Electrolysis

1. Electrolytes: These are the substances which allow the electricity to pass through them in their molten states or in the form of their aqueous solution and undergo chemical decomposition. Examples-acids, bases & salts.

2. Strong electrolytes: The electrolytes which are almost completely dissociated into ions in solution are called strong electrolytes. Examples-NaCl, KCl, HCl, NaOH etc.

3. Weak electrolytes: The electrolytes which do not ionise completely in solution are called weak electrolytes. Examples-CH₃COOH, H₂CO₃, HCN, ZnCl₂, NH₄OH etc.

4. Electrolysis: The process of chemical decomposition of an electrolyte by passage of electric current through its molten state or its solution is called electrolysis.

5. Electrodes: In order to pass the current through an electrolytes in molten state or in aqueous solution, two rods or plates are needed to connect with the terminal of a battery. These rods or plates are called electrodes.

Anode: The electrode which is attached to positive terminal of battery is called anode. Oxidation occurs at anode.

Cathode: The electrode which is attached to negative terminal of batteries is called Cathode. Reduction occurs at cathode.

Examples Electrolysis of molten NaCl

At anode: Cl^– – e → Cl

Cl + Cl → Cl₂

At cathode: Na⁺ + e → Na

So, Cl₂ gas occurs at anode while Na at cathode.

⇒ Molten NaCl is good conductor of electricity due to free ions.

⇒ When electrictiy is passed throught brine (aqueous Nacl) H₂ is appeared at cathude Cl₂ is appeared at anode. The solution becomes alkaline due to presence of NaOH.

The relation between the quantity of charged passed through an electrolyte and the amount of the substance deposited at electrode is obtained from Faraday’s Laws of electrolysis.

The amount of the substance (w) deposited or liberated at any electrode is directly proportional to the quantity of charge (Q) passed

w ∝ Q (Q = charge in coulomb)

w ∝ I·t (Q = current (I) x time (t))

[w = Z·I·t]

Where Z is a proportionality constant known as electrochemical equivalent.

The charge carried in one mole electrons is normally taken as 96500 C mol-¹ which is called Faraday’s constant (F) i.e.

[1F = 96500 C mol^-1]

When some quantity of charge is pass through the solutions of two different electrolytes connected in series the masses of substances deposited at respective electrodes are directly proportional to their equivalent masses.

Conductance is the ease with which the current flows through the electrolyte. It is the reciprocal of the resistance (R) i.e. C = 1/R. The unit of conductance is ohm^-1 or  m^-1 Siemens (S)

The conductance of the solution of an electrolyte enclosed in a cell with two electrodes of unit area of crosssection separated by one centimetre is called specific conductance. The unit of specific conductance is ohm-¹cm-¹ or ohm¹m¹ or S m-¹.

The conductance of all the ions prodnced by one gram equivalent of electrolyte in solution is known as equivalent conductance. The unit of equivalent conductance is ohm-¹ cm² (g-equiv)-¹ or S cm² (g-equiv)-¹.

When a metal is attacked by substances around it such as moisture, acid etc. it is said to corrode and this process is called corrosion. The black coating on silver and green coating on copper are example of corrosion: Corrosion slowly coats the surface of metallic objects with oxides or other salts of the metal. Iron articles are shiny when new, but they get coated with a reddish brown powder when left for some time. This process is commonly known as rusting of iron. Rust is hydrated ferric oxide (Fe₂O₃·xH₂O).

Corrosion causes damage to car bodies, bridges, iron railing, ships and to all objects made of metals specially those of iron.

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