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) \(\mathrm{NH}_{3}\) (l) (c) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\) (1) (d) \(\mathrm{H}_{2} \mathrm{O}\)

Step-by-Step Solution

Answer

NH_3 is the most volatile and has the highest vapor pressure.

Step 1: Understanding Volatility and Vapour Pressure

Volatility refers to how readily a substance vaporizes. A more volatile substance has a higher vapor pressure at a given temperature because it evaporates more easily.

Step 2: Consider Intermolecular Forces

The amount of vapor pressure a liquid exerts depends on the intermolecular forces between its molecules. Weaker intermolecular forces result in a higher vapor pressure and increased volatility.

Step 3: Analyze Each Compound's Intermolecular Forces

- HF has hydrogen bonding which is relatively strong. - NH_3 has hydrogen bonding but is weaker compared to HF. - C_2H_5OH (ethanol) has hydrogen bonding as well, typically stronger than in NH_3. - H_2O (water) has strong hydrogen bonds. Among these, NH_3's hydrogen bonding is the weakest.

Step 4: Determine the Most Volatile Liquid

Given the weaker hydrogen bonds, NH_3 is the most volatile and hence has the highest vapor pressure compared to the other liquids listed.

Key Concepts

Volatility

In chemistry, volatility is an important concept that refers to the tendency of a substance to vaporize, or turn from a liquid into a gas. This characteristic is strongly linked to vapour pressure, which is the pressure exerted by the vapor above a liquid in a closed container. Substances that vaporize easily at a specific temperature exhibit higher vapor pressures and are thus considered more volatile.

Volatility largely depends on the nature of the intermolecular forces present in the substance.

• If a liquid has weak intermolecular forces, it will require less energy to enable the molecules to break free into the vapor phase.
• Therefore, such substances are typically more volatile, resulting in higher vapor pressures.
• This principle can be used to predict the relative volatility of different compounds at a given temperature.

Understanding volatility helps in making sense of many practical and industrial processes, such as distillation, where separation of components is based on differences in boiling points due to varying volatilities.

Intermolecular Forces

Intermolecular forces are the forces that exist between molecules, affecting properties such as boiling point, melting point, and, importantly, vapor pressure. These forces determine how tightly molecules are held together in a liquid.

Molecules can interact through different types of intermolecular forces:

• Dispersion forces: These are the weakest and exist between all molecules, whether polar or non-polar.
• Dipole-dipole interactions: These occur between polar molecules, where positive and negative ends attract each other more strongly than dispersion forces.
• Hydrogen bonding: A special, stronger type of dipole-dipole interaction involving hydrogen atoms and more electronegative atoms such as oxygen, nitrogen, or fluorine. It plays a crucial role in determining the physical properties of substances.

Substances with strong intermolecular forces tend to have lower vapor pressures because its molecules are more strongly attracted to one another. This means they require more energy to escape into the vapor phase, resulting in lower volatility.

Hydrogen Bonding

Hydrogen bonding is a particularly strong type of dipole-dipole attraction that occurs when hydrogen is bonded directly to either fluorine, oxygen, or nitrogen. This subgroup of intermolecular forces significantly affects the physical characteristics of a molecule, especially its boiling and melting points.

The strength of hydrogen bonds varies among different compounds:

• In water (\(H_2O\)), hydrogen bonds are notably strong due to the high electronegativity of oxygen, leading to high boiling and melting points.
• In hydrogen fluoride (\(HF\)), hydrogen bonding is strong but less extensive compared to water due to the linear arrangement.
• Ammonia (\(NH_3\)), although forming hydrogen bonds, does so with lesser efficiency than water or HF, making them relatively weaker.
• Ethanol (\(C_2H_5OH\)) also forms hydrogen bonds, facilitated by its hydroxyl group, which significantly impacts its volatility and solubility.

The weaker the hydrogen bonds, the lower the boiling point and the higher the volatility of the substance, as seen in ammonia having weaker hydrogen bonds compared to the others, making it more volatile with a higher vapour pressure.