
Ions in solution half cell
Conventional Representation of Cells
Example: Writing a cell diagram
If you connect an aluminium electrode to a zinc electrode, the voltmeter reads 0.94V and the aluminium is the negative. Write the conventional cell diagram to the reaction.
Al (s)∣Al3+ (aq) ∥ Zn2+ (aq)∣Zn (s) Ecell = +0.94 V
It is also acceptable to include phase boundaries on the outside of cells as well:
∣ Al (s)∣Al3+ (aq) ∥ Zn2+ (aq)∣Zn (s) ∣ Ecell = +0.94 V
Example: Calculating the standard cell potential
Calculate the standard cell potential for the electrochemical cell below and explain why the Cu2+ / Cu half-cell is the positive pole. The half-equations are as follows:
Cu2+(aq) + 2e- ⇌ Cu(s) Eꝋ = +0.34 V
Zn2+(aq) + 2e- ⇌ Zn(s) Eꝋ = −0.76 V
Step 1: Calculate the standard cell potential. The copper is more positive so must be the right hand side.
Ecellꝋ = Erightꝋ - Eleftꝋ
Ecellꝋ = (+0.34) - (-0.76)
= +1.10 V
The voltmeter will therefore give a value of +1.10 VStep 2: Determine the positive and negative poles
The Cu2+ / Cu half-cell is the positive pole as its Eꝋ is more positive than the Eꝋ value of the Zn2+ / Zn half-cell
Feasibility
A reaction is feasible when the standard cell potential Eꝋ is positive
Limitations of Eθ to predict reactions
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