Phase Changes

Phase Changes Matter exists in three physical states:

Solid Liquid Gas

When matter is converted from one physical state to another, a phase change occurs. A change in the physical state of a

substance

Phase Changes

You should know what phase change occurs for each of the following: Melting Freezing Vaporization Condensation Sublimation Deposition

Phase Changes Melting:

Solid Liquidendothermic

Freezing: Liquid Solid

exothermic

Vaporization Liquid Gas

endothermic

Condensation Gas Liquid

exothermic

Phase Changes Sublimation

Solid Gasendothermic

Deposition Gas Solid

exothermic

Phase Changes In order for a phase change to occur,

the molecules must gain enough kinetic energy to overcome the intermolecular forces between the molecules.

As the strength of the IMF increases, more energy is needed for a phase change to occur.

Higher boiling point and higher melting point occur.

Phase Changes Every phase change is accompanied by

a change in the energy of the system. Heat of fusion (Hfus)

Enthalpy change required to change a solid at its melting point to a liquid at the same temperature

Heat of vaporization (Hvap) Enthalpy change required to

change a liquid at its boiling point to a gas at the same temperature

Phase Changes Heat of Sublimation (Hsub)

Enthalpy change required to transform a solid directly into the gas phase

You should be able to use the heat of fusion, heat of vaporization, or the heat of sublimation and the mass (or moles) of a substance to calculate the amount of heat needed to accomplish a specified phase change at constant temperature.

qphase change = Hphase change x # moles

Phase Changes & Heating Curves

During a phase change, the two phases exist in equilibrium. At the melting point, solid and liquid

phases are in equilibrium. At the boiling point, liquid and gas

phases are in equilibrium.

The heat added to the system at the melting point and boiling point goes into pulling the molecules further apart from each other (i.e. overcoming the intermolecular forces).

Phase Changes & Heating Curves

During a phase change the temperature of the substance is constant.

The temperature does not increase until the phase change is complete. A

heating/cooling curve for water

Phase Changes & Heating Curves

A general heating or cooling curve for a solid below the MP being converted to a gas above the boiling point:

Csolid

Cliq

Cgas

Hfus

Hvap(g, BP)(l, BP)

(g, Tf)

(l, MP)

(s, Ti)

(s, MP)

Phase Changes & Heating Curves

The amount of heat needed to covert a solid below its melting point to a gas above its boiling point is found by calculating the amount of heat needed for each step:

qtotal = q(Ti MP) + q(sl) + q(MPBP) + q(lg) + q(BPTf)

To calculate the amount of heat needed when the temperature changes (same physical state) q = C x mass x T

To calculate the amount of heat needed during a phase change (temperature is constant) q = H x moles

Phase Changes & Heating Curves

Example: Calculate the enthalpy change associated with converting 12.0 g of ice at -15oC to steam (water vapor) at 115oC under a constant pressure of one atmosphere. The specific heats of ice, water, and steam are 2.09J/g-K, 4.18 J/g-K, and 1.84 J/g-K, respectively. For water, Hfus = 6.01 kJ/mol and Hvap = 40.67 kJ/mol.

Phase Changes & Heating Curves

Phase Changes & Heating Curves

Phase Changes & Phase Diagrams

What would you do differently if the problem asked to calculate the enthalpy change to convert 12.0 g of steam at 115oC to ice at -15oC?

You should be able to handle both types of problems.

Phase Changes & Heating Curves

Example: Benzene (C6H6) has a boiling point of 80.1 oC and a melting point of 5.5oC. The specific heat of benzene is 1.516 J/gK in the solid phase, 1.726 J/gK in the liquid phase, and 1.055 J/gK in the gas phase. Its heat of fusion is 9.90 kJ/mole while its heat of vaporization is 30.77 kJ/mol. Calculate the amount of heat (in kJ) needed to heat 150.0 g of C6H6 from 22.0oC to 90.0oC.

Vapor Pressure What happens to a

glass of water that is left sitting on a table for an extended period of time?

Vapor Pressure Some of the water molecules have enough

kinetic energy to escape the intermolecular forces that are “holding” them in the liquid phase.Evaporation takes place.

In a closed container, molecules in the gas phase cannot escape, and the pressure exerted by the vapor in the “headspace” begins to increase.

Vapor Pressure In a closed container, the liquid and

vapor reach a state of dynamic equilibrium:Liquid molecules evaporate and

vapor molecules condense at the same rate.

The vapor pressure in the container reaches a constant value.

Vapor pressure: the pressure exerted by a vapor when the liquid and vapor states are in dynamic equilibrium.

Vapor Pressure Hotter liquids

evaporate more easily because they have higher vapor pressures than colder liquids. VP as T

Substances with higher vapor pressures are more volatile.

Volatile: evaporates easily.

Phase Changes and Phase Diagrams

Example: Which of the compounds on the previous slide is most volatile at room temperature: diethyl ether, ethyl alcohol, water or ethylene glycol? Which one has the strongest intermolecular forces? (How do you know?) What types of intermolecular forces exist in each molecule.

Vapor Pressure At the same

temperature, compounds with different boiling points will have different vapor pressures. High BP = low VP

Low BP = high VP

Vapor Pressure & BP Boiling point:

The temperature at which the vapor pressure of a liquid equals atmospheric pressure.

Normal boiling point: The temperature at

which the vapor pressure of a liquid equals 760.torr or 1 atm.

Phase Changes and Phase Diagrams

Example: Use the graph of vapor pressure as a function of temperature to predict the normal boiling point of diethyl ether and its approximate boiling point at 400 torr and at 1000 torr.

Normal BP =

BP @ 200 torr =

BP @ 1000 torr =

Phase Changes and Phase Diagrams

Example: Bromine has a normal boiling point of 332.0K while iodine has a normal boiling point of 457.6K. Which is more volatile at room temperature? Which one has the weakest intermolecular forces at RT?

Phase Changes and Phase Diagrams

Dynamic equilibria can exist between other states of matter.

A phase diagram for a substance illustrates: its physical state at various T and P the equilibria that exist between

phases

Phase Changes and Phase Diagrams

The AB line is the liquid-vapor interface.

It starts above the triple point (A) the point at which all three states

are in equilibrium.

Phase Changes and Phase Diagrams

It ends just below the critical point (B); the temperature (critical temperature)

and pressure (critical pressure) above which the liquid and vapor are indistinguishable from each other

Phase Changes and Phase Diagrams

Each point along this line (EXCEPT THE CRITICAL POINT or TRIPLE POINT) is the boiling point of the substance at that pressure.

Phase Changes and Phase Diagrams

The AD line is the interface between liquid and solid.

The melting point at each pressure can be found along this line (excluding the triple point).

Phase Changes and Phase Diagrams

Below A the substance cannot exist in the liquid state.

Along the AC line the solid and gas phases are in equilibrium

The sublimation point at each pressure is along this line (excluding the triple point).

Phase Changes and Phase Diagrams

Example: Given the phase diagram for water, which point represents: the normal boiling point of water

the normal melting point of water

the critical point of water

the triple point of water

Phase Changes and Phase Diagrams

Example: Given the phase diagram for carbon dioxide, which line represents the equilibrium between: Solid and liquid?

Liquid and gas?

Solid and gas?

Phase Changes and Phase Diagrams

Example: Using this phase diagram for water, describe the changes in physical state that occur when water is maintained at 0oC while the pressure decreases from that at point 5 to that at point 1.

5:4:3:2:1:

Phase Changes and Phase Diagrams

Example: Using this phase diagram for water, describe the changes in physical state that occur when water is kept at 1 atm pressure while the temperature decreases from that at point 9 to that at point 6.

9:8:7:4:6: