Standard electrode potentials for four redox systems are shown below:
| Redox system | Half-equation | Eθ/VE^\theta / \text{V}Eθ/V |
|---|---|---|
| 1 | CO2(g)+2H+(aq)+2e−⇌HCOOH(aq)\text{CO}_2(\text{g}) + 2\text{H}^+(\text{aq}) + 2\text{e}^- \rightleftharpoons \text{HCOOH}(\text{aq})CO2(g)+2H+(aq)+2e−⇌HCOOH(aq) | -0.18 |
| 2 | HCOOH(aq)+2H+(aq)+2e−⇌HCHO(aq)+H2O(l)\text{HCOOH}(\text{aq}) + 2\text{H}^+(\text{aq}) + 2\text{e}^- \rightleftharpoons \text{HCHO}(\text{aq}) + \text{H}_2\text{O}(\text{l})HCOOH(aq)+2H+(aq)+2e−⇌HCHO(aq)+H2O(l) | -0.03 |
| 3 | Fe3+(aq)+e−⇌Fe2+(aq)\text{Fe}^{3+}(\text{aq}) + \text{e}^- \rightleftharpoons \text{Fe}^{2+}(\text{aq})Fe3+(aq)+e−⇌Fe2+(aq) | +0.77 |
| 4 | MnO4−(aq)+8H+(aq)+5e−⇌Mn2+(aq)+4H2O(l)\text{MnO}_4^-(\text{aq}) + 8\text{H}^+(\text{aq}) + 5\text{e}^- \rightleftharpoons \text{Mn}^{2+}(\text{aq}) + 4\text{H}_2\text{O}(\text{l})MnO4−(aq)+8H+(aq)+5e−⇌Mn2+(aq)+4H2O(l) | +1.51 |
Formic acid, HCOOH\text{HCOOH}HCOOH, can be used in a direct formic acid fuel cell. As with all fuel cells, the fuel (HCOOH\text{HCOOH}HCOOH) 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 1.38 V.
The overall reaction is shown below:
HCOOH(aq)+12O2(g)→CO2(g)+H2O(l) \text{HCOOH}(\text{aq}) + \frac{1}{2}\text{O}_2(\text{g}) \rightarrow \text{CO}_2(\text{g}) + \text{H}_2\text{O}(\text{l}) HCOOH(aq)+21O2(g)→CO2(g)+H2O(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.
