Problem 116 - A Complete Resource Book in Chemistry for JEE Main

Question

Consider the following transformations: (1) \(\mathrm{XeF}_{6}+\mathrm{NaF} \longrightarrow \mathrm{Na}^{+}\left[\mathrm{XeF}_{7}\right]^{-}\) (2) \(2 \mathrm{PCl}_{5}(\mathrm{~s}) \longrightarrow\left[\mathrm{PCl}_{4}\right]^{+}\left[\mathrm{PCl}_{6}\right]^{-}\) (3) \(\left[\mathrm{Al}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}+\mathrm{H}_{2} \mathrm{O} \longrightarrow\) \(\left[\mathrm{Al}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5} \mathrm{OH}\right]^{2+}+\mathrm{H}_{3} \mathrm{O}^{+}\) The possible transformations are (a) 1,2 and 3 (b) 1 and 3 only (c) 1 and 2 only (d) 2 only

Step-by-Step Solution

Answer

(a) 1, 2, and 3.

Step 1: Identifying Transformation Types

First, identify the types of transformations. (1) involves ion formation with xenon hexafluoride reacting with sodium fluoride. (2) shows solid phosphorus pentachloride dissociating into ionic species. (3) describes an acid-base reaction where a water molecule reacts with a hexa-aqua aluminum ion to form a hydroxide complex and hydronium ion.

Step 2: Analyzing Each Transformation

Examine each transformation individually. (1) is a Lewis acid-base reaction where xenon hexafluoride accepts a fluoride ion from sodium fluoride forming a heptafluoroxenate ion. (2) phosphorus pentachloride, a solid molecular compound, dissociates into \[\mathrm{PCl}_4\]^{+} and \[\mathrm{PCl}_6\]^{-} ions. This is a process of de-molecularization into ionic species. (3) involves the transfer of a proton from a water molecule to the complex ion, resulting in the formation of hydronium and hydroxide complexes, a typical acid-base equilibrium.

Step 3: Evaluating Possible Transformations

Look at the provided options to evaluate which combination of transformations can occur based on our analysis. Transformations (1), (2), and (3) match the criteria of their types of reaction, all are feasible under their conditions.

Step 4: Choosing the Correct Option

Since all 1, 2, and 3 transformations are possible as analyzed earlier, the correct choice is the option that includes all three. Therefore, the answer is (a) 1, 2, and 3.

Key Concepts

Lewis acid-base reaction

In a Lewis acid-base reaction, we encounter the exchange of electron pairs between molecules. The basic idea revolves around Lewis acids and bases:

• Lewis acid: An electron pair acceptor. It is often a molecule that lacks electrons, hence it strives to gain them.
• Lewis base: An electron pair donor. Typically, it has excess electrons and readily donates them.

In the exercise, the first transformation of xenon hexafluoride (\(\text{XeF}_6\)) reacting with sodium fluoride (\(\text{NaF}\)) is a clear example of a Lewis acid-base reaction. Here, \(\text{XeF}_6\) is the Lewis acid. It accepts a fluoride ion, \(\text{F}^-\), from sodium fluoride, the Lewis base, resulting in the formation of a heptafluoroxenate ion, \([\text{XeF}_7]^−\). This process highlights the basic principle of electron pair donation and acceptance, facilitating the transformation.

Ion formation

Ion formation is a key concept in understanding chemical transformations where neutral compounds break apart or combine to form ions. In the exercise, the second transformation highlights this phenomenon with phosphorus pentachloride (\(\text{PCl}_5\)).
Phosphorus pentachloride, which is initially a solid molecular compound, undergoes segregation into its ionic constituents:

• Positive ion: \([\text{PCl}_4]^+\)
• Negative ion: \([\text{PCl}_6]^-\)

This occurs as the bond within \(\text{PCl}_5\) breaks, leading to the separation of positive and negative ions. This process exemplifies the transition from a covalent molecular structure to an ionic form, a process that is critical in the formation of ionic compounds and is vital for understanding reaction mechanisms in chemistry.

Acid-base equilibrium

Acid-base equilibrium involves the balance between acids and bases in a reaction, often tied to the transfer of protons (H+ ions). In the exercise, the third transformation showcases this with \(\left[\mathrm{Al}\left(\mathrm{H}_2 \mathrm{O}\right)_6\right]^{3+}\) reacting with water.
Here, a proton from a water molecule is transferred to the aluminum aqua complex, resulting in:

• Hydroxide complex: \(\left[\mathrm{Al}\left(\mathrm{H}_2 \mathrm{O}\right)_5 \mathrm{OH}\right]^{2+}\)
• Hydronium ion: \(\mathrm{H}_3 \mathrm{O}^+\)

This transformation is a typical example of a reversible acid-base reaction. It emphasizes the dynamic nature of such reactions where an equilibrium between the reactants and products forms. This equilibrium is a central concept in understanding the behavior and properties of acids and bases in chemical systems.