Chemical Forums
Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: Needaask on July 06, 2013, 02:48:56 AM
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Why is it that having stronger intermolecular forces (IMF) would mean a greater boiling and melting point?
Reading up online what I understood is that having stronger IMF would mean that at a given temperature, the potential energy is more negative than a weaker IMF substance. So when heating it up, more energy would be required to increase the PE to the point where melting/boiling can occur.
However, I was thinking if A has stronger IMF than B, then at a given temperature their average KE is the same but the PE of A is more negative than B. So, when heating both of them, each joule of energy would distribute evenly between the KE and the PE so for A there would be a smaller temperature change than B. But it doesn't tell me why A would have a greater melting/boiling point than B. I would only understand why A would have a greater specific molar capacity and not why a greater BP.
Can someone explain this to me? Thanks :)
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Overthinking it.
To boil, you need to break intermolecular bonds. Breaking a bond takes energy. The stronger the bonds, the more energy it takes to break them. So substances with stronger intermolecular bonds take more energy to boil. A higher temperature is therefore required to provide the requisite energy to break those bonds. Therefore more and stronger intermolecular bonds results in a higher boiling point.
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Melting points are extremely complicated and not predictable up to now. While there is conceptually a link with intermolecular forces, you can completely forget and abandon any simple relation like "the stronger, the hotter".
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Overthinking it.
To boil, you need to break intermolecular bonds. Breaking a bond takes energy. The stronger the bonds, the more energy it takes to break them. So substances with stronger intermolecular bonds take more energy to boil. A higher temperature is therefore required to provide the requisite energy to break those bonds. Therefore more and stronger intermolecular bonds results in a higher boiling point.
Hi Corribus :)
I agree that I'm overthinking it. So for now i should just understand this with the basic explanation? Cos I'm quite curious about it but i feel like i'm lacking in knowledge to comprehend it completely.
Thanks :)