Liquid-Vapor Equilibrium Section 17.1 (AHL)

Vaporization You have a container with a volatile liquid in the bottom Molecules of the liquid will escape from the surface and enter the vapor phase Molecules in the vapor phase will collide with the walls of the container and exert a pressure

Continued Some of the molecules in the vapor phase will strike the surface of the liquid and condense back into the liquid phase As more and more molecules escape into the vapor phase, the pressure increases The rate of return also increases After time the rate of vaporization equals the rate of condensation

Dynamic Equilibrium The system just described is in a state of dynamic equilibrium The pressure exerted by the particles in the vapor phase is known as the vapor pressure of the liquid Change the surface area and both rates are affected equally, so the vapor pressure is unaffected (equilibrium will be established faster if the surface area is increased)

Kinetic Energy Consideration The molecules on the surface of the liquid need a certain amount of KE to escape from the attractive forces of the other surface molecules The amount of KE needed depends on the strength of the intermolecular forces Vaporization is an endothermic process (requires overcoming of the attractive forces between particles)

Enthalpy of Vaporization Enthalpy of vaporization: the amount of energy required to convert one mole of the substance from the liquid to the gaseous state Dependent mostly on overcoming the intermolecular forces, but also to do work against the atmosphere The atmosphere is pushing down on the surface

Strength of the Forces The stronger the intermolecular forces: The greater the enthalpy of vaporization The lower the vapor pressure at a given temperature The higher the boiling point The temperature of a substance does not increase as it boils, stays same until all particles are in the gas phase

More The energy absorbed is involved in increasing potential energy by overcoming attractive forces between the particles

Temperature Considerations At a higher temperature, more molecules will have the required KE to escape into the vapor phase and the rate of vaporization will increase More molecules are required in the gas phase for the rate of condensation to equal the rate of vaporization Higher temperature = increased vapor pressure

Boiling Point Boiling point: the temperature at which the vapor pressure is equal to the standard atmospheric pressure (101.3 kPa) A liquid will boil when this occurs because bubbles of vapor will form in the body of the liquid This means that vaporization can occur throughout the liquid and proceeds rapidly

Pressure Consideration A liquid will boil at a lower temperature if the external pressure is reduced This is because it will take less KE for the vapor pressure to equal the atmospheric pressure (Everest base camp) If the external pressure is increased, the boiling point of the liquid increases (pressure cooker)