The Particulate Nature of Matter: Thermal Physics, Ideal Gases and Thermodynamics
Particulate nature of matter covers thermal energy, ideal gases and thermodynamics. The exam tests kinetic models, the gas laws, specific and latent heat, the first law, and entropy as a measure of disorder. HL questions often ask you to interpret a p–V diagram or justify why a process is adiabatic, isothermal or isochoric.
Core rules
- Ideal gas assumptions: molecules in random motion, negligible volume, no intermolecular forces except during elastic collisions, and collision time negligible.
- PV = nRT = NkT; temperature must be in kelvin.
- Kinetic theory gives P = (1/3)ρ⟨c²⟩ and average molecular kinetic energy = (3/2)kT.
- Heating: Q = mcΔT; phase change: Q = mL, with no temperature change.
- First law: Q = ΔU + W, where W is work done by the gas. W = pΔV for constant pressure.
- Entropy increases in a spontaneous process; the second law sets the direction.
Worked example
A gas expands at constant pressure 1.0 × 10⁵ Pa from 2.0 × 10⁻³ m³ to 5.0 × 10⁻³ m³ while absorbing 900 J. The work done by the gas is W = pΔV = (1.0 × 10⁵)(3.0 × 10⁻³) = 300 J. The first law gives ΔU = Q − W = 900 − 300 = 600 J. The temperature rises because internal energy increases.
Traps
Using degrees Celsius in PV = nRT is a common error. Keep the sign of W consistent: work done by the gas is positive in the IB convention. Internal energy of an ideal gas depends on temperature, not pressure alone. During a phase change, added energy changes potential energy, not kinetic energy, so temperature stays constant.