Problem 62 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following liquid has the highest vapour pressure or is most volatile? (a) HF (1) (b) \(\mathrm{NH}_{3}\) (1) (c) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}(\mathrm{l})\) (d) \(\mathrm{H}_{2} \mathrm{O}\)

Step-by-Step Solution

Answer

Ammonia (N_3) is the most volatile because it has the highest vapour pressure.

Step 1: Understanding Volatility and Vapour Pressure

Volatility describes how easily a liquid transforms into a vapor; it is directly related to the liquid's vapour pressure. A substance with a higher vapour pressure at a given temperature is considered more volatile because its molecules escape more readily into the gas phase.

Step 2: Analyzing Molecular Interactions

The intermolecular forces within a liquid determine its vapour pressure. Stronger intermolecular forces mean lower vapour pressure as molecules are held more tightly and do not vaporize easily. The substances include: - (a) HF: exhibits hydrogen bonding, strong intermolecular forces. - (b) NH₃: also capable of hydrogen bonding, with moderate strength intermolecular forces. - (c) C₂H₅OH: has hydrogen bonding as well, forming strong intermolecular attractions. - (d) H₂O: known for strong hydrogen bonds, leading to strong intermolecular forces.

Step 3: Comparing Intermolecular Forces

Water (H₂O) has the strongest intermolecular forces due to its extensive hydrogen bonding, followed by ethanol (C₂H₅OH). Hydrogen fluoride (HF) has strong hydrogen bonds, but not as extensive as water. Ammonia (NH₃), while capable of forming hydrogen bonds, has weaker interactions compared to the others.

Step 4: Conclusion on Vapour Pressure and Volatility

Since NH₃ (ammonia) has the weakest intermolecular forces among the options, it will have the highest vapour pressure. Thus, ammonia is the most volatile substance in the given list of liquids.

Key Concepts

Volatility

Volatility is a term that describes how easily a liquid can turn into a gas. This transformation is closely linked to a property called vapor pressure. A liquid with high vapor pressure at a specific temperature tends to be more volatile. This is because its molecules can escape into the atmosphere more swiftly. When comparing different liquids, the one with the highest vapor pressure is generally the most volatile. For example, in a comparison between HF, NH₃, C₂H₅OH, and H₂O, ammonia (NH₃) is found to have the highest volatility due to its weaker intermolecular forces. This is why NH₃ evaporates more readily than the others.

Intermolecular Forces

Intermolecular forces are the mysterious glue that holds molecules in a liquid together. They play a key role in influencing a liquid's vapor pressure and volatility.
These forces can vary from strong to weak:

• Strong Intermolecular Forces: When present, they result in lower vapor pressure because they hold the molecules firmly together, preventing evaporation. Examples include hydrogen bonding, where you find liquids like water (H₂O) having strong bonds.
• Weak Intermolecular Forces: These result in higher vapor pressure, allowing more molecules to escape into the gas phase. Ammonia (NH₃), for instance, has weaker intermolecular forces, leading to its classification as highly volatile.

Understanding these forces helps in grasping why some liquids transform into vapor more readily than others.

Hydrogen Bonding

Hydrogen bonding is one of the strongest types of intermolecular forces. It occurs when hydrogen is bonded to electronegative atoms such as oxygen, nitrogen, or fluorine. This special bond significantly impacts a liquid's properties, including its vapor pressure.
Liquids exhibiting hydrogen bonding tend to have:

• Lower vapor pressure: Molecules are tightly held together, making evaporation a challenge. Water (H₂O) is an excellent example due to its extensive hydrogen bonding network.
• Greater surface tension: Alongside low volatility, these liquids resist surface molecule escape.

When analyzing liquids like HF, NH₃, and C₂H₅OH, hydrogen bonding plays a crucial role in determining their volatility, with water displaying the strongest hydrogen bonds.