Standard electrode potentials for four redox systems are shown below:
| Redox system | Half-equation | Eθ/VE^\theta / \text{V}Eθ/V |
|---|---|---|
| 1 | CO2(g)+6H+(aq)+6e−⇌CH3OH(aq)+H2O(l)\text{CO}_2(\text{g}) + 6\text{H}^+(\text{aq}) + 6\text{e}^- \rightleftharpoons \text{CH}_3\text{OH}(\text{aq}) + \text{H}_2\text{O}(\text{l})CO2(g)+6H+(aq)+6e−⇌CH3OH(aq)+H2O(l) | -0.02 |
| 2 | HCHO(aq)+2H+(aq)+2e−⇌CH3OH(aq)\text{HCHO}(\text{aq}) + 2\text{H}^+(\text{aq}) + 2\text{e}^- \rightleftharpoons \text{CH}_3\text{OH}(\text{aq})HCHO(aq)+2H+(aq)+2e−⇌CH3OH(aq) | +0.13 |
| 3 | Ag+(aq)+e−⇌Ag(s)\text{Ag}^+(\text{aq}) + \text{e}^- \rightleftharpoons \text{Ag}(\text{s})Ag+(aq)+e−⇌Ag(s) | +0.80 |
| 4 | Cr2O72−(aq)+14H+(aq)+6e−⇌2Cr3+(aq)+7H2O(l)\text{Cr}_2\text{O}_7^{2-}(\text{aq}) + 14\text{H}^+(\text{aq}) + 6\text{e}^- \rightleftharpoons 2\text{Cr}^{3+}(\text{aq}) + 7\text{H}_2\text{O}(\text{l})Cr2O72−(aq)+14H+(aq)+6e−⇌2Cr3+(aq)+7H2O(l) | +1.33 |
Methanol, CH3OH\text{CH}_3\text{OH}CH3OH, can be used in a direct methanol fuel cell. As with all fuel cells, the fuel (CH3OH\text{CH}_3\text{OH}CH3OH) is supplied at one electrode and the oxidant (oxygen, O2\text{O}_2O2) at the other electrode.
The standard cell potential for this fuel cell is 1.25 V.
The overall reaction is shown below:
CH3OH(aq)+32O2(g)→CO2(g)+2H2O(l) \text{CH}_3\text{OH}(\text{aq}) + \frac{3}{2}\text{O}_2(\text{g}) \rightarrow \text{CO}_2(\text{g}) + 2\text{H}_2\text{O}(\text{l}) CH3OH(aq)+23O2(g)→CO2(g)+2H2O(l)Using the information above, deduce the half-equation for the reaction at the oxygen electrode, and calculate the standard electrode potential for the oxygen half-cell.
