The products of electrolysis of molten NaCl are Na metal and Cl 2  gas, while the products of electrolysis of aqueous solution of NaCl are NaOH, Cl 2  and H 2 . Please explain this in detail with the appropriate reactions.

Dear Student,

The products of electrolysis of molten NaCl and aqueous solution of NaCl can be explained as follow

• Electrolysis of molten NaCl:

Molten NaCl contains only NaCl so gives Na+ and Cl- ions only as

NaCl -----------> Na+ + Cl-

Reaction as cathode:  Na+ + e- ----------> Na

Reaction as anode:  Cl- ----------> Cl + e-

Cl + Cl ---------------> Cl2

So, the product of electrolysis are Na at cathode and Cl2 at anode.

• Electrolysis of aqueous  NaCl:

Here, NaCl and Water H2O both are present and both dissociate as

NaCl -----------> Na+ + Cl-

H2O--------------> H+ + OH-

Reaction as cathode: Both Na+ and H+ will compete for cathode but reaction with higher E0 is preferred

Na+ + e- ----------> Na  E0 = -2.71 V  (1)

H++ e- -----------> 1/2 H2   E0 = 0.00 V  (2)

Since, (2) has higher value of E0 So, H2 is product deposited at cathode.

Note-The eqn 2 can also be written as  H2O + e- --------------> 1/2 H2 + OH-

Reaction as anode: Both Cl- and OH- will compete for anode but reaction with lower E0 is preferred

Cl-------------------> 1/2Cl2 + e-    E0 = +1.36 V    (3)

2H2O --------------> O2+ 4H+ + 4 e-  E0 = +1.23 V  (4)

The eqn 4 is the reaction of OH- at anode. The (4) has lower  E0, it should be preferred. But due to overpotential Cl2 is preferred product at anode.

Hence,  the product of electrolysis are H2 at cathode and Cl2 at anode.

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It has been clearly explained in NCERT text.....chap 3: electrochemistry..

• -29

Hello my Friend,

During the electrolysis of Molten NaCl, since it is in solid state, some electrons are ejected.  The Na atom is more in number in the molten state than the H+ ions.  So Na+ ions are ejected than the H+ ions.

But in the aqueous solution H+ ions dominate the ejection of electrons than Na+ ions and H+ and Cl- are evolved

The one phrase answer to this is" whoever wins the race to cathode is the product"

in Molten phase Na+ wins the race and in aqueous phase H+ wins the race.

This is the easy way to remember which is the product

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