chemical cell x reduction x Na

In a chemical cell, reduction occurs at the cathode only.

Consider a chemical cell with the cathode dipped in the electrolyte of Na2SO4(aq). The ions that migrate to the anode are Na+(aq) ions from the dissociation of Na2SO4 and H+(aq) ions from the self-ionization of H2O(l). H+(aq) ions are reduced much more readily than Na+(aq) (actually Na+(aq) CANNOT be reduced to Na(s) in aqueous solution). Therefore, H+(aq) ions are preferentially discharged at the cathode to give hydrogen gas.
2H+(aq) + 2e- → H2(aq)

Hence, when Na2SO4 is used as the electrolyte, the product of reduction is hydrogen gas.

2009-10-06 19:19:23 補充：
Sorry.
The second paragraph should read:
"Consider a chemical cell with the anode ......."

2009-10-06 19:22:45 補充：
不是「H+ 會self ionize to carry reduction」。而是：

H^+ ions are formed in the self ionization of water; and
H^+ ions undergo reduction at the cathode.

2009-10-06 19:26:37 補充：
Sorry again. The second paragraph should be:

Consider a chemical cell with the cathode dipped in the electrolyte of Na2SO4(aq). The anions in the half cell include Na^+(aq) ions from the dissociation of Na2SO4 and H^+(aq) ions from the self-ionization of H2O(l).

2009-10-06 19:26:44 補充：
H+(aq) ions are reduced much more readily than Na+(aq) (actually Na+(aq) CANNOT be reduced to Na(s) in aqueous solution). Therefore, H^+(aq) ions are reduced at the cathode to give hydrogen gas.
2H^+(aq) + 2e^- → H2(aq)