Chemical Bonds

TLDR: More Vapor pressure = more evaporates without boiling

Less IMF = more vapor pressure

Vapor Pressure

Vapor pressure is a fundamental concept in chemistry, describing the pressure exerted by a vapor in equilibrium with its condensed phase (liquid or solid) at a given temperature. It’s a measure of the tendency of a substance to evaporate.

Understanding Vapor Pressure

Imagine a closed container containing a liquid. Some molecules at the liquid’s surface possess enough kinetic energy to escape into the gaseous phase, creating a vapor above the liquid. As more molecules vaporize, the pressure exerted by the vapor increases. Simultaneously, some vapor molecules collide with the liquid surface and condense back into the liquid phase.

When the rate of vaporization equals the rate of condensation, a dynamic equilibrium is established. At this point, the pressure exerted by the vapor is called the vapor pressure.

Factors Affecting Vapor Pressure

• Temperature: Increasing temperature increases the kinetic energy of molecules, leading to more molecules escaping into the vapor phase and thus a higher vapor pressure. This relationship is described by the Clausius-Clapeyron Equation:

$ ln(P_2/P_1) = - \Delta H_{vap}/R (1/T_2 - 1/T_1) $

where:

• $ P_1 $ and $ P_2 $ are the vapor pressures at temperatures $ T_1 $ and $ T_2 $ respectively.

• $ \Delta H_{vap} $ is the Enthalpy of vaporization.

• R is the ideal gas constant.

• intermolecular forces: Stronger intermolecular forces (like hydrogen bonding) require more energy to overcome, resulting in lower vapor pressure at a given temperature. For example, water has a higher boiling point and lower vapor pressure than diethyl ether due to its strong hydrogen bonding.

• Surface Area: A larger surface area of the liquid exposes more molecules to the vapor phase, leading to a higher vapor pressure.

Examples

• Evaporation of Water: At room temperature, water molecules constantly escape from the liquid surface, creating a small vapor pressure. This is why a puddle of water eventually dries up.
• Boiling Point: The boiling point of a liquid is the Temperature at which its vapor pressure equals the external atmospheric pressure. For example, water boils at 100°C at standard atmospheric pressure (1 atm).
• Volatile Liquids: Liquids with high vapor pressures at room Temperature are called volatile liquids. Examples include acetone, ethanol, and gasoline.

Applications

• Distillation: Vapor pressure differences are used in Distillation to separate components of a mixture based on their boiling points.
• Refrigeration: Refrigerants utilize the vapor pressure of a fluid to absorb heat and cool a system.
• Atmospheric Chemistry: Vapor pressure plays a crucial role in understanding the behavior of volatile organic compounds in the atmosphere.