Of Cr $(\mathrm{VI})$ as $\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}$ and $\mathrm{CrO}_{4}^{2-}$, which is better…

Of Cr $(\mathrm{VI})$ as $\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}$ and $\mathrm{CrO}_{4}^{2-}$, which is better oxidising agent?
  1. $\mathrm{CrO}_{4}^{2-}$, basic medium
  2. $\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}$, basic medium
  3. $\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}$, acidic medium
  4. $\mathrm{CrO}_{3}$, basic medium

Solution

$\mathrm{CrO}_{4}^{2-}(\mathrm{aq})+4 \mathrm{H}_{2} \mathrm{O}+3 \mathrm{e}^{-} \longrightarrow$
$\mathrm{Cr}(\mathrm{OH})_{3}+5 \mathrm{OH}^{-}$,
$\mathrm{E}^{\mathrm{o}}=-0.13 \mathrm{~V}$
$\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}$ (aq.) $+14 \mathrm{H}^{+}$ (aq.) $+6 \mathrm{e}^{-} \longrightarrow$
$2 \mathrm{Cr}^{3+}$ (aq.) $+7 \mathrm{H}_{2} \mathrm{O}$,
$\mathrm{E}^{\mathrm{o}}=1.33 \mathrm{~V}$ .

Asked in: JEE-TOPICTESTS-CHEMISTRY

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