Question

Discuss the interpretation of each of the following observations: (a) Although the hydrogen bonding in HF is stronger than that in \(\mathrm{H}_{2} \mathrm{O}\), water has much the higher boiling point. (b) Silver chloride and silver iodide are soluble in saturated aqueous KI, but insoluble in saturated aqueous KCl.

Short answer

(a) Water forms a more extensive hydrogen bonding network than HF. (b) Solubility is influenced by complex ion formation in KI, absent in KCl.

Step-by-step solution

Analyzing HF and H2O Boiling Points

Hydrogen fluoride (HF) has strong hydrogen bonds due to the high electronegativity of fluorine compared to oxygen in water (H2O). However, water exhibits the capacity to form more extensive hydrogen bonding networks due to its bent structure and capacity to form up to four hydrogen bonds per molecule. This extensive bonding network in water creates stronger overall intermolecular forces, leading to a higher boiling point.

Solubility of AgCl and AgI in KI and KCl

Silver chloride (AgCl) and silver iodide (AgI) are more soluble in saturated aqueous KI due to the formation of soluble complexes with iodide ions, such as \( \text{[AgI}_2\text{]}^- \). These complexes reduce the concentration of free Ag+ ions, driving the dissolution process forward. In contrast, KCl does not form such complexes with silver ions, thus keeping AgCl and AgI insoluble in saturated KCl solutions.

Key concepts

Hydrogen Bonding

Hydrogen bonding is a special type of intermolecular force that occurs when hydrogen is covalently bonded to a highly electronegative atom, like oxygen or fluorine.
This bond is uniquely strong and influences many properties of compounds, such as boiling points and solubility.
Although one might expect that stronger hydrogen bonds always result in higher boiling points, the situation is more nuanced. Take, for example, HF and water (H₂O).

• In HF, fluorine's high electronegativity pulls electrons more strongly, creating strong hydrogen bonds.
• However, water can form up to four hydrogen bonds per molecule due to its bent structure, allowing for a more extensive hydrogen bonding network.

This extensive network makes the overall intermolecular forces in water much stronger than those in HF, contributing to water's high boiling point. Understanding hydrogen bonding helps explain complex phenomena in inorganic chemistry.

Boiling Points

The boiling point of a substance is the temperature at which its vapor pressure equals the atmospheric pressure. Intermolecular forces play a crucial role in determining these boiling points. Substances with stronger intermolecular forces have higher boiling points because more energy is needed to separate the molecules.
In the case of water (H₂O) and hydrogen fluoride (HF), even though HF has strong hydrogen bonds due to fluorine's electronegativity, water's ability to form a more extensive hydrogen network gives it a higher boiling point. Here's why:

• Water molecules can bond with up to four other water molecules, creating a strong and stable three-dimensional network.
• This network increases the amount of energy required to turn water from liquid to vapor.

Hence, despite HF having strong bonds per molecule, water's collective bonding results in a much higher boiling point.

Solubility

Solubility refers to the ability of a substance to dissolve in a solvent, creating a homogeneous mixture at a molecular level. Many factors influence solubility, including the nature of the solute and solvent, temperature, and the presence of other substances.
For instance, let’s examine why silver chloride (AgCl) and silver iodide (AgI) are soluble in saturated aqueous KI but not in KCl:

• In aqueous KI, iodide ions ( I^- ) form complexes with silver, such as [AgI₂]⁻, which stabilize the ions in solution.
• This complex formation decreases the concentration of free Ag⁺ ions, allowing more of the solid to dissolve.
• In contrast, KCl does not facilitate such complex formation, leaving AgCl and AgI largely undissolved.

This example illustrates how complex ion formation can drastically affect solubility.

Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion that act between neighboring particles—these play a critical role in determining the physical properties of a substance, such as boiling point, melting point, and solubility.
There are several types of intermolecular forces, including:

• Dispersion forces or London forces, which are weak interactions caused by temporary changes in electron density.
• Dipole-dipole interactions, which occur between molecules with permanent dipoles.
• Hydrogen bonds, which are particularly strong dipole-dipole interactions when hydrogen is bonded to N, O, or F.

The strength and number of these forces in a substance determine how much energy is needed for phase changes. In cases like water, multiple hydrogen bonds create significant intermolecular forces that result in high boiling points. By mastering intermolecular forces, students can better predict and understand the behavior of different substances in various conditions.