Question

The decreasing order of the boiling points of the following hydrides is: 1\. \(\mathrm{NH}_{3}\) 2\. \(\mathrm{PH}_{3}\) 3\. \(\mathrm{AsH}_{3}\) 4\. \(\mathrm{SbH}_{3}\) 5\. \(\mathrm{H}_{2} \mathrm{O}\) (a) \(5>4>1>3>2\) (b) \(5>1>2>3>4\) (c) \(2>4>3>1>5\) (d) \(4>3>1>2>5\)

Short answer

The correct order is (a) 5>4>1>3>2.

Step-by-step solution

Understanding the Problem

We are given a series of hydrides and need to determine the order of their boiling points from highest to lowest. The options are structured such that by analyzing their boiling points we can find the correct sequence.

Analyze Boiling Points

The boiling point of a substance depends heavily on the type of intermolecular forces present. Generally, compounds with hydrogen bonding will have higher boiling points than those with Van der Waals forces. - H2O and NH3 can form hydrogen bonds due to their electronegative atoms (oxygen and nitrogen, respectively). SbH3, AsH3, and PH3 experience only Van der Waals forces. Since H2O can form stronger hydrogen bonds than NH3, it will have the highest boiling point among these. - Considering Van der Waals forces, increasing molecular weight generally increases boiling point. Therefore, SbH3 > AsH3 > PH3 in terms of boiling point.

Determine the Sequence

Based on the above analysis: - H2O has the highest boiling point. - NH3 comes next due to hydrogen bonding. - Among the remaining hydrides, which rely on Van der Waals forces, SbH3 has the highest boiling points followed by AsH3 and PH3 due to increasing molecular mass.

Select the Correct Option

Combining all the observations: - H2O > NH3 > SbH3 > AsH3 > PH3. Thus, among the options given, (a) corresponds to the correct order: 5>4>1>3>2.

Key concepts

Hydrogen Bonding

Hydrogen bonding is a special type of intermolecular force that significantly affects boiling points. This force occurs when hydrogen is covalently bonded to highly electronegative atoms such as oxygen or nitrogen. The large difference in electronegativity causes a strong attraction between molecules, giving rise to hydrogen bonds.

In the context of hydrides like \(\begin{align*}\text{NH}_3 &\quad \text{(ammonia)} \\text{H}_2\text{O} &\quad \text{(water)} \end{align*}\), water and ammonia have high boiling points because they can establish hydrogen bonds. Water, with its oxygen atom, forms stronger hydrogen bonds than ammonia, which accounts for water's higher boiling point compared to ammonia.

This property of hydrogen bonding is crucial in determining the boiling points in hydrides, making it a powerful factor when deciding the order of boiling points.

Van der Waals Forces

Van der Waals forces, also known as London dispersion forces, are weak intermolecular forces caused by temporary fluctuations in electron density. These forces are present in all molecules, but are particularly significant in molecules without hydrogen bonding, such as \(\begin{align*}\text{PH}_3 &\quad \text{(phosphine)} \\text{AsH}_3 &\quad \text{(arsine)} \\text{SbH}_3 &\quad \text{(stibine)} \end{align*}\).

The strength of Van der Waals forces increases with molecular size and mass. As a result, heavier hydrides like stibine and arsine have higher boiling points than phosphine. In the absence of significant hydrogen bonding, the boiling points of these hydrides are predominantly determined by Van der Waals forces.

This is why in the list of hydrides, though they lack hydrogen bonds, the ones with heavier molecular weights boil at higher temperatures.

Intermolecular Forces

Intermolecular forces are the forces between molecules that determine the physical properties of substances, such as boiling points. They encompass several types of forces, including hydrogen bonding and Van der Waals forces.

Understanding intermolecular forces is essential for predicting how substances behave in different states.

- **Hydrogen Bonding**: Strong and present in molecules with hydrogen and electronegative atoms (e.g., \(\text{H}_2\text{O}, \text{NH}_3\)). - **Van der Waals Forces**: Weak and present in all molecules but primary in those without hydrogen bonds (e.g., \(\text{PH}_3, \text{AsH}_3, \text{SbH}_3\)).

These forces explain why water (\text{H}_2\text{O}) has the highest boiling point among the given hydrides, due to its strong hydrogen bonding, while heavier hydrides like stibine (\text{SbH}_3) boil at higher temperatures than lighter ones like phosphine (\text{PH}_3), even though they rely on Van der Waals forces.