Which liquid has the highest vapor pressure at 25 °C?
13% of national qualifiers got this right
Explanation
draftVapor pressure depends on the strength of intermolecular forces (mainly London dispersion forces for these nonpolar/weakly polar molecules). Weaker intermolecular attractions → more molecules escape to the gas phase → higher vapor pressure.
Dispersion forces increase with molecular size/polarizability (roughly with molar mass and number of electrons):
- : g/mol
- : g/mol, but F is small and only weakly polarizable, and this molecule has the smallest surface area/polarizability among the heavier halogens present
- : g/mol
- : g/mol (largest, most polarizable, strongest dispersion forces)
Comparing the four, has the largest, most polarizable electron cloud (heaviest halogens), giving the strongest dispersion forces and lowest vapor pressure. (Freon-11) contains a small, weakly polarizable F atom in place of one Cl, reducing the overall polarizability/surface area compared to or , and it also lacks the H-bond-like dipole enhancement seen in . Experimentally, (bp ≈ 24 °C) is the most volatile of the four, giving it the highest vapor pressure at 25 °C.
Thus the correct order of vapor pressure is , matching the trend of increasing molar mass/polarizability with decreasing vapor pressure.
Student assumes 's slight polarity (dipole moment) makes it the most volatile, overlooking that its dispersion forces plus dipole-dipole interactions actually give it a higher boiling point and lower vapor pressure than .
Correct: has the weakest net intermolecular attractions among the four (small, weakly polarizable F atom lowers dispersion forces relative to the heavier all-Cl/Br analogs), giving it the lowest boiling point (~24°C) and thus highest vapor pressure at 25°C.
Student picks reasoning that it's simplest/symmetric and nonpolar, ignoring that its larger polarizable Cl atoms (compared to ) create stronger dispersion forces and a higher boiling point.
Student mistakenly assumes greater halogen substitution/molar mass correlates with higher volatility, but heavier, more polarizable Br atom actually strengthens dispersion forces, making the least volatile of the four.
USNCO 2020 National Part I exam, question 13. © American Chemical Society; reproduced for non-commercial practice.