Electrolysis





The process of chemical decomposition of the electrolyte by the passage of electricity through its molten or dissolved state is termed as electrolysis. It is the chemical change (Redox reaction occurring at electrode on passing current). The device in which this process is carried out is called electrolytic cell.

In this process first of all electrolyte is ionised. The cations deposit on cathode where reduction takes place. The anions deposit on anode where oxidation takes place. The conversion of ions in the neutral particles at the respective electrodes is referred to as primary change. The product obtained on primary change may be collected as such or may undergo secondary change to form final products. The reactions taking place in these changes are called electrochemical reactions.

It is interesting to note that liberation of a particular ion depends upon the nature of electrode, discharged potential and concentration of ions in a solution. According to the preferential discharge theory, if more than one type of cations or anions or both are present in the electrolyte, then the ion is discharged which requires least energy or discharged potential.

The decreasing order of discharged potential or the increasing order of deposition of some ions is given below:

Cations: Na^+, ca^+, Mg^{++}, Al^{3+}, Zn^{++}, Fe^{++}, H^+, Cu^{++}, Hg^{++}, Ag+, Ag^{3+} etc.

Anions: SO^{-2}_4 , NO^-_3 , OH^- , Cl^-, Br^-, I^- etc.

 

To explain the above, we consider the following examples:

 

1. Electrolysis of molten PbBr_2: Molten PbBr_2 consists of Pb^{++}  and Br^- ions which deposit on cathode and anode respectively on passing electricity. The changes may be represented as:

PbBr_2 \leftrightharpoons Pb^{++} + 2Br^-

 

At cathode Pb^{++} + 2e^- \to Pb

 

At anode Br^- e^- \to Br (Primary change)

Br + Br \to Br_2 (Primary change)

 

2. Electrolysis of aqueous solution of NaCl: Aqueous NaCl consists of the following ions in solution:

NaCl(aq) + H_2O (l) \leftrightharpoons Na^+(aq) + H^+(aq) + Cl^-(aq) + OH^-(aq)

 

Thus aqueous solution of NaCl consists of

Na^+ \text{and} \hspace{3mm}cations H^+ , \text{Cations} , Cl^- \text{and} \hspace{3mm} OH^- anions.

On passing electricity H^+ions deposit at the cathode H^+ in preference and produce H_2 gas. ThusH_2O molecules reduce at cathode as follows:

2H_2O + 2e^-\to 2OH^-(aq) + H_2(g)

 

On the other hand Cl^- ions deposit at the anode in preference and produce Cl_2 gas as:

Cl^- \to Cl + e^+ (Primary change)

Cl + Cl \to Cl_2(g) (Secondary change)

Thus overall reaction taking place on electrolysis of aqueous solution of NaCl may be given as:

2Na^+ + 2Cl^- + 2H_2O \overset{Electricity}{\rightarrow} 2Na^+ +2OH^- + H_2(g) ++ Cl_2(g)

 

I.e.

Na^+ \text{and} OH^- ions remain in the solution and the solution on evaporation gives crystals of NaOH.

  • Due to over voltage concept in place of O_2 here Cl_2 is liberated at anode, i.e., forO_2 additional voltage is needed here.

 

3. Electrolysis of aqueous solution of CuSO_4 using Pt electrodes (Inert electrodes): Aqueous CuSO_4 consists following ions:

CuSO_4 + H2O \leftrightharpoons + Cuy^{++} + H^+ + OH^- + SO^{-2}_4

 

On passing electricity Cu^{++} ions deposit at the cathode in preference due to higher electrode potential than ion.

I.e.

Cu^{++} + 2e^- \to Cu

 

On the other hand OH‘ ions deposit at the anode in preference and produce O_2 gas.

4OH^- \to 2H_2O + O_2 + 4e^- (At anode)

Thus overall reaction taking place on electrolysis of aqueous solution of CuSO_4 using Pt-electrodes may be given as:

2Cu^{++} + 2SO^{-2}_4 + 2H_2O \overset{Electricity}{\rightarrow} 2Cu +O_2 + 4H^+ + 2SO^{-2}_4

 

4. Electrolysis of aqueous solution of CuSO_4 using Cu electrodes: Thus ionisation of CuSO4 may be given as:

CuSO_4 + H_2O \leftrightharpoons Cu^{++} + H^+ + OH^- + SO^{-2}_4

It is the concept of electroplating that Cu-metal is liberated at cathode. Here blue colour of the solution gets faded due to electrolysis.

On passing electricity Cu^{++}ions deposit at the cathode as follows.

Cu^{++} + 2e^- \to Cu (At cathode)

On the other hand, anode itself undergoes loss of electrons to form Cu^{++} ions which go into the solution.

Cu \to Cu^{++} + 2e^-

 

Thus the net concentration of Cu^{++} ions remains the same.

Here Cu is transferred from the anode to cathode so the solution concentration does not change.

  • This concept develops the concept of refining used for purification of metals.

 

5.       Electrolysis of CuCl_2using inert electrodes:

At anode 2Cl^- (aq.) \to Cl_2(g) + 2e^-

At anode Cu^{+2}(aq.) \to 2e^- + Cu(s)

Net reaction Cu^{2+}(aq.) + 2Cl^-(aq.) \to Cu(s) + Cl_2(g)



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