Question

In which case there is change in oxidation number:
A.Aqueous solution of $CrO_4^{2 - }$ is acidified
B.$S{O_2}$ gas is passed into $C{r_2}O_7^{2 - }$
C.$C{r_2}O_7^{2 - }$ solution is made alkaline
D.$Cr{O_2}C{l_2}$ is dissolved in $NaOH$

Answer 1

Hint: To answer this question, you need to calculate the change occurring in the oxidation state of the central metal ion in the reactant state to that in the product state. You must recall the reactions of coordination complexes.

Complete step by step solution:
In option A, the reaction is the acidification of $CrO_4^{2 - }$ solution.
The chemical reaction can be written as: $2{\left[ {Cr{O_4}} \right]^{2 - }} + 2{H^ + } \to {\left[ {C{r_2}{O_7}} \right]^{2 - }} + {H_2}O$
The oxidation state of chromium in ${\left[ {C{r_2}{O_7}} \right]^{2 - }}$ is equal to $ + 6$ and that in $CrO_4^{2 - }$ is also $ + 6$
Thus, we can say there is no change in the oxidation number.
In option B, $S{O_2}$ gas is passed into $C{r_2}O_7^{2 - }$
The chemical reaction can be written as: \[{\left[ {C{r_2}{O_7}} \right]^{2 - }} + 3S{O_2} + 2{H^ + } \to 2C{r^{3 + }} + 3SO_4^{2 - } + {H_2}O\]
The oxidation state of chromium in ${\left[ {C{r_2}{O_7}} \right]^{2 - }}$ is equal to $ + 6$ which changes to $ + 3$. Thus, there is a change in the oxidation number.

Thus, the correct answer is B.

Additional Information:
- Coordination compounds are a type of additional compounds. Coordination compounds are the compounds in which a central metal atom is linked by coordination bond to a number of ligands, which may either be ions or neutral molecules, i.e. by donation of lone pairs by the ligands to the central metal atom.
- If this compound carries a positive or negative charge, it is called a complex ion. These complex ions are relatively stable and they do not lose their identity in aqueous solution like double salts.
- Apart from coordination compounds, the other types of addition compounds are double salts. Unlike coordination compounds, double salts are stable only in crystalline state and lose their identity in solution form.

Note: If there is an increase in the oxidation number of an element, the element is said to have been oxidized and if there is a decrease in the oxidation number, the element is said to be reduced. The sum of the oxidation states of all the atoms in a compound must be zero and equal to the charge in case of ions.