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.