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Occurrence and extraction of sodium metal

Answer Text:
The metals
1. Sodium
Sodium occurrence and extraction
i. Rock salt / sodium chloride; NaCl
ii. Chile saltpetre / sodium Nitrate; #NaNO_3#
iii. Soda ash/sodium carbonate; #Na_2CO_3#.
Other ores include;
(i). Borax; #Na_2B_4O_7 .10H_2O#
(ii). Sodium Sulphate, #Na_2SO_4#;
Extraction;
- Sodium is obtained by the electrolysis of fused sodium chloride in the electrolytic cell.
- Calcium chloride and calcium fluoride are added to the electrolyte.
Reasons;
- To lower the melting point of sodium chloride from# 800^oC# to# 600^oC#;
- Once molten, the electrical resistance within the cell is sufficient to maintain the temperature without external heating.
- Steel or iron is used as the cathode, while carbon/graphite is used as the anode.
- Thus steel is not used as the anode.
Reason;
- At high temperatures, steel would react with chloride formed at the anode, but graphite is inert even at high temperatures.
- Steel wire gauze separates the electrodes.
Reason;
- To prevent products of electrolysis (sodium and chlorine) from mixing and reacting to form sodium chloride.
- The electrolytic apparatus used in sodium extraction is called the Downs cell.
Diagram: The Downs cell.

- During electrolysis, fused sodium chloride dissociates according to the equation;
#NaCl(l) to Na^(+) (l)+ Cl^(-)(l)#
At the cathode:
Observation;
- Soft silvery metal
Explanation
- #Na^(+)# ions are attracted and undergo reduction (accept electrons) to form/ produce molten sodium metal.
Equation;
#Na^(+)(l) +e^(-) to Na(l)#
- Molten sodium is lighter than fused sodium chloride and floats on the surface where it overflows into a separate container / sodium reservoir.
Note;
- The resultant sodium is usually collected in liquid / molten state, floating on top of the electrolyte.
Reasons;
- Less dense than molten sodium chloride
- Has a low melting point.
At the anode;
Observations;
- Evolution of a greenyellow gas.
Explanation:
- Chlorine gas is evolved as a by – product and collected
separately.
- Negatively charged #Cl^(-)# ions migrate to the positive anode and undergo oxidation to form chlorine gas;
Equation:
#2Cl^(-) to Cl_2 (g) + 2e^(-)#