Galvanic Cell

Dry cell compared to Button cell.
  The common dry cell battery is a galvanic cell that uses a zinc casing as an anode and a manganese dioxide (MnO2) and carbon (graphite) as a cathode. The carbon in the cathode does not involve the reaction, it only conducts the electricity. The electrolyte consists of a paste of ammonium chloride and zinc chloride rather than a solution. The paste prevents the cell contents from mixing and thus a salt bridge is not required. The diagram is shown as below:

Oxidation reaction at the anode ( left ):
      Zn (s) → Zn2+ (aq) + 2e-.
Reduction reaction at the cathode ( right ):
      2MnO2 (s) + 2NH4+ (aq) + 2e- → Mn2O3 (s) + H2O (l) + 2NH3 (g).
The overall equation is:
      2MnO2 (s) + 2NH4+ (aq) + Zn (s) → Mn2O3 (s) + H2O (l) + Zn(NH3)22+ (aq).
  However, in the reduction reaction, NH4+ ions only provide H+ ions needed for cathode process. In fact, only MnO2 gains the electrons. So when we calculate the potential difference, the 2 half equations are:
      MnO2 + 4H+ + e- → Mn3+ + 2H2O.     Eo = +0.95 V
      Zn (s) → Zn2+ (aq) + 2e-.             Eo = +0.76 V
So that the net ionic equation is MnO2 + 4H+ + Zn → Mn3+ + 2H2O + Zn2+ .
        Eo = 0.95 + 0.76 = 1.71 V
It is little higher than the usual data given (1.5 V) because during the whole process of producing the galvanic cell, some of the energy is lost or the conditions of the cells do   not meet the standards such as temperature errors.
  The standard dry cell is very cheap to produce, and can be made in a variety of sizes so it is very as flashlights, remote controls, portable radios, many toys, etc. It has a poor shelf life as reactions with its zinc casing slowly occur. Sometimes the battery 'leaks' when this casing has been perforated. It is also easy to transport, especially when compared to liquid cells such as lead-acid cell.
  In fact, the dry cell encouraged the development of all these devices. Small portable electrical devices would never have...