19.7 Electrolytic Cells
PRE-HEALTH PREP
Prediciting the Products of Molten Electrolysis
Predicting the products during molten electrolysis of a binary salt is relatively straightforward. The cation is reduced to its elemental form at the cathode, and the anion is oxidized to its elemental form at the anode. For example, in the molten electrolysis of AgCl(l), elemental silver is produced at the cathode, and elements chlorine (Cl2) is produced at the anode.
Ag+ + e- --> Ag (cathode)
2Cl- --> Cl2 + 2e- (anode)
a) What are the products of electrolysis of NaBr(l)?
Prediciting the Products of Aqueous Electrolysis
Predicting the products during aqueous electrolysis is more challenging than predicting the products of molten electrolysis due to the presence of water. The cation may still be reduced to its elemental form at the cathode, but water might be reduced to H2 and OH- instead. And the anion may still be oxidized to its elemental form at the anode, but water might be oxidized to O2 and H+ instead. Typically, only one reduction half reaction will occur at the cathode; whichever has the higher (more positive or less negative) reduction potential. And only one oxidation half reaction will occur at the anode; whichever has the higher (more positive or less negative) oxidation potential.
b) What are the products of electrolysis of a solution of NaBr?
| Na+ + 1e- → Na | E°red = -2.71V |
|---|---|
| 2H2O + 2e- → H2 + 2OH- | E°red = -0.83V |
| 2Br- → Br2 + 2e- | E°ox = -1.07V |
| 2H2O → O2 + 4H+ 4e- | E°ox = -1.23V |