Electrochemistry
Electrochemistry deals with the relation of the flow of electric current to chemical
change, and the conversion of electrical and chemical energy.
There are two main branches:
electrolysis
electrochemical cells
Electrolysis is a non-spontaneous reaction where electrical energy is converted
to chemical energy.
Common examples: metal plating (silver, gold, rhodium)
decomposition of water
refining of pure metals and chemicals
charging a battery
In an electrochemical cell, a spontaneous reaction occurs where chemical
energy is converted to electrical energy.
Common examples
Component
Electrodes
car battery
dry cell (acidic or alkaline)
rechargeable batteries
galvanic corrosion
Electrolytic Device
Electrochemical Device
usually a metal, but may be another conductor
such as graphite
Anode
the electrode at which oxidation takes place
has positive
charge
outside
current
forces
Cathode
the electrode at which reduction takes place
has negative charge
Electrolyte
Curcuit
has positive charge
ions dissolved in a water solution
ions in melted ionic solid
External
has any
negative
charge
includes
device
that
moist paste
electrons flow anode to cathode
not spontaneous Rx’n
spontaneous Rx’n
Electrolysis - is a process by which an external electric current causes a redox
reaction to occur in a water solution of an electrolyte or in a pure electrolyte in the
liquid phase. The reaction will not occur without the external circuit.
There are 3 essential parts to an electrolytic device:
- a source of direct current
- 2 electrodes, the cathode (negative) and anode (positive)
- an electrolyte
The negative anions in the electrolyte are attracted to the anode. And the positive
cations are attracted to the cathode.
Electrons are lost at the anode (LEO) and gained at the cathode (GER).
Electroplating
In the process of electroplating (silver), the object to be plated (a spoon) is set up
as the cathode. This is so the negative charge of the electrode will attract the
positive metal (silver) ions in solution.
The electrolyte must be a soluble salt of the metal (silver nitrate), and the anode
must be a solid strip of the metal being plated. The strip’s function is to replace
the positive ions in solution that have “plated out”.
-
+
Rx’n at the anode (strip) :
Ag(s) ➩ Ag+ + e-
Ag+
NO3-
Power
Source
+
+
+
-
Rx’n at the cathode (spoon):
Ag+ + e- ➩ Ag(s)
The silver ions formed at the anode replace those from the electrolyte that are
plated on the spoon. Thus the anode loses mass and the cathode gains in mass.
Decomposition of Water
An electric current is applied to water which decomposes it into the elements
hydrogen and oxygen.
Rx’n at the anode (LEO)
Rx’n at the cathode (GER)
2H2O ➩ O2 + 4H+ + 4e- (acidic)
4H2O + 4e- ➩ 2H2 + 4OH- (basic)
6 H2O ➩ 2H2 + O2 + (4H+ + 4OH-)
2H2O ➩ 2H2 + O2
Electrolysis of Molten Sodium Chloride
An electric current is passed through molten (liquid) sodium chloride. The result
is pure sodium metal and pure chlorine gas.
Rx’n at the anode (LEO)
Rx’n at the cathode (GER)
1(2Cl- ➩ Cl2(g) + 2e-)
2(Na+ + e- ➩ Na(s))
2NaCl ➩ 2Na(s) + Cl2(g)
Any pure metal can be obtained from its soluble salt using a similar process.
Na+ + e- ➩ Na(s)
Ni2+ + 2e- ➩ Ni(s)
Al3+ + 3e- ➩ Al(s)
Notice that each metal must gain a different number of electrons. The correct
amount of electricity must be calculated and carefully supplied to have maximum
effectiveness.