Which of the following compounds of mercury is commonly known as calomel ?

Options

A. `Hg_(2)F_(2)`
B. `Hg_(2)CI_(2)`
C. `Hg_(2)Br_(2)`
D. `Hg_(2)I_(2)`

Answer

B

Explanation

Mercury forms halides in the two oxidation states, `+1` and `+2`. All the mercury (I) halides are known. They contain the mercurous ion `[Hg-Hg]^(2+)`, not `Hg^(+)` . The two Hg atoms are bonded togher using the 6s orbitals. Mercury is unique in forming stable dinuclear metal ions. The only other metals which form dinuclear ions are `(Zn - Zn)^(2+)` and `(Cd - Cd)^(2+)`, but these ions are unstable and have only been detected spectoscopically. The most common among mercury (I) halides is white mercury (I) chloride (mercurous chlonely) or calomel, `Hg_(2)CI_(2)` . It is prepared by heating a mixture of mercury (II) chloride (mercuric chloride) and mercuty:
`HgCI_(2)` `overset (Delta) rarr` `Hg_(2)CI_(2)`
The calomel sublimes along with mercuric chloride. The latter is removed by extracting the mixture with water which removed the sparingly soluble mercuric chloride. If can also be obtained by the reduction of mercuric chloride by a reducing agent, e.g. tin (II) chloride in limiting quantity:
`2HgCI_(2) + SnCI_(2) rarr Hg_(2)CI_(2) + SnCI_(4)`
Excess of stannous chloride reduces mercurous chloride to mercury:
`Hg_(2)CI_(2) + SnCI_(2) rarr 2Hg + SnCI_(4)`
When heated, mercrous chloride decompose into mercuric chloride and mercury:
`Hg_(2)CI_(2)` `overset (Delta) rarr` `Hg + HgCI_(2)`
The action of aqueous ammonia `(NH_(4)OH)` on the solid mercurous chloride gives a black substance which is a mixture of black finely divided mercury and white mercury aminochloride. It is disproportionation reaction:
`Hg_(2)CI_(2) + 2NH_(3) rarr Hg + Hg(NH_(2))CI + NH_(4)CI`
or `Hg_(2)CI_(2) + 2NH_(4)OH rarr Hg(NH_(2)CI + NH_(4)CI + 2H_(2)O`
Mercury chloride is used to make the calomel electrode (a secondary reference): `Hg|Hg_(2)CI_(2)|KCI`. It is also used as a purgative in medicine.