Problem 8 - Advanced Problems in Physical Chemistry for Competitive Examinations

Question

Which of the following is not a characteristic property of the polar liquids? (a) They have high boiling points. (b) They have high heat of vaporization. (c) They have low viscosity. (d) They have low vapour pressure.

Step-by-Step Solution

Answer

Polar liquids are characterized by higher boiling points, higher heats of vaporization, and lower vapour pressures due to their strong intermolecular forces. Thus, the property of having low viscosity is not a characteristic of polar liquids.

Step 1: Understanding characteristic properties of polar liquids

Polar liquids are characterized by the strong intermolecular forces between the molecules due to the presence of a permanent dipole. This is because polar molecules typically have a significant difference in electronegativity between the atoms, leading to partial positive and negative charges and therefore stronger intermolecular attractions.

Step 2: Analyzing the options given

Based on the understanding of intermolecular forces in polar molecules: (a) Polar liquids typically have higher boiling points since more energy is required to break the stronger intermolecular forces. (b) A high heat of vaporization is also characteristic of polar liquids, again due to the strong intermolecular forces that require more energy to overcome. (d) Polar liquids generally have low vapour pressure as the strong intermolecular forces keep the molecules from escaping into the vapor phase easily.

Step 3: Identifying the incorrect characteristic

Option (c) states that polar liquids have low viscosity. However, the strong intermolecular forces in polar liquids tend to make them more viscous. Therefore, polar liquids typically do not have low viscosity. Option (c) is the incorrect characteristic of polar liquids.

Key Concepts

Intermolecular Forces

Intermolecular forces are the attractive or repulsive forces between molecules, playing a crucial role in determining the physical properties of substances. In the context of polar liquids, these forces are particularly strong because of the permanent dipoles that exist within their molecules. The dipoles arise when there is an uneven distribution of electrons between atoms with different electronegativities, resulting in partial charges.

These partial charges create electrostatic attractions between molecules, known as dipole-dipole interactions. Polar molecules can also engage in hydrogen bonding when they contain hydrogen atoms bonded to highly electronegative atoms like oxygen, nitrogen, or fluorine. Hydrogen bonds are a powerful type of dipole-dipole interaction and contribute significantly to the unique properties of polar liquids such as water.

Boiling Points

The boiling point of a liquid is the temperature at which its vapor pressure equals the atmospheric pressure, causing the liquid to transition to gas. Polar liquids typically have high boiling points, and this is attributable to their strong intermolecular forces. To reach the boiling point, ample energy must be supplied to overcome the attractions between molecules. This fact explains why substances like water, which exhibits hydrogen bonding, require considerable amounts of energy to boil.

The high boiling point is a distinctive trait of polar substances, and this property finds practical applications in daily life, such as the boiling of water for cooking and for sterilization processes, where the reliable high boiling point ensures consistent outcomes.

Heat of Vaporization

The heat of vaporization is the amount of energy required to turn a liquid into a vapor without changing its temperature. For polar liquids, the heat of vaporization is high due to the energy needed to break the strong intermolecular forces. The extensive hydrogen bonding in water, for example, demands substantial energy to vaporize, which is why it absorbs a lot of heat when it evaporates.

This property is vital for processes like cooling systems where the evaporation of a liquid absorbs heat from the surroundings, and also for biological mechanisms like sweating, where the high heat of vaporization of water helps to effectively cool the body through evaporation.

Vapour Pressure

Vapour pressure is the pressure exerted by the vapor of a liquid in a closed system when the liquid and vapor are in equilibrium. Polar liquids, with their intensive intermolecular forces, exhibit low vapour pressures because fewer molecules have the required energy to escape from the liquid to the vapor phase. This is because the strong attractions between molecules of polar liquids necessitate higher energy to overcome those forces, making it less likely for the molecules to evaporate.

As a result, polar liquids will evaporate more slowly compared to nonpolar liquids at the same temperature, which affects many aspects ranging from the drying of paint to the preservation of food.

Viscosity

Viscosity is a measure of a fluid's resistance to flow and is directly related to the intermolecular forces within the liquid. Polar liquids are known to have higher viscosity due to stronger intermolecular attractions that resist the flow of molecules past each other. This increased resistance translates to a thicker or more 'syrupy' consistency.

It plays a critical role in numerous applications such as the design of lubricants for engines and the formulation of syrups in pharmaceuticals. Incorrectly assuming that polar liquids have low viscosity can lead to misunderstandings in practical applications, where the flow behavior of a liquid can be essential for its use.