Intermolecular Forces 11.2

• A change in the physical state of a substance is called a phase change.
• Some phase changes are endothermic: melting, vaporization, and sublimation.
• The reverse phase changes are exothermic: freezing, condensation, and deposition.
• The enthalpy change that occurs when one molecule of a pure solid is completely melted is called the heat of fusion, /_Hfus.
• The term fusion in chemistry means melting.
• The enthalpy change that occurs when mole of a pure liquid is completely vaporized is called the heat of vaporization, /_Hvap.
• Substances with stronger intermolecular attractive forces generally have higher values of /_Hfus and /_Hvap.
• The heat of vap of a given substance is generally much higher than its heat of fusion.
• A HEATING CURVE for a substance is a plot of temperature as a function of the amount of heat absorbed by the substance.
• The MELTING POINT of a substance (which is the same as its FREEZING POINT) is the temperature at which its solid and liquid states coexist in equilibrium (indicated by the two-way equilibrium arrow).
• The heating curve remains flat at the melting point because even though heat continues to be added, all of the additional heat goes into giving the solid-state molecules sufficient energy to partially overcome the intermolecular forces and go into the liquid phase.
• VAPOR PRESSURE is the pressure exerted by a vapor in equilibrium with its liquid (or solid) state.
• Vapor pressure is a dynamic equilibrium meaning that liquid molecules evaporate (liquid ->gas) and gas molecules condense (gas -> liquid) at the same rate.
• Under equilibrium conditions, the rate of sublimation of solid naphthalene (solid->gas) equals the rate of deposition of napthalene vapor (gas->solid) and the pressure exerted by the napthalene vapor is called its vapor pressure.
• If the container is small enough to prevent evaporation of all the water, liquid water continues to evaporate as water vapor condenses, with both processes occurring at the same rate.
• The vapor pressure of a substance does not depend on the surface area or the volume of the liquid (or solid).
• The vapor pressure of a substance does depend on temperature.
• At a higher temperature, more liquid molecules can acquire sufficient kinetic energy to escape the surface of the liquid and go into the gas phase.
• Vapor pressure does depend on the strength of intermolecular forces.
• The stronger the intermolecular attractive forces, the lower the vapor pressure and the less volatile a substance is.
• The vapor pressure of water at 100C is 760 torr illustrating the important relationship between boiling point and vapor pressure.
• BOILING POINT is defined as the temperature at which the vapor pressure of liquid equals the external pressure.
• When a liquid such as water boils, its molecules possess enough kinetic energy to overcome the intermolecular attractions holding them in the liquid states.
• The NORMAL BOILING POINT of a liquid is the temperature at which the vapor pressure of a liquid equals 1 atm.
• Like vapor pressure, the boiling point of a substance is related to the strength of the intermolecular forces holding the molecules together in the liquid phase.
• The stronger the intermolecular forces, the higher the boiling point.