So I have heat of formation data for H2, HD, and D2, and I'm being asked to find the difference in vibrational frequency and zero point energies between H-H and H-D.

I know that i need to use the bond enthalpy as the force constant in v=(1/2pi) * sqrt(k/mu). I'm slightly confused since frequency needs to be in wavenumbers (cm-1) but the enthalpies are in kj/mol. Would I maybe use Planck's constant (J*s) to convert to hertz, and then speed of light to convert to wavenumbers...?

The second part asks to determine whether H2, HD, or D2 has the highest heat capacity at 1000 Kelvin. I'm not too sure about this one... I was reading somewhere that classical physics fails at high temperatures because heat capacity does not stay constant, and it has to do with the molecule's degrees of freedom. But don't all three molecules have the same degrees of freedom?

Thank you for any help.

  • $\begingroup$ The equation for frequency $\nu=...$ you quote is already in $s^{-1}$,if you want it in wavenumbers then divide by $c$ in cm/s. This is a frequency not energy. The heat capacity is greatest for the molecule that has the largest number of energy levels occupied at the given temperature. $\endgroup$ – porphyrin Apr 25 '20 at 8:07

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