Post on 22-Dec-2015
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Dry cells
Simple chemical cell
Zinc
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
Daniell cell
Daniell cell Anode (oxidation / -ve ele
ctrode):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)
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
Zinc-carbon cell
Anode (oxidation / -ve electrode): zinc Cathode (reduction / +ve electrode): carbo
n Electrolyte: moist paste of ammonium chlo
ride Oxidizing agent: manganese(IV) oxide Additive: carbon powder is added to increa
se the conductivity
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()
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.
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
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) + 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)
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
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) + 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)
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.