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Question
Describe the electrolysis of molten NaCl using inert electrodes.
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Solution
1. The electrolytic cell consists of two iron electrodes dipped in molten sodium chloride and they are connected to an external DC power supply via a key.
2. The electrode which is attached to the negative end of the power supply is called the cathode and the one is which attached to the positive end is called the anode.
3. Once the key is closed, the external DC power supply drives the electrons to the cathode and at the same time pulls the electrons from the anode.
Cell reactions:
Na+ ions are attracted towards the cathode, where they combine with the electrons and are reduced to liquid sodium.
Cathode (reduction)
\[\ce{Na^+_{ (l)} + e^- -> Na_{(l)}}\]
E0 = – 2.71 V
Similarly, Cl– ions are attracted towards anode where they losses their electrons and oxidised to chlorine gas.
Anode (oxidation)
\[\ce{2Cl^-_{ (l)} -> Cl2_{(g)} + 2e^-}\]
E0 = – 1.36 V
The overall reaction is,
\[\ce{2Na^+_{ (l)} + 2Cl^-_{ (l)} -> 2Na_{(l)} + Cl2_{(g)}}\]
E0 = 4.07 V
Electrolysis of molten NaCl
The negative E0 value shows that the above reaction is a non spontaneous one. Hence, we have to supply a voltage greater than 4.07 V to cause the electrolysis of molten NaCl.
In an electrolytic cell, oxidation occurs at the anode and reduction occurs at the cathode as in a galvanic cell, but the sign of the electrodes is the reverse i.e., in the electrolytic cell cathode is −ve and the anode is +ve.
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