\( \mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7} \) acts as a good oxidizing agent in acidic med...
0\( \mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7} \) acts as a good oxidizing agent in acidic medium.
In alkaline solution, orange colour of \( \mathrm{Cr}_{2} \mathrm{O}_{7}^{2-} \) changes to yellow colour due to formation of \( \mathrm{CrO}_{4}^{2-} \) and again yellow colour changes to orange colour on changing the solution to acidic medium.
\( \mathrm{CrO}_{4}^{2-} \) and \( \mathrm{Cr}_{2} \mathrm{O}_{7}^{2-} \) exist in equilibrium at \( \mathrm{pH}=4 \) and are interconvertible by altering the \( \mathrm{pH} \) of the solution. When heated with \( \mathrm{H}_{2} \mathrm{SO}_{4} \) and metal chloride, \( \mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7} \) gives vapours of chromyl chloride \( \left(\mathrm{CrO}_{2} \mathrm{Cl}_{2}\right) \), Chromyl chloride \( \left(\mathrm{CrO}_{2} \mathrm{Cl}_{2}\right) \) when passed into aqueous \( \mathrm{NaOH} \) solution, yellow colour solution of \( \mathrm{CrO}_{4}^{2-} \) is obtained. This on reaction with lead acetate gives yellow \( \mathrm{ppt} \). of \( \mathrm{PbCrO}_{4} \).
When \( \mathrm{H}_{2} \mathrm{O}_{2} \) is added to an acidified solution of dichromate ion, a complicated reaction occurs. The products obtained depends on the \( \mathrm{pH} \) and concentration of \( \mathrm{Cr} \).
\[
\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}+2 \mathrm{H}^{+}+4 \mathrm{H}_{2} \mathrm{O}_{2} \longrightarrow 2 \mathrm{CrO}\left(\mathrm{O}_{2}\right)_{2}+5 \mathrm{H}_{2} \mathrm{O}
\]
A deep blue-violet coloured peroxo compound, \( \mathrm{CrO}\left(\mathrm{O}_{2}\right)_{2} \), called chromic peroxide is formed. This decomposes rapidly in aqueous solution into \( \mathrm{Cr}^{3+} \) and oxygen.
The \( \mathrm{CrO}_{3} \) on reaction with \( \mathrm{HCl} \) and \( \mathrm{NaOH}(a q) \) gives respectively :
(A) \( \mathrm{CrO}_{2} \mathrm{Cl}_{2}, \mathrm{CrO}_{4}^{2-} \)
(B) \( \mathrm{Cr}(\mathrm{OH})_{2}, \mathrm{CrO}_{4}^{2-} \)
(C) \( \mathrm{Cl}_{2}, \mathrm{Cr}_{2} \mathrm{O}_{7}^{2-} \)
(D) \( \mathrm{Cl}_{2}, \mathrm{Cr}(\mathrm{OH})_{3} \)
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