`C_(v)` values of He is always `(3R)/(2)` but `C_(v)` values of `H_(2)` is `(3R)/(2)`
at low temperature and `(5R)/(2)` at higher temperature explain .

Explanation

Helium is monoatomic gas and thus only translational energy (three degrees of freedom) contributes for kinetic energy Thus `C_(V)` for He comes out to be `[(3)/(2)R(T+1)-(3)/(2)RT]=(3R)/(2)` Hydrogen being diatomic (five degree of freedom) and thus vibrational and rotational energies also contributes for its kinetic energy Thus `C_(V)` value of `H_(2)` comes out to be `(3R)/(2) +X`
where `X` is contribution of vibrational and rotational energies change At low temperature `X ~0` thus `C_(V) = (3R)/(2)` At moderate temperature rotational energy contribution becomes significant and `X` equal to `R` Thus `C_(V) == (5R)/(2)` Also at high temperature vibrational energy contribution becomes significant and thus `C_(V)` more than `(5R)/(2)` .