Courtesy: labinitio.com

Vaporization or evaporation: molecules of a liquid escaping the liquid’s surface and forming a gas. Vaporization is endothermic because energy is required to overcome the relatively strong intermolecular forces in the liquid. The energy required to vaporize 1 mole of a liquid at a pressure of 1 atm is called the heat of vaporization, or the enthalpy of vaporization, and is usually symbolized as ΔH vap. Why is it important to us that water has a unusually high heat of vaporization?

The equilibrium vapor pressure is the vapor pressure measured when a dynamic equilibrium exists between condensation and evaporation. H 2 O (l) H 2 O (g) Rate of condensation Rate of evaporation = Dynamic Equilibrium

The rates of condensation and evaporation over time for a liquid sealed in a closed container. The rate of evaporation remains constant and the rate of condensation increases as the number of molecules in the vapor phase increases, until two rates become equal. At this point, the equilibrium vapor pressure is attained.

A simple barometer can measure vapor pressure of a liquid. The liquid is injected at the bottom of the tube of mercury and floats to the top. Part of the liquid evaporates and produces a vapor that pushes some mercury from the tube.

High vapor pressure = volatile = evaporate readily Strong intermolecular forces = low vapor pressure Which of the three liquids below is the most volatile? (C 2 H 4 ) 2 O = diethyl ether

Vapor pressure increases with increasing temperature

What happens when a solid is heated? Typically, it will melt to form a liquid. What if the heat is continued? The liquid will boil and form the vapor phase. We can represent this process by a heating curve.

As energy flows into ice, the vibrations of the water molecules increase as temperature rises. The plateau at 0 o C indicates the molecules are so energetic they can break lose from lattice and change from solid to liquid occurs. This temperature is the melting point and all added energy is used to break the hydrogen bonds and increase the potential energy of the molecules.

The enthalpy change that occurs at melting point when a solid melts is called the heat of fusion, or the enthalpy of fusion, ΔH fus. In other words, it is the heat absorbed by one mole of the solid when it melts to give a liquid at the same temperature and pressure. Note as the melting point increases the energy required increases. The normal melting point is defined as the temperature at which the solid and liquid states have the same vapor pressure under conditions where the total pressure is 1 atmosphere.

The temperature remains constant until the solid has completely changed to liquid and then increases. At 100 o C the liquid water reaches the boiling point and the temperature remains constant as added energy is used to vaporize the liquid. Boiling occurs when the vapor pressure of a liquid becomes equal to the pressure of its environment. The normal boiling point of a liquid is the temperature at which the vapor pressure of the liquid is exactly 1 atmosphere.

The heat of vaporization, ΔH vap, is the heat absorbed when one mole of a liquid is changed to one mole of vapor at a constant temperature and pressure. The heat of sublimation, ΔH sublimation, is the heat absorbed by one mole of a solid when it sublimes to give one mole of vapor at a constant temperature and pressure. Note that the ΔH values for fusion, vaporization, and sublimation are all positive because the phase change in each case is accompanied by an increase in potential energy.

Changes of state do not always occur exactly at the boiling point or melting point. For example, water can be supercooled; that is, it can be cooled below 0 o C at 1 atm pressure and remain in the liquid state. Occurs because as it cools, the water may not achieve the degree of organization necessary to form ice at 0 o C and continues to exist as liquid. Eventually the correct ordering occurs, energy is released and the remainder of the water freezes.