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Difficulty: MediumAlkali Metals: Sodium Extraction, Properties, and Compounds

In the industrial extraction of sodium metal using the Downs process, anhydrous molten sodium chloride is electrolyzed instead of an aqueous sodium chloride solution. Why is an aqueous solution of sodium chloride unsuitable for extracting sodium metal?

  1. Hydrogen ions are preferentially discharged at the cathode because they have a higher reduction potential than sodium ions.Answer
  2. B
    Sodium ions migrate to the anode in aqueous solution and form sodium hydroxide.
  3. C
    Water acts as a reducing agent at the cathode, converting sodium metal into sodium oxide.
  4. D
    Water elevates the melting point of sodium chloride, preventing electrolysis from occurring efficiently.

Answer

Hydrogen ions are preferentially discharged at the cathode because they have a higher reduction potential than sodium ions.
When an aqueous solution of sodium chloride is electrolyzed, both Na+Na^+ and H+H^+ cations migrate to the cathode. Because H+H^+ lies lower in the electrochemical series and has a much higher reduction potential (0.00 V0.00\text{ V}) than Na+Na^+ (−2.71 V-2.71\text{ V}), H+H^+ ions are preferentially reduced to form hydrogen gas (H2H_2). To isolate metallic sodium, water must be absent, which is why molten (fused) sodium chloride is used in the Downs process.

Step-by-Step Solution

1
Identify all cations present in an aqueous solution of sodium chloride
The solution contains Na+Na^+ ions from sodium chloride and H+H^+ ions from the self-ionization of water.
Electrolysis of aqueous ionic solutions involves competing ions from both the dissolved salt and water.
2
Compare the standard reduction potentials of the competing cations at the cathode
H+H^+ has a reduction potential of 0.00 V0.00\text{ V}, whereas Na+Na^+ has a standard reduction potential of −2.71 V-2.71\text{ V}.
The cation with the higher (more positive) reduction potential gains electrons more readily at the cathode.
3
Determine the cathode product and deduce why molten NaCl is required
In aqueous solution, H+H^+ ions are reduced to H2(g)H_2(g) at the cathode instead of Na+Na^+ forming Na(s)Na(s). Therefore, molten NaClNaCl must be used to exclude water entirely.
Extracting reactive metals like sodium requires an electrolyte free of water so that Na+Na^+ is the only cation available for reduction.

Key Concept

Preferential discharge of ions during electrolysis and extraction of reactive metals
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Topics

Alkali Metals: Sodium Extraction, Properties, and Compounds

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