Question

Why do liquids have a vapor pressure? Do all liquids have vapor pressures? Explain. Do solids exhibit vapor pressure? Explain. How does vapor pressure change with changing temperature? Explain.

Short answer

Liquids have vapor pressure because of the varying kinetic energy of their molecules, allowing some to escape into the gas phase. All liquids have vapor pressures, but the values differ due to the properties of the liquids and their temperatures. Solids can also exhibit vapor pressure through sublimation, although typically lower than liquids. Vapor pressure increases with temperature because more molecules have enough energy to escape the condensed phase. The relationship can be described by the Clausius-Clapeyron equation: \[\ln{ \frac{P_2}{P_1} } = - \frac{\Delta H_{vap}}{R} ( \frac{1}{T_2} - \frac{1}{T_1} )\]

Step-by-step solution

Understanding Vapor Pressure

Vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. It occurs because molecules within a substance have varying levels of kinetic energy. Molecules on the surface of a liquid that have enough energy can escape the liquid phase and enter the gas phase, creating a vapor.

Vapor Pressure in Liquids

All liquids have vapor pressures, although they may vary significantly depending on the liquid and its temperature. In a closed container, some of the liquid's molecules will evaporate into the gas phase, while others from the gas phase will condense back into the liquid phase. When the rate of evaporation equals the rate of condensation, the system reaches an equilibrium, and the pressure exerted by the vapor in the gas phase is known as the vapor pressure.

Vapor Pressure in Solids

Yes, solids can also exhibit vapor pressure, although it is usually much lower than that of liquids due to the stronger intermolecular forces in solids. This phenomenon is called sublimation, where a solid directly transitions into a gaseous state without going through the liquid phase. This process typically occurs only in specific conditions, such as low pressure or when the solid is heated.

Vapor Pressure and Temperature

Vapor pressure increases as temperature increases because as the temperature rises, more molecules have enough kinetic energy to escape the condensed phase (liquid or solid) and enter the gas phase. In other words, with rising temperature, the rate of evaporation increases, leading to a higher equilibrium vapor pressure. This relationship between temperature and vapor pressure can be described by the Clausius-Clapeyron equation: \[\ln{ \frac{P_2}{P_1} } = - \frac{\Delta H_{vap}}{R} ( \frac{1}{T_2} - \frac{1}{T_1} )\] Here, \(P_1\) and \(P_2\) are the vapor pressures at temperatures \(T_1\) and \(T_2\), respectively, \(\Delta H_{vap}\) is the heat of vaporization, \(R\) is the gas constant, and \(T_1\) and \(T_2\) are given in Kelvin. In summary, vapor pressure is a property exhibited by liquids and solids, and it is affected by temperature. With increasing temperature, the vapor pressure of a substance increases due to an increase in the number of molecules with enough energy to enter the gas phase.