Question

${\text{C}}{{\text{r}}_2}{\text{O}}_7^{2 - }\xrightarrow{{{\text{pH = x}}}}{\text{CrO}}_4^{2 - }\xrightarrow{{{\text{pH = y}}}}{\text{C}}{{\text{r}}_2}{\text{O}}_7^{2 - }$
What are the pH values $x$ and $y$ ?
A. $4{\text{ and }}5$
B. $4{\text{ and 8}}$
C. ${\text{8 and 4}}$
D. ${\text{8 and 9}}$

Answer 1

The Dichromate ion $\left( {{\text{C}}{{\text{r}}_2}{\text{O}}_7^{2 - }} \right)$ changes into Chromate ion $\left( {{\text{CrO}}_4^{2 - }} \right)$ in alkaline medium whereas the Chromate ion $\left( {{\text{CrO}}_4^{2 - }} \right)$ changes into Dichromate ion $\left( {{\text{C}}{{\text{r}}_2}{\text{O}}_7^{2 - }} \right)$ in acidic medium.
An acidic medium has pH value less than $7$ and an alkaline medium has pH value greater than $7$ .

Complete step by step answer:
The two ions given in the question are commonly known as Dichromate ion $\left( {{\text{C}}{{\text{r}}_2}{\text{O}}_7^{2 - }} \right)$ and Chromate ion $\left( {{\text{CrO}}_4^{2 - }} \right)$ . They change into one another depending upon the pH of the medium.
Both the ions exists in equilibrium in aqueous solution as
$2{\text{CrO}}_4^{2 - } + 2{H^ + } \rightleftharpoons {\text{C}}{{\text{r}}_2}{\text{O}}_7^{2 - } + {{\text{H}}_2}{\text{O}}$
The oxidation state of both the dichromate and the chromate ion is $+ 6$ .
In an alkaline medium, the concentration of ${H^ + }$ ions will be less which shifts the equilibrium to backward direction and favours the formation of Chromate ion $\left( {{\text{CrO}}_4^{2 - }} \right)$ whereas in an acidic medium, the concentration of ${H^ + }$ ions will be high which shifts the equilibrium to forward direction and favours the formation of Dichromate ion $\left( {{\text{C}}{{\text{r}}_2}{\text{O}}_7^{2 - }} \right)$ .
Therefore, the Dichromate ion $\left( {{\text{C}}{{\text{r}}_2}{\text{O}}_7^{2 - }} \right)$ changes into Chromate ion $\left( {{\text{CrO}}_4^{2 - }} \right)$ in alkaline medium whereas the Chromate ion $\left( {{\text{CrO}}_4^{2 - }} \right)$ changes into Dichromate ion $\left( {{\text{C}}{{\text{r}}_2}{\text{O}}_7^{2 - }} \right)$ in acidic medium.
We know that an acidic medium has pH value less than $7$ and an alkaline medium has pH value greater than $7$ .
So, the value of $x$ should be greater than $7$ and the value of $y$ should be less than $7$ .
Hence option C is correct.

Note:
Both the dichromate and the chromate ions are strong oxidizing agents. Due to this property they have several applications. They are used in the process of chrome plating which protect the metals from corrosion and also improve their paint adhesion. The chromium compounds are mostly toxic in nature due to their oxidizing properties.