Question

Explain how each of the following affects the vapor pressure of a liquid: (a) volume of the liquid, (b) surface area, (c) intermolecular attractive forces, (d) temperature, (e) density of the liquid.

Short answer

The factors affecting the vapor pressure of a liquid are volume, surface area, intermolecular attractive forces, temperature, and density. The volume of the liquid does not affect the vapor pressure at equilibrium, while surface area impacts the rate of evaporation but not the vapor pressure at equilibrium. Stronger intermolecular forces lead to lower vapor pressures, while weaker intermolecular forces result in higher vapor pressures. As temperature increases, vapor pressure also increases due to increased kinetic energy of molecules. Lastly, the density of the liquid correlates indirectly with vapor pressure, as denser liquids tend to have stronger intermolecular forces and lower vapor pressures, but other factors also contribute to this relationship.

Step-by-step solution

(a) Volume of the liquid

The volume of a liquid itself does not affect the vapor pressure. However, it is important to note that the amount of vapor above the liquid in the container will depend on the volume of the container (if it is closed). But when the liquid reaches equilibrium, the vapor pressure will be the same, regardless of the volume of the liquid.

(b) Surface area

Surface area affects the rate at which vaporization/evaporation occurs. With a greater surface area, a greater amount of the liquid is exposed to its surroundings, and thus evaporation occurs more quickly. However, this does not affect the vapor pressure once equilibrium is reached. At equilibrium, vapor pressure will be the same for a given liquid at a specific temperature, regardless of the surface area.

(c) Intermolecular attractive forces

Intermolecular attractive forces play a significant role in determining vapor pressure. Stronger intermolecular forces lead to lower vapor pressures, as molecules are more strongly attracted to each other, making it more difficult for them to leave the liquid and become vapor. Inversely, a substance with weaker intermolecular forces will have a higher vapor pressure, as molecules can more easily escape into the vapor phase.

(d) Temperature

Temperature has a substantial effect on vapor pressure. As temperature increases, the kinetic energy of the liquid molecules also increases. This causes more molecules to have enough energy to overcome the intermolecular attractive forces and escape into the vapor phase. As a result, vapor pressure increases with higher temperatures.

(e) Density of the liquid

Density of the liquid does not directly impact the vapor pressure. However, there may be a correlation between density and vapor pressure due to physical properties. In general, denser liquids have stronger intermolecular forces, leading to lower vapor pressures. Conversely, substances with lower density often have weaker intermolecular forces, resulting in higher vapor pressures. Nevertheless, this is not a direct causal relationship, as other factors like molecular weight and composition also contribute to intermolecular forces, the density of the liquid, and the vapor pressure.