Storage cells and fuel cells are types of electrochemical cells.
The standard electrode potentials for five redox systems are shown in the table below:
| Redox system | Half-equation | Eθ/VE^\theta / \text{V}Eθ/V |
|---|---|---|
| 1 | Zn2+(aq)+2e−⇌Zn(s)\text{Zn}^{2+}(\text{aq}) + 2\text{e}^- \rightleftharpoons \text{Zn}(\text{s})Zn2+(aq)+2e−⇌Zn(s) | −0.76-0.76−0.76 |
| 2 | O2(g)+4H+(aq)+4e−⇌2H2O(l)\text{O}_2(\text{g}) + 4\text{H}^+(\text{aq}) + 4\text{e}^- \rightleftharpoons 2\text{H}_2\text{O}(\text{l})O2(g)+4H+(aq)+4e−⇌2H2O(l) | +1.23+1.23+1.23 |
| 3 | Au3+(aq)+2e−⇌Au+(aq)\text{Au}^{3+}(\text{aq}) + 2\text{e}^- \rightleftharpoons \text{Au}^+(\text{aq})Au3+(aq)+2e−⇌Au+(aq) | +1.41+1.41+1.41 |
| 4 | Au+(aq)+e−⇌Au(s)\text{Au}^+(\text{aq}) + \text{e}^- \rightleftharpoons \text{Au}(\text{s})Au+(aq)+e−⇌Au(s) | +1.69+1.69+1.69 |
| 5 | Ce4+(aq)+e−⇌Ce3+(aq)\text{Ce}^{4+}(\text{aq}) + \text{e}^- \rightleftharpoons \text{Ce}^{3+}(\text{aq})Ce4+(aq)+e−⇌Ce3+(aq) | +1.70+1.70+1.70 |
A student sets up an electrochemical cell based on redox systems 1 and 5.
Describe the key components of a labelled diagram showing how this cell could be set up in the laboratory.
Construct the equation for the overall cell reaction.
Gold(I) ions, Au+(aq)\text{Au}^+(\text{aq})Au+(aq), undergo disproportionation in solution to form gold metal, Au(s)\text{Au}(\text{s})Au(s), and gold(III) ions, Au3+(aq)\text{Au}^{3+}(\text{aq})Au3+(aq).
Explain, in terms of oxidation numbers, why disproportionation has taken place.
Explain, in terms of electrode potentials and equilibrium shifts, why Au+(aq)\text{Au}^+(\text{aq})Au+(aq) disproportionates. Use the information in the table.
An acidic hydrogen-oxygen fuel cell is set up.
The overall equation for the cell reaction is:
2H2(g)+O2(g)→2H2O(l) 2\text{H}_2(\text{g}) + \text{O}_2(\text{g}) \rightarrow 2\text{H}_2\text{O}(\text{l}) 2H2(g)+O2(g)→2H2O(l)Redox system 2 in the table is the positive electrode of this cell.
Write the half-equation at the negative electrode.
The overall cell potential is 1.23 V1.23\text{ V}1.23 V. Calculate the standard electrode potential of the negative electrode.
State one important feature of a fuel cell that is different from a conventional storage cell.
