Page 225 - Theory and Problems of BEGINNING CHEMISTRY

214 OXIDATION AND REDUCTION [CHAP. 14

Ans. HNO 3 + Mn −→ NH 4 NO 3 + Mn(NO 3 ) 2
The manganese goes up two oxidation numbers, and some of the nitrogen atoms are reduced from +5to −3:

(0 −→ +2) =+2
| |
HNO 3 + Mn −→ NH 4 NO 3 + Mn(NO 3 ) 2
| |
(+5 −→ −3) =−8
It takes four Mn atoms per N atom reduced:

HNO 3 + 4Mn −→ 1NH 4 NO 3 + 4Mn(NO 3 ) 2
There are other N atoms that were not reduced, but still are present in the nitrate ions. Additional HNO 3 is
needed to provide for these.

10 HNO 3 + 4Mn −→ NH 4 NO 3 + 4Mn(NO 3 ) 2
Water is also produced, needed to balance both the H and O atoms:
10 HNO 3 + 4Mn −→ NH 4 NO 3 + 4Mn(NO 3 ) 2 + 3H 2 O
The equation is now balanced, having 10 H atoms, 10 N atoms, 30 O atoms, and 4 Mn atoms on each side.

ELECTROCHEMISTRY
14.19. Even if sodium metal were produced by the electrolysis of aqueous NaCl, what would happen to the
sodium produced in the water?
Ans. The sodium is so active that it would react immediately with the water. No elementary sodium should ever
be expected from a water solution.

14.20. Explain why Al cannot be produced from its salts in aqueous solution.
Ans. Al is too active; the water will be reduced instead.

14.21. Can a Daniell cell be recharged?
Ans. No. If the Daniell cell were to be recharged, the Cu 2+ ions would get into the zinc half-cell through the salt
bridge. There, they would react directly with the zinc electrode, and the cell would be destroyed.

14.22. Explain why a Daniell cell cannot be placed in a single container, like the lead storage cell.

Ans. The copper(II) ions would migrate to the zinc electrode and be reduced to copper metal directly. The zinc
electrode would become copper-plated, and the cell would not function.

14.23. Write a balanced chemical equation for (a) the direct reaction of zinc with copper(II) sulfate and (b) the
overall reaction in a Daniell cell containing ZnSO 4 and CuSO 4 .
Ans. (a) and (b) Zn + CuSO 4 −→ ZnSO 4 + Cu

14.24. The electrolysis of brine (concentrated NaCl solution) produces hydrogen at the cathode and chlorine at
the anode. Write a net ionic equation for each half-reaction and the total reaction. What other chemical
is produced in this commercially important process?
Ans. 2H 2 O + 2 e −→ H 2 + 2OH −
−
2Cl −→ Cl 2 + 2 e −
−
−
2Cl + 2H 2 O −→ Cl 2 + H 2 + 2OH −
The other product is NaOH.
14.25. Must two oxidation-reduction half-reactions be carried out (a) in the same location and (b) at the same
time?

Ans. (a) No, they may be in different locations, as in electrochemical processes (b) Yes. (The electrons cannot
appear from nowhere or go nowhere.)