OFB Chapter 6 Condensed Phases and Phase Transitions

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1 OFB Chapter 6 Condensed Phases and Phase Transitions 6-1 Intermolecular Forces: Why Condensed Phases Exist 6- The Kinetic Theory of Liquids and Solids 6-3 Phase Equilibrium 6-4 Phase Transitions 6-5 Phase Diagrams 6-6 Colligative Properties of Solutions 6-7 Mixtures and Distillation 6-8 Colloidal Dispersions /3/003 OFB Chapter 6 1

2 Intermolecular Forces: Why Condensed Phases Exist Intermolecular Forces Chemical bonds Strong Directional Short Range (relative) Intermolecular Forces Weaker than chemical bonds, usually much weaker Less directional than covalent bonds, more directional than ionic bonds Longer range than covalent bonds but at shorter range than ionic bonds Condensed Phases Solids and Liquids Intermolecular forces: mutual attractions hold the molecules closer together than gases Potential Energy Curves Distinction between intramolecular and intermolecular Forces /3/003 OFB Chapter 6

3 Potential Energy Curves /3/003 OFB Chapter 6 3

4 Potential Energy Curves Atoms or molecules approach at large distance (zero PE (energy of position) PE goes negative as intermolecular forces come into play PE minimum energy well is a balance of Intermolecular forces and repulsive forces PE goes positive as repulsive forces dominate /3/003 OFB Chapter 6 4

5 Types of on-bonded (Intermolecular) Attractions 1. Dipole-Dipole Interactions e.g., Hydrogen Bonding. Ion-Dipole Interactions 3. Induced Dipole Attractions 4. Dispersive Forces temporary fluctuations in electron distribution /3/003 OFB Chapter 6 5

6 Types of on-bonded (Intermolecular) Attractions 1. Dipole-Dipole Interactions e.g., Hydrogen Bonding /3/003 OFB Chapter 6 6

7 Types of on-bonded (Intermolecular) Attractions 1. Dipole-Dipole Interactions e.g., Hydrogen Bonding. Ion-Dipole Interactions /3/003 OFB Chapter 6 7

8 Types of on-bonded (Intermolecular) Attractions 1. Dipole-Dipole Interactions e.g., Hydrogen Bonding. Ion-Dipole Interactions 3. Induced Dipole Attractions Argon Atom Argon Atom /3/003 OFB Chapter 6 8

9 Types of on-bonded (Intermolecular) Attractions 1. Dipole-Dipole Interactions e.g., Hydrogen Bonding. Ion-Dipole Interactions 3. Induced Dipole Attractions 4. Dispersive Forces temporary fluctuations in electron distribution /3/003 OFB Chapter 6 9

10 Types of on-bonded (Intermolecular) Attractions 1. Dipole-Dipole Interactions e.g., Hydrogen Bonding. Ion-Dipole Interactions 3. Induced Dipole Attractions 4. Dispersive Forces temporary fluctuations in electron distribution /3/003 OFB Chapter 6 10

11 Types of on-bonded (Intermolecular) Attractions 1. Dipole-Dipole Interactions e.g., Hydrogen Bonding Between H O to H O or R-OH to H O or R-OH to R-OH (R is an organic unit) /3/003 OFB Chapter 6 11

12 Kinetic Theory of Liquids and Solids /3/003 OFB Chapter 6 1

13 6-3 Phase Equilibrium Phase: A sample of matter that is uniform throughout, both in its chemical constitution and in its physical state. Coexisting phase equilibrium Vapor pressure /3/003 OFB Chapter 6 13

Correcting for the Vapor Pressure of Water in Wet Gases Many chemical reactions conducted in aqueous solution often generate gaseous products The gas

14 Correcting for the Vapor Pressure of Water in Wet Gases Many chemical reactions conducted in aqueous solution often generate gaseous products The gas collected will be humid or wet due to the coexistence of the gas and water vapor. A correction is made to account for the water vapor present /3/003 OFB Chapter 6 14

15 Exercise page 6-: Passage of an electric current through a dilute aqueous solution of sodium sulfate produces a mixture of gaseous hydrogen and oxygen according to the following equation: H O(l) H (g) + O (g) One liter of the mixed gases is collected over water at 5 o C and under a total pressure of torr. Calculate the mass of oxygen that is present. The vapor pressure of water is 3.8 torr at 5 o C. /3/003 OFB Chapter 6 15

16 Exercise page 6-: For ideal gas mixtures P of a gas is proportional to the number of moles of gas H O(l) H (g) + O (g) /3/003 OFB Chapter 6 16

17 Exercise page 6-: /3/003 OFB Chapter 6 17

18 6-4 Phase Transitions Boiling liquid equilibrium gas Boiling Point Vapor press external pressure ormal boiling point Vap press. 1 atm Melting solid equilibrium liquid Melting Point Vapor press external pressure ormal melting point Vap press. 1 atm /3/003 OFB Chapter 6 18

19 Intermolecular Forces and Phase Transitions The stronger the intermolecular attractions in a liquid, the lower the vapor pressure at any temperature /3/003 OFB Chapter 6 19

20 Special Case of Water Due to Hydrogen Boiling 1. Water expands when it freezes. Water has a higher Boiling point and higher melting pint than expected for its Molar Mass, cf. other Group VI 3. The energy input to boil is exceptional high (45kJ) and the reverse (condensation of steam) release the same large amount. /3/003 OFB Chapter 6 0

21 Trends in Boiling Points /3/003 OFB Chapter 6 1

22 Exercise 6-3: Rank the following from lowest to highest in terms of expected normal boiling point: abr, Ar, HCl. /3/003 OFB Chapter 6

23 6-5 Phase Diagrams Phase diagram: Triple point: /3/003 OFB Chapter 6 3

24 Phase diagram: /3/003 OFB Chapter 6 4

25 Critical point: Critical pressure: Critical temperature: /3/003 OFB Chapter 6 5

26 Exercise page 6-4 Estimate the boiling temperature of diethyl ether (C H 5 OC H 5 ) atop a tall mountain (under a pressure of 0.50 atm), using the vapor pressure curve of diethyl ether in Figure 6-3 /3/003 OFB Chapter 6 6

27 Exercise 6-5: A sample of solid argon is heated at a constant pressure of 0 atm from 50 K to 150 K. Describe any phase transitions, and give the approximate temperatures at which they occur. /3/003 OFB Chapter 6 7

28 6-6 Colligative Properties of Solutions For some properties, the amount of difference between a pure solvent and dilute solution depend only on the number of solute particles present and not on their chemical identify. Called Colligative Properties Examples Vapor Pressure Melting Point Boiling Point Osmotic Pressure /3/003 OFB Chapter 6 8

29 Colligative Properties of Solutions Mass percentage (weight percentage): mass percentage of the component mass of component total mass of mixture 100% Mole fraction: X 1 n 1 /(n 1 + n ) The chemical amount (in moles) divided by the total amount (in moles) X n /(n 1 + n ) 1 - X 1 /3/003 OFB Chapter 6 9

30 Molality m solute moles solute per kilograms solvent mol kg -1 Molarity c solute moles solute per volume solution mol L -1 /3/003 OFB Chapter 6 30

31 m Exercise 6-7: Suppose that 3.6 g of acetic acid, CH 3 COOH, is dissolved in 83.8 g of water, giving a total solution volume of 11.1 ml. Calculate the molality and molarity of acetic acid (molar mass g mol -1 ) in this solution. Molality m solute solute 3.6 g g mol 83.8 g g 1000 kg moles solute kg solvent mol kg mol kg mol kg 1 /3/003 OFB Chapter 6 31

32 Exercise 6-7: Suppose that 3.6 g of acetic acid, CH 3 COOH, is dissolved in 83.8 g of water, giving a total solution volume of 11.1 ml. Calculate the molality and molarity of acetic acid (molar mass g mol -1 ) in this solution. Molarity c c solute solute 3.6 g g mol 11.1 ml ml 1000 L moles solute volume solvent mol mol 6.48 mol /3/003 OFB Chapter 6 3 L 1 L 1

33 Converting among Mass Fraction, Mole Fraction, Molality, Molarity /3/003 OFB Chapter 6 33

34 Ideal Solutions and Raoult s Law Consider a non-volatile solute in a solvent X 1 mole fraction of solvent P 1 X 1 P 1 Positive deviation solute-solvent attractions < solvent-solvent attractions egative deviation solute-solvent attractions > solvent-solvent attractions /3/003 OFB Chapter 6 34

Ideal Solutions and Raoult s Law Lowering of Vapor Pressure Vapor Pressure of a solvent above a dilute solution is always less than the vapor pressure above the pure solvent.

35 Ideal Solutions and Raoult s Law Lowering of Vapor Pressure Vapor Pressure of a solvent above a dilute solution is always less than the vapor pressure above the pure solvent. Elevation of Boiling Point The boiling point of a solution of a nonvolatile solute in a volatile solvent always exceeds the boiling point of a pure solvent /3/003 OFB Chapter 6 35

36 Elevation of Boiling Point T b mk b The Effect of Dissociation i the number of particles released into the solution per formula unit of solute Depression of Freezing Point T f -mk b /3/003 OFB Chapter 6 36

37 Osmotic Pressure Fourth Colligative Property Important for transport of molecules across cell membranes Osmotic Pressure Π g d h Π c RT ΠV n RT ΠV i c RT /3/003 OFB Chapter 6 37

38 Exercise 6-17 A dilute aqueous solution of a nondissociating compound contains 1.19 g of the compound per liter of solution and has an osmotic pressure of atm at a temperature of 37 C. Compute the molar mass of the compound Strategy 1.) use Π.) 3.) Recall Rearrange crt to find the c in mol/l that mole M g Molar mass g mole g mole l l Given c /3/003 OFB Chapter 6 38

39 Exercise 6-17 A dilute aqueous solution of a nondissociating compound contains 1.19 g of the compound per liter of solution and has an osmotic pressure of atm at a temperature of 37 C. Compute the molar mass of the compound Solution 1.) use Π c c Π RT 1.13x10 3.) Rearrange M crt or c (0.080 L atm mol 3 mol/l M 1.19 g l 1.13x10 mol/l Π RT atm g mole g mole l x10 g/mol /3/003 3 OFB Chapter K 1 l )( K)

solute) P 1 X 1 P 1 Ideal Solutions One volatile (the

40 6-7 Mixtures and Distillations Raoult s Law Ideal Solutions One volatile (the solvent) One non-volatile (the solute) P 1 X 1 P 1 Ideal Solutions One volatile (the solvent) One volatile (the solute) P tot P 1 +P /3/003 OFB Chapter 6 40

41 Ideal Solutions One volatile (the solvent) One volatile (the solute) But. For Real Solutions Henry s Law (for dilute solutions) The vapor pressure of a volatile in a sufficiently dilute solution is proportional to the mole fraction of the solute in the solution. If component is a sufficiently small value of the mole fraction, then k H is Henry s Law constant f (Temp, and the solute-solvent interactions) /3/003 OFB Chapter 6 41

$as the prssure of a gas above a solution is increased, the mole fraction of the gas in solution increases in direct proportion /3/003 OFB Chapter 6$

42 Henry s Law (for dilute solutions) P k H X Dilute Solutions One volatile (the solvent) One dilute volatile (the solute) According to Henry s Law, as the prssure of a gas above a solution is increased, the mole fraction of the gas in solution increases in direct proportion /3/003 OFB Chapter 6 4

43 Exercise 6-18 When the partial pressure of nitrogen over a sample of water at 19.4 C is 9.0 atm, then the concentration of nitrogen in the water is 5.76 x 10-3 mol L -1. Compute Henry s law constant for nitrogen in water at this temperature. Given P c P X n Henry's Law 9.0 atm [ k ] X n 5.76x10 + n H O 3 H O /3/003 OFB Chapter 6 43 n n mol/l

44 Given P c P X Exercise 6-18 When the partial pressure of nitrogen over a sample of water at 19.4 C is 9.0 atm, then the concentration of nitrogen in the water is 5.76 x 10-3 mol L -1. Compute Henry s law constant for nitrogen in water at this temperature. [ k n Henry's Law 9.0 atm ] X n 5.76x10 + n H O 3 n n mol/l H O P rearrange k k P X X Given Find ext assume 1 Liter /3/003 OFB Chapter 6 44

45 Exercise 6-18 When the partial pressure of nitrogen over a sample of water at 19.4 C is 9.0 atm, then the concentration of nitrogen in the water is 5.76 x 10-3 mol L -1. Compute Henry s law constant for nitrogen in water at this temperature. P X k n n P X H O k x10 X Given Find /3/003 OFB Chapter x x10 mol/l 1000g/l 18g/mol Given Find 4 4 atm atm x10 4

46 6-8 Colloidal Dispersions Colloids are large particles dispersed in solution 1nm to 1000 nm in size E.g., Globular proteins 500nm Examples Opal (water in solid SiO ) Aerosols (liquids in Gas) Smoke (solids in Air) Milk (fat droplets & solids in water) Mayonnaise (water droplets in oil) Paint (solid pigments in liquid) Biological fluids (proteins & fats in water) Characteristics Large particle size colloids: translucent, cloudy, milky) Small particle size colloids: can be cle /3/003 OFB Chapter 6 46

47 6-8 Colloidal Dispersions Tyndall Effect Light Scattering /3/003 OFB Chapter 6 47

48 Chapter 6 Condensed Phases and Phase Transitions Examples / Exercises All (6-1 thru 6-18) Problems 4, 0, 6, 38, 44, 6 /3/003 OFB Chapter 6 48

A) sublimation. B) liquefaction. C) evaporation. D) condensation. E) freezing. 11. Below is a phase diagram for a substance.

A) sublimation. B) liquefaction. C) evaporation. D) condensation. E) freezing. 11. Below is a phase diagram for a substance. PX0411-1112 1. Which of the following statements concerning liquids is incorrect? A) The volume of a liquid changes very little with pressure. B) Liquids are relatively incompressible. C) Liquid molecules