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  • What are the different properties that intermolecular forces and Intramolecular forces attribute to? i.e. intermolecular forces contribute to BPs, vapor pressures, etc.

Specific problem: why does n-butylamine have lower vapor pressure at room temperature than methanol? Explain in regards to IMFs.

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  • $\begingroup$ I added a specific Question regarding vapor pressure. $\endgroup$ – CocoxXxPuffs May 3 '15 at 20:44
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What are intermolecular molecular forces?
The forces(electrostatic force) between molecules of a compound. These forces are responsible for intermolecular distances and how tightly the molecules are held together. If the intermolecular forces(van der Waals force) are large, it means the molecules are tightly held together in a group and they are fixed in their position.

Now what is boiling point? It's a temperature at which liquid state of a compound changes into its gas state. What's the difference between gaseous and liquid phase? Gaseous phase has very large intermolecular distance when compared to that in liquid phase. If a large force exists between the molecules and they are quite fixed in their position, then large amount energy or heat will be required to move them away from each other. Large heat means greater temperature and hence greater boiling point. So the lesson is that, the stronger the intermolecular forces, the higher the boiling point. The similar concept in applicable in melting point. the stronger the intermolecular forces, the higher will be the melting point.

You will recall that the vapor pressure is a direct measure of escaping tendency, so we can use these terms more or less interchangeably(In case you may be wondering, how are they related. Click here). If the intermolecular forces are large, then molecule in liquid phase are not able to leave to liquid easily, hence lowering the escaping tendency and therefore lower vapour pressure.

Note: This is a very basic explanation.

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  • $\begingroup$ @ShiddharthYadav I added a specific vapor pressure problem in my question. Why is n-Butylamine lower? Doesn't it have a lot of LDFs? $\endgroup$ – CocoxXxPuffs May 3 '15 at 20:46
  • $\begingroup$ @Loong what I think is that was a typo from the answerer. Everywhere else he mentions intermolecular forces. So that seems more like a mistake than deliberate action. $\endgroup$ – M.A.R. Jun 9 '15 at 18:50
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Butylamine does not exhibit H-bonding between each other, where as alcohols do. This added force from the hydrogen bond makes the boiling point of alcohols higher than those of most other ORGANIC molecules. Ionic bonds have the most strength.

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    $\begingroup$ I disagree. Nitrogen does participate in hydrogen bonding. But I do agree with your second point, the added force from hydrogen bonding would decrease vapour pressure, thereby increasing boiling point. $\endgroup$ – Bob Mar 2 '17 at 22:31

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