Question

Aqueous solution of ${{\text{K}}_{2}}\text{C}{{\text{r}}_{2}}{{\text{O}}_{7}}$ (orange) changes to yellow on the addition of an alkali. This is because of the formation of:
(A) $\text{C}{{\text{r}}^{6+}}$
(B) $\text{CrO}_{4}^{-2}$
(C) $\text{C}{{\text{r}}_{2}}\text{O}_{7}^{-2}$
(D) None of the above

Answer 1

For this problem, firstly we have to write the complete reaction and then the ions which will be released by the reactant species will affect the solution colour. The final product form will have a different colour.

Complete step by step solution:
-In the given question, we have to explain the change in colour of the aqueous solution of potassium dichromate when alkali is added in the solution.
-Now, it is observed that when the electronic configuration of an atom is either half-filled and full-filled then the colour of the complex is colourless.
-So, the different configuration of the atom will show different colours of the solution.
-Now, in the question potassium dichromate is given which has an orange colour. When the aqueous solution of potassium dichromate reacts with the alkali such as potassium hydroxide.
-Then it yields a compound known as potassium chromate which is obtained by the addition of hydrogen atom into the dichromate ion.
-The balanced chemical reaction is:
$\text{C}{{\text{r}}_{2}}\text{O}_{7}^{2-}\text{ + }{{\text{H}}^{+}}\to \text{ C}{{\text{r}}_{2}}\text{O}_{4}^{2-}$
-So, as we can see that dichromate ion is converted into chromate ion by the removal of oxygen atom due to which the colour solution also changes from orange to yellow.

Therefore, option (B) is the correct answer.

Note: In the above reaction, the pH of the solution also changes which can be easily determined by using a suitable indicator such as methyl orange, litmus paper, etc. Methyl orange is usually in the titration process.