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Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: iyc on August 14, 2006, 12:50:48 AM

Title: Is there any differences in melting point/ boiling point between isotopes?
Post by: iyc on August 14, 2006, 12:50:48 AM
Is there any differences in m.p./b.p. between 35Cl2 and 37Cl2?

In general, van der Waals' forces between the molecules increase with increasing molecular mass.
But, in the case of isotopes, I don't know whether it is true or not.

As both 35Cl2 and 37Cl2 have the same number of electron, the fluctuation in symmetry of charge cloud will be almost the same, resulting similar strength of instantaneous dipole- induced dipole atrraction. Therefore, I think they will have the same m.p./ b.p.

But this contradicts with that van der Waals' forces between the molecules increase with increasing molecular mass.

Can anybody help?
Title: Re: Is there any differences in melting point/ boiling point between isotopes?
Post by: Mitch on August 14, 2006, 01:35:06 AM
There will be a slight difference, the lighter isotope will boil at a lower temperature.
Title: Re: Is there any differences in melting point/ boiling point between isotopes?
Post by: iyc on August 14, 2006, 02:06:21 AM
is it due to the slight difference in surface area between the isoptopes?
Title: Re: Is there any differences in melting point/ boiling point between isotopes?
Post by: Yggdrasil on August 14, 2006, 02:17:13 AM
As both 35Cl2 and 37Cl2 have the same number of electron, the fluctuation in symmetry of charge cloud will be almost the same, resulting similar strength of instantaneous dipole- induced dipole atrraction.

I think this explanation is right on and the two isotopes will have the same van der Waals forces between them.  However, I think there will be a slight difference in the thermal energy needed to free the heavier 37Cl2 from the solid and/or liquid phase during the phase change, which accounts for the slight increase in bp/mp.