2.6 Batteries

. Batteries . . Primary Batteries Fig. . : A commercial dry cell consists of a graphite (carbon) cathode in a zinc container; the latter acts as the anode. . If a current of . ampere flows through a metallic wire for hours, then how many electrons would flow through the wire? . Suggest a list of metals that are extracted electrolytically. . Consider the reaction: Cr O – + 14H + + 6e – ® 2Cr + + 7H O What is the quantity of electricity in coulombs needed to reduce mol of Cr O – ? Fig. . : The Lead storage battery. The overall reaction is represented by Zn(Hg) + HgO(s) ¾® ZnO(s) + Hg(l) The cell potential is approximately . V and remains constant during its life as the overall reaction does not involve any ion in solution whose concentration can change during its life time. A secondary cell after use can be recharged by passing current through it in the opposite direction so that it can be used again. A good secondary cell can undergo a large number of discharging and charging cycles. The most important secondary cell is the lead storage battery (Fig. . ) commonly used in automobiles and invertors. It consists of a lead anode and a grid of lead packed with lead dioxide (PbO ) as cathode. A % solution of sulphuric acid is used as an electrolyte. The cell reactions when the battery is in use are given below: Anode: Pb(s) + SO – (aq) ® PbSO (s) + 2e – Cathode: PbO (s) + SO – (aq) + 4H + (aq) + 2e – ® PbSO (s) + 2H O (l) i.e., overall cell reaction consisting of cathode and anode reactions is: Pb(s) + PbO (s) + 2H SO (aq) ® 2PbSO (s) + 2H O(l ) On charging the battery the reaction is reversed and PbSO (s) on anode and cathode is converted into Pb and PbO , respectively. Fig.