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| Chemical Effects of Current (Contd…) |
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| Michael Faraday |
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| On the basis of various experiments conducted, Faraday arrived at two laws of electrolysis, which are as follows: |
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| 1. The mass of a substance liberated or deposited at an electrode during electrolysis is directly proportional to the quantity of charge passed through the electrolyte. i.e., |
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| m µ q |
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| i.e. m µ It |
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| or m = ZIt |
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| where m is the mass of the substance, q is the charge (= It, a current I flowing for time t) passing through the electrolyte. |
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| Z is known as the electrochemical equivalent (E.C.E) of the substance. |
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| If q = 1C, then m = Z. |
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| Hence, electrochemical equivalent of a substance is the mass of the substance liberated or deposited in electrolysis by the passage of 1 coulomb of charge. The S.I. Unit of Z is kg/coulomb (kg C-1). |
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| 2. When the same amount of charge is made to pass through any number of electrolytes, the masses of the substances liberated or deposited at the electrodes are proportional to their chemical equivalents. |
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| i.e., if m1 and m2 are the masses and E1 and E2 are the chemical equivalents of the two substances, then |
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| Faraday's laws can be explained as follows. Suppose one mole of a substance of mass M and valency p is deposited on an electrode, i.e., an Avogadro number NA of atoms gets deposited. To deposit one atom, a charge pe is required. |
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| Therefore, for a mole (of mass M), the charge required is NA Pe. Then, from the equation m = Zq, we have |
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| M = Z NA pe |
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| The quantity M/p is a constant for a
substance and is known as the chemical equivalent E. The E.C.E (Z) of a substance is thus directly proportional to its chemical equivalent (E); this is evident also from the following table. |
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| Electrochemical and Chemical Equivalents |
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| where F(=NAe) is a fundamental constant and is called Faraday's constant. Its value is 96487 C mol-1. |
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| Faraday's constant is equal to the amount of charge required to liberate the mass of a substance during electrolysis equal to its chemical equivalent (in gm). |
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| Equation (1) is the combined form of Faraday's laws of electrolysis. |
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| For a given electrolyte, |
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Faraday's laws have a lot of significance. They imply that to liberate
one atom of a substance, the charge required is  That
is, the charge per ion of any substance is  Two
important results follow from this: |
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| 1. The chemical concept of valency is related to electric charge. |
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| 2. Since p is an integer, all charges are multiples of an elementary charge |
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| The quantitative significance is that the value of e can be calculated by using values of F from electrolysis experiments. The value of e comes out to be ~1.6 x 10-19 C. |
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| The phenomenon of electrolysis has many scientific and commercial applications. |
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| Electroplating |
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| Electroplating of objects by nickel, silver and gold is very common. The conducting material to be electroplated is made the cathode of an electrolytic cell. A strip of metal whose coating is required on the cathode material is used as the anode, while a soluble salt of the same anode material is taken as the electrolyte. Below figure shows an experimental set-up used for electroplating. When the current is passed through the circuit, a thin film of the metal deposits on the cathode. To make the electroplating uniform and firmly adherent, a suitable current strength is used. If the current strength is very high, the plating may become brittle. For gold plating, we need a current from 1V to 3V batteries; and for copper, current is drawn from a battery of 5V to 10V. |
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| Extraction of metals from ores |
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| Certain metals are extracted from their ores using electrolysis. For example, aluminium is obtained by passing an electric current through fused bauxite (Al2O3) and cryolite (Na3AlF6). Active metals like sodium, calcium and magnesium are also extracted from their ores using electrolysis. |
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| Purification of Metals |
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| For this purpose, the impure metal is made the anode, and a pure metallic strip is used as cathode. A soluble salt of pure metal is taken as the electrolyte. On passing current, the impure metal anode dissolves but only the pure metal deposits on the cathode. Many metals like copper are purified up to 99.99% using electrolysis. |
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| Electrolytic Capacitors |
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| These capacitors consist of two aluminium electrodes placed in an electrolytic mixture of ammonium borate for sodium phosphate in glycerine. When a steady current is passed, a thin layer of dielectric aluminium oxide (or hydroxide) is formed on the anode. Such very thin films can offer large values of capacitance. Modern capacitors use electrolytes in the form of a paste or a solution soaked in paper placed between two aluminium foils. If the potential across the two electrodes becomes excessively high, this dielectric layer breaks down and temporarily ceases to function. However, it is possible to regenerate this layer and repair the damage. Such capacitors are very common in power circuits. |
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