Question

Why is zinc is used as battery covering?

Answer 1

A zinc–carbon battery is a dry cell primary battery that delivers about 1.5 volts of direct current from the electrochemical reaction between zinc and manganese dioxide. A carbon rod collects the current from the manganese dioxide electrode, giving the name to the cell. A dry cell is usually made of a zinc can which also serves as the anode with a negative potential, while the inert carbon rod is the positive cathode. General purpose batteries may use an aqueous paste of ammonium chloride as electrolyte, possibly mixed with some zinc chloride solution. Heavy duty types use a paste primarily composed of zinc chloride. Zinc–carbon batteries were the first commercial dry batteries, developed from the technology of the wet Leclanché cell. They made flashlights and other portable devices possible, because the battery can function in any orientation. They are still useful in low drain or intermittent use devices such as remote controls, flashlights, clocks or transistor radios. Zinc–carbon dry cells are single-use primary cells. electrode. Zinc chloride cells (usually marketed as "heavy duty" batteries) use a paste primarily composed of zinc chloride, which gives a longer life and steadier voltage output compared with ammonium chloride electrolyte. Manufacturers must now use more highly purified zinc to prevent local action and self-discharge. In a zinc–carbon dry cell, the outer zinc container is the negatively charged terminal. The zinc is oxidised according to the following half reactions: Anode (oxidation reaction, marked -) Zn(s) → Zn2+(aq) + 2 e− [E° = −0.7626 V] Cathode (reduction reaction, marked +) 2 MnO2(s) + 2 e− + 2 NH4Cl(aq) → Mn2O3(s) + 2 NH3(aq) + H2O(l) + 2 Cl−(aq) [E° ≈ +0.5 V] Other side-reactions are possible, but the overall reaction in a zinc–carbon cell can be represented as: Zn(s) + 2 MnO2(s) + 2 NH4Cl(aq) → Mn2O3(s) + ZnCl2 (aq) + 2NH3(aq) {or Zn(NH3)2Cl2(s)} + H2O(l) If zinc chloride is substituted for ammonium chloride (i.e. replaces ammonium chloride) as the primary electrolyte, the anode reaction remains the same but the cathode reaction is: MnO2(s) + H2O(l) + e− → MnO(OH)(s) + OH−(aq) and the overall reaction: 4 Zn(s) + 8 MnO2(s) + ZnCl2(aq) + 9 H2O(l) → 8 MnO(OH)(s) + Zn(OH)Cl(aq) + 4 H2O(l) +4 ZnO(s) + HCl(aq) The battery has an electromotive force (e.m.f.) of about 1.5 V. The approximate nature of the e.m.f is related to the complexity of the cathode reaction. The anode (zinc) reaction is comparatively simple with a known potential. Side reactions and depletion of the active chemicals increases the internal resistance of the battery, which causes the e.m.f. to drop under load. Zinc chloride cell The zinc chloride cell is an improvement on the original zinc–carbon cell, using purer chemicals and giving a longer service life and steadier voltage output as it is used. These cells are usually marketed as heavy-duty, extra-heavy-duty, or even super-heavy-duty batteries, and offer about twice the service life of general purpose zinc–carbon cells, or up to four times in continuous-use or high-drain applications.[1] Alkaline batteries[6] offer up to eight times the battery life of zinc chloride batteries,[7] especially in continuous-use or high-drain applications.[1]Manufacturers recommend storage of zinc–carbon batteries at room temperature; storage at higher temperatures reduces the expected service life.[8] While batteries may be frozen without damage, manufacturers recommend that they be returned to normal room temperature before use, and that condensation on the battery jacket must be avoided. By the end of the 20th century, the storage life of zinc–carbon cells had improved fourfold over expected life in 1910.[1] Zinc carbon cells have a short shelf life as the zinc is attacked by ammonium chloride. The zinc container becomes thinner as the cell is used, because zinc metal is oxidized to zinc ions. When the zinc case thins enough, zinc chloride begins to leak out of the battery. The old dry cell is not leak proof and becomes very sticky as the paste leaks through the holes in the zinc case. The zinc casing in the dry cell gets thinner even when the cell is not being used, because the ammonium chloride inside the battery reacts with the zinc. An "inside-out" form with a carbon cup and zinc vanes on the interior, while more leak resistant, has not been made since the 1960s.[1] This picture shows the zinc container of fresh batteries at (a), and discharged batteries at (b) and (c). The battery shown at (c) had a polyethylene protection film (mostly removed in the photo) to keep the zinc oxide inside the casing.