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

Question

KF combines with HF to form \(\mathrm{KHF}_{2}\). The compound contains the species (a) \(\mathrm{K}^{+}, \mathrm{F}\) and \(\mathrm{H}^{\prime}\) (b) \(\mathrm{K}^{+}, \mathrm{F}^{-}\)and \(\mathrm{HF}\) (c) \(\mathrm{K}^{\prime}\) and \(\left[\mathrm{HF}_{2}\right]\) (d) \([\mathrm{KHF}]^{+}\)and \(\mathrm{F}_{2}\)

Step-by-Step Solution

Answer

The correct answer is (c) \( \mathrm{K}^{+} \) and \( [\mathrm{HF}_{2}]^{-} \).

Step 1: Understanding KF and HF

First, we need to identify the ions and molecules in potassium fluoride (KF) and hydrogen fluoride (HF). Potassium fluoride dissociates into ions: \( \mathrm{K}^{+} \) and \( \mathrm{F}^{-} \). Hydrogen fluoride (HF) is a polar covalent molecule that can participate in hydrogen bonding and can also exist in ionic forms.

Step 2: Analyzing Structure of \( \mathrm{KHF}_{2} \)

\( \mathrm{KHF}_{2} \) is a complex compound. Here, hydrogen fluoride (HF) can form a bifluoride ion \( [\mathrm{HF}_{2}]^{-} \), where an extra fluoride ion \( \mathrm{F}^{-} \) bonds with HF. The potassium ion \( \mathrm{K}^{+} \) balances out this structure.

Step 3: Determining the Ions and Species in \( \mathrm{KHF}_{2} \)

Given the structure of \( \mathrm{KHF}_{2} \), it contains \( \mathrm{K}^{+} \) due to the dissociation of KF. The complex \( [\mathrm{HF}_{2}]^{-} \) forms from the combination of HF with extra fluoride resulting in the stable ion \([\mathrm{HF}_{2}]^{-}\). Thus, \( \mathrm{KHF}_{2} \) can be viewed as containing \( \mathrm{K}^{+} \) and \([\mathrm{HF}_{2}]^{-}\).

Step 4: Determining the Correct Option

The correct option will include \( \mathrm{K}^{+} \) and \([\mathrm{HF}_{2}]^{-}\) as individual ions. Option (c) '\( \mathrm{K}^{+} \) and \([\mathrm{HF}_{2}]^{-}\)' correctly represents the composition of \( \mathrm{KHF}_{2} \).

Key Concepts

Ionic Compounds

Ionic compounds form when atoms transfer electrons to achieve a full outer shell of electrons. This usually happens between metals and non-metals. In the case of potassium fluoride (KF), the metal potassium (K) donates an electron to the non-metal fluoride (F), resulting in the formation of positive potassium ions (\( \mathrm{K}^{+} \)) and negative fluoride ions (\( \mathrm{F}^{-} \)).
This electron transfer leads to the formation of strong ionic bonds due to the electrostatic attraction between the oppositely charged ions, creating a stable ionic compound.

• These compounds typically have high melting and boiling points.
• They often dissolve in water, forming electrolyte solutions that conduct electricity.
• The ionic compounds appear solid and have crystal lattice structures.

Understanding ionic compounds like KF helps in identifying how they combine with other substances to form more complex structures, such as \( \mathrm{KHF}_{2} \), illustrating the diversity in chemical compound formations.

Hydrogen Bonding

Hydrogen bonding is a special type of dipole-dipole interaction that occurs when hydrogen is bonded to a highly electronegative atom, such as fluorine, oxygen, or nitrogen. In hydrogen fluoride (HF), hydrogen bonds are particularly strong because of the significant electronegativity difference between hydrogen (H) and fluorine (F).
These bonds are not as strong as covalent or ionic bonds, but they play a crucial role in determining the physical properties of compounds.
Hydrogen bonds can cause molecules to group together, significantly affecting their boiling and melting points. In water, for example, hydrogen bonds lead to its exceptional properties like high surface tension and the ability to dissolve many substances.
In \( \mathrm{KHF}_{2} \), hydrogen bonding facilitates the formation of the bifluoride ion \( [\mathrm{HF}_{2}]^{-} \), where an additional fluoride ion links through hydrogen bonding, enhancing the stability of the compound. This interaction illustrates the complexity and importance of hydrogen bonds in chemical formations.

Complex Ions

Complex ions are large, structured arrangements formed when central atoms or ions are surrounded by other ions or molecules. These other molecules are called ligands. A good example of complex ions is found in \( \mathrm{KHF}_{2} \), where the bifluoride ion \( [\mathrm{HF}_{2}]^{-} \) acts as a complex ion. This complex arises when HF interacts with an extra fluoride ion (\( \mathrm{F}^{-} \)) through hydrogen bonding to form a stable arrangement.

• Complex ions can be positively or negatively charged.
• The nature and charge of the ligands impact the overall charge of the complex ion.
• Complex formation can influence the solubility and stability of the compound.

In \( \mathrm{KHF}_{2} \), the presence of \( [\mathrm{HF}_{2}]^{-} \) demonstrates how complex ions play a crucial role in the stability and formation of chemical structures, showcasing chemical bonding's rich and dynamic nature. By understanding how complex ions function, you can appreciate how varied and adaptable chemical compounds can be, beyond simple ionic or covalent bonds.