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 | Cr3+(aq)+3e−⇌Cr(s)\text{Cr}^{3+}(\text{aq}) + 3\text{e}^- \rightleftharpoons \text{Cr}(\text{s})Cr3+(aq)+3e−⇌Cr(s) | −0.74-0.74−0.74 |
| 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 | MnO2(s)+4H+(aq)+e−⇌Mn3+(aq)+2H2O(l)\text{MnO}_2(\text{s}) + 4\text{H}^+(\text{aq}) + \text{e}^- \rightleftharpoons \text{Mn}^{3+}(\text{aq}) + 2\text{H}_2\text{O}(\text{l})MnO2(s)+4H+(aq)+e−⇌Mn3+(aq)+2H2O(l) | +0.95+0.95+0.95 |
| 4 | Mn3+(aq)+e−⇌Mn2+(aq)\text{Mn}^{3+}(\text{aq}) + \text{e}^- \rightleftharpoons \text{Mn}^{2+}(\text{aq})Mn3+(aq)+e−⇌Mn2+(aq) | +1.51+1.51+1.51 |
| 5 | VO2+(aq)+2H+(aq)+e−⇌V3+(aq)+H2O(l)\text{VO}^{2+}(\text{aq}) + 2\text{H}^+(\text{aq}) + \text{e}^- \rightleftharpoons \text{V}^{3+}(\text{aq}) + \text{H}_2\text{O}(\text{l})VO2+(aq)+2H+(aq)+e−⇌V3+(aq)+H2O(l) | +0.34+0.34+0.34 |
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.
Manganese(III) ions, Mn3+(aq)\text{Mn}^{3+}(\text{aq})Mn3+(aq), undergo disproportionation in acidic solution to form manganese(IV) oxide, MnO2(s)\text{MnO}_2(\text{s})MnO2(s), and manganese(II) ions, Mn2+(aq)\text{Mn}^{2+}(\text{aq})Mn2+(aq).
Explain, in terms of oxidation numbers, why disproportionation has taken place.
Explain, in terms of electrode potentials and equilibrium shifts, why Mn3+(aq)\text{Mn}^{3+}(\text{aq})Mn3+(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.
