States of Matter and Kinetic Molecular Theory
Kinetic molecular theory and gas laws
KMT assumes gas particles are in constant random motion, have negligible volume, collide elastically, and exert no attractive forces. Average kinetic energy is proportional to Kelvin temperature. The ideal gas law is PV = nRT; R = 0.0821 L·atm/(mol·K). Boyle's law (P–V inverse), Charles's law (V–T direct), and Gay-Lussac's law (P–T direct) are special cases. Standard molar volume is 22.4 L/mol at STP.
Condensed phases
In liquids and solids, intermolecular forces dominate. Stronger forces raise boiling point, viscosity, and surface tension and lower vapor pressure. Phase diagrams show melting, boiling, and triple points; the critical point ends the liquid–gas boundary. Colligative properties depend on particle count: vapor-pressure lowering, boiling-point elevation, freezing-point depression, and osmosis.
Worked example
A gas occupies 2.0 L at 300 K and 1.0 atm. Find its volume at 400 K and 1.5 atm. Use P1V1/T1 = P2V2/T2. V2 = P1V1T2/(T1P2) = (1.0)(2.0)(400)/((300)(1.5)) = 1.78 L. The gas expands with heating but is compressed by the higher pressure; the net result is a smaller volume.
Traps
Using Celsius in gas-law calculations; assuming ideal behavior at high pressure or low temperature; confusing vapor pressure with total pressure; and forgetting that colligative properties depend on the number of particles, not their identity.