1. Intermolecular Forces

Hey guys, I had a quick chemistry question:

The melting point of $H_{2}$ is -129 degrees celcius. The melting point of $C_3H_8$ is -190 degrees celcius. Explain why $H_2$ has a higher melting point than $C_3H_8$.

When i read this question, I was under the impression that C3H8 would have a higher melting point, due to the fact that it has a higher molecular weight than H2, thus increasing the London forces. I would appreciate it if anyone could help me out with this problem.

Thank You.

2. Originally Posted by rtblue
Hey guys, I had a quick chemistry question:

The melting point of $H_{2}$ is -129 degrees celcius. The melting point of $C_3H_8$ is -190 degrees celcius. Explain why $H_2$ has a higher melting point than $C_3H_8$.

When i read this question, I was under the impression that C3H8 would have a higher melting point, due to the fact that it has a higher molecular weight than H2, thus increasing the London forces. I would appreciate it if anyone could help me out with this problem.

Thank You.
Things have really advanced since my days of taking college chemistry (as London forces weren't taught back then). I suspect the answer lies with the geometry of the differing molecules.

3. The mass have nothing to do with the intermolecular bonding, which however is directly associated to melting/boiling points. In non-polar substances (like in your question), the predominating form of intermolecular forces is London forces, and the latter depends on the number of electrons present in each molecule (it's a induced dipole interaction). Therefore, propane (C3H8) has a higher melting point.

Note that the melting point of hydrogen is -259 C and not -129 C and has boiling point -253 C.

4. Well, Unknown008, increasing mass, generally, means that the number of electrons is increasing (I don't see how we can only add protons and neutrons to a compound). That is essentially what I was saying when I referred to mass. I believe there was a typo in the question, since, from your data the melting point of H2 should be -259 degrees Celsius.

Thanks for pointing that out.

5. Originally Posted by rtblue
Well, Unknown008, increasing mass, generally, means that the number of electrons is increasing (I don't see how we can only add protons and neutrons to a compound). That is essentially what I was saying when I referred to mass. I believe there was a typo in the question, since, from your data the melting point of H2 should be -259 degrees Celsius.

Thanks for pointing that out.
Not totally true, since neutrons also affect the mass, while they won't affect the number of electrons. In that sense, you can end up with two substance with identical mass, yet one has more electrons that the other and has hence a greater melting/boiling point.