Question

When aqueous sodium hydroxide is added to an aqueous solution of ${{Chromium}}\left( {{{{\rm I}{\rm I}{\rm I}}}} \right){{ions}}$, a green blue precipitate is first formed which re-dissolves to give a green solution. This green colour is due to:
A \[{\left[ {{{Cr}}{{\left( {{{{H}}_{{2}}}{{O}}} \right)}_{{6}}}} \right]^{{{3 + }}}}\]
B ${{C}}{{{r}}_{{4}}}^{{{2 - }}}$
C \[{\left[ {{{Cr}}{{\left( {{{OH}}} \right)}_{{4}}}} \right]^{{ - }}}\]
D $\left[ {{{Cr}}{{\left( {{{OH}}} \right)}_{{3}}}{{\left( {{{{H}}_{{2}}}{{O}}} \right)}_{{3}}}} \right]$

Answer 1

Hint: Aqueous ${{Chromium}}\left( {{{{\rm I}{\rm I}{\rm I}}}} \right){{ions}}$ on reaction with sodium hydroxide which is a strong base forming blue coloured solution which then turns into green colour with the chromium in same oxidation state i.e., +3.

Complete step by step answer:
Chromium is a transition metal having an atomic number 24. It is hard, lustrous and steely grey in colour
Reaction of hydroxide ions with aqueous solution of ${{Chromium}}\left( {{{{\rm I}{\rm I}{\rm I}}}} \right){{ions}}$
${\left[ {{{Cr}}{{\left( {{{{H}}_{{2}}}{{O}}} \right)}_{{6}}}} \right]^{{{3 + }}}}{{ + 3O}}{{{H}}^{{ - }}}\xrightarrow{{}}\left[ {{{Cr}}{{\left( {{{{H}}_{{2}}}{{O}}} \right)}_{{3}}}{{\left( {{{OH}}} \right)}_{{3}}}} \right]{{ + 3}}{{{H}}_{{2}}}{{O}}$
This will result in green blue colour precipitate which will then re-dissolve to form a green coloured solution
$\left[ {{{Cr}}{{\left( {{{{H}}_{{2}}}{{O}}} \right)}_{{3}}}{{\left( {{{OH}}} \right)}_{{3}}}} \right]{{ + O}}{{{H}}^{{ - }}}\xrightarrow{{}}{\left[ {{{Cr}}{{\left( {{{OH}}} \right)}_{{4}}}} \right]^{{ - }}}{{ + 3}}{{{H}}_{{2}}}{{O}}$
Thus the green colour is because of ${\left[ {{{Cr}}{{\left( {{{OH}}} \right)}_{{4}}}} \right]^{{ - }}}$.

So, the correct answer is Option C.

Additional information:
The colour of ${\left[ {{{Cr}}{{\left( {{{{H}}_{{2}}}{{O}}} \right)}_{{6}}}} \right]^{{{3 + }}}}$ is violet. However solutions often contain mixed complexes of ${{Chromium}}\left( {{{{\rm I}{\rm I}{\rm I}}}} \right){{ions}}$ which are green. It is common for ${{Cr}}\left( {{{{\rm I}{\rm I}{\rm I}}}} \right)$ to appear green in solution. Green is the expected observation when${{Chromium}}\left( {{{{\rm I}{\rm I}{\rm I}}}} \right){{ions}}$ are produced in the oxidation of alcohols and aldehydes using acidified dichromate.
Addition of aqueous ammonia to ${\left[ {{{Cr}}{{\left( {{{{H}}_{{2}}}{{O}}} \right)}_{{6}}}} \right]^{{{3 + }}}}$ initially results in the formation of grey-green precipitate${{Cr}}{\left( {{{OH}}} \right)_{{3}}}$. On further adding ammonia${{N}}{{{H}}_{{3}}}$ the precipitate dissolves to form \[{\left[ {{{Cr}}{{\left( {{{N}}{{{H}}_{{3}}}} \right)}_{{6}}}} \right]^{{{3 + }}}}\] complex which is purple in colour.Some of the chromium based reagents for oxidation are Potassium dichromate, Pyridinium chlorochromate (PCC), Collins reagent, Chromic acid $\left( {{{{H}}_{{2}}}{{Cr}}{{{O}}_{{4}}}} \right)$, Pyridinium dichromate (PDC), PFC eltc. One of the important uses of chromic acid in the synthesis is the oxidation of secondary alcohols to ketones. Tertiary alcohols are not affected by chromic acid and oxidation with acid solution of chromic acid is unsuitable for alcohols which contain acid sensitive groups or other easily oxidisable groups.

Note: The main additive in stainless steel is chromium having a melting point of \[{{1907}}{{{ }}^{{o}}}{{C}}\] with Van der waals radius ${{200 pm}}$. Chromium has got some most common oxidation states as ${{ + 6, + 3, + 2}}$. Chromium can be polished highly while resisting tarnishing.