From a pearson chemistry textbook I am studying, the boiling point and Hvap of both water and isopropanol is detailed. How is the Hvap of isopropanol so similar to water, yet has a lower boiling point? I read that as intermolecular forces are increased, so does the boiling point. Wouldn’t the same be true for the Hvap?
|Chemical|Boiling point Celsius|Hvap (Boiling Point)|Hvap (25C)|
|:-|:-|:-|:-|
|Water|100|40.7|44|
|Isopropanol|82.3|39.9|45.4|
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Because your reference is KJ/Mol. Isopropanol has a higher molar mass. when you evaporate 1 Mol of isopropanol you evaporate 60g and for 1 Mol water you evaporate only 18g.
A counter question, just to jog some connections:
Boiling temperature at what pressure? Water doesn’t boil at 100C in vacuum.
Enthalpy of vapourisation is a measure of how much energy you need provide to liberate molecules from the liquid into a free, unbound (gaseous) state. While it depends on the temperature, it doesn’t care about the boiling point. Evaporation occurs even below boiling point, that’s why stuff dries. The individual molecules just need that energy surplus from somewhere.
The boiling point meanwhile is partially related to enthalpy, as they both are affected by strength of intermolecular bonding, but not analogous. Liquids boil when their vapour pressure equals the pressure of the atmsphere.
The vapour pressure of isopropanol is higher than water’s (because water’s bonding strength is higher so ot holds onto its molecules stronger in a liquid), therefore it can boil at a higher atmospheric pressure than water can for the same temperature. Water needs to be heated to a higher temperature overcome the same atmospheric pressure.
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