Problem 54 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

An acidified solution of potassium chromate was layered with an equal volume of amyl alcohol. When it was shaken after the addition of \(1 \mathrm{~mL}\) of \(3 \% \mathrm{H}_{2} \mathrm{O}_{2}\), a blue alcohol layer was obtained. The blue color is due to the formation of a chromium (VI) compound ' \(\mathbf{X}\). What is the number of oxygen atoms bonded to chromium through only single bonds in a molecule of \(\mathbf{X}\) ?

Step-by-Step Solution

Answer

The molecule of chromium compound 'X' has 4 oxygen atoms bonded through single bonds.

Step 1: Understand Reaction Conditions

An acidified potassium chromate solution is layered with amyl alcohol, creating two layers where the added hydrogen peroxide reacts with chromate (CrO_4^{2-}) ions. Under acidic conditions, chromate converts to dichromate (Cr_2O_7^{2-}).

Step 2: Analyze Further Reaction

When hydrogen peroxide is added to dichromate in acidic conditions, a blue chromium (VI) peroxide complex, often considered to be chromium trioxide peroxide (CrO_5), forms.

Step 3: Determine Structure of Chromium Compound 'X'

The presumed structure of CrO_5 consists of a central chromium atom bonded to three peroxo groups (O-O) and one additional oxygen. The structure is: (O=)O-Cr-(O-O)-(O-O)-(O-O).

Step 4: Count Single-bonded Oxygen Atoms

Within the CrO_5 molecule, the chromium atom is connected to three peroxo groups through single bonds, resulting in four oxygen atoms (one from each of the three O-O bonds) that are bonded to chromium through only single bonds.

Key Concepts

Chromium Trioxide Peroxide

Chromium trioxide peroxide, often referred to as CrO₅, is a fascinating compound observed when hydrogen peroxide is added to dichromate in acidic solutions. This compound is responsible for the captivating blue color displayed in the alcohol layer during the reaction process. The formation of CrO₅ involves a chromium atom bonded to peroxo groups derived from hydrogen peroxide.

Chromium in this compound is in the +6 oxidation state, indicating how it forms bonds with oxygen atoms. In CrO₅, the chromium atom binds with three peroxo (O-O) groups and one oxo (O=) group resulting in a unique blue-colored complex. This specific arrangement leads to CrO₅ being a key compound in detecting the reactivity of chromium species with peroxides.

This molecule provides an excellent example of the intriguing chemistry of transition metals, showcasing how chromium's oxidation state can facilitate interactions with oxygenated species such as peroxides.

Peroxide Complexes

Peroxide complexes are compounds that contain a bond between two oxygen atoms (O-O), known as peroxo linkage. Peroxide complexes, like CrO₅, are often characterized by intense colors. In the case of chromium trioxide peroxide, the peroxo linkage is vital for the distinctive blue color observed.

These complexes are generally formed when metals, in higher oxidation states, react with hydrogen peroxide under specific conditions such as in acidic environments. The peroxo group acts as a ligand, bonding to the metal center and stabilizing its high oxidation state.

Peroxide complexes are not only crucial in understanding metal-peroxide interactions but also highlight the role of peroxides in facilitating unique chemical transformations. They often require careful handling due to their potential to release reactive oxygen species upon decomposition.

Oxygen Single Bonds

In the structure of chromium trioxide peroxide (CrO₅), single bonds play a pivotal role in determining the molecular geometry. Specifically, each peroxo group in CrO₅ contains a pair of oxygen atoms bonded through an O-O single bond. These oxygen atoms, in turn, form single bonds with the central chromium atom.

Overall, CrO₅ features four oxygen atoms that are bonded to chromium solely through single bonds - one from each of the three peroxo attachments, where the terminal oxygen in each O-O group attaches directly to the chromium.

This single-bond organization is crucial for maintaining the stability and color properties of the CrO₅ complex. It also indicates how chromium can facilitate multiple bonding interactions simultaneously, a characteristic feature of transition metals.

Chromate-Dichromate Equilibrium

The chromate-dichromate equilibrium is a well-known chemical equilibrium that involves the interconversion between chromate ( CrO₄²⁻ ) and dichromate ( Cr₂O₇²⁻ ) ions. This equilibrium is pH-dependent and shifts towards dichromate in acidic conditions.

In the context of the exercise, when the potassium chromate solution is acidified, it facilitates the conversion to dichromate ions. This shift is necessary for the subsequent formation of the blue-colored chromium trioxide peroxide.

Understanding this equilibrium is crucial for numerous chemical processes, as it lays the groundwork for reactions involving chromium compounds, including the specialized conditions under which different species form. The dynamic nature of chromium species in solutions emphasizes the importance of precise pH control to manipulate and observe specific chemical phenomena.