• SLC Science
  • Physics
    • Gravitational force
      • Law of gravitation
      • Acceleration(g)
      • Free fall
      • Numerical
    • Pressure
      • Def.Pressure
      • Principles
      • Upthrust
      • Numerical
    • Heat
      • Heat Energy
      • Anomalous expansion of water
      • Specific heat capacity
      • Numerical
    • Electricity
      • Electricity
      • Fuse
      • Faraday'sPrinciple
      • Transformer
      • Numerical
  • Chemistry
  • Biologi
• SLC Maths
• slc.Opt.Math
• +2Science
  • Wave Motion

---

Chemical Effect of Current :
Theory of Electrolysis :
Faraday's Law of Electrolysis:

Chemical Effect of Current (Short Question and Answer) click here

Introduction

When an electric current is passed through solid metallic conductors, it gets heated up as well as it produces a magnetic field in the surrounding medium. Thus, solid conductors show both the heating and magnetic effect of current. On the other hand, when current is passed through liquid, different liquids behave differently. Some liquids like distilled water, vegetable oil do not allow current to pass through them and are called insulators. Liquids like molten metal, mercury allow the passage of current without dissociating into ions. There are some liquids like NaCl solutions, AgNO₃ solution, CuSO₄ solution etc. that allow the current to pass through them by dissociating into ions. Such liquids are called electrolystes. This phenomenon of it, is called chemical effect of current. This effect was studied by M Faraday in 1833.

Some Important Terms

Electrolysis : It is the process of decomposition of an electrolyte into its constituent ions by the passage of electric current through it.
Voltameter: The vessel in which electrolysis is carried out is called voltameter. It is also called electrolytic cell.
Electrodes: The metal rods or plates which are partially dipped in the electrolyte solution for passing the current through it are called electrodes.
Anode : The electrode connected to the positive terminal of an external battery is called anode.
Cathode : The electrode connected to the negative terminal of an external battery is called cathode.
Ionisation: The process of decomposition of a compound into its constituent ions is called ionisation
Anions: The negatively charged ions which move towards the anode during electrolysis are called anions.
Cations: The ions which carry positive charge and move towards cathode during electrolysis are called cations.
Chemical equivalent: It is the ratio of atomic mass and valency of a substance,
                  chemical equivalent = $\frac{\mathrm{atomic\; mass}}{\mathrm{valency}}$

Theory of electrolysis

Arrhenius in 1887 put forward an ionic dissociation theory to explain the process of electrolysis. According to this theory, the molecules of an electrolyte exist anelectolyte exist in the form of ions even if the electrolyte is in the solid state. For example a common salt molecule (NaCl) exists in the form of ions even in crystalline state as NaCl → Na⁺ + Cl^-. These ions are held together by the electronstatic force of attraction.
When NaCl is dissolved in water, the force between them is lesser (1/80 times) and hence they become nearly free from each other's attraction and therefore they get separated. when a potential difference is applied across the electrolyte, the cations, (Na⁺) move towards cathode and anion (Cl^-) move towards anode. One reaching to the respective electrodes, the ions get discharged (becomes neutral) and them appear as deposits on the electodes or get liberated as free gas. This expalins the conduction of electric current through an electrolyte.
The electrolytes conduct electricity, but their conductivity is very low (about 10^-6 times that of good conductor) because of the following reasons:
(a) Ions have very large mass compared to electons. So they drift slowly under an electric field.
(b) Number of ions per unit volume in an electrolyte is much less than the number of free electrons in a metallic conductor.
(c) Electrolyte solution is dense and disordered and therefore ions have difficulty to drift through it.

Faraday's Laws of Electrolysis

Michael Faraday performed a number of experiments of electolysis and summarized his conclusions in the following two laws, known as the Faraday's laws of electrolysis.

First Law

The mass of a substance liberated or deposited on an electrode during electrolysis is directly proportional to the quantity of electric charge passed through the electolyte.
If m is the mass of the substance deposited on the cathode when an electric charge Q is passed through the electolyte, then
                m α Q
                m = Z Q
                m = Z I t
where Z is a constant of proportionality called electrochemical equivalent of the substance.

Second Law

If the same amount of electric charge is passed through different electrolytes, the masses of the substances liberated or deposited are proportional to their chemical equivalents.
Let m₁, m₂ and m₃ be the masses of the substances liberated in three different voltameters when the same electric current is passed through them and E₁, E₂ and E₃ be their respective chemical equivalents. Then
                $\frac{\mathrm{m₁}}{\mathrm{E₁}}=\frac{\mathrm{m₂}}{\mathrm{E₂}}=\frac{\mathrm{m₃}}{\mathrm{E₃}}$
                or $\frac{m}{E}$ = constant.

Electrochemical Equivalent and its Experimental Determination

From Faraday's first law of electrolysis,
                m = Z Q
If Q = 1 coulomb, then Z = m
Hence electrochemical equivalent (z) of a substance is defined as the mass of the substance liberated or deposited in electrolysis by the passage of 1 coulomb of charge. Its SI unit is kg/C. Its value for copper is 3.29 × 10 ^{− 7} kg/C, for silver 1.18 × 10 ^{− 6} kg/C, for hydrogen 1.05 × 10^{− 11} kg/C.

Relation between Electrochemical Equivalent Z and Chemical Equivalent (E) of a substance

Let m₁ and m₂ be the mass of the substance liberated when the same quantity of charge Q is passed. If Z₁ and Z₂ be their respective electrochemical equivalents, then from Faraday's first law of electrolysis,
          $\frac{\mathrm{m₁}}{\mathrm{m₂}}=\frac{\mathrm{Z₁}}{\mathrm{Z₂}}$ .............(i)
If E₁ and E₂ be their respective chemical equivalents, then from Faraday's second law,
          $\frac{\mathrm{m₁}}{\mathrm{E₁}}=\frac{\mathrm{m₂}}{\mathrm{E₂}}$
          or, $\frac{\mathrm{m₁}}{\mathrm{m₂}}=\frac{\mathrm{E₁}}{\mathrm{E₂}}$ .............(ii)
From (i) and (ii), we get
          $\frac{\mathrm{E₁}}{\mathrm{E₂}}=\frac{\mathrm{Z₁}}{\mathrm{Z₂}}$ .............(iii)
          Therefore, E α Z
Hence the electrochemical equivalent of an element is directly proportional to its chemical equivalent
          or, E = F Z ..................(iv)
where F is constant and is called Faraday's constant.
This is the relationship between Z and E.

Faraday's Constant

Faraday's constant is defined as the ratio between the chemical equivalent of a substance to its electrochemical equivalent.

Important Practical Application of Electrolysis

Following are some important application of electrolysis
I. Electroplating : This is the process of coating of one metal (Gold, Copper, silver etc.) over another metal by electrolysis. The particles os cheap metals can be coated with precious metals to make them more attractive or to prevent corrosion.
2. Purification of metals : This method is used in the refining of metals like copper, zinc, tin etc. The anode is made of impure metal and the cathode is made of pure metal. The electrolyte used is soluble salt of pute metal. On passing the current, the pure metal is deposited on the cathode.
3. Manufacture of chemicals: By electrolysis chemical, like sodium hydroxide, sodium chloride, pure sodium etc. can be manufactured.
4. Chemical analysis: The chemical composition of salts can be found out by electrolysis.
5. Medical application: Electrolysis is used for stimulating nerves especially fr treating polio and for removal of unwanted hair from any part of body.
6. Printing industry: The metal copies of types used in printing books, gramophones, records, block etc. can be made by using the process of electrolysis.
7. Production of gases for commercial use: Oxygen and hydrogen are obtained by the electrolysis of acidulated water, on commercial scales.
8. Electrolysis capacitors: are prepared by depositing a thin film of aluminium oxide and aluminium anode during electrolysis which acts as dielectric between the two electrodes. The electrolysis is a mixture of boric acid, glycerin and ammonium hydroxide due to oxide deposit, capacitors has very large capacitance.