• Electrolysis is the passage of a direct current through a substance that is either molten or dissolved in a suitable solvent, resulting in chemical reactions at the electrodes and separation of mixtures.

Principles of electrolysis

• An ion-electron half reaction occurs at each electrode.

• The overall reaction is a redox reaction.

• The terminal at which oxidation occurs is the anode whereas the terminal at which reduction occurs is the cathode .

• The electrode connected to the positive terminal of the external source of electricity is the positive electrode. The electrode connected to the negative terminal is the negative electrode.

• The positive ions in the solution are attracted by the negative electrode. The negative ions are attracted by the positive electrode.

• Under certain conditions, a different species in the medium/set up may be oxidized or reduced in preference to the relevant ions.

Electrolysis of water (acidic or basic) using inert (carbon or platinum) electrodes

At anode; 2H₂O(l) → O₂(g) + 4H⁺(aq) + 4e
At cathode; 2H₂O(l) + 2e → H₂(g) + 2OH^–(aq)

Electrolysis of aqueous CuSO4 solution using copper electrodes

At anode; Cu(s) → Cu²⁺(aq) + 2e
At cathode; Cu²⁺(aq) + 2e → Cu(s)

Electrolysis of aqueous CuSO₄ solution using inert electrodes

At anode; 2H₂O(l) → O₂(g) + 4H⁺(aq) + 4e
At cathode; Cu²⁺(aq) + 2e → Cu(s)

Electrolysis of aqueous NaCl solution using inert electrodes

At anode; 2Cl^–(aq) → Cl₂(g) + 2e
At cathode; 2H₂O(l) + 2e → H₂(g) + 2OH^–(aq)

Electrolysis of molten NaCl using inert electrodes
At anode; 2Cl^–(l) → Cl₂(g) + 2e
At cathode; Na⁺(l) + e → Na(l)

Faraday’s Laws

(1) The quantity of a substance liberated at an electrode is directly proportional to the quantity of electric charge that has flowed in the circuit.

(2) For a given quantity of electric charge, the amount of any metal deposited is proportional to its equivalent weight (atomic weight devided by the charge on the metal ion)
Faraday’s constant (F) = Molar charge of a proton                                                                                         =  1.602 x 10-19 C x 6.022 x 1023 mol-1= 96 484 C mol^-1≅96 500 C mol^-1

Electroplating

• Electroplating means coating a metal with another metal using electricity. This is different from the deposition of a less reactive metal on a more reactive metal.

In order to have a quality coating during electrolysis, the coating should strongly bind with the metal. In addition, it should have the following qualities.
• Strength

• Lustre

• Chemical inertness

• Good mechanical properties

• Absence of cracks and holes

• Uniformity in thickness and appearance

To get a quality metal coating the following factors should be appropriately controlled.
• The nature and purity of the electrolyte

• Concentration of ions

• Temperature

• Current density

• pH value

• Nature of the other ions presents

• Potential difference

• Purity of the anode

• Relative positioning of the anode and the cathode

• Cleanliness of the object and the nature of its surface

Two or more reactions may also occur simultaneously on the electrodes. They can be controlled by changing the temperature, concentration, voltage and the nature of electrodes.