Question 25.8: Use the data in Table 25.3 to determine whether Co^3+(aq), a......
Use the data in Table 25.3 to determine whether Co^{3+}(aq), a fairly strong oxidizing agent, is capable of oxidizing water to O_2(g) in acidic aqueous solution under standard conditions at 25.0°C, according to the equation
4\ Co^{3+}(aq) + 2\ H_2O(l) → 4\ Co^{2+}(aq) + O_2(g) + 4\ H^+(aq)| TABLE 25.3 Standard reduction voltages at 25.0°C for aqueous solutions (see also Appendix G)* | |||
| Electrode half reaction | E^{\circ}_{red}/V | ||
|
\uparrow |
Acidic solutions |
increasing strength |
|
| F_{2}(g) + 2\ e^− → 2\ F^−(aq) | +2.866 | ||
| O_{3}(g) + 2\ H^+(aq) + 2\ e^− → O_{2}(g) + H_2O(l ) | +2.076 | ||
| Co^{3+}(aq) + e^− → Co^{2+}(aq) | +1.92 | ||
| Cl_{2}(g) + 2\ e^− → 2\ Cl^−(aq) | +1.358 | ||
| O_{2}(g) + 4\ H^+(aq) + 4\ e^− → 2\ H_2O(l ) | +1.229 | ||
| Pt^{2+}(aq) + 2\ e^– → Pt(s) | +1.18 | ||
| NO_{3}^{–}(aq) + 4\ H^+(aq) + 3\ e^– → NO(g) + 2\ H_2O(l ) | +0.957 | ||
| Ag^+(aq) + e^− → Ag(s) | +0.7996 | ||
| Cu^+(aq) + e^− → Cu(s) | +0.521 | ||
| Cu^{+2}(aq) +2\ e^− → Cu(s) | +0.342 | ||
| Hg_2Cl_2(s) + 2\ e^− → 2\ Hg(l ) + 2\ Cl^−(aq) | +0.268 | ||
| AgCl(s) + e^− → Ag(s) + Cl^−(aq) | +0.2223 | ||
| Cu^{2+}(aq) + e^− → Cu^+(aq) | +0.153 | ||
| 2\ H^+(aq) + 2\ e^− → H_2(g) | +0.0 | ||
| Pb^{2+}(aq) + 2\ e^− → Pb(s) | -0.126 | ||
| V^{3+}(aq) + e^− → V^{2+}(aq) | -0.255 | ||
| Fe^{2+}(aq) + 2\ e^– → Fe(s) | –0.447 | ||
| Zn^{2+}(aq) + 2\ e^− → Zn(s) | -0.762 | ||
| Mn^{2+}(aq) + 2\ e^– → Mn(s) | –1.185 | ||
| Al^{3+}(aq) + 3\ e^− → Al(s) | -1.662 | ||
| H_2(g) + 2\ e^− → 2\ H^−(aq) | -2.23 | ||
| Mg^{2+}(aq) + 2\ e^− → Mg(s) | -2.372 | ||
| Na^+(aq) + e^− → Na(s) | -2.71 | ||
| Ca^{2+}(aq) + 2\ e^– → Ca(s) | –2.868 | ||
| K^+(aq) + e^– → K(s) | –2.931 | ||
| Li^+(aq) + e^− → Li(s) | -3.0401 | ||
| Basic solutions | |||
| O_2(g) + 2\ H_2O(l ) + 4\ e^− → 4\ OH^−(aq) | +0.401 | ||
| Cu(OH)_2(s) + 2\ e^− → Cu(s) + 2\ OH^−(aq) | -0.222 | ||
| Fe(OH)_3(s) + e^– → Fe(OH)_2(s) + OH^–(aq) | –0.56 | ||
| 2\ H_2O(l ) + 2\ e^− → H_2(g) + 2\ OH^−(aq) | -0.8277 | ||
| 2\ SO_{3}^{2−}(aq) + 2\ H_2O(l) + 2\ e^− → S_{2}O_{4}^{2−}(aq) + 4\ OH^−(aq) | -1.12 | ||
| *Data from CRC Handbook of Chemistry and Physics, 87th ed., ed. David R. Lide, CRC Press, 2006–2007 | |||
The oxidation of H _{2}O(l ) to O_2(g) by Co^{3+}(aq) under standard conditions (Q = 1) will be a spontaneous process if E^{\circ}_{cell} = E^{\circ}_{red}+E^{\circ}_{ox} is greater than zero. The two half reaction equations are
4\ Co^{3+}(aq) + 4\ e^− → 4\ Co^{2+}(aq) \qquad E^{\circ}_{red} = +1.92\ V
2\ H_2O(l) → O_2(g) + 4\ H^+(aq) + 4\ e^− \qquad E^{\circ}_{ox} = –E^{\circ}_{red} = –1.23\ V
The E^{\circ}_{red} values for the two half reactions were obtained from Table 25.3. The value of E^{\circ}_{cell} is
E^{\circ}_{cell} = E^{\circ}_{red}[Co^{3+}|Co^{2+}] + E^{\circ}_{ox}[H_2O|O_2] = (1.92\ V) + (–1.23\ V) = +0.69\ VAgain note that we do not multiply the value of E^{\circ}_{red}[Co^{3+}|Co^{2+}] by 4 because the magnitude of a cell voltage or half-cell voltage is independent of the quantity of material involved or how we choose to (arbitrarily) write the equations that describe the cell reaction. The positive value of E^{\circ}_{cell} means that Co^{3+}(aq) is capable of oxidizing water at 25.0°C under standard conditions. The rate of oxidation of water by Co^{3+}(aq) is fairly rapid at 25.0°C, and so Co^{3+}(aq) does not persist at appreciable concentrations in water.