AP CHEMISTRY

Phase Diagrams

Phase DiagramsPhase diagrams display the state of a substance at various pressures and temperatures and the places where equilibria exist between phases.

Phase Diagrams

The AB line is the liquid-vapor interface.It starts at the triple point (A), the point

at which all three states are in equilibrium (boiling ice water!).

Phase DiagramsIt ends at the critical point (B); intermediate “fluid” region critical temperature (above this temperature condensation is impossible at any pressure) and critical pressure (pressure required to condense at critical temperature) .

Phase DiagramsEach point along this line is the boiling point of the substance at that pressure.

Phase DiagramsThe AD line is the interface between

liquid and solid.The melting point at each pressure can

be found along this line.

Phase DiagramsBelow 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.

Phase Diagram of Water

Note the high critical temperature and critical pressure: These are due to the

strong van der Waals forces between water molecules.

Phase Diagram of Water

The slope of the solid–liquid line is negative. This means that as the

pressure is increased at a temperature just below the melting point, water goes from a solid to a liquid.

Also, liquid water is more dense than solid water!

Phase Diagram of Carbon Dioxide

Carbon dioxide cannot exist in the liquid state at pressures below 5.11 atm; CO2 sublimes at normal pressures.

Phase Diagram of Carbon Dioxide

The low critical temperature and critical pressure for CO2 make supercritical CO2 a good solvent for extracting nonpolar substances (such as caffeine).

Phase Diagram for Carbon:

1. Can we change pencil graphite into diamonds?

2. How many triple points?

Phase Diagram for Sulfur

1. What phase is sulfur @ STP?

Of the following substances, predict which has the highest boiling point based on intermolecular forces.

1. Propane, C3H8

2. Dimethyl ether, CH3OCH3

3. Methyl chloride, CH3Cl4. Acetaldehyde, CH3CHO5. Acetonitrile, CH3CN

Correct Answer:

Each of these molecules has almost the same molecular weight; however, acetonitrile has the largest dipole moment (3.9 D) and hence the largest dipole-dipole forces. Thus it has the highest boiling point.

1. Propane, C3H8

2. Dimethyl ether, CH3OCH3

3. Methyl chloride, CH3Cl4. Acetaldehyde, CH3CHO5. Acetonitrile, CH3CN

Of the following substances, predict which has the lowest boiling point based on London dispersion forces.

1. He2. Ne3. Ar4. Kr5. Xe

Correct Answer:

More massive species have more polarizability and stronger London dispersion forces; consequently, amongst the noble gases He has the lowest boiling point.

1. He2. Ne3. Ar4. Kr5. Xe

Of the following substances, predict which has the highest boiling point based upon intermolecular forces?

1. CH4

2. H2O3. H2S4. SiH4

5. H2Se

NH ……. O=C

Correct Answer:

Of these, only H2O has any hydrogen bonding. Hydrogen bonding substantially increases the intermolecular forces, and hence the boiling point.

1. CH4

2. H2O3. H2S4. SiH4

5. H2Se

Which one of the following phase changes is an exothermic process?

1. Sublimation2. Vaporization3. Condensation4. Melting

Correct Answer:

All the other phase-change processes listed are endothermic.

1. Sublimation2. Vaporization3. Condensation4. Melting

Temperature

Pre

ssu

reAA

DD

CC

BB

In the unlabeled phase diagram below, the line segment from A to B separates which two phases?

1. Gas-liquid2. Liquid-solid3. Solid-gas

Correct Answer:

1. Gas-liquid2. Liquid-solid3. Solid-gas

Temperature

Pre

ssu

reAA

DD

CC

BB

Quartz is an example of which type of solid: crystalline or amorphous?

1. Crystalline2. Amorphous

Correct Answer:

In quartz, the SiO bonds are arranged in regular, defined arrays.1. Crystalline

2. Amorphous

Diamond and graphite are examples of which type of crystalline solids?

1. Molecular2. Covalent network3. Ionic4. Metallic

Correct Answer:

Diamond and graphite are both forms of carbon, and consist of a network of covalent bonds (hence covalent network).

1. Molecular2. Covalent network3. Ionic4. Metallic

SAMPLE EXERCISE 11.6 Interpreting a Phase Diagram

Referring to Figure 11.28, describe any changes in the phases present when is (a) kept at 0°C while the pressure is increased from that at point 1 to that at point 5 (vertical line), (b) kept at 1.00 atm while the temperature is increased from that at point 6 to that at point 9 (horizontal line).

Solve: (a) At point 1, H2O exists totally as a vapor. At point 2 a solid-vapor equilibrium exists. Above that pressure, at point 3, all the H2O is converted to a solid. At point 4 some of the solid melts and an equilibrium between solid and liquid is achieved. At still higher pressures all the H2O melts, so only the liquid phase is present at point 5.

Figure 11.28  Phase diagram of H2O.

SAMPLE EXERCISE 11.6 continued

(b) At point 6 the H2O exists entirely as a solid. When the temperature reaches point 4, the solid begins to melt and an equilibrium exists between the solid and liquid phases. At an even higher temperature, point 7, the solid has been converted entirely to a liquid. A liquid-vapor equilibrium exists at point 8. Upon further heating to point 9, the H2O is converted entirely to the vapor phase.

PRACTICE EXERCISEUsing Figure 11.27(b), describe what happens when the following changes are made in a CO2 sample initially at 1 atm and –60ºC: (a) Pressure increases at constant temperature to 60 atm. (b) Temperature increases from –60ºC to –20ºC at constant 60 atm pressure.

Supercooling: Temperature goes lower than freezing point until molecules align in lattice structure then temperature rises to normal freezing point.

Superheated: usually use boiling chips to prevent popping!