If a diatomic gas like Hydrogen has 6 maximum degrees of freedom why its molar heat capacity reaches at high temperatures $C_V = \frac{7}{2} R$ and not $C_V = \frac{6}{2} R= 3R$?
[Physics] Why does Hydrogen molar heat capacity reach 7/2 R
thermodynamics
Best Answer
The degrees of freedom of a diatomic gas are as follows:
2 rotational: The molecule can in principle rotate around each axis. But consider rotations around the molecular axis (connecting the H atoms): in this case, the physics doesn't change. Another way of thinking about it is that the axial rotation mode only can store a vanishing amount of energy, compared to the others.
The rotational modes are only available at higher temeratures, since the molecule has a moment of inertia that has to be overcome to start rotating.
2 vibrational: The atoms can wiggle together and apart, which is one degree of freedom. But there is also another one, which is harder to see: Think of the molecule as an harmonic oszillator with kinetic and potential energy.
If both atoms are displaced towards the middle, the molecule has a higher potential energy. The equations of the harmonic oscillator would normally fix the kinetic energy of the atoms in this case. But in a gas with its random kinematics it is totally possible that they have the "wrong" kinetic energies for their relative displacement. So, for the purposes of statistical mechanics, these are 2 further DOFs, making seven.