Liquids, Solids, and Intermolecular Forces: Structured Study Notes

Liquids, Solids, and Intermolecular Forces

Interactions Between Molecules

Intermolecular forces are the attractive forces that exist between molecules and are responsible for many observable phenomena in chemistry and biology. For example, the spherical shape of a water drop is due to these forces among water molecules.

• Intermolecular forces are responsible for the existence of liquids and solids.

• They play a crucial role in physiological processes in living organisms.

• All molecules and atoms experience some form of intermolecular force.

• These forces are distinct from the chemical bonds within molecules (intramolecular forces).

• Example: The interaction between bitter molecules in coffee and taste receptors is due to intermolecular forces.

States of Matter and Intermolecular Forces

The physical state of a substance—solid, liquid, or gas—is determined by the balance between intermolecular forces and thermal energy.

• Thermal energy is the energy associated with the random motion of molecules and atoms, which increases with temperature.

• If intermolecular forces are weak compared to thermal energy, the substance is likely to be a gas.

• If intermolecular forces are strong compared to thermal energy, the substance is likely to be a liquid or solid.

Comparison of Solids, Liquids, and Gases

Solids, liquids, and gases differ in the arrangement and movement of their constituent particles.

• Gases: Particles are far apart, move freely, have low density, indefinite shape and volume, and weak intermolecular forces.

• Liquids: Particles are close together but can move past each other, have high density, indefinite shape, definite volume, and moderate intermolecular forces.

• Solids: Particles are closely packed and fixed in place (though they vibrate), have high density, definite shape and volume, and strong intermolecular forces. Solids may be crystalline (ordered) or amorphous (disordered).

• Examples: CO2 (gas), H2O (liquid), C12H22O11 (solid sugar).

Shape and Movement in Liquids and Solids

The ability of molecules to move determines the shape and flow of liquids and solids.

• In liquids, molecules are free to move around each other, allowing them to flow and take the shape of their container.

• In solids, molecules are fixed in place but vibrate about those positions, resulting in a rigid structure.

Manifestations of Intermolecular Forces: Surface Tension and Viscosity

Intermolecular forces give rise to important properties in liquids, such as surface tension and viscosity.

• Surface tension is the tendency of a liquid to minimize its surface area, creating a 'skin' that resists penetration. This explains why objects denser than water can float if they do not break the surface.

• Viscosity is the resistance of a liquid to flow. Liquids with strong intermolecular forces (e.g., maple syrup) are more viscous and flow more slowly than those with weaker forces (e.g., water).

• Long-chain molecules (e.g., in motor oil) increase viscosity due to entanglement.

Evaporation and Vaporization

Evaporation is the process by which molecules at the surface of a liquid gain enough energy to escape into the gas phase.

• The rate of vaporization increases with increasing surface area, increasing temperature, and decreasing strength of intermolecular forces.

• Volatile liquids evaporate easily; nonvolatile liquids do not.

• Only the most energetic molecules at the surface can escape into the gas phase.

• At higher temperatures, more molecules have sufficient energy to evaporate.

Condensation and Dynamic Equilibrium

Condensation is the reverse of evaporation, where gas molecules lose energy and return to the liquid phase.

• When the rates of evaporation and condensation are equal, the system reaches dynamic equilibrium.

• Vapor pressure is the pressure exerted by the vapor in equilibrium with its liquid.

• Vapor pressure increases with temperature and decreases with stronger intermolecular forces.

• Vapor pressure is independent of surface area at equilibrium.

Boiling and Heating Curves

Boiling occurs when the vapor pressure of a liquid equals the external pressure.

• At the boiling point, molecules throughout the liquid (not just at the surface) can vaporize, forming bubbles.

• Further heating increases the rate of boiling but does not raise the temperature above the boiling point until all liquid is vaporized.

• Heating curves show temperature remains constant during phase changes (e.g., boiling or melting).

Energetics of Phase Changes

Phase changes involve energy transfer.

• Evaporation is endothermic: energy is absorbed to break intermolecular forces ($q = + ext{heat}$).

• Condensation is exothermic: energy is released as molecules form intermolecular attractions ($q = - ext{heat}$).

• Evaporation cools the body (sweating); condensation releases heat (e.g., steam burns).