Zinc And Its Compounds
Category : JEE Main & Advanced
(1) Occurrence of zinc: Zinc does not occur in the native form since it is a reactive metal. The chief ores of zinc are (i) Zinc blende \[(ZnS)\] (ii) Calamine or zinc spar \[(ZnC{{O}_{3}})\] and (iii) Zincite \[(ZnO)\]
(2) Extraction of zinc : Zinc blende, after concentration by Froth floatation process, is roasted in air to convert it into \[ZnO\]. In case of calamine, ore is calcined to get \[ZnO\]. The oxide thus obtained is mixed with crushed coke and heated at 1673 K in fire clay retorts (Belgian Process) when \[ZnO\] gets reduced to metallic zinc. Being volatile at this temperature, the metal distils over and is condensed leaving behind Cd, Pb and Fe as impurities. The crude metal is called spelter. The metal may be refined either by electrolysis or by fractional distillation.
Properties of Zn : Zinc is more reactive than mercury. It is a good conductor of heat and electricity. Zinc readily combines with oxygen to form \[ZnO\]. Pure zinc does not react with non-oxidising acids (\[HCl\] or \[{{H}_{2}}S{{O}_{4}})\] but the impure metal reacts forming \[Z{{n}^{2+}}\] ions and evolving \[{{H}_{2}}\] gas.
\[Zn+2HCl\to ZnC{{l}_{2}}+{{H}_{2}}\uparrow \]
Hot and conc. \[{{H}_{2}}S{{O}_{4}}\] attacks zinc liberating \[S{{O}_{2}}\] gas
\[Zn+2\,{{H}_{2}}S{{O}_{4}}\to ZnS{{O}_{4}}+S{{O}_{2}}+2{{H}_{2}}O\]
Zinc also reacts with both dilute (hot and cold) \[HN{{O}_{3}}\] and conc. \[HN{{O}_{3}}\] liberating nitrous oxide \[({{N}_{2}}O)\], ammonium nitrate \[(N{{H}_{4}}N{{O}_{3}})\] and nitrogen dioxide \[(N{{O}_{2}})\] respectively.
\[4Zn+10HN{{O}_{3}}\] (warm, dilute) \[\to \] \[4\,Zn{{(N{{O}_{3}})}_{2}}+{{N}_{2}}O+5\,{{H}_{2}}O\]
\[4Zn+10HN{{O}_{3}}\] (coldvery dilute)\[\to \] \[4Zn{{(N{{O}_{3}})}_{2}}+N{{H}_{4}}N{{O}_{3}}+3{{H}_{2}}O\]
\[Zn+4HN{{O}_{3}}\](hot and conc.)\[\to Zn{{(N{{O}_{3}})}_{2}}+2N{{O}_{2}}+2{{H}_{2}}O\]
Zinc dissolves in hot concentrated \[NaOH\] forming the soluble sod. Zincate
\[Zn+2NaOH+2{{H}_{2}}O\to N{{a}_{2}}[Zn{{(OH)}_{4}}]+{{H}_{2}}\]
or \[Zn+2NaOH\to N{{a}_{2}}Zn{{O}_{2}}+{{H}_{2}}\]
(3) Special varieties of zinc. (i) Zinc dust : It is prepared by melting zinc and then atomising it with a blast of air.
(ii) Granulated zinc : It is prepared by pouring molten zinc into cold water.
Both these varieties of zinc are used as reducing agents in laboratory.
Compounds of zinc
(1) Zinc oxide (Zinc white or Chinese white), ZnO : It is obtained by burning zinc in air or by heating zinc carbonate or zinc nitrate.
\[2Zn+{{O}_{2}}\xrightarrow{\text{Heat}}2ZnO\]
\[ZnC{{O}_{3}}\xrightarrow{\text{Heat}}ZnO+C{{O}_{2}}\]
\[2Zn{{(N{{O}_{3}})}_{2}}\xrightarrow{\text{Heat}}2ZnO+4N{{O}_{2}}+{{O}_{2}}\]
It is a white powder but becomes yellow on heating and again white on cooling.
It is insoluble in water and is very light and hence commonly known as philosopher’s wool.
It is amphoteric in nature.
\[\underset{\text{(Basic)}}{\mathop{ZnO}}\,+2HCl\to ZnC{{l}_{2}}+{{H}_{2}}O\]
\[\underset{\text{(Acidic)}}{\mathop{ZnO}}\,+2NaOH\to \underset{\text{Sod}\text{.zincate}}{\mathop{N{{a}_{2}}Zn{{O}_{2}}}}\,+{{H}_{2}}O\]
or \[ZnO+2NaOH+{{H}_{2}}O\to \underset{\text{Sod}\text{. tetrahydroxozincate (II)}}{\mathop{N{{a}_{2}}[Zn{{(OH)}_{4}}]}}\,\]
It is reduced both by carbon and \[{{H}_{2}}\] and is used as a white paint
\[ZnO+C\to Zn+CO\]; \[ZnO+{{H}_{2}}\to Zn+{{H}_{2}}O\]
(2) Zinc chloride, \[\mathbf{ZnC}{{\mathbf{l}}_{\mathbf{2}}}\] : It is obtained when \[Zn\] metal, \[ZnO\] or \[ZnC{{O}_{3}}\] is treated with dil. HCl. It crystallizes as \[ZnC{{l}_{2}}.2{{H}_{2}}O\] and becomes anhydrous on heating. \[ZnC{{l}_{2}}\] is highly deliquescent and is highly soluble in \[{{H}_{2}}O\] and also readily dissolves in organic solvents like acetone, alcohol, ether etc. its aqueous solution is acidic due to hydrolysis.
\[ZnC{{l}_{2}}+{{H}_{2}}O\to Zn(OH)Cl+HCl\]
Anhydrous \[ZnC{{l}_{2}}\] is used as a Lewis acid catalyst in organic reactions. Mixed with moist zinc oxide, it is used for filling teeth and its solution is used for preserving timber. Anhydrous \[ZnC{{l}_{2}}\] used as a Lucas reagent with conc. HCl.
(3) Zinc sulphide, ZnS : It is a white solid. It is soluble in dil. HCl and thus does not get precipitated by \[{{H}_{2}}S\] in the acidic medium.
\[ZnS+2HCl\to ZnC{{l}_{2}}+{{H}_{2}}S\].
It is a constituent of lithopone \[(ZnS+BaS{{O}_{4}})\]
(4) Zinc sulphate, \[\mathbf{ZnS}{{\mathbf{O}}_{\mathbf{4}}}\mathbf{.7}{{\mathbf{H}}_{\mathbf{2}}}\mathbf{O}\] : It is commonly known as white vitriol and is obtained by the action of dil. \[{{H}_{2}}S{{O}_{4}}\] on zinc metal, ZnO or \[ZnC{{O}_{3}}\]. On heating, it first loses six molecules of water of crystallization at 373 K. At 723 K, it becomes anhydrous and on further heating, it decomposes.
\[ZnS{{O}_{4}}.7{{H}_{2}}O\xrightarrow{373\,K}ZnS{{O}_{4}}.{{H}_{2}}O\xrightarrow{723\,K}\]
\[2ZnS{{O}_{4}}\xrightarrow{1073\,K}2ZnO+2S{{O}_{2}}+{{O}_{2}}\]
It is used to prepare lithopone \[(BaS{{O}_{4}}+ZnS)\], a white paint and also in galvanising iron.
\[ZnS{{O}_{4}}+BaS\to ZnS+BaS{{O}_{4}}\]
It is also used as an eye lotion.
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