Gas molecules move randomly, have elastic collisions, are point-like, and have no interactions {kinetic theory}|. Ideal gases follow kinetic theory. Gas molecules have cross-sectional area, and hydrogen bonds and van der Waals forces make molecules slightly attract, so real gas molecules do not move completely randomly and have somewhat inelastic collisions.
molecular collisions
In gases, one cubic centimeter has 10^28 molecular collisions per second. Collision frequency increases as mass decreases, temperature increases, cross-sectional area increases, and density increases.
molecular velocity
Gas-molecule collisions distribute speeds and directions. Molecular-velocity distributions are Boltzmann distributions. Some molecules have low velocity. Most molecules are near average velocity. Few molecules have very high velocities. Average gas-molecule velocity at room temperature is 500 meters per second. Molecular velocity increases as mass decreases or temperature increases.
Maxwell envisioned a demon {Maxwell's demon} {Maxwell demon} that can see particle motions and act on particles individually, so perpetual motion of second kind can happen. However, demon, light, and energy are all system parts, so perpetual motion cannot happen.
On average, particles travel short distances {mean free path}| between collisions. Mean free path is collision-frequency inverse and measures average distance between gas molecules. Mean free path decreases as mass decreases, temperature increases, cross-sectional area increases, and density increases.
Systems have different motions and kinetic energies {degrees of freedom, partition}, such as translations, rotations, and vibrations.
translation
All particles can have translations. Average random translational kinetic energy determines temperature.
rotations
Spherically symmetric molecules cannot have net rotational motion. Linear molecules can have one rotational motion state. Two-dimensional molecules can have two rotational motion states. Three-dimensional molecules can have three rotational motion states.
vibrations
Molecules with chemical bonds can have vibration states. Vibrations can involve one bond and be along bond axis. Vibrations can involve two bonds and be across bond axes. Molecule symmetries can cancel vibration states.
partition
Heat can go equally into all available energy states {partition of energy, heat}|. If molecule has more rotation and/or vibration states, raising temperature requires more energy, because some heat does not become average random translation kinetic energy.
partition: heat capacity
Material heat capacity depends on molecular-motion degrees of freedom. Molecules with more rotation and/or vibration states have higher heat capacity.
partition: equipartition
Motion-type average kinetic energies must be the same {equipartition, energy} {energy equipartition} {principle of equipartition of energy}, because energy transfers freely among states by collisions.
amount
Partition average kinetic energy KE is half Boltzmann constant k times temperature T: KE = 0.5 * k * T.
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Date Modified: 2022.0225