Problem 5 - A Complete Resource Book in Chemistry for JEE Main

Question

Which type of ion does the anhydrons mixture of KF and HF contain? (a) \(\mathrm{KH}^{+}, \mathrm{F}\) (b) \((\mathrm{KF})^{+}(\mathrm{HF})\) (c) \(\mathrm{K}^{+}, \mathrm{H}^{+}, \mathrm{F}^{-}\) (d) \(\mathrm{K}^{+}, \mathrm{HF}_{2}^{-}\)

Step-by-Step Solution

Answer

The anhydrons mixture of KF and HF contains K⁺ and HF₂⁻ ions (Option d).

Step 1: Understanding Anhydrons Mixture

When dealing with an anhydrons mixture of KF and HF, the lack of water indicates possible unusual ionic forms due to their interactions. Specifically, HF can form hydrogen bonds and change into other ions with fluorides.

Step 2: Analyze KF and HF Ions Individually

Potassium fluoride (KF) forms ions such as K⁺ and F⁻ when dissolved, while hydrogen fluoride (HF) typically forms H⁺ and F⁻ ions, although in particular conditions like in mixtures it can form complex ions or remain as HF.

Step 3: Presence of Complex Ions in the Mixture

In the special conditions of an anhydrons mixture, HF can interact with F⁻ from KF to form a bifluoride ion, HF₂⁻, rather than dissociating into H⁺ and F⁻ ions. Therefore, the mixture can involve complex ions like HF₂⁻.

Step 4: Identify Correct Ion Composition

Based on the interactions and complex ions formed, the ion composition of the anhydrons mixture of KF and HF commonly includes K⁺ ions and HF₂⁻ ions formed by the interaction of HF with F⁻ ions.

Key Concepts

Anhydrons Mixture

In chemistry, an "anhydrons mixture" refers to a mixture that does not contain water. When we talk about anhydrous mixtures involving ionic compounds, we consider how these substances interact without water's influence. Specifically, for the mixture of potassium fluoride (KF) and hydrogen fluoride (HF), the absence of water leads to unique interactions.
In a typical aqueous solution, ionic compounds dissociate fully into their respective ions. However, in an anhydrous mixture, the behavior changes. Without water, ions might form unusual species due to direct interaction. For KF and HF, this means potential formation of complex ions. HF can particularly participate in hydrogen bonding or other transformations when interacting with KF. This interaction is crucial because it results in the formation of ions that wouldn't normally exist in a regular aqueous solution.
Keeping these interactions in mind helps us understand the resulting ionic composition present in the mixture.

Bifluoride Ion

The bifluoride ion, represented as \( \text{HF}_2^- \), is a complex ion formed under specific conditions, like in an anhydrous mixture of KF and HF. This ion results from the reaction where hydrogen fluoride (HF) interacts with fluoride ions (F⁻).

• Formation: In an anhydrous environment, HF does not fully dissociate into H⁺ and F⁻ ions. Instead, a unique reaction occurs. HF can accept another F⁻ ion from KF, forming the bifluoride ion \( \text{HF}_2^- \).
• Stability: The stabilization comes from hydrogen bonding within the structure between HF molecules and F⁻ ions.

The formation of \( \text{HF}_2^- \) illustrates the remarkable way in which ions can rearrange in the absence of water, showcasing the diverse nature of chemical bonding and interactions.

Dissociation of HF

Hydrogen fluoride (HF) behaves differently in terms of dissociation depending on its environment. In aqueous solutions, HF is recognized for its weak acid properties, not completely dissociating into hydrogen (H⁺) and fluoride (F⁻) ions.
However, in anhydrous conditions, like the mixture with potassium fluoride (KF), HF's dissociation is altered. While some HF molecules might remain intact, others can take part in forming the bifluoride ion \( \text{HF}_2^- \). This interaction limits the production of free H⁺ ions since HF prefers to engage in complex ion formations rather than dissociating completely.
This behavior underscores how the dissociation of HF is heavily influenced by the surrounding environment. In the absence of water, HF interacts in unexpected ways, emphasizing the flexibility and variability in chemical bonding.