Mercury And Its Compounds
Category : JEE Main & Advanced
(1) Occurrence and extraction of mercury : Cinnabar (HgS) is the only important ore of Hg. It is concentrated by froth floatation method and mercury is extracted from this ore by heating it in air at 773-873 K (auto reduction).
\[HgS+{{O}_{2}}\xrightarrow{273-873\,K}Hg+S{{O}_{2}}\]
The mercury vapours thus obtained are condensed to give liquid metal. Hg thus obtained contains impurities of Zn, Sn and Pb. These are removed by treating the impure metal with dil \[HN{{O}_{3}}\], mercurous nitrate, \[H{{g}_{2}}{{(N{{O}_{3}})}_{2}}\] thus formed react with metals present as impurities forming their nitrates which pass into solution leaving behind pure mercury. However, it is best purified by distillation under reduced pressure.
\[6Hg+8HN{{O}_{3}}(dil.)\xrightarrow{\text{warm}}\]\[3H{{g}_{2}}{{(N{{O}_{3}})}_{2}}+4{{H}_{2}}O+2NO\]
\[Zn+H{{g}_{2}}{{(N{{O}_{3}})}_{2}}\to Zn{{(N{{O}_{3}})}_{2}}+2Hg\]
Similar reaction is given by Pb and Sn.
Properties of mercury : Mercury is less reactive than Zn. It is a liquid at room temperature and has low thermal and electrical conductivity. Mercury forms dimeric mercury (I) ions, \[Hg_{2}^{+2}\] in which the two atoms are bonded by a covalent bond. It is slowly oxidised to HgO at about its boiling point. Hg does not react with dil. HCl or dil.\[{{H}_{2}}S{{O}_{4}}\] but reacts with hot concentrated \[{{H}_{2}}S{{O}_{4}}\] to form \[HgS{{O}_{4}}\], it reacts with both warm dil. and conc. \[HN{{O}_{_{3}}}\] evolving NO and \[N{{O}_{2}}\] respectively.
\[Hg+2{{H}_{2}}S{{O}_{4}}(\text{hot, conc}\text{.})\to HgS{{O}_{4}}+S{{O}_{2}}+2{{H}_{2}}O\]
\[Hg+4HN{{O}_{3}}(\text{conc}\text{.})\to Hg{{(N{{O}_{3}})}_{2}}+2N{{O}_{2}}+2{{H}_{2}}O\]
Hg does not react with steam or water hence can’t form any hydroxide.
Compounds of mercury
(1) Mercuric oxide, HgO : It is obtained as a red solid by heating mercury in air or oxygen for a long time at 673 K
\[2Hg+{{O}_{2}}\xrightarrow{673\,K}2HgO(\text{red})\]
or by heating mercuric nitrate alone or in the presence of Hg
\[2Hg{{(N{{O}_{3}})}_{2}}\xrightarrow{\text{Heat}}\underset{\text{red}}{\mathop{2HgO}}\,+4N{{O}_{2}}+{{O}_{2}}\]
When NaOH is added to a solution of \[HgC{{l}_{2}}\], yellow precipitate of HgO are obtained.
\[H{{g}_{2}}C{{l}_{2}}+2NaOH\xrightarrow{{}}\underset{\text{(yellow)}}{\mathop{HgO\downarrow }}\,+{{H}_{2}}O+2NaCl\]
Red and yellow forms of HgO differ only in their particle size. On heating to 673 K, yellow form changes to red form.
\[\underset{\text{yellow}}{\mathop{HgO}}\,\xrightarrow{673\,K}\underset{\text{red}}{\mathop{HgO}}\,\]
It is used in oil paints or as a mild antiseptic in ointments.
(2) Mercuric chloride, HgCl2 : It is obtained by treating Hg with \[C{{l}_{2}}\] or by heating a mixture of NaCl and \[HgS{{O}_{4}}\] in presence of small amount of \[Mn{{O}_{2}}\] (which oxidises any Hg(I) salts formed during the reaction).
\[HgS{{O}_{4}}+2NaCl\underset{Mn{{O}_{2}}}{\mathop{\xrightarrow{\text{Heat}}}}\,HgC{{l}_{2}}+N{{a}_{2}}S{{O}_{4}}\]
It is a white crystalline solid and is commonly known as corrosive sublimate. It is a covalent compound since it dissolves in organic solvents like ethanol and ether.
It is extremely poisonous and causes death. Its best antidote is white of an egg.
When treated with stannous chloride, it is first reduced to white ppt. of mercurous chloride and then to mercury (black).
\[2HgC{{l}_{2}}+SnC{{l}_{2}}\to \underset{\text{white ppt}\text{.}}{\mathop{H{{g}_{2}}C{{l}_{2}}}}\,+SnC{{l}_{4}}\]
\[H{{g}_{2}}C{{l}_{2}}+SnC{{l}_{2}}\to \underset{\text{grey}}{\mathop{2Hg}}\,+SnC{{l}_{4}}\]
With ammonia it gives a white ppt. known as infusible white ppt.
\[HgC{{l}_{2}}+2N{{H}_{3}}\to Hg(N{{H}_{2}})Cl+N{{H}_{4}}Cl\]
A dilute solution of \[HgC{{l}_{2}}\] is used as an antiseptic.
(3) Mercuric iodide, \[\mathbf{Hg}{{\mathbf{I}}_{\mathbf{2}}}\] : It is obtained when a required amount of KI solution is added to a solution of \[HgC{{l}_{2}}\].
\[HgC{{l}_{2}}+2KI\to \underset{\text{(red)}}{\mathop{Hg{{I}_{2}}}}\,+2KCl\]
Below 400 K, \[Hg{{I}_{2}}\] is red but above 400 K, it turns yellow
\[\underset{\text{(red)}}{\mathop{Hg{{I}_{2}}}}\,\underset{\text{(yellow)}}{\mathop{Hg{{I}_{2}}}}\,\]
\[Hg{{I}_{2}}\] readily dissolves in excess of KI solution to form the \[{{(Hg{{I}_{4}})}^{2-}}\] complex ion.
\[\underset{\text{Red ppt}\text{.}}{\mathop{Hg{{I}_{2}}}}\,+2KI\ \ \ \ \ \ \to \underset{\text{soluble colourless solution}}{\mathop{{{K}_{2}}Hg{{I}_{4}}}}\,\]
An alkaline solution of \[{{K}_{2}}[Hg{{I}_{4}}]\] is called Nessler’s reagent and is used to test \[NH_{4}^{+}\] ions.
It gives a brown ppt. of \[N{{H}_{2}}-Hg-O-Hg-I\] (Iodide of Millon’s base) with \[NH_{4}^{+}\] ions.
\[2{{K}_{2}}[Hg{{I}_{4}}]+N{{H}_{3}}+3KOH\to \]\[N{{H}_{2}}.HgO.Hgl+7KI+2{{H}_{2}}O\]
It is used in ointments for treating skin infections.
(4) Mercurous chloride, \[\mathbf{H}{{\mathbf{g}}_{\mathbf{2}}}\mathbf{C}{{\mathbf{l}}_{\mathbf{2}}}\] : It is obtained as under :
(a)\[H{{g}_{2}}{{(N{{O}_{3}})}_{2}}+2NaCl\to \underset{\text{white ppt}\text{.}}{\mathop{H{{g}_{2}}C{{l}_{2}}}}\,+2NaN{{O}_{3}}\]
(b) \[HgC{{l}_{2}}+Hg\xrightarrow{\text{Heat in an iron retort }}H{{g}_{2}}C{{l}_{2}}\] (condenses on cooling)
It is purified by sublimation.
Mercurous chloride is also called calomel. It is a white powder insoluble in \[{{H}_{2}}O\]. On heating, it decomposes to give \[HgC{{l}_{2}}\] and Hg.
\[H{{g}_{2}}C{{l}_{2}}\xrightarrow{\text{Heat}}HgC{{l}_{2}}+Hg\]
It dissolves in chlorine water forming mercuric chloride.
\[H{{g}_{2}}C{{l}_{2}}+C{{l}_{2}}\to 2HgC{{l}_{2}}\]
With ammonia, it turns black due to the formation of a mixture of finely divided black Hg and mercuric amino chloride.
\[H{{g}_{2}}C{{l}_{2}}+2N{{H}_{3}}\to \] \[\underbrace{Hg+N{{H}_{2}}HgCl}_{\text{(black)}}+N{{H}_{4}}Cl\]
It is used to prepare standard calomel electrode and as a purgative in medicine.
(5) Mercuric sulphide, HgS : The solubility product of HgS is lower than that of ZnS and hence it gets precipitated as black solid when \[{{H}_{2}}S\] is passed through an acidic solution of any mercury (II) salt.
\[HgC{{l}_{2}}+{{H}_{2}}S\to HgS+2HCl\]
It is insoluble in water and HCl but dissolves in aqua regia (1 part conc. \[HN{{O}_{3}}+\]3 parts conc. HCl)
\[\underset{\text{Aqua regia}}{\mathop{3HCl+HN{{O}_{3}}}}\,\to \underset{\text{Nitrosyl chloride}}{\mathop{NOCl}}\,+2{{H}_{2}}O+\underset{\text{Nascent chlorine}}{\mathop{2[Cl]}}\,\]
\[HgS+2|Cl|\ \to \underset{\text{(Soluble)}}{\mathop{HgC{{l}_{2}}}}\,+S\downarrow \]
On sublimation, its colour changes to red and hence it is used as a red pigment.
(6) Mercuric sulphate, \[\mathbf{HgS}{{\mathbf{O}}_{\mathbf{2}}}\]: It is obtained when HgS is treated with conc.\[{{H}_{2}}S{{O}_{4}}\].
\[Hg+2{{H}_{2}}S{{O}_{4}}\to HgS{{O}_{4}}+S{{O}_{2}}+2{{H}_{2}}O\]
It is a white solid which decomposes on heating to give mercurous sulphate.
\[3HgS{{O}_{4}}\underset{675\,K}{\mathop{\xrightarrow{\Delta }}}\,H{{g}_{2}}S{{O}_{4}}+Hg+2S{{O}_{2}}+2{{O}_{2}}\]
It is used as a catalyst in the hydration of alkynes to give aldehydes or ketones. It is also used as a cosmetic under the name Vermillon and in ayurvedic medicine as makardhwaj.
(7) Amalgams : Mercury forms alloys commonly known as amalgams, with all metals except iron and platinum. Hence it is transported in iron containers.
(8) Alloy of transition metal : See in table discuss earlier in metallurgy.
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