Vapor Pressure, Boiling Point, and Molar Heat of Vaporization PDF

*Learning Objectives:* *Describe the following properties of liquids, an explain the effect of intermolecular forces on these properties: surface tension, viscosity, vapor pressure, boiling point, and molar heat of vaporization (STEM\_GC11IMFIIIa-c-102)* **Properties of Liquid and Intermolecular F...

*Learning Objectives:* *Describe the following properties of liquids, an explain the effect of intermolecular forces on these properties: surface tension, viscosity, vapor pressure, boiling point, and molar heat of vaporization (STEM\_GC11IMFIIIa-c-102)* **Properties of Liquid and Intermolecular Forces** The properties of liquids that were observed are consequences of the interactions of particles that make up the liquid. I. **Vapor Pressure** - The equilibrium vapor pressure is the maximum vapor pressure of a liquid at a given temperature and that it is constant at a constant temperature. It increases with temperature. - When temperature is high, more molecules have enough energy to escape from the liquid. At a lower temperature, fewer molecules have sufficient energy to escape from the liquid - The stronger the intermolecular forces of attraction, the lower the vapor pressure of a liquid. II. **Boiling point**- is the temperature at which the vapor pressure of a liquid is equal to the external pressure. - A liquid boil when its vapor pressure equals the pressure acting on the surface of the liquid. - The normal boiling point is the temperature at which the liquid converts to a gas when the external pressure is 1 atm. The normal boiling point of water is 100^o^C. - The boiling points of substances often reflect the strength of the intermolecular forces operating among the molecules. At the BP, enough energy must be supplied to overcome the attractive forces among molecules before they can enter the vapor phase. III. **Molar Heat of Vaporization** - The molar heat of vaporization (*ΔH~vap~*) is the energy required to vaporize 1 mole of a liquid at a given temperature. H is the symbol for enthalpy, which means heat content at a given standard condition. - The heat of vaporization may be considered a measure of the strength of intermolecular forces in a liquid. If the intermolecular attraction is strong, it takes a lot of energy to free the molecules from the liquid phase and the heat of vaporization will be high. - It is easier to vaporize acetone (lower *H~vap~*) than water (higher *H~vap~*) at a given temperature, and more acetone escapes into the vapor phase at a given temperature. Acetone is a polar substance but has no H-bonding. It has weaker intermolecular forces than water, and therefore acetone molecules are held less tightly to one another in the liquid phase. - A practical way to demonstrate differences in the molar heat of vaporization is by rubbing acetone on your hands. Compare what is felt when water is used. Acetone has a lower *ΔH~vap~* than water so that heat from our hands is enough to increase the kinetic energy of these molecules and provide additional heat to vaporize them. As a result of the loss of heat from the skin, our hands feel cool. **Activity/Exercises:** **True or False** 1. The vapor pressure is higher for a substance with stronger IMFA. 2. The boiling point of a substance is directly proportional to its heat of vaporization. 3. The normal boiling point is the boiling point of the liquid when the vapor pressure is 1 torr. 4. Acetone has a higher *ΔH~vap~* than water. 5. When temperature is high, more molecules have enough energy to escape from the liquid