Electrolysis
The phenomenin of decomposition of an electrolyte by passing electric current through its solution is termed electrolysis (lyo= breaking).
Mechanism of electrolysis
The cations migrate to the cathode and form a neutral atom by accepting electrons from it. The anions migrate to the anode and yield a neutral particle by transfer of electrons to it. As a result of the loss of electrons by anions and gain of electrons by cations at their respective electrodes chemical reactions takes place.
For example, in solution, HCl is ionized,
HCl = H+ + Cl-
At anode: Cl- = Cl + e-
At cathode: H+ + e- = H
Electrical units
Coulomb
A coulomb (unit quantity of electricity) is the amount of electricity which will deposit 0.001118g of silver from a 15% solution of silver nitrate in a coulometer.
Ampere
An ampere (unit rate of flow of electricity) is that current which will deposit 0.001118g of silver in one second.
Ampere = current of 1C/sec.
Ohm
An ohm (unit of electrical resistance) is the resistance offered at 0oC to current by a column of mercury 106.3 cm long of about 1 sq mm cross-sectional area and weighing 14.4521grams.
Volt
A volt (unit of electromotive force) is the difference in electrical potential required to send a current of one ampere through a resistance of one ohm.
Faraday’s 1st law
The amount of a given product liberated at an electrode during electrolysis is directly proportional to the quantity of electricity which passes through the electrolyte solution.
If w is the mass of substance (in gram) deposited on electrode by passing Q coulombs of electricity then,
w ∞ Q
We know, Q = It; I= current in ampere, and t= time in second
Therefore, w ∞ It
Or, w= ZIt; Z= constant known as the electrochemical equivalent
Electrochemical equivalent
From the 1st law, we get, w= ZIt;
Now if I=1amp, t= 1sec, then, w= Z
Thus, the electrochemical equivalent is the amount of a substance deposited by 1 ampere current passing for 1 second (i.e., 1 coulomb).
Faraday (the electrical unit)
It has been found experimentally that the quantity of electricity required to liberate one gram-equivalent of a substance is 96,500 coulombs. This quantity of electricity is known as Faraday and is denoted by F.
How can we calculate the charge of an electron by Faraday’s 1st law?
The quantity of electricity needed to deposit 1 mole of the substance is given by the expression, Quantity of electricity = n X F; n= valency of its ion
Let, number of ions in 1 mole ion = N
Again, if the value charge of every ion of 1 valency becomes equal with the charge of an electron ‘e’ then, the charge conducted by N ions= Ne.
So, Ne= n X F coulomb
=F coulomb; (for 1 valency radical n=1)
Or, e = F/N C
=96500/ (6.023X1023) C
=1.6 X 10-19 C
| 1 Faraday= 96500 C= 1 mole electrons |
Application/ importance of 1st law
With the help of the first law of electrolysis we are able to calculate:
1. the value of electrochemical equivalents of different substances; and
2. the masses of different substances produced by passing a known quantity of electricity through their solutions
Faraday’s 2nd law
When the same quantity of electricity passes through solutions of different electrolytes, the amounts of the substances liberated at the electrodes are directly proportional to their chemical equivalents.
Thus, w ∞ E; w= mass of deposited substance
E= chemical equivalent
Chemical equivalent
The chemical equivalent is the amount of a substance (in gram) deposited at an electrode by 1.0F current passing through the electrolyte solution. It is denoted by E.
Application/ importance of 2nd law
The 2nd law of electrolysis helps to calculate:
1. the equivalent weights of metals
2. the unit of electric charge
3. the Avogadro’s number
Electrolyte
Electrolytes are electrovalent substances that form ions in solution which conduct an electric current.
Electrolytes may be divided into two classes:
1. strong electrolytes
2. weak electrolytes
Strong electrolytes
A strong electrolyte is a substance that gives a solution in which almost all the molecules are ionized.
Example- HCl, H2SO4, HNO3, HClO4, HBr , HI
Weak electrolytes
A strong electrolyte is a substance that gives a solution in which only a small portion of the solute molecules are ionized.
Example- acetic acid, oxalic acid, sulphurous acid (H2SO3); alkyl amines; mercury (ll) chloride, lead (ll) chloride.
Conductance
The power of electrolytes to conduct electric currents is known as conductivity or conductance.
Like metallic conductors, electrolytes obey Ohm’s law. According to this law, the current I flowing through a metallic conductor is given by the relation,
I= E/R; E= potential difference at two ends
R= resistance
Again we know that, R∞ l/A; l= length, and A= cross-sectional area
Or, R = ρ X l/A; ρ = constant of proportionality and is called resistivity or specific resistance.
Specific resistance/ conductance
We know, ρ = R X A/l
If l= 1 cm and A= 1 sq cm, then ρ= R
Thus, specific resistance of a conductor is the resistance in ohms which one centimeter cube of it offers to the passage of electricity.
The reciprocal of specific resistance is termed as specific conductance or specific conductivity. It is the conductance of one cm cube of a soln of an electrolyte. It is denoted by κ.
κ = 1/ρ
Its unit is ohm-1cm-1
Equivalent conductance
It is defined as the conductance of an electrolyte obtained by dissolving one gram-equivalent of it in V cc (the volume of a solution) of water. It is denoted by Λ. Its unit is ohm-1cm2eqvt-1.
Molar conductance
It is the ratio of the conductivity of an electrolytic solution to the concentration of electrolyte in moles per unit volume. It is denoted by µ. Mathematically,
µ = κ X V
Its unit is ohm-1cm2mol-1.
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