Chemical Properties and Structure of Dichromate Ions

To determine the correct statements, let us analyze each statement individually based on the chemical properties and structure of chromium compounds:

1. Statement A: Potassium dichromate ($\text{K}_2\text{Cr}_2\text{O}_7$) is a well-known strong oxidizing agent in acidic medium. It oxidizes ferrous sulphate ($\text{FeSO}_4$, where Fe is in $+2$ oxidation state) to ferric sulphate ($\text{Fe}_2(\text{SO}_4)_3$, where Fe is in $+3$ oxidation state). The balanced ionic equation for this redox reaction is: $\text{Cr}_2\text{O}_7^{2-} + 14\text{H}^+ + 6\text{Fe}^{2+} \longrightarrow 2\text{Cr}^{3+} + 6\text{Fe}^{3+} + 7\text{H}_2\text{O}$ Therefore, Statement A is correct.

2. Statement B: Sodium dichromate ($\text{Na}_2\text{Cr}_2\text{O}_7$) is hygroscopic in nature (it readily absorbs moisture from the atmosphere) and is more soluble than potassium dichromate. Hence, its exact mass cannot be accurately weighed to prepare a standard solution directly. Thus, sodium dichromate cannot be used as a primary standard in volumetric analysis, whereas potassium dichromate is used. Therefore, Statement B is incorrect.

3. Statement C: Chromate ($\text{CrO}_4^{2-}$) and dichromate ($\text{Cr}_2\text{O}_7^{2-}$) ions exist in a dynamic equilibrium in aqueous solution, which depends strongly on the pH of the medium: $2\text{CrO}_4^{2-} + 2\text{H}^+ \rightleftharpoons \text{Cr}_2\text{O}_7^{2-} + \text{H}_2\text{O}$

   • In acidic medium (low pH), yellow chromate ion converts into orange dichromate ion.
   • In basic medium (high pH), dichromate ion converts back to chromate ion. Therefore, Statement C is correct.

4. Statement D: The dichromate ion ($\text{Cr}_2\text{O}_7^{2-}$) consists of two tetrahedral $\text{CrO}_4$ units sharing one oxygen atom at a corner. The bridging $\text{Cr-O-Cr}$ bond angle in the dichromate ion is experimentally found to be $126^\circ$. Therefore, Statement D is correct.

Thus, statements A, C, and D are correct.

Correct Option: B (A, C and D Only)