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(Pvap) = - [ΔHvap/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.