Intermolecular Forces, Phase Changes, and Phase Diagrams – Lecture Review

A set of question-and-answer flashcards summarizing the major ideas, definitions, equations, and relationships from the lecture on intermolecular forces, phase changes, vapor pressure, and phase diagrams.

What is a phase in chemistry?

A physically distinct, homogeneous part of a system representing a particular physical state of matter.

Which two forms of energy determine the properties of a phase?

Potential energy (attractive forces) and kinetic energy (motion of particles).

How do intramolecular (bonding) forces differ from intermolecular (non-bonding) forces?

Intramolecular forces occur within a molecule and determine chemical behavior, whereas intermolecular forces act between molecules and determine physical behavior.

Why does the same chemical substance exhibit identical chemical behavior in all phases?

Because the same constituent particles (e.g., H₂O molecules) are present, bonded by the same intramolecular forces.

Describe the relative strength of attractive forces and kinetic energy in a gas.

Attractive forces are weak relative to kinetic energy; particles are far apart, giving a gas no fixed shape or volume.

Describe the balance of forces in a liquid.

Attractive forces are stronger (relative to a gas) because particles have lower kinetic energy; a liquid has fixed volume but variable shape.

Describe the balance of forces in a solid.

Attractions dominate motion; particles are fixed in place, giving a solid fixed shape and volume.

Give the heat-flow equation used within a single phase.

q = (amount) × (heat capacity) × ΔT

Give the heat-flow equation used during a phase change.

q = (amount) × (ΔH of phase change)

During a phase change, how does temperature behave?

Temperature remains constant while heat is absorbed or released.

Define vapor pressure.

The pressure exerted by a vapor in dynamic equilibrium with its liquid (or solid) at a given temperature in a closed system.

How does increasing temperature affect vapor pressure?

It increases the fraction of molecules with enough energy to enter the vapor phase, raising the vapor pressure.

How does intermolecular force strength affect vapor pressure?

Stronger intermolecular forces lower vapor pressure; weaker forces raise it.

Define boiling point.

The temperature at which a liquid’s vapor pressure equals the external pressure.

Define the normal boiling point.

The temperature at which a liquid’s vapor pressure equals 1 atm (760 mm Hg).

What relationship does the Clausius-Clapeyron equation describe?

It relates vapor pressure to temperature, enabling calculation of enthalpy of vaporization from two P–T data points.

Define critical temperature (T_c).

The temperature above which a gas cannot be liquefied, regardless of pressure.

Define critical pressure (P_c).

The minimum pressure required to liquefy a gas at its critical temperature.

What is the critical point?

The unique combination of critical temperature and critical pressure where distinct liquid and gas phases cease to exist.

Define triple point.

The temperature and pressure at which three phases of a substance coexist in equilibrium.

What is a supercritical fluid?

A substance above its critical temperature and pressure that shows properties of both gases (diffusion) and liquids (solvation).

Why does the solid–liquid equilibrium line for water have a negative slope?

Because liquid water is denser than ice; increasing pressure favors the denser liquid phase, lowering the melting point.