1994 CE CHEM MC no.6

Question

When a substance X is electrolysed using platinum electrodes, a gas is collected at each electrode. X may be
A. silver nitrate solution
B potassium chloride solution
C molten sodium chloride
D molten copper(II) chloride

The answer is B but why not C?

this is no 13 sorry

Answer 1

B.
In KCl solution, KCl gives K+(aq) and Cl-(aq) ions, while H2O(l) is ionized to give a small amount of H+(aq) and OH-(aq) ions.

Anode : Cl-(aq) and OH-(aq) ions migrate to the anode. Cl-(aq) is preferentially discharged, because the concentration of Cl-(aq) ion is higher than that of OH-(aq) ion.
2Cl-(aq) → Cl2(g) + 2e-

Cathode : K+(aq) and H+(aq) ions migrate to the cathode. H+(aq) is preferentially discharged, because H+(aq) ion is reduced more readily than K+(aq) ion.
2H+(aq) → H2(g) + e-
or 2H2O(l) → H2(g) + 2OH-(aq) + 2e-

Conculsion : Cl2 gas and H2 gas are given off at the anode and the cathode respectively.

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C.
Molten NaCl does not contain water. It is the liquid form of sodium chloride, which is formed by heating sodium chloride solid. In molten NaCl, NaCl gives K+(l) and Cl-(l) ions, without any other ions.

Anode : Only Cl-(l) ion migrates to the anode. The Cl-(l) ion is discharged to give Cl2 gas.
2Cl-(l) → Cl2(g) + 2e-

Cathode : Only Na+(l) ion migrates to the cathode. The Na+(l) ion is discharged to give Na liquid (because temperature of molten NaCl is high, and the melting point of Na is 98oC).
Na+(l) + e- → Na(l)

Conclusion : Cl2 gas is given off at the anode, while Na (which is not a gas) is formed at the cathode.

Answer 2

因為c會出Na

Na+ +e- --> Na

雖然Na+ 好難reduce ，但係佢係molten state，所以可以reduce