Intermolecular Forces: Liquids and Solids

Intermolecular Forces: Liquids and Solids ● Phases and Phase Diagrams ● Liquids and Liquid Properties ● Intermolecular Forces ● Heating Curves ● Introduction to Solids ● Cubic Packing Arrangements ● Closest-Packed Structures ● Density of a Crystalline Solid ● Ionic Solids and Interstitial Sites ● The Born-Haber Cycle

Phases and Phase Diagrams very compressible d ≈ 1 – 10 g L−1 at SATP condensation vaporization sublimation deposition fusion incompressible d ≈ 1 – 10 g mL−1 freezing

Phases and Phase Diagrams A few definitions: STP vs SATP STP = Standard Temperature and Pressure (0 oC, 100 kPa) SATP = Standard Ambient Temperature and Pressure (25 oC, 100 kPa) 100 kPa = 1 bar

Phases and Phase Diagrams Differences between the different states of Matter: • Gas: Molecules move randomly and the intermolecular separations are large (i.e. most of a gas is empty space). • Liquids and Solids: the molecular motions are quite restricted and the intermolecular separations are small. • Solids: The molecules are often, but not always, arranged in regular, repeating patterns. • Substances exist in different phases and phase changes occur because molecules exert forces on each other. (Without intermolecular forces, all substances would behave as ideal gases!!) It takes energy to overcome the attractive intermolecular forces that cause molecules to aggregate. Therefore, sublimation, fusion and evaporation are all endothermic processes • A given substance will exist as a solid, liquid or gas depending on the temperature and pressure of the sample. A phase diagram shows the stable phases at each temperature and pressure.

Phases and Phase Diagrams Phase diagram of I2

Phases and Phase Diagrams Take note of the following points: • 1. Solid is the most stable phase at low T and high P. Gas is the stable phase at high T and low P.

Phases and Phase Diagrams Take note of the following points: 2. The S-L line shows the T’s and P’s at which both solid and liquid are stable and can coexist. It also shows us how the melting temperature changes with pressure.

Phases and Phase Diagrams Take note of the following points: 3. The L-G line curve shows the T’s and P’s at which both liquid and gas are stable and can coexist. It also shows us how the boiling temperature changes with pressure.

Phases and Phase Diagrams Take note of the following points: 4. For most substances, the S-L line has a positive slope, but for a few substances (most notably, water but also bismuth and antimony), it has a negative slope! For most substances

Phases and Phase Diagrams Take note of the following points: 4. For most substances, the S-L line has a positive slope, but for a few substances (most notably, water but also bismuth and antimony), it has a negative slope! For water

Phases and Phase Diagrams Take note of the following points: 5. At the triple point, all three phases are stable and coexist.

Phases and Phase Diagrams Take note of the following points: • 6. The G-L line ends abruptly at the critical point (Tc, Pc)

Phases and Phase Diagrams Phase diagram of I2 What is the phase of I2 at 25 oC and 1 atm? We are dealing with a solid. 25 oC and 1 atm 25 oC

Phases and Phase Diagrams Phase diagram of CO2 What is the phase of CO2 at 25 oC and 1 atm? We are dealing with a gas. 25 oC and 1 atm 1 atm 25 oC

Phases and Phase Diagrams • Phase diagram of H2O • What is the phase of H2O at 25 oC and 1 atm? We are dealing with a liquid. 25 oC and 1 atm 1 atm 25 oC

Phases and Phase Diagrams Take note of the following points: 4. For most substances, the S-L line has a positive slope, but for a few substances (most notably, water but also bismuth and antimony), it has a negative slope! The slope of the S-L line is negative!

Phases and Phase Diagrams Take note of the following points: 1 m A column of water 1 m ×1 m × 10 m occupies a volume of 10 m3 or 10,000 L. 1 L of water weighs 1 kg. 10,000 L of water weigh 10,000 kg. The pressure exerted by 10,000 kg of water equals: 9.8 m2/s×10,000 kg/(1 m×1 m)  105 Pa  1 atm 10 m of water generates a 1 atm additional pressure. 1 m 10 m

Phases and Phase Diagrams Take note of the following points: We find liquid water at the bottom of the ocean. The slope of the S-L line is negative!

Phases and Phase Diagrams Take note of the following points: Polymorphism: The existence of a solid substance in more than one form. Other forms of ice obtained at several thousands of atmospheres

Phases and Phase Diagrams supercritical fluid solid-liquid coexistence line critical point Solid B Pc P Liquid A 1 atm liquid-vapourcoexistence line “normal” melting point Gas “normal” boilingpoint Tfus Tvap Tc triple point T To summarize, a typical phase diagram looks like this:

Phases and Phase Diagrams solid-liquid coexistence line supercritical fluid P critical point (S) B Pc (L) A liquid-vapourcoexistence line (G) Tc T Supercritical fluid: As one moves from A to B, the pressure increases and the density of the gas increases until it equals the density of the liquid. At this point, gas and liquid are indistinguishable, the interface between liquid and gas vanishes and we have a supercritical fluid. • If a gas is at T > TC (Point A in diagram), increasing the pressure of the gas does not yield a liquid but rather a supercritical fluid (Point B). To take a gas at T > TC and transform it into a liquid, the temperature must first be reduced below TC. Then the pressure is increased to pass the liquid-vapor coexistence curve.

Phases and Phase Diagrams solid-liquid coexistence line supercritical fluid P critical point (S) B Pc (L) A liquid-vapourcoexistence line (G) Tc T Supercritical fluid: Below Tc, the phase boundary is clearly visible. T increases from below TC to above TC Just below Tc, the phase boundary is barely visible. The phase boundary between liquid benzene and its vapour disappears at Tc. At Tc, the phase boundary disappears.

Phases and Phase Diagrams solid-liquid coexistence line supercritical fluid P critical point (S) Pc (L) liquid-vapourcoexistence line (G) Tc T Supercritical fluid: Did you know? Supercritical CO2 is used to extract caffeine from coffee beans. The extracted caffeine can be sold to pharmaceutical or beverage companies. The critical point for CO2 is fairly low (Tc = 31 oC) and so, supercritical CO2 can be used at ambient temperatures without causing decomposition or “denaturing” of other compounds. Because it has low toxicity, a low critical temperature and is nonflammable, supercritical CO2 is becoming an increasingly important industrial and commercial solvent.

Phases and Phase Diagrams solid-liquid coexistence line supercritical fluid P critical point (S) Pc (L) liquid-vapourcoexistence line (G) Tc T • Examples: For a particular substance, the S-L coexistence curve has a negative slope. PT TT • What phase changes are possible if the pressure is increased at constant temperature T? Assume that T is less than Ttp, where Ttp is the triple point temperature. • Gas  deposition  solid  melting  liquid

Phases and Phase Diagrams solid-liquid coexistence line supercritical fluid P critical point (S) Pc (L) liquid-vapourcoexistence line (G) Tc T • Examples: For a particular substance, the S-L coexistence curve has a negative slope. PT TT • What phase changes are possible if the pressure is increased at constant temperature T, assuming Ttp < T < Tc, where Ttp and Tc are the triple point and critical point temperatures, respectively. • Gas  condensation  liquid

Phases and Phase Diagrams solid-liquid coexistence line supercritical fluid P critical point (S) Pc (L) liquid-vapourcoexistence line (G) Tc T • Examples: For a particular substance, the S-L coexistence curve has a negative slope. Tm PT TT • c) True or False? The melting temperature increases as the pressure increases. • False

Phases and Phase Diagrams solid-liquid coexistence line supercritical fluid P critical point (S) Pc (L) liquid-vapourcoexistence line (G) Tc T • Examples: For a particular substance, the S-L coexistence curve has a negative slope. PT TT • d) True or False? The solid is more dense than the liquid. • At a given temperature, when we increase the pressure, the density increases and the solid becomes a liquid. False

Phases and Phase Diagrams • Examples: For a particular substance, the triple point is at 57 ºC and 5.1 atm, and the critical point is at 31oC and 73 atm. 25 oC and 73 atm 73 atm • a) What is the phase of this substance at 25oC and 73 atm? • We are dealing with a liquid. 25 oC

Phases and Phase Diagrams • Examples: For a particular substance, the triple point is at 57 ºC and 5.1 atm, and the critical point is at 31oC and 73 atm. -60 oC and 75 atm -60 oC and 0.001 atm • b) What phase changes occur if the pressure is decreased from 75 atm to 0.001 atm at −60 oC? Assume that the solid-liquid line has a positive slope. • Solid  sublimation  gas -60 oC

REVIEW Phases and Phase Diagrams very compressible d ≈ 1 – 10 g L−1 at SATP condensation vaporization sublimation deposition fusion incompressible d ≈ 1 – 10 g mL−1 freezing

REVIEW Phases and Phase Diagrams supercritical fluid P Solid Liquid Gas T To summarize, a typical phase diagram looks like this:

REVIEW Phases and Phase Diagrams supercritical fluid solid-liquid coexistence line critical point Solid B Pc P Liquid A 1 atm liquid-vapourcoexistence line “normal” melting point Gas “normal” boilingpoint Tfus Tvap Tc triple point T To summarize, a typical phase diagram looks like this:

Phases and Phase Diagrams solid-liquid coexistence line supercritical fluid P critical point (S) Pc (L) liquid-vapourcoexistence line (G) Tc T • Examples 12-45: Which substances listed in the table can exist as liquids at room temperature (~ 20 oC)? T = 20 oC

Phases and Phase Diagrams solid-liquid coexistence line supercritical fluid P critical point (S) Pc (L) liquid-vapourcoexistence line (G) Tc= -240 oC • Examples 12-45: Which substances listed in the table can exist as liquids at room temperature (~ 20 oC)? T = 20 oC

Phases and Phase Diagrams solid-liquid coexistence line supercritical fluid P critical point (S) Pc (L) liquid-vapourcoexistence line (G) Tc T • Examples 12-45: Which substances listed in the table can exist as liquids at room temperature (~ 20 oC)? T = 20 oC 20 oC < TC or TC > 293.15 K Gases that can be liquified at room temperature are said to be “non-permanent gases”. Gases that cannot be liquified at room temperature are said to be “permanent gases”.

Phases and Phase Diagrams • Examples 12-51: Phase diagram of phosphorous • a) Indicate the phases present in the regions labeled with a question mark? P (L) ? (S) 43 atm (G) ? 590 oC T

Phases and Phase Diagrams • Examples 12-51: Phase diagram of phosphorous • b) A sample of solid red phosphorous cannot be melted by heating in a container open to the atmosphere. Explain why this is so? P (L) (S) 43 atm (G) 1 atm 590 oC T Solid phosphorous can only be sublimed (S  G) if it is heated at P = 1 atm.

Phases and Phase Diagrams • Examples 12-51: Phase diagram of phosphorous • c) Trace the phase changes that occur when the pressure on a sample is reduced from Point A to B, at constant temperature. A P (L) (S) 43 atm (G) B 590 oC T Solid  condensation  Liquid  vaporization  Gas

Intermolecular Forces: Liquids and Solids ● Phases and Phase Diagrams ● Liquids and Liquid Properties ● Intermolecular Forces ● Heating Curves ● Introduction to Solids ● Cubic Packing Arrangements ● Closest-Packed Structures ● Density of a Crystalline Solid ● Ionic Solids and Interstitial Sites ● The Born-Haber Cycle

Liquids and Liquid Properties supercritical fluid P solid-liquid coexistence line critical point (S) (L) liquid-vapourcoexistence line (G) T Condensation We know that if the temperature of a gas is lowered sufficiently, the gas will condense to a liquid. Why is this? As T is lowered, the average kinetic energy of the molecules decreases. At some point, the molecules will no longer have enough kinetic energy to overcome the attractive forces that draw the molecules together. Consequently, the molecules cluster together to form a liquid.

Liquids and Liquid Properties supercritical fluid P solid-liquid coexistence line critical point (S) (L) liquid-vapourcoexistence line (G) T Freezing The freezing of a liquid can be explained in the same way: If the temperature of a liquid is lowered sufficiently, the molecules will not have enough kinetic energy to overcome attractive forces that draw the molecules closer together  the liquid freezes.

Liquids and Liquid Properties The physical properties of a liquid depend on the strength and nature of the intermolecular forces. We shall examine why the following physical properties are different from substance to substance. vapour pressure = equilibrium pressure of vapour that forms above a liquid in a closed container normal boiling point (Tvap) = temperature at which the vapour pressure of the liquid equals 1 atm surface tension (g) = energy required to increase the surface area of a liquid viscosity (η) provides a measure of a fluid’s resistance to flow; the speed of flow through a tube is inversely proportional to the viscosity In general, the stronger the intermolecular attractions, the higher the boiling point, the greater the surface tension, the higher the viscosity and the lower the vapour pressure.

Liquids and Liquid Properties vapour pressure = equilibrium pressure of vapour that forms above a liquid in a closed container to vacuum air liquid

Liquids and Liquid Properties vapour pressure = equilibrium pressure of vapour that forms above a liquid in a closed container to vacuum air liquid Liquid N2

Liquids and Liquid Properties vapour pressure = equilibrium pressure of vapour that forms above a liquid in a closed container to vacuum air solid Liquid N2

Liquids and Liquid Properties vapour pressure = equilibrium pressure of vapour that forms above a liquid in a closed container to vacuum vacuum solid Liquid N2

Liquids and Liquid Properties vapour pressure = equilibrium pressure of vapour that forms above a liquid in a closed container to vacuum vacuum solid

Intermolecular Forces: Liquids and Solids