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

(d) \(\mathrm{K}^{+}, \mathrm{HF}_{2}^{-}\)

Step 1: Understand the Components of the Mixture

The mixture consists of potassium fluoride (KF) and hydrogen fluoride (HF). KF is a salt that dissociates into potassium ions (\(\mathrm{K}^{+}\)) and fluoride ions (\(\mathrm{F}^{-}\)). HF, being a weak acid, can undergo partial ionization.

Step 2: Identify Ionization of HF

HF can ionize to produce \(\mathrm{H}^{+}\) ions and release\(\mathrm{F}^{-}\) ions. However, due to its weak nature, HF also tends to associate with fluoride ions.

Step 3: Consider the Formation of Complex Ions

In the presence of excess \(\mathrm{F}^{-}\) ions from KF, HF can form a complex ion called bifluoride,\(\mathrm{HF}_{2}^{-}\), which indicates the combination of a hydrogen ion with two fluoride ions.

Step 4: Match the Ions with the Answer Choices

Based on the identified ions, \(\mathrm{K}^{+}\) ions come from KF, and\(\mathrm{HF}_{2}^{-}\) ions form due to the presence of both KF and HF. The remaining ions from HF are incorporated into the \(\mathrm{HF}_{2}^{-}\) ion.

Step 5: Determine the Correct Answer

Given the ions formed, we match these to the choice provided: \(\mathrm{K}^{+}\) and \(\mathrm{HF}_{2}^{-}\), which corresponds to choice (d).

Key Concepts

Potassium Fluoride (KF)

Potassium fluoride (KF) is a compound primarily composed of potassium (K) and fluoride (F) ions. When KF is mixed with water, it dissociates completely into its constituent ions:

• Potassium ions (\(\mathrm{K}^{+}\)), which are positively charged.
• Fluoride ions (\(\mathrm{F}^{-}\)), which carry a negative charge.

This dissociation occurs because KF is an ionic compound, meaning it readily separates into ions when dissolved in water.
Potassium ions are very stable in solution and do not react further, whereas fluoride ions can participate in further reactions, especially in the presence of other chemical species.

Hydrogen Fluoride (HF)

Hydrogen fluoride (HF) is a unique compound that serves as a weak acid. Unlike strong acids, HF does not fully ionize in solution. Instead, it partially dissociates, which means only some HF molecules release ions:

• \(\mathrm{H}^{+}\) ions, contributing to acidity.
• \(\mathrm{F}^{-}\) ions, similar to those from KF.

However, the weak nature of HF means it can associate back, forming intact HF molecules. This behavior affects its reactivity and the resulting solutions, making it an interesting subject in acid chemistry.
When HF is in excess, it can interact with any fluoride ions already present, leading potentially to complex ion formation.

Complex Ion Formation

In mixtures containing both KF and HF, complex ion formation can occur due to the presence of excess fluoride ions. The most notable complex ion formed here is the bifluoride ion (\(\mathrm{HF}_{2}^{-}\)). This ion forms when HF interacts with excess fluoride ions in solution:

• The bifluoride ion consists of a hydrogen ion and two fluoride ions.
• This results in a stable ion due to the hydrogen bonding between elements.

The formation of \(\mathrm{HF}_{2}^{-}\) demonstrates how complex ions can stabilize certain elements or compounds in a mixture.
In our specific exercise with KF and HF, identifying the resulting ions guides us to understand potential reactions and stability within the mixture.