Dry cells

Simple chemical cell Zinc

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 Cu 2+ (aq) + 2e -  Cu(s) Reduction

Daniell cell

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

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

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)

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

Zinc-carbon cell Anode (oxidation / -ve electrode): Zn(s)  Zn 2+ (aq) + 2e - Cathode (reduction / +ve electrode): 2NH 4 + (aq) + 2e –  2NH 3 (aq) + H 2 (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. 2MnO 2 (s) + H 2 (g)  Mn 2 O 3 (s) + H 2 O( )

Zinc-carbon cell Ammonia is taken up by zinc ions. Zn 2+ (aq) + 2NH 3 (aq) + 2Cl – (aq)  Zn(NH 3 ) 2 Cl 2 (s) Overall cell equation: 2MnO 2 (s) + 2NH 4 Cl(aq) + Zn(s)  Zn(NH 3 ) 2 Cl 2 (s) + H 2 O( ) + Mn 2 O 3 (s) The overall voltage of this cell is 1.5 volts.

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.

Alkaline manganese cell

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

Alkaline manganese cell Anode (oxidation / -ve electrode): Zn(s) + 2OH – (aq)  ZnO(s) + H 2 O(l) + 2e – Cathode (reduction / +ve electrode): 2MnO 2 (s) + H 2 O(l) + 2e –  Mn 2 O 3 (s) + 2OH – (aq) The overall cell reaction is: Zn(s) + 2MnO 2 (s)  ZnO(s) + Mn 2 O 3 (s)

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.

Silver oxide cell

Anode (oxidation / -ve electrode): zinc powder Cathode (reduction / +ve electrode): silver oxide Electrolyte: potassium hydroxide

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

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.

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

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.

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.