Question

${\mathrm{Cr}}_2{{\mathrm{O}}_7}^{2-}\stackrel{\mathrm{pH}=\mathrm{x}}{\to }{{\mathrm{CrO}}_4}^{2-}\stackrel{\mathrm{pH}=\mathrm{y}}{\to }{\mathrm{Cr}}_2{{\mathrm{O}}_7}^{2-}$
pH values x and y can be:

Answer 1

Change in pH results in change in concentration:-

• ${\mathrm{Cr}}_2{{\mathrm{O}}_7}^{2-}$is called the Dichromate ion and $\mathrm{Cr}{{\mathrm{O}}_4}^{2-}$is called the Chromate ion.
• Depending upon the pH of the solution, the ions can change into each other.
• In equilibrium they exist in aqueous solution:

$2{\mathrm{CrO}}_4^{2-}+2{\mathrm{H}}^{+}\rightleftharpoons {\mathrm{Cr}}_2{\mathrm{O}}_7^{2-}+{\mathrm{H}}_2\mathrm{O}$

• However, the oxidation state in both the ions is the same $+6$.
• When the reaction takes place in an alkaline medium(base), then the concentration of ${\mathrm{H}}^{+}$ions decreases.
• As a result of this, the equilibrium of the solution shifts in the backward direction, and hence Chromate ion is formed.
• On the other hand, when the reaction takes place in an acidic medium, then the concentration of ${\mathrm{H}}^{+}$ions increases in the solution.
• As a result of this, the equilibrium of the solution shifts in the forward direction, and hence Dichromate ion is formed.
• Thus we can say that, for ${\mathrm{Cr}}_2{{\mathrm{O}}_7}^{2-}\stackrel{\mathrm{pH}=\mathrm{x}}{\to }{{\mathrm{CrO}}_4}^{2-}$, the value of pH should be more than 7, as the reaction is taking place in an alkaline medium.
• Thus we can say that, for ${{\mathrm{CrO}}_4}^{2-}\stackrel{\mathrm{pH}=\mathrm{y}}{\to }{\mathrm{Cr}}_2{{\mathrm{O}}_7}^{2-}$, the value of pH should be less than 7, as the reaction is taking place in an acidic medium.

Therefore, the value of $\mathrm{x}$ should be more than 7$(\mathrm{x}>7)$and the value of $\mathrm{y}$should be less than 7$(\mathrm{y}<7)$.