A5.1 Mean kinetic energy
Consider a system of particles, each of mass m but with differing values of speed v; the kinetic energy of each particle is mv2/2, which may be resolved along mutually perpendicular x, y and z axes. The mean value of any distribution of the form is given by:
where the integration is taken over the range of the variable. Assuming that the energies of the particles follow a Boltzmann distribution, then from Appendix A4, the mean value for the kinetic energy uK for a single particle is:
which simplifies to:
and for the system of n particles, the average kinetic energy is given by:
A5.2 Equipartition theorem
A molecule possesses energy in terms of translation, rotation and vibration. The equipartition theorem, which derives from the Boltzmann distribution (Appendix A4), states that for a system of particles at thermal equilibrium under a temperature T, the mean value of each quadratic contribution to the energy of the system is kT/2. A quadratic contribution is one that can be expressed as the square of a property such as position or velocity.
Translational energy is kinetic and involves three quadratic terms:
with each quadratic term in this expression equal to kT/2 Hence, the total classical kinetic energy is (3/2)kT; it is a mean value, since all directions of translation are equally probable.
A rigid rotor has two perpendicular axes about which it can rotate, each leading to a quadratic kinetic energy contribution; rotations about the axis of the rotor have no effect on the energy. The key term in a rotor is the moment of inertia; it has the form mr2 in the simplest case and constitutes a quadratic term. In accordance with the equipartition theorem, each such term contributes kT/2 to the energy; thus, the total mean rotational energy is kT for two quadratic terms. The vibrational energy of an atomic oscillator involves quadratic terms in both kinetic and potential energy; hence, there are again two quadratic terms and a total mean vibrational energy of kT.
A monatomic species such as argon possesses only energy of translation of total mean value (3/2)RT per mole. A diatomic molecule like oxygen may rotate about two directions perpendicular to each other and to the molecular axis, with each rotation contributing to the energy of the molecule. Rotation about the molecular axis does not contribute to the energy of the molecule which has the total mean value of (5/2)RT per mole, that is, (3/2)RT for translation plus RT for rotation.