Question

1. The pure substances below are listed in order of increasing evaporation rate. Why does evaporation rate increase among these substances in this order?

lowest    H2O   < C2H6O   < Br2   < (C2H5)2O    highest

2. Volatile organic compounds (VOCs) are released into the air from materials found in buildings, such as paint, wood preservatives, cleansers, disinfectants, and many building materials, furnishings, and office equipment. People living and working in buildings are exposed to VOCs every day, which can cause health problems. Which of these VOCs will provide the highest levels of exposure to humans? Which will provide the lowest exposure levels? Use the three factors affecting intermolecular force strength to explain.

Answer 1

1. The rate of evaporation of a compound is dependent on the strength of the intermolecular forces in play within the molecules or atoms. The strongest of intermolecular forces give the compound the slowest evaporation rate with the weakest give fastest evaporation. The strongest intermolecular force is hydrogen-bonding, which occurs when a hydrogen attached to an electronegative atom forms a strong electrostatic interaction with an electronegative atom with available lone pairs of electrons. The strength of this force depends on the tendency of the atom to adopt H-bonding between its molecules. The next lowest are dipole-dipole interactions, which as the name suggests is an interaction occurring between two dipoles. The lowest of these forces is the van der Waals force which exists between uncharged and non-polar molecules, with no dipole moment.

Water, with two electron-deficient hydrogens attached to an oxygen has much higher degree of H-bonding than ethanol, which has only one such hydrogen. Also, the ethyl group repulses the polar interaction, preferring hydrophobic interactions like van der Waals, making it boil faster than water. Next, bromine is uncharged and non-polar and possesses van der Waals forces while ether has dipole-dipole interactions. However, the large size of bromine atoms enable it to be polarized better than O atoms in ether and so have the weaker forces in large quantities, giving it a slower evaporation rate. Finally, ether's dipole moment is non-existent as it has equal dipole vectors in opposite directions, coupled with destabilizing van der Waals forces from the ethyl groups. The lack of such a destabilizing force can also be attributed to bromine's lower evaporation rate.