Question

Does the strength of the intermolecular forces in a liquid change as the liquid is heated? Explain. Why does a liquid's viscosity decrease with rising temperature?

Short answer

Heating a liquid doesn't change the strength of intermolecular forces but makes molecules overcome them more easily, decreasing viscosity.

Step-by-step solution

Understanding Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion between neighboring particles (atoms, molecules, or ions). These forces include van der Waals forces, dipole-dipole interactions, hydrogen bonding among others.

Effect of Heating on Intermolecular Forces

As a liquid is heated, the kinetic energy of its molecules increases. However, the intrinsic strength of the intermolecular forces themselves does not change. What changes is the relative influence of these forces compared to the kinetic energy of the molecules.

Balancing Kinetic Energy and Intermolecular Forces

With increased temperature, molecules move more rapidly and are better able to overcome the intermolecular forces that hold them together in the liquid.

Understanding Viscosity

Viscosity is a measure of a liquid's resistance to flow. It depends on the strength of intermolecular forces and the temperature of the liquid.

Temperature and Viscosity Relationship

As temperature rises, the increased kinetic energy of the molecules allows them to move past one another more easily. Thus, the viscosity of the liquid decreases.

Key concepts

Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion that act between neighboring particles. These forces play a crucial role in determining the physical properties of substances. These forces include:

• Van der Waals forces
• Dipole-Dipole interactions
• Hydrogen bonding

Each type of intermolecular force has its own unique characteristics and strengths, which we will explore further in upcoming sections.

Kinetic Energy

Kinetic energy refers to the energy that a body possesses due to its motion.
In the context of liquids, kinetic energy is the energy that the molecules have due to their movement. When a liquid is heated, the kinetic energy of its molecules increases. Despite the increasing kinetic energy, the inherent strength of the intermolecular forces remains unchanged.
The increased kinetic energy allows molecules to move more easily and potentially overcome the intermolecular forces binding them together.

Viscosity

Viscosity is a measure of a liquid's resistance to flow. High viscosity means the liquid flows slowly (like honey), while low viscosity means the liquid flows easily (like water).
Viscosity is influenced by:

• Temperature
• Strength of intermolecular forces

The stronger the intermolecular forces, the higher the viscosity. Higher temperatures increase kinetic energy, helping molecules overcome these forces, thus reducing viscosity.

Van der Waals Forces

Van der Waals forces are the weakest type of intermolecular force, but they are present in all molecules. They arise due to temporary dipoles created when the electron distribution around a molecule becomes asymmetrical.
Though individually weak, Van der Waals forces collectively have a significant impact on properties like boiling and melting points. They become especially noticeable in larger molecules where these forces can be quite strong.

Dipole-Dipole Interactions

Dipole-dipole interactions occur between molecules that have permanent dipoles. These are stronger than Van der Waals forces.
In polar molecules, the positive end of one dipole is attracted to the negative end of another, leading to the formation of dipole-dipole interactions. These interactions significantly affect properties such as boiling points, with stronger dipole-dipole interactions leading to higher boiling points.

Hydrogen Bonding

Hydrogen bonding is a special type of dipole-dipole interaction that occurs when hydrogen is directly bonded to a highly electronegative element like nitrogen, oxygen, or fluorine.
This force is much stronger than regular dipole-dipole interactions and has profound effects on the physical properties of substances. For example, hydrogen bonding is responsible for the high boiling point of water and the unique properties of DNA. These strong interactions lead to higher viscosity and greater resistance to flow in substances where hydrogen bonding is prevalent.