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Dry cells

Dry cells. Zinc. Simple chemical cell. Simple chemical cell. Overall equation (Redox reaction): Zn(s) + CuSO 4 (aq)  ZnSO 4 (aq) + Cu(s) Ionic equation: Zn(s) + Cu 2+ (aq)  Zn 2+ (aq) + Cu(s) Half equations: Zn (s)  Zn 2+ (aq) + 2e - Oxidation

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Dry cells

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  1. Dry cells

  2. Zinc Simple chemical cell

  3. Simple chemical cell • Overall equation (Redox reaction): Zn(s) + CuSO4(aq)  ZnSO4(aq) + Cu(s) • Ionic equation: Zn(s) + Cu2+(aq)  Zn2+(aq) + Cu(s) • Half equations: Zn(s)  Zn2+(aq) + 2e- Oxidation Cu2+(aq) + 2e-  Cu(s) Reduction

  4. Daniell cell

  5. Daniell cell • Anode (oxidation / -ve electrode): Zn(s)  Zn2+(aq) + 2e- • Cathode (reduction / +ve electrode): Cu2+(aq) + 2e-  Cu(s) • Overall cell equation (Redox reaction): Zn(s) + Cu2+(aq)  Zn2+(aq) + Cu(s)

  6. Daniell cell • Disadvantages • Liquid electrolyte may spill out  inconvenient to be used • Require porous pot for separation of two different electrolytes • Low voltage ~ 1.1V

  7. Dry cell • Use a paste of electrolyte instead of aqueous electrolyte. • Zinc-carbon cell • Alkaline manganese cell • Silver oxide cell / Button cell • Nickel-cadmium cell (rechargeable)

  8. Zinc-carbon cell

  9. Zinc-carbon cell • Anode (oxidation / -ve electrode): zinc • Cathode (reduction / +ve electrode): carbon • Electrolyte: moist paste of ammonium chloride • Oxidizing agent: manganese(IV) oxide • Additive: carbon powder is added to increase the conductivity

  10. Zinc-carbon cell • Anode (oxidation / -ve electrode): Zn(s)  Zn2+(aq) + 2e- • Cathode (reduction / +ve electrode): 2NH4+(aq) + 2e– 2NH3(aq) + H2(g) • Hydrogen accumulates at electrode and decreases the current of the cell. This problem is solved by manganese(IV) oxide, an oxidizing agent that removes the hydrogen. 2MnO2(s) + H2(g)  Mn2O3(s) + H2O()

  11. Zinc-carbon cell • Ammonia is taken up by zinc ions. Zn2+(aq) + 2NH3(aq) + 2Cl–(aq)  Zn(NH3)2Cl2(s) • Overall cell equation: 2MnO2(s) + 2NH4Cl(aq) + Zn(s)  Zn(NH3)2Cl2(s) + H2O() + Mn2O3(s) • The overall voltage of this cell is 1.5 volts.

  12. Zinc-carbon cell • Disadvantages • If current is drawn from the cell rapidly, the gaseous product cannot be removed fast enough. The voltage drops as a result. It is restored after standing. • The lifetime of the cell is relatively short. There is a slow direct reaction between the zinc electrode and ammonium ions. After some time, the zinc case becomes thinner and the paste leaks out. The leakage problem can be solved by enclosing the whole cell in a steel or plastic case.

  13. Alkaline manganese cell

  14. Alkaline manganese cell • Anode (oxidation / -ve electrode): zinc powder • Cathode (reduction / +ve electrode): manganese(IV) oxide • Electrolyte: potassium hydroxide

  15. Alkaline manganese cell • Anode (oxidation / -ve electrode): Zn(s) + 2OH–(aq)  ZnO(s) + H2O(l) + 2e– • Cathode (reduction / +ve electrode): 2MnO2(s) + H2O(l) + 2e– Mn2O3(s) + 2OH–(aq) • The overall cell reaction is: Zn(s) + 2MnO2(s)  ZnO(s) + Mn2O3(s)

  16. Alkaline manganese cell • Its lifetime is longer than that of a zinc-carbon cell. • The outer steel case is not involved in the reaction. Therefore, this cell does not leak. • It is used when larger currents are needed, for example in motorized toys. • It is much more expensive than a zinc-carbon cell. • It gives 1.5 V.

  17. Silver oxide cell

  18. Silver oxide cell • Anode (oxidation / -ve electrode): zinc powder • Cathode (reduction / +ve electrode): silver oxide • Electrolyte: potassium hydroxide

  19. Silver oxide cell • Anode (oxidation / -ve electrode): Zn(s) + 2OH–(aq)  ZnO(s) + H2O(l) + 2e– • Cathode (reduction / +ve electrode): Ag2O(s) + H2O(l) + 2e– 2Ag(s) + 2OH–(aq) The overall cell reaction is: Zn(s) + Ag2O(s)  ZnO(s) + 2Ag(s)

  20. Silver oxide cell • The silver oxide cell is small. • It lasts for a long time. • It also gives a steady current. • It is more expensive than other types of dry cell. • It gives 1.5 V.

  21. Nickel-cadmium cell (Ni-Cd) • Anode (oxidation / -ve electrode): cadmium • Cathode (reduction / +ve electrode): nickel(IV) oxide • Electrolyte: potassium hydroxide

  22. Nickel-cadmium cell (Ni-Cd) • It is classified as a secondary cell (rechargeable cell). • It gives a larger current. • It is more expensive. • It gives 1.25 volts. • When the cell is recharged, an electric current is passed through it in the direction opposite to that of the cell reaction.

  23. Pollution problems • Cells contain toxic materials. • mercury in zinc-carbon cell • cadmium in nickel-cadmium cell • Materials inside the cells do not decompose even after a long time.

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