Animated Solution for Chemistry - d and f-Block Elements: An acidified solution of potassium chromate was layered with an equal volume of amyl alcohol. When it was shaken after the addition of 1 mL of 3% H2O2, a blue alcohol layer was obtained. The blue color is due to the formation of a chromium (VI) compound 'X' . What is the number of oxygen atoms bonded to chromium through only single bonds in a molecule of X?

The magic of chemistry often reveals itself in striking colors, and the reaction described in this problem is one of the most visually captivating tests in qualitative analysis. When an acidified solution of potassium chromate ($K_{2}Cr{O}_4$) is treated with hydrogen peroxide ($H_2{O}_2$), a deep blue color emerges. But what exactly is this blue compound, and why does its structure matter so much? Let's dive into the fascinating world of chromium chemistry.

In an acidic medium, the chromate ion ($Cr{O}_4^{2-}$) is in equilibrium with the dichromate ion ($C{r}_2{O}_7^{2-}$). When hydrogen peroxide is added, it acts as an oxidizing agent in a very specific way, leading to the formation of a new compound: Chromium Pentoxide ($Cr{O}_5$).

The reaction can be summarized as:

This newly formed $Cr{O}_5$ is the mysterious compound 'X' responsible for the brilliant blue color. However, there is a catch. Chromium pentoxide is highly unstable in aqueous solutions. If left alone in water, it rapidly decomposes into chromium(III) ions ($C{r}^{3+}$) and oxygen gas, causing the blue color to quickly fade into a pale green.

To capture and observe this blue compound, we use a clever trick: solvent extraction. By layering the aqueous solution with an organic solvent like amyl alcohol (or diethyl ether) and shaking the mixture, the $Cr{O}_5$ is extracted into the organic layer.

In the non-polar environment of amyl alcohol, the $Cr{O}_5$ molecules are stabilized, allowing the blue layer to persist. This is why the problem specifically mentions the formation of a "blue alcohol layer."

Now, let's address the core of the question: the structure of $Cr{O}_5$. If we were to blindly calculate the oxidation state of chromium in $Cr{O}_5$ using standard rules (assuming all oxygens are $-2$), we would get:

This is chemically impossible! Chromium belongs to Group 6 of the periodic table, meaning it only has 6 valence electrons ($4s^{1}3d^5$). Its maximum possible oxidation state is $+6$. The paradox of a $+10$ oxidation state tells us that not all oxygen atoms in $Cr{O}_5$ are standard oxide ions ($O^{2-}$).

The reality is that $Cr{O}_5$ adopts a unique and beautiful geometry known as the butterfly structure.

In this structure, the central chromium atom is bonded to five oxygen atoms in two distinct ways: 1. One Oxo Linkage: The chromium atom forms a standard double bond ($Cr=O$) with one oxygen atom. This oxygen has an oxidation state of $-2$. 2. Two Peroxy Linkages: The remaining four oxygen atoms form two peroxy groups ($-O-O-$). In a peroxy linkage, the oxygen atoms are bonded to each other via a single bond, and each oxygen has an oxidation state of $-1$.

Let's recalculate the oxidation state with this new structural knowledge: This perfectly aligns with chromium's maximum oxidation state, confirming that the butterfly structure is correct.

The question specifically asks for the number of oxygen atoms bonded to chromium through only single bonds. Looking at the butterfly structure, the "wings" are formed by the four peroxy oxygen atoms. Each of these four atoms is connected to the central chromium atom via a single bond.

Therefore, there are exactly 4 oxygen atoms bonded to chromium through single bonds.

This problem is a beautiful reminder that chemical formulas don't always tell the whole story. Sometimes, you have to look at the geometry and the types of bonds to truly understand the nature of a molecule!