Electrochemical Cells - Also called voltaic or galvanic cells, these cells produce an
electric current as a result of a redox reaction that converts chemical energy into
electrical energy. These reactions are spontaneous.
These reactions occur because electrons flow from a higher electrical potential to a
lower one. This technology was developed using a Zn//Cu cell called a Daniell cell.
Zn(s) + Cu2+(aq) ➩
Cu(s) + Zn2+(aq)
For the energy to be useful, the two half reactions must occur in separate
compartments. The electrons are forced to migrate through an external circuit. A
porous barrier allows for ion migration to neutralize cell charge.
Rx’n at the anode (LEO)
Rx’n at the cathode (GER)
Zn(s) ➩ Zn2+(aq) + 2eCu2+(aq) + 2e- ➩ Cu(s)
The Daniell Cell was invented in 1836 by
John Fredrick Daniell, a British chemist.
See
(http://en.wikipedia.org/wiki/History_of_the
_battery) for a full history.
Lead-Storage Battery (Car Battery)
A 12 volt battery consists of 6 cells of 2 volts each. The anode is Pb(s) and the cathode
is PbO2(s). The electrolyte is H2SO4 (aq).
Pb(s) + SO42-(aq) ➩ PbSO4(s) + 2ecathode PbO2(s) + SO42-(aq) + 4H+(aq) + 2e- ➩ PbSO4(s) + 2H2O
overall Rx’n PbO2(s) + Pb(s) + 2H2SO4(aq) ➩ PbSO4(s) + 2H2O(l)
anode
Because both reactants and products are solids, this battery can be recharged.
However if the battery is charged too quickly the water can be decomposed into H2
and O2. Aside from their explosive nature, these gases dislodge the solid reactant and
product which forms a sludge at the bottom of the battery. This can cause a “short
circuit” and the battery won’t hold a charge.
Dry Cell (Acidic Form)
The cell commonly used in flashlights was invented by Leclanche in 1866.
The anode is a zinc cup. The cathode is an inert graphite rod. The electrolyte is a
paste of MnO2, NH4Cl and carbon.
anode
Zn(s) ➩ Zn2+(aq) + 2ecathode
2NH4+(aq) + 2MnO2(s) + 2e- ➩ Mn2O3(s) + 2NH3(aq) + H2O(l)
The cathode reactions are complex and seem to vary with the rate of discharge. This
cell cannot be recharged.
Dry Cell (Alkaline Form)
In an alkaline battery, used in calculators and watches the NH4Cl is replaced with
KOH. These cells are more costly, but have improved performance.
NiCad Batteries (Rechargeable)
The anode is Cd(s) and the cathode is NiO2(s). Like the lead storage battery, reactants
and products are solids so the cell can be recharged.
anode
cathode
Cd(s) + 2OH-(aq) ➩ Cd(OH)2(s) + 2eNiO2(s) + H2O(l) +2e- ➩ Ni(OH)2(s) + 2OH-(aq)