Question

Which of the following statement is false? (1) The boiling point of a compound is raised by intermolecular hydrogen bonding. (2) o-nitrophenol is more volatile than p-nitrophenol because of intramolecular hydrogen bonding. (3) Lower alcohols such as methanol and ethanol arc miscible in water due to hydrogen bonding. (4) Intramolecular hydrogen bond enhances the boiling point.

Short answer

Statement 4 is false.

Step-by-step solution

Identify the Correct Statements

Review each given statement and determine if it is true or false based on your knowledge of hydrogen bonding and its effects.

Analyze Statement 1

The statement, 'The boiling point of a compound is raised by intermolecular hydrogen bonding,' is generally true. Intermolecular hydrogen bonding increases the energy required to break bonds, thus raising the boiling point.

Analyze Statement 2

The statement, 'o-nitrophenol is more volatile than p-nitrophenol because of intramolecular hydrogen bonding,' is true. Intramolecular hydrogen bonding in o-nitrophenol makes it less likely to form intermolecular hydrogen bonds, increasing its volatility.

Analyze Statement 3

The statement, 'Lower alcohols such as methanol and ethanol are miscible in water due to hydrogen bonding,' is true. Hydrogen bonding between the hydroxyl groups of alcohols and water makes them miscible.

Analyze Statement 4

The statement, 'Intramolecular hydrogen bond enhances the boiling point,' is false. Intramolecular hydrogen bonding does not significantly affect boiling point as it does not contribute to the energy required to break intermolecular forces.

Conclusion

Based on the analysis, statements 1, 2, and 3 are true and statement 4 is false.

Key concepts

intermolecular forces

Intermolecular forces are the forces of attraction or repulsion between molecules. They play a crucial role in determining the physical properties of substances. There are several types of intermolecular forces:

• Hydrogen bonding: A special type of dipole-dipole interaction that occurs when hydrogen is bonded to a highly electronegative atom, such as oxygen, nitrogen, or fluorine.
• Dipole-dipole interactions: Occur between molecules that have permanent dipoles, meaning they have regions of partial positive and partial negative charges.
• London dispersion forces (van der Waals forces): These are weak, temporary forces that result from fluctuations in the electron distribution within molecules.

Intermolecular hydrogen bonding can significantly increase the boiling point of compounds because it requires additional energy to break these interactions. Understanding these forces helps explain properties like boiling point, miscibility, and volatility.

boiling point

The boiling point of a substance is the temperature at which its vapor pressure equals the external pressure. This causes the liquid to turn into vapor. Factors affecting boiling points include:

• Intermolecular Forces: Stronger intermolecular forces result in higher boiling points. Hydrogen bonds, for example, require substantial energy to break, thus raising the boiling point.
• Molecular Weight: Generally, heavier molecules have higher boiling points because they have more electrons, leading to stronger London dispersion forces.
• Structure of the Molecule: Linear molecules tend to have higher boiling points due to increased surface area facilitating stronger intermolecular forces compared to branched molecules.

For instance, hydrogen bonding in water and alcohols increases their boiling points compared to hydrocarbons of similar molecular weight. Yet, intramolecular hydrogen bonding, like in o-nitrophenol, does not increase the boiling point significantly as it does not add to the energies required to break the intermolecular forces.

volatility

Volatility refers to how easily a substance vaporizes. It is inversely related to boiling point. Several factors influence volatility:

• Intermolecular Forces: Substances with weaker intermolecular forces are more volatile because less energy is needed for molecules to escape into the vapor phase.
• Intramolecular Hydrogen Bonding: In cases like o-nitrophenol, intramolecular hydrogen bonding reduces the ability to form intermolecular hydrogen bonds, thus increasing volatility.
• Molecular Size and Structure: Smaller and less complex molecules tend to be more volatile as they require less energy to vaporize.

o-nitrophenol is more volatile than p-nitrophenol because the former has intramolecular hydrogen bonds, which reduces intermolecular hydrogen bonding, making it easier for the substance to vaporize.

miscibility in water

Miscibility refers to the ability of a substance to mix thoroughly with another. Water is a polar solvent, and its miscibility with other substances depends largely on hydrogen bonding and polarity:

• Hydrogen Bonding: Lower alcohols like methanol and ethanol are highly miscible in water due to their ability to form hydrogen bonds with water molecules. The hydroxyl groups (-OH) in alcohols interact strongly with water's hydrogen bonds.
• Polarity: Polar substances mix well with water. Substances that can form hydrogen bonds with water are typically polar, enhancing their miscibility.
• Molecular Size: Smaller alcohols are more miscible compared to larger hydrocarbons, which are less polar and unable to form hydrogen bonds with water.

Thus, methanol and ethanol dissolve well in water due to their strong intermolecular hydrogen bonding with water molecules. On the contrary, larger, non-polar molecules don't mix well with water due to lack of hydrogen bonding.