Question

When hydrogen peroxide is added to acidified potassium dichromate, the blue colour produced is due to the formation of

• A. ${\mathrm{Cr}}_2{\mathrm{O}}_3$
• B. ${\mathrm{CrO}}_5$
• C. ${\mathrm{CrO}}_3$
• D. ${\mathrm{Cr}}_2{{\mathrm{O}}_7}^{2-}$

Answer 1

The correct option is B

${\mathrm{CrO}}_5$

Explanation for Correct option: (b) ${\mathrm{CrO}}_5$

Step 1:

• Acidified potassium dichromate on reaction
• ${\mathrm{K}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7\left(\mathrm{aq}\right)+{\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)\to {\mathrm{K}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)+{\mathrm{H}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7\left(\mathrm{aq}\right)$
• ${\mathrm{H}}_2{\mathrm{O}}_2$on breaking down we get:
• $[{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{H}}_2\mathrm{O}+(\mathrm{O})]*4$

Step 2:

• $$\begin{gathered} {\mathrm{H}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7\left(\mathrm{aq}\right)+4\left(\mathrm{O}\right)\left(\mathrm{g}\right)\to 2{\mathrm{CrO}}_5\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{aq}\right) \\ \left(\mathrm{Blue}\right) \end{gathered}$$

Step 3:

• Chromium Pentaoxide production results in the development of blue color from orange (as potassium dichromate is orange in color).
• Because chromium pentaoxide is bluish, the reaction results in blue color.
• $$\begin{gathered} {\mathrm{K}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7\left(\mathrm{aq}\right)+{\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)+4{\mathrm{H}}_2{\mathrm{O}}_2\left(\mathrm{aq}\right)\to 2{\mathrm{CrO}}_5\left(\mathrm{aq}\right)+{\mathrm{K}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)+5{\mathrm{H}}_2\mathrm{O}\left(\mathrm{aq}\right) \\ \left(\mathrm{orange}\right)\left(\mathrm{Blue}\right) \end{gathered}$$
• Hence is the correct option.

Explanation for incorrect options:

(a) ${\mathrm{Cr}}_2{\mathrm{O}}_3$

• ${\mathrm{Cr}}_2{\mathrm{O}}_3$ is yellowish-green in color
• Hence this option is not correct

(c) ${\mathrm{CrO}}_3$

• ${\mathrm{CrO}}_3$ is dark purplish in color
• Hence this option is not correct

(d) ${\mathrm{Cr}}_2{{\mathrm{O}}_7}^{2-}$

• ${\mathrm{Cr}}_2{{\mathrm{O}}_7}^{2-}$ is yellow in color
• Hence this option is not correct

Hence the correct option is (b) ${\mathrm{CrO}}_5$