We performed an experiment in chemistry where tissue paper soaked in various chemicals was wrapped around the end of a temperature probe.

The chemicals with stronger intermolecular forces had a lower drop in temperature (from room temperature to final, after most of the alcohol had evaporated off). For reference, two of the chemicals were ethanol with hydrogen bonding and n-pentane with only London dispersion forces.

My question is why.

Below are just my initial attempts to understand

My initial feeling was that the molecules with stronger intermolecular forces would take more energy to evaporate and would have a larger temperature change than those chemicals with weaker bonds, but we see the opposite in the data.

I was also thinking that maybe that went the other way: The chemicals with stronger forces had fewer molecules evaporate off, so the temperature drop was smaller. Is this more correct?

Thank you!

  • 1
    $\begingroup$ It's a matter of kinetics, not thermodynamics. If you measured heat not temp. you should get expected results. $\endgroup$ – Mithoron Oct 31 '18 at 23:18
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    $\begingroup$ The heat (enthalpy) needed vaporise 1 g of each solvent is approx 840 J for ethanol and approx 360 for the pentane. So unless you have equal or at least known quantities in your tissue your experiment can give misleading results. However, your initial idea about stronger forces needing more energy to evaporate makes sense. $\endgroup$ – porphyrin Nov 1 '18 at 9:37

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