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Answer the following in brief. Write electrode reactions for the electrolysis of aqueous NaCl. - Chemistry

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Answer in Brief

Answer the following in brief.

Write electrode reactions for the electrolysis of aqueous NaCl.

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  • Reduction half-reaction at cathode:

At cathode, two reduction reactions compete.

i. Reduction of sodium ions.

`"Na"_(("aq"))^+ + "e"^(-) -> "Na"_("s"), "E"^circ = - 2.71 "V"`

ii. Reduction of water to hydrogen gas.

2H2O(l) + 2e- → H2(g) + `2"OH"_(("aq"))^-,` E° = - 0.83 V

The standard potential for the reduction of water is higher than that for the reduction of Na+. Hence, water has a much greater tendency to get reduced than the Na+ ion. Therefore, reduction of water is the cathode reaction when the aqueous NaCl is electrolysed.

  • Oxidation half-reaction at anode: 

At anode, there will be competition between oxidation of Cl ion to Cl2 gas as in the case of molten NaCl and the oxidation of water to O2 gas.

i. Oxidation of Cl- ions to chlorine gas

`2 "Cl"_(("aq"))^(-) -> "Cl"_(2("g")) + 2"e"^(-), "E"_"oxd"^circ` = - 1.36 V

ii. Oxidation of water to oxygen gas.

`2"H"_2"O"_(("l")) -> "O"_(2("g")) + 4"H"_(("aq"))^+ + 2 "e"^(-),  "E"_"oxd"^circ` = - 0.4 V

The standard electrode potential for the oxidation of water is greater than that of Cl ion or water has a greater tendency to undergo oxidation. Hence, an anode half-reaction would be oxidation of water. However, experiments have shown that the gas produced at the anode is Cl2 and not O2. This suggests that anode reaction is oxidation of Cl to Cl2 gas. This is because of overvoltage.

  • Net cell reaction:

The net cell reaction is the sum of two electrode reactions.

`2 "Cl"_(("aq"))^(-) -> "Cl"_(2("g")) + 2"e"^(-)`                     (Oxidation half reaction at anode)

`2"H"_2"O"_(("l")) + 2 "e"^(-) -> "H"_(2("g")) + 2"OH"_(("aq"))^-`    (Reductionhalfreactionat cathode)


`2 "Cl"_(("aq"))^(-) + 2"H"_2"O"_(("l")) -> "Cl"_(2("g")) + "H"_(2("g"))  + 2"OH"_(("aq"))^-`  (Overall cell reaction)

Concept: Electrolytic Cell
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Balbharati Chemistry 12th Standard HSC for Maharashtra State Board
Chapter 5 Electrochemistry
Exercises | Q 3.03 | Page 118
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