KEY2CHEM

Physical Properties and Intermolecular Forces

Physical properties such as state of matter, vapor pressure, and viscosity can be directly related to the intermolecular forces of a substance. Intermolecular forces are attractive interactions holding collections of particles together, and energy input is required to break up intermolecular forces. Stronger intermolecular forces require the input of higher amounts of energy in order to break them up. Substances with stronger intermolecular forces are more likely to be liquids or solids, since more energy is required to break up particles in order to enter the gas phase. Because of this, substances with stronger intermolecular forces have lower vapor pressures (the pressure of the gas phase above a substance) since fewer particles have sufficient energy to overcome the intermolecular forces to enter the gas phase. Substances with stronger intermolecular forces also have higher viscosity (resistance to flow) since the particles interact with each other more strongly.

Summary Table

 Intermolecular Forces State of Matter Vapor Pressure Viscosity Stronger Likely solid, liquid Lower Higher Weaker Likely gas Higher Lower

Example 1.

Which substance is predicted to have the lowest vapor pressure? Assume all samples are at the same temperature.

A. $$\require{mhchem}\ce{CH4}$$

B. $$\require{mhchem}\ce{CH2F2}$$

C. $$\require{mhchem}\ce{CH3OH}$$

Solution

C. $$\require{mhchem}\ce{CH3OH}$$. The substance with the strongest intermolecular forces will have the lowest vapor pressure. $$\require{mhchem}\ce{CH4}$$ has dispersion forces, $$\require{mhchem}\ce{CH2F2}$$ is a polar molecular with dipole-dipole interactions (note: even though both H and F are present, they are not directly bonded to one another so there is no hydrogen bonding), and $$\require{mhchem}\ce{CH3OH}$$ is a polar molecule with hydrogen bonding. Since hydrogen bonding is the strongest type of intermolecular force, $$\require{mhchem}\ce{CH3OH}$$ will have the lowest vapor pressure.

Example 2.

Which substance is predicted to have the lowest viscosity (e.g., is the least viscous)? Assume all samples are at the same temperature.

A. $$\require{mhchem}\ce{C8H18}$$

B. $$\require{mhchem}\ce{CH4}$$

C. $$\require{mhchem}\ce{C5H12}$$

Solution

B. $$\require{mhchem}\ce{CH4}$$. The substance with the weakest intermolecular forces will have the lowest viscosity (be the least viscous). These molecules ($$\require{mhchem}\ce{C8H18}$$, $$\require{mhchem}\ce{CH4}$$, $$\require{mhchem}\ce{C5H12}$$) are all nonpolar hydrocarbons (molecules that contain only hydrogen and carbon), so they all have dispersion forces as the strongest type of intermolecular forces. However, since $$\require{mhchem}\ce{CH4}$$ has the smallest mass, it is the least polarizable and has the weakest intermolecular forces. Therefore, $$\require{mhchem}\ce{CH4}$$ will have the lowest viscosity.

Example 3.

At a temperature of $$80^{\circ}\text{C}$$, one of the following substances is a liquid and the other two are gases. Which substance is the liquid at $$80^{\circ}\text{C}$$?

A. $$\require{mhchem}\ce{H2O}$$

B. $$\require{mhchem}\ce{CH3OCH3}$$

C. $$\require{mhchem}\ce{CHCl3}$$

Solution

A. $$\require{mhchem}\ce{H2O}$$. The substance with the strongest intermolecular forces will be the liquid. Since $$\require{mhchem}\ce{H2O}$$ has hydrogen bonding and the other substances do not (both $$\require{mhchem}\ce{CH3OCH3}$$ and $$\require{mhchem}\ce{CHCl3}$$ are polar molecules with dipole-dipole interactions but not hydrogen bonding), $$\require{mhchem}\ce{H2O}$$ has strong enough intermolecular forces to keep it in the liquid phase, while the other substances, with their weaker intermolecular forces, will be gases.