CHAPTER 5. THE CHEMICAL INDUSTRY 5.5
The paste of ammoniumchloride reacts according to the following half-reaction:
2NH+ 4 (aq) + 2e−→ 2NH 3 (g) + H 2 (g) (a)
Manganese(IV) oxide isincluded in the cell to remove the hydrogen produced during
half-reaction (a), according to the following reaction:
2MnO 2 (s) + H 2 (g)→ Mn 2 O 3 (s) + H 2 O(l) (b)
The combined result ofthese two half-reactionscan be represented by the following
half reaction:
2NH+ 4 (aq) + 2MnO 2 (s) + 2e−→ Mn 2 O 3 (s) + 2NH 3 (g) + H 2 O(l) (c)
- Explain why it is important that the hydrogenproduced in half-reaction (a) is re-
moved by the manganese(IV) oxide.
In a zinc-carbon cell,such as the one above, half-reaction (c) andthe half-
reaction that takes placein the Zn/Zn2+half-cell, produce an emf of 1,5 V under
standard conditions.
- Write down the half-reaction occurring at theanode.
- Write down the net ionic equation occurringin the zinc-carbon cell.
- Calculate the reduction potential for the cathode half-reaction.
- When in use the zinc casing of the dry cellbecomes thinner, because it is oxi-
dised. When not in use,it still corrodes. Give a reason for the latter observation.
- Dry cells are generally discarded when ’flat’. Why is the carbon rod the most
useful part of the cell, even when the cell is flat?
(DoE Exemplar Paper 2,2007)
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(1.) 01rs (2.) 024q (3.) 024r (4.) 024s (5.) 024t (6.) 024u
Chapter 5 — Summary
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