Chemistry · General chemistry II · Worked example
Find a standard cell potential
A galvanic cell pairs Zn(s) | Zn²⁺(aq) with Cu²⁺(aq) | Cu(s). With E° = +0.34 V for Cu²⁺/Cu and −0.76 V for Zn²⁺/Zn, find E°cell and ΔG°, and say which way electrons flow.
Pick the cathode
Copper has the more positive reduction potential, so Cu²⁺ is reduced at the cathode and zinc is oxidized at the anode.
Subtract the potentials
Cathode minus anode, both as reduction potentials.
Write the overall reaction
Two electrons move per zinc atom, so n = 2.
Find ΔG°
With F = 96,485 C/mol, and 1 C·V = 1 J.
Interpret
Electrons flow through the wire from the zinc anode to the copper cathode. E°cell is positive and ΔG° negative, so the reaction is spontaneous; its equilibrium constant at 298 K is about 10³⁷.
Result
E°cell = 1.10 V and ΔG° = −212 kJ; electrons flow from zinc to copper.
Your turn
A cell pairs Ag⁺/Ag (E° = +0.80 V) with Cu²⁺/Cu (+0.34 V). Find E°cell.
Show the answer and explanation
0.46 V.
Silver has the higher reduction potential, so it is the cathode: 0.80 V − 0.34 V = 0.46 V. The silver half-reaction is doubled to balance the electrons, but its potential is not.
Keep exploring
In Electrochemistry & charge, swap the two electrodes. E°cell becomes −1.10 V and ΔG° is +212 kJ: written that way, the reaction is not spontaneous.
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