Question

When hydrogen peroxide is added to acidified potassium dichromate, a blue colour
is produced due to the formation of?
A) \[Cr{{O}_{3}}\]
B) \[C{{r}_{2}}{{O}_{3}}\]
C) \[Cr{{O}_{5}}\]
D) \[Cr{{O}_{4}}^{2-}\]
E) \[Cr{{O}_{7}}^{2-}\]

Answer 1

Hint: Potassium dichromate is a very strong oxidizing agent that oxidises primary alcohols, secondary alcohols and aldehydes. However, it is not as strong an oxidizing agent as potassium permanganate.

Complete answer: Potassium dichromate is an orange crystalline ionic solid. When it is added in alkali solutions, its colour changes from orange to yellow as a result of the redox reaction of dichromate ions.

Now, when we add hydrogen peroxide to acidified potassium dichromate, hydrogen peroxide will act as an oxidizing agent, as it gets reduced, and its oxidation number changes to -2 from -1 and the chromium present in +6 oxidation state changes to +3 oxidation state. As a final result, we will get chromium (VI) pentoxide, also known as chromium (VI) peroxide. Chromium (VI) pentoxide is blue in color, so we get a blue colour after the reaction. This decomposes rapidly in aqueous solution into \[C{{r}^{3+}}\] and dioxygen.
\[{{K}_{2}}C{{r}_{2}}{{O}_{7}}+4{{H}_{2}}{{O}_{2}}+{{H}_{2}}S{{O}_{4}}\to 2Cr{{O}_{5}}+{{K}_{2}}S{{O}_{4}}+5{{H}_{2}}O\]


Additional Information:
>In an alkaline solution with 30% hydrogen peroxide, a red-brown compound \[{{K}_{2}}C{{r}_{3}}{{O}_{8}}\] is formed which is a tetra peroxo species \[\,{{[Cr{{({{O}_{2}})}_{4}}]}^{-}}\] and contains Cr(+V).
>In ammonia solution the dark red brown compound \[{{(N{{H}_{3}})}_{3}}Cr{{O}_{4}}\] is formed which contains Cr(+IV).

So, the correct option is (c).

Note: Potassium dichromate, as other Cr (VI) compounds, is toxic, mutagenic and carcinogenic. It is also extremely dangerous to the environment. The mechanism of reaction of decomposition of hydrogen peroxide catalyzed by potassium dichromate.