Kinetic Theory: Gas, Liquid, and Solid State Principles

Kinetic Theory and States of Matter Fundamentals

Kinetic Molecular Theory (KMT) of Gases

• The particles in a gas are considered to be small, hard spheres with an insignificant volume.
• The motion of the particles in a gas is rapid, constant, and random.
• All collisions between particles in a gas are perfectly elastic.

Gas Pressure and Atmospheric Conditions

• Gas pressure is the result of billions of rapidly moving particles simultaneously colliding with an object.
• Air exerts pressure on Earth because gravity holds the particles in air within Earth’s atmosphere.

Temperature, Energy, and Standard Conditions

• STP (Standard Temperature and Pressure) is defined as 0ºC and 101.3 kPa.
• Average kinetic energy is directly related to the substance’s temperature. Higher temperature means more kinetic energy.
• Absolute zero (0 K or -273.15 ºC) is a condition where no particle motion occurs.
• Kelvin temperature is directly proportional to the average kinetic energy.
• Standard pressure conversions: 1 atm = 101.3 kPa = 760 mmHg.

Properties of Liquids

• The interplay between the disruptive motions of particles in a liquid and the attractions among the particles determines the physical properties of liquids.
• According to the kinetic theory, there is attraction between particles of a liquid.

Evaporation and Vapor Pressure

• During evaporation, only those molecules with a certain minimum kinetic energy can escape from the surface of the liquid.
• The evaporation of a liquid in a closed system differs from evaporation in an open system, as no particles can escape into the outside air from the sealed container.
• In a system at constant vapor pressure, a dynamic equilibrium exists between the vapor and the liquid. This occurs because the rate of evaporation equals the rate of condensation.
• Boiling occurs when a liquid is heated to a temperature at which particles throughout the liquid have enough kinetic energy to vaporize.

Characteristics of Solids and Phase Changes

Solid Structure and Form

• The general properties of solids reflect the orderly arrangement and fixed locations of their particles.
• The shape of a crystal reflects the arrangement of the particles within the solid.
• Some solid substances can exist in more than one form (polymorphism).
• When a crystal is shattered, the fragments retain the same surface angle as the original. In contrast, shattered glass (an amorphous solid) yields fragments with irregular angles and jagged edges.

Sublimation and Phase Diagrams

• Sublimation can occur because solids, like liquids, have a vapor pressure.
• Sublimation occurs in solids with vapor pressures that exceed atmospheric pressure at or near room temperature.
• The conditions of pressure and temperature at which two phases can exist in equilibrium are indicated on a phase diagram by a line separating the two regions representing the phases.