We are asked to calculate ΔG°rxn for each of the following reactions at 25 °C.
O2(g) + 2H2O(l) + 2Cu(s) → 4OH−(aq) + 2Cu2+(aq)
The relationship between ΔG and Ecell is
Step 1. Write the two half-cell reactions
O2(g) + 2 H2O(l) → 4OH−(aq)
Cu(s) → 2Cu2+(aq)
Step 2. Identify the oxidation half-reaction (anode) and the reduction half-reaction (cathode)
When a balanced reaction is given, identify the anode and the cathode half-reactions by determining the changes in oxidation states of each species based on the given reaction:
Recall:
Lose Gain
Electron Electrons
Oxidation Reduction
cathode → reduction → oxidation number decreases
anode → oxidation → oxidation number increases
For ions: charge of the ion = oxidation number
For neutral atoms/compound: oxidation number = 0
Cu(s) → 2Cu2+(aq)
• oxidation number increased → lost electrons → oxidized → anode
O2(g) + 2 H2O(l) → 4OH−(aq)
• oxidation number decreased → reduced → cathode
Step 3. Determine the half-cell potentials (refer to the Standard Reduction Potential Table)
2 Cu(s) → 4 e- + 2Cu2+(aq) E°cell = 0.342 V
O2(g) + 2 H2O(l) + 4 e-→ 4OH−(aq) E°cell = 0.401 V
Step 4. Calculate E°cell.
E°cell = 0.059 V
Use tabulated half-cell potentials to calculate ΔG°rxn for each of the following reactions at 25 °C.
O2(g) + 2H2O(l) + 2Cu(s) → 4OH−(aq) + 2Cu2+(aq)
Express the energy change in kilojoules to one significant figure.
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