There are large number of chemical reactions which are initiated by heat energy. Heat provides energy for a chemical reaction to occur. Electricity is also another factor for particular chemical reaction to occur. Such reactions always occur when particular reacting substance is in a liquid state i.e. molten form or in the aqueous solution form. During such a state they get ionized into their constituent ions. Different substances behave differently with electricity. Some of them pass electricity through them and some do not and other get decomposed while passing electricity through them.
1. Conductors: Those substances that conduct electricity through them.
Eg: metals, alloy aqueous solution of salt, acid or base.
2. Insulators: Those substance that do not conduct electricity through them.
Eg: glass, plastic, etc.
3. Non- electrolyte: The liquid i.e. solution or molten compound that neither passes electricity nor undergoes any chemical change is called non- electrolyte.
4. Electrolyte: The liquid substance that conducts electricity and undergoes chemical change on passing electricity through it is called electrolyte.
Eg: aqueous solution of acid, base or salt or their molten state. An electrolyte is thus a compound in aqueous solution or molten compound which conducts electric current with decomposition at electrodes.
5. Electrodes: The two poles of carbon or metal dipped in electrolyte through which either electricity enters or leaves the electrolyte is called electrodes.
6. Anode: The positive electrode by which the electrons leave the electrolyte is called anode.
It is connected with the positive terminal of battery.
7. Cathode: The negative electrode by which the electron enter the electrolyte is called cathode.
It is connected with the negative terminal of battery.
In terms of conductivity the substance may be classified as follows:
Ions and Ionic Theory
Electrolytes contain electrically charged particles called ions. Ions are derived from atom or groups of atom and they differ in possessing charge. These charges are positive for the ions derived from metals, ammonium (NH4+) or hydrogen and negative for ions derived from non-metals or acid radicals. The number of charges carried by an ion is equal to the valency of the corresponding atom or group of atoms.
Ions are electrically charged particles with either positive or negative charges. Ions can be categorized into two types on the basis of the types of charge they have, they are :
1. Cations The positively charged ions are called cations. They are also called basic ions.
For eg: Na+, K+, Ca+ +, Mg++, Cu++ etc.
2. Anion The negatively charged ions are called anion. They are also called acidic ions.
For eg: SO4--, Cl-, ClO3-, PO4---, NO3- CH3COO- etc.
Some examples of ionization in aqueous solution is given below:
H2SO4 ionized as 2H+ + SO4- -
NaCl ionized as Na+ + C -
NaOH ionized as Na+ + OH-
CuSO 4 ionized as Cu++ + SO4- -
HCL ionized as H+ + Cl-
CaCl2 ionized as Ca++ + 2Cl-
KI ionized as K+ + I-
The phenomenon of electrolysis in terms of ionic theory was first put forward by Arrhenius called Arrhenius theory of Ionization.
According the theory when an electrolyte is dissolved in water it ionises to produce ions in solution. The charged particles obtained by dissociating a compound are called ions. These ions are free to move about the solution. And the process of dissociating a compound into ions is called ionization.
eg: NaCl dissociates as Na+ + Cl-
The two oppositely charged ions are named as cations and anions. A positively charged ion formed by removal of electrons is called cation. A negatively charged ion formed by addition of electron is called anion.
Discharging of Ions
When ions move to the electrodes, they get discharged selectively. If a number of ions are present in a solution some may discharge and other remain in the solution. If the ions get discharged at electrodes, they release the charge either by gaining or losing electrons to form neutral species. Thus, in order to discharge by cation, it has to lose electron and anion needs to gain electrons.
The discharging of ions when two or more ions of the similar charge are present in the electrolyte depends on the following factors :
1. Position of metal or radicals in electrochemical series.
2. Concentration of ions.
3. Nature of electrodes.
An electrolytic solution or the molten electrolyte contains free ions. These ion tend to move freely in solution. The electrolytes conduct electricity due to ions. These ions are attracted to the oppositely charged electrodes. Cations move towards the cathode or negative electrode and anions move towards anode or positive electrode. When electricity passes through it, the ions may change into either of the three ways at electrodes.
* Deposits on the surface of electrode
* May be released as a gas.
* May dissolve into solution as ions.
Electrolysis is carried out in an especially designed vessel or apparatus called electrolytic cell or Voltameter. The current passes through graphite rods or metal plates.
Some important questions:
1. What is ionization? How does it occur?
2. How do you obtain sodium from sodium chloride ?
3. What are ions? Write its types.
Electrolysis of water
Reaction at anode
OH- and SO4- - ions are attracted towards anode. Here OH- is discharged in preference to SO4- - as the position of OH- ion is lower in the electrochemical series i.e. the case of discharge of OH- ions is higher than SO4- -. The discharging occurs when electron is lost by it.
OH- ----> OH + e-
OH + OH ----> H2O + O
O + O ----> O2(g)
It can be summarized as,
4OH- ----> 4OH + 4e-
4OH ----> 2H2O + O2(g)
Reaction at anode
H ions formed after the dissociation of water or acid migrate to cathode where they gain electrons to form hydrogen gas.
4H+ + 4e - ----> 4H
4H ----> 2H2(g)
H+ ions on gaining electron forms nascent hydrogen which is unstable and combines further to form molecular hydrogen.
Thus, in the electrolysis of water, we can see that hydrogen gas is collected at cathode and oxygen gas at anode.
Electrolysis of copper sulphate solution:
Reaction at cathode:
Cu++(aq) and H+(aq) are available at cathode. Cu++(aq) being lower in the electrochemical series gets discharged in preference to H+.
Cu++(aq) + 2e ----> Cu(s)
Reaction at anode:
Anode gets SO4- -(aq) and OH-(aq) ions. None of these ions is discharged at anode and remains in solution. Instead of discharging of any ions copper passes into solution from anode as Cu++ ions by losing electrons.
Cu(s) - 2e ----> Cu ++(aq)
Thus copper anode gets dissolved and metallic copper is deposited on cathode. In the electrolysis of copper sulphate there is a transfer of copper from anode to cathode. The total concentration of Cu++ and SO4 - - solution remains unchanged.
Electroplating : Electroplating is the process of despositing a thin layer of one metal, usually a finer one over the surface of a baser metal by the process of electrolysis. Electroplating of a metal is done to give the impression of finer metal, to protect metal surface from corrosion and for attractive appearance.
The points to be remembered during electroplating:
1. The article to be electroplated is cleaned thoroughly with acid in an acid bath.
2. The article to be electroplated is to be connected to the negative terminal of cell i.e. it is connected as cathode electrode.
3. The metal with which the electroplating is done is made anode i.e. connected with the positive terminal of cell.
4. Water soluble salt of the metal kept as anode should be used as electrolyte.
5. Small but steady current should pass during the process.
Electroplating of copper
For electroplating of copper, a piece of copper sheet is used as anode and an article such as iron nail or spoon as cathode. The soluble salt such as copper sulphate is used as electrolyte. The ionization of the solution takes place as below.
After passing electricity, Cu++ (aq) migrates towards cathode where it gains electrons to form metallic copper and gets deposited on the surface of irons.
CuSO4 ----> Cu++(aq) + SO4- -(aq)
It makes the anode to be thin and cathode to be plated with copper.
Copper gets dissolved to form Cu2+(aq) ions.
Cu ----> Cu++ + 2e-(aq)
Metallic copper is deposited on the article to be plated.
Cu2+(aq) + 2e- ----> Cu(s)