Phase Changes Chapter 11

Vaporization An endothermic process in which the intermolecular attractions of a liquid are broken releasing molecules as a gas. ΔH vap, The heat of vaporization is the heat required to vaporize one mole of a liquid at 1 atm.

Vapor Pressure In a closed container, some liquid is initially changed to a gas. Eventually the rate of evaporation equals the rate of condensation. Vapor Pressure is at equilibrium.

Vapor Pressure Vapor pressure varies depending on the volatility of a liquid. Volatility is the ease of a liquid to form a gas. Liquids with low IMF’s have high volatility. Large molecules tend not to be volatile. Water is a small molecule, but is not highly volatile. Why?

Vapor Pressure increases with temperature Increasing the average kinetic energy of the molecules, means they require less additional energy to overcome their intermolecular forces to become a gas.

Calculating Enthalpy of vaporization ln(P vap ) = - [ΔH vap /R] x (1/T) +C R is universal gas constant C is the constant characteristic of a given liquid. ln is the natural log. y=mx+b Plotting the natural log versus 1/T in kelvin gives a straight line.

Calculating the ΔH vap

Changes of State Vaporization Condensation Fusion Solidification Sublimation Deposition

Heating curve Energy can be converted into a heat change of a substance. Energy is also required to change the state of a substance. This change in energy is plotted in a Heating Curve.

Phase Diagrams These are a convenient way to represent the phases of any substance as a function of temperature and pressure.

Phase Diagrams The Triple Point is the temperature and pressure that a substance can exist as all three states of matter (solid, liquid and gas) as all three have the same vapor pressure. The Critical Temperature is defined as the temperature above which the vapor cannot exist as a liquid no matter what pressure is applied. The Critical Pressure is the pressure required to condense a vapor at the critical temperature. Together the critical temperature and critical pressure make the Critical Point. The melting point at 1 atm is the Normal Melting Point.

Phase Diagram for Carbon

Application for Phase Diagrams Note: on the phase diagram for water that the melting point of ice decreases as the external pressure increases. This is opposite to observations for most other liquids. Why is this possible?

Phase Diagrams For ice skating, the narrow blade exerts a large external energy on the ice melting it beneath the blade, and refreezing as the blade leaves the surface. This allows the smooth gliding action.

Phase diagrams As pressure decreases so does the boiling point of water. This is why Doug in Caldwell, NJ requires more heat to boil water than Kristen in Boulder, CO.