Question

For $Cr{{O}_{3}}$, which of the following is not a true statement?
(a)- It is called chromic acid
(b)- It is colorless due to 3d-configuration
(c)- It is a bright orange solid and color areas are due to charge transfer
(d)- It is toxic and corrosive

Answer 1

Hint: $Cr{{O}_{3}}$ is called Chromium trioxide. It has two color forms, when it is in the anhydrous condition it has a different color and when it is wet it has a different color. It is carcinogenic which means it causes cancer.

Complete step by step answer:
$Cr{{O}_{3}}$ is an inorganic compound which is called Chromium trioxide. It is also known as chromium(VI)oxide or chromic anhydride. The oxidation number of chromium in $Cr{{O}_{3}}$ is +6.
It is marketed under the name of chromic acid because it is the acidic anhydride of chromic acid.
This compound has two colors: when it is in the anhydrous form it is a dark-purple solid and in the wet form it is bright orange solid. The color is due to charge transfer even though the chromium has no electrons in d-orbital.
It is carcinogenic (it causes cancer) and is a powerful oxidizer, hence it is highly toxic and corrosive.
It will ignite organic compounds such as alcohol on contact because it is a powerful oxidizer.
It is mainly used for electroplating and a huge amount of $Cr{{O}_{3}}$ is produced annually only for electroplating.
When sodium dichromate is treated with sulfuric acid chromium trioxide is formed. The reaction is given below:
${{H}_{2}}S{{O}_{4}}+N{{a}_{2}}C{{r}_{2}}{{O}_{7}}\to 2Cr{{O}_{3}}+N{{a}_{2}}S{{O}_{4}}+{{H}_{2}}O$

From the above discussion, the option (b)- It is colorless due to 3d-configuration is incorrect about $Cr{{O}_{3}}$. So, the correct answer is “Option B”.

Note: In organic chemistry, it is used for the synthesis of acetic acid or acetone. Since it is a powerful oxidizing agent it is used directly to convert alcohols to carboxylic acid or ketones. It is also known as John’s reagent.