Liquid ethanol is placed in an open beaker in a laboratory maintained at 20 °C
CHEMISTRY · Multi-part · AP · Chemistry · Properties of Substances and Mixtures · Properties of Substances and Mixtures
Problem
Liquid ethanol is placed in an open beaker in a laboratory maintained at 20 °C. The beaker is then warmed to 60 °C, and the measured vapour pressure of ethanol triples.
Answer the following questions about the underlying molecular-energy distributions of ethanol molecules in the liquid phase.
(1) Describe qualitatively how the distribution of molecular kinetic energies in the liquid changes as the temperature rises from 20 °C to 60 °C.
(2) Explain why the observed increase in vapour pressure requires that the fraction of ethanol molecules with energies sufficient to escape the liquid surface increases significantly.
(3) Assuming that vapour pressure is directly proportional to the fraction of surface molecules whose energies exceed the escape threshold, estimate by what factor that high-energy fraction increases between 20 °C and 60 °C. Justify any assumptions you make.
(4) Identify one limitation of using the proportionality assumption in part (c) when applying it to real liquids.