Faraday’s Laws Of Electrolysis

CONTENT

• Faraday First and Second laws of electrolysis.
• Verification of Faraday’s Laws of Electrolysis.
• Calculations based on Faraday’s Laws.

FARADAY’S FIRST LAW OF ELECTROLYSIS

Faraday’s first law states that the mass (m) of an element discharged during an electrolysis of an electrolyte is directly proportional to the quantity of electricity (Q) passing through it.

Faraday’s first law can be expresses in the following mathematical way

M α Q

But Q = It

            M α It

            M =EIt   or                   M = EQ

Where E is the constant of proportionality known as Electrochemical equivalent of a substance.

M is measured in gram (g)

I is measured in Amperes (A)

t is measured in seconds (s)

Q is measured in Coulomb(C)               

Therefore E is measured in gA^-1S^-1 or gC^-1, for a particular electrolytic process at different time, the law can also be expressed as M₁/Q₁ = M₂/ Q₂

VERIFICATION OF FARADAY’S FIRST LAW OF ELECTROLYSIS

VERIFICATION OF FARADAY’S SECOND LAW OF ELECTROLYSIS

EVALUATION

1. What mass of aluminum will be deposited if 5.6A is passed through its solution for 10minutes?

2. Calculate the mass of silver deposited when a current of 2.6A is passed through a solution    of a silver salt for 70 minutes. (Ag = 108; 1 Faraday = 96500C)

WORKED EXAMPLES

1. How many moles of electrons are required to discharge 2.5 moles of silver ion during electrolysis?

SOLUTION:

Ag⁺+  e^–          →        Ag

1 Mole Ag⁺ is discharged by 1 mole e^–

2.5 moles Ag⁺ are discharged by 2.5 mole e^–

                 2.5 moles of electrons are required

2.         If 6 moles of electrons are passed through different solutions of copper and silver, calculate the moles of silver deposited by the time 3 moles of copper have been deposited

SOLUTION

            Cu²⁺     +   2e^–  →       Cu

            1 mole of Cu is deposited by 2 moles of e^–

            3 moles Cu is deposited by 6 moles of e^–

For silver:

       Ag⁺    +   e^–     →     Ag

            1 mole of e^– deposits 1 moles of Ag

            6 moles of e^–will deposit 6 moles of Ag

EVALUATION

1. How many Faraday’s of a quantity electricity are required to produce 2 moles of oxygen gas during electrolysis?
2. A current of 0.25A flows for 16 minutes 5 seconds and deposits 0.163g of a metal X during the electrolysis of an aqueous solution of a salt of the metal. What is the charge on the metal ion?

APPLICATIONS OF ELECTORLYSIS

Electrolysis is applied in the industries in the following areas e.g

1.         Extraction of elements:  Na, K, Mg, Ca, Al, Zn and non metale.g H₂, F₂, Cl₂ are obtained either by electrolysis of their ores or by the electrolysis of their fused compounds or their aqueous solutions.

2.         Purification of metals e.g Cu, Hg, Ag, Au

3.         Electroplating of one metal by another

4.         Preparation of certain important compounds such as sodium hydroxide, and sodium trioxochlorate(V)

EXTRACTION AND PURIFICATION

Very electropositive metalse.gCa, Mg, and Al and electronegative non-metal e.g F and Cl are extracted from their compounds by electrolysis.

The less electropositive metalse.g copper, mercury, silver and gold occur in the uncombined ores mixed with impurities. The pure metals are obtained from the ore by electrolytic purification.

 During electrolytic purification of copper ores, the impure copper is made the anode while a pure copper is made the cathode. The electrolyte is usually copper (II)tetraoxosulphate(VI) solution.

At the anode :- The metallic copper atoms in the ore give up two electrons each forming copper II ions and move into the solution.

   Cu_(s)      →        Cu²⁺_(aq)   +      2e^–

Electrolytic Purification of Copper

At the Cathode. The copper ions in the solution are discharged at the cathode by receiving electrons to become neutral solid copper.

        Cu²⁺_(aq)    + 2e^–    →         Cu_(s)     

The net result is that pure copper atoms are transferred from impure ore (anode) to the cathode. The impurities are collected at the base of the cell used.

EVALUATION

1. Mention two metal extracted by electrolysis
2. Give two industrial application of electrolysis

ELECTROPLATING

Electroplating is a method of coating the surface of one metal with another metal, usually copper, silver, chromium, nickel, or gold by means of electrolysis.

The main objectives of electroplating are

1.   Decoration                        

2.   Protection against corrosion.

Article which are usually plated include cutlery, jewellery and iron or steel objects such as the metallic parts of motor cars

GENERAL EVALUATION/REVISION

1. In silver – plating an iron spoon, which electrode is the spoon made?
2. Electrolytes used in silver – plating is.
3. Mention two metal extracted by electrolysis
4. Give two industrial application of electrolysis
5. Briefly describe purification of impure copper ore.

READING ASSIGNMENT

New School Chemistry for Senior Secondary Schools by O. Y. Ababio (6^th edition) pages 210-212.

WEEKEND ASSIGNMENT

SECTION A: Write the correct option ONLY

1          In silver plating a spoon, the spoon must be made the

            a.         Anode  b.         Cathode           c.          Electrode          d.         Electrolyte

2          A current of 20A is passed for 30 minutes during the electrolysis of gold (III) tetraoxosulphate (VI). The mass of gold deposited at the cathode is

            a.         24.4g               b.         26.5g               c.          28.9g   d.         32.6g

3          What quantity of chlorine in moles is liberated when a current of 15A is passed through molten potassium chloride in 1930 seconds?

            a.         0.05                 b.         0.10                 c.          0.15                 d.         0.20

4          What quantity of silver is deposited when 96500C of electricity is passed through a solution containing silver ions [Ag = 108, F = 96500C]

            a.         1.08g               b.         10.8g               c.          108g                d.         5.40g

5          Calculate the volume of oxygen liberated at s.t.p when 96500C of electricity is passed through aqueous solution of H₂S0₄. [IF = 96500C, Volume at s. t. p = 22.4 dm³]

a.         5.6dm³ b.         0.56dm³           c.          56dm³  d.         560dm³

SECTION B

1.           In the electrolysis of dilute CuSO₄ solution using platinum electrodes, 2.0A current passed for 2.50hrs and deposited 5.85g of copper

a.  Calculate the relative atomic mass of copper from the experiments

b.  What volume of gas is liberated at s.t.p ?

2.           a.   Using a well-labeled diagram only, explain the process of silver plating a spoon.

b.  Give three uses of electrolysis