5.5 CHAPTER 5. THE CHEMICAL INDUSTRY
The manganese(IV) oxide in the cell removes thehydrogen produced above, according
to the following reaction:
2MnO 2 (s) + H 2 (g)→ Mn 2 O 3 (s) + H 2 O(l)
The combined result ofthese two reactions canbe represented by the following half
reaction, which takes place at the cathode:
Cathode: 2NH+ 4 (aq) + 2MnO 2 (s) + 2e−→ Mn 2 O 3 (s) + 2NH 3 (g) + H 2 O(l)
The anode half reactionis as follows:
Anode: Zn(s)→ Zn2++ 2e−
The overall equation forthe cell is:
Zn(s) + 2MnO 2 (s) + 2NH+ 4 → Mn 2 O 3 (s) + H 2 O + Zn(NH 3 )2+ 2 (aq) (E^0 = 1.5 V)Alkaline batteries are almost the same aszinc-carbon batteries, except that the elec-
trolyte is potassium hydroxide (KOH), rather than ammonium chloride. The two half
reactions in an alkalinebattery are as follows:
Anode: Zn(s) + 2OH−(aq)→ Zn(OH) 2 (s) + 2e−
Cathode: 2MnO 2 (s) + H 2 O(l) + 2e−→ Mn 2 O 3 (s) + 2OH−(aq)
Zinc-carbon and alkaline batteries are cheap primary batteries and aretherefore very
useful in appliances such as remote controls, torches and radios where the power drain
is not too high. The disadvantages are that these batteries can’t be recycled and can
leak. They also have ashort shelf life. Alkalinebatteries last longer thanzinc-carbon
batteries.
Environmental considerations ESCCV
While batteries are veryconvenient to use, theycan cause a lot of damage to the en-
vironment. They use lots of valuable resourcesas well as some potentially hazardous
chemicals such as lead,mercury and cadmium.Attempts are now beingmade to recy-
cle the different parts ofbatteries so that they arenot disposed of in the environment,
where they could get into water supplies, riversand other ecosystems.
Exercise 5 - 7
A dry cell does not contain a liquid electrolyte.The electrolyte in a typical zinc-carbon
cell is a moist paste of ammonium chloride andzinc chloride.