Metals secondary chemistry

Notes on Metals

Mode of extraction depends on reactivity of the metal

Potassium, sodium, calcium, magnesium and aluminium are very reactive and never found as free element.

They are extracted by electrolysis



Zinc, lead and iron are moderately reactive and found as oxides, carbonates or sulphides

They are extracted by reducing their metal oxides with either carbon or carbon (II) oxide

Copper, mercury, silver and gold may be found in uncombined form

Sodium

Occurrence (ores)

  1. Sodium chloride dissolved in sea water and salt lakes
  2. Trona as double salt in rift valley lakes such as lake magandi
  3. Sodium nitrate(salt petre)
  4. Rock salt(sodium chloride)

Extraction

Done through electrolysis of fussed (molten) sodium chloride using downs cell

Sodium chloride has a melting point of 8000C. Calcium chloride is added to lower the melting point of sodium chloride to about 6000C

This lowers the cost of electricity used to heat sodium chloride to melt it.

Downs cell

Downs cell

 

Has outer shell lined with fire bricks on the outside, this fire resistant wall maintains high temperature so that the electrolyte does not crystallize.

A diaphragm of iron gauze screens (separates) the graphite (carbon) from the ring shaped iron cathode that surrounds it.

This allows chlorine formed at the anode to escape through the hood without recombining with sodium formed at the cathode.

 Chemistry of the reactions

Molten sodium chloride contains Na+ and Cl

At the cathode

 

Sodium ions move to the cathode where they are reduced to molten sodium metal by gaining electron

Na+(s)   +  e-   →  Na (l) (reduction)

Sodium formed is less dense than sodium chloride (the electrolyte) hence floats on it over the inverted trough from where it is tapped.

At the anode

Chloride ions move to the anode being negatively charged where they are oxidized to chlorine gas by losing electrons

2Cl(l)   →   Cl2 (g)   +  2e   (oxidation)

Chlorine is a valuable by product of the process

NB

Calcium may also be produced at the cathode but it crystallizes (forms a solid) before sodium when the mixture is cooled leaving liquid sodium which is tapped



 

 

METALS WORD FILE