Liquids and Solids. Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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1 Liquids and Solids Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1

2 Gases, Liquids and Solids Gases are compressible fluids. They have no proper volume and proper shape. Liquids are relatively not compressible fluids. They have a proper volume, but not a proper shape. Solids are not compressible and rigid species.

3 Properties of Liquids Surface tension is the amount of energy required to stretch or increase the surface of a liquid by a unit area. Strong intermolecular forces High surface tension 3

4 Properties of Liquids Cohesion is the intermolecular attraction between like molecules Adhesion is an attraction between unlike molecules Adhesion Cohesion 4

5 Properties of Liquids Viscosity is a measure of a fluid s resistance to flow. Strong intermolecular forces High viscosity 5

A crystalline solid possesses rigid and long-range order. In a crystalline solid, atoms, molecules or ions occupy specific (predictable) positions.

6 A crystalline solid possesses rigid and long-range order. In a crystalline solid, atoms, molecules or ions occupy specific (predictable) positions. An amorphous solid does not possess a well-defined arrangement and long-range molecular order. A unit cell is the basic repeating structural unit of a crystalline solid. lattice point At lattice points: Atoms Unit Cell Unit cells in 3 dimensions Molecules Ions 6

7 Seven Basic Unit Cells 7

8 Three Types of Cubic Unit Cells 8

9 Arrangement of Identical Spheres in a Simple Cubic Cell 9

10 Arrangement of Identical Spheres in a Body-Centered Cubic Cell 10

11 Number of Atoms Per Unit Cell 1 atom/unit cell (8 x 1/8 = 1) 2 atoms/unit cell (8 x 1/8 + 1 = 2) 4 atoms/unit cell (8 x 1/8 + 6 x 1/2 = 4) 11

electrostatic attraction Hard, brittle, high melting

12 Types of Crystals Ionic Crystals Lattice points occupied by cations and anions Held together by electrostatic attraction Hard, brittle, high melting point Poor conductor of heat and electricity CsCl ZnS CaF 2 12

13 Types of Crystals Covalent Crystals Lattice points occupied by atoms Held together by covalent bonds Hard, high melting point Poor conductor of heat and electricity carbon atoms diamond graphite 13

14 Types of Crystals Molecular Crystals Lattice points occupied by molecules Held together by intermolecular forces Soft, low melting point Poor conductor of heat and electricity water benzene 14

15 Types of Crystals Metallic Crystals Lattice points occupied by metal atoms Held together by metallic bonds Soft to hard, low to high melting point Good conductors of heat and electricity nucleus & inner shell e - Cross Section of a Metallic Crystal mobile sea of e - 15

16 Phase Changes Least Order Greatest Order 16

17 The equilibrium vapor pressure is the vapor pressure measured when a dynamic equilibrium exists between condensation and evaporation H 2 O (l) H 2 O (g) Dynamic Equilibrium Rate of condensation = Rate of evaporation 17

$A fraction of the molecules present at the liquid surface has enough kinetic energy to overcome the molecular$

18 Speed distribution Liquid molecules have a distribution of kinetic energy. A fraction of the molecules present at the liquid surface has enough kinetic energy to overcome the molecular attraction and to escape to the gaseous phase 18

19 Distribution and Temperature Dario Bressanini 19

20 Dario Bressanini 20

21 Vapor pressure If the recipient is open, it is impossible to reach the equilibrium condition and we observe the liquid evaporation 21

22 Vapor pressure Dario Bressanini 22

23 Boiling Point The liquid boils when the vapor pressure reaches the external pressure The boiling point increases with the increasing pressure. " Normal Boiling Point: pressure = 1 atm " Standard Boiling Point: pressure = 1 bar Dario Bressanini 23

24 Vapor pressure Evaporation: the molecules escape from the surface Ebullition: the gas is formed also within the liquid Dario Bressanini 24

25 Ebullition When the vapor pressure reaches the external pressure, the vapor bubbles are formed inside the liquid. It is possible to boil a liquid by increasing the temperature or decreasing the pressure. 25

26 Ebullition 26

27 Pressure Cooker Denis Papin in 1682 discovered the Pressure Cooker, including the pressure valve. In a normal cooker, the water temperature cannot go over 100 C. Dario Bressanini 27 In the pressure cooker, the water evaporation increases the pressure. The water temperature reaches 120 C and 2 Atm.

28 Ebullition at Low Pressure Place Altitude (m) boiling Point H 2 O ( o C) Rimini Courmayeur Mt. Everest

29 Ebullition at Low Pressure MARS Average Temperature: 218 K (-55 C) Range: 140/300 K (-133/27 C) Atmosphere: CO % N 2 2.7% Ar 1.6% O % H 2 O 0.03% Average Pressure: 6 millibar In these conditions water is a solid or a vapor. In the zones with higher pressures, water boils at 10 C. the Martians cannot cook pasta, other than to use the pressure cooker!! Dario Bressanini 29

30 The boiling point is the temperature at which the (equilibrium) vapor pressure of a liquid is equal to the external pressure. The normal boiling point is the temperature at which a liquid boils when the external pressure is 1 atm. 30

31 The critical temperature (T c ) is the temperature above which the gas cannot be made to liquefy, no matter how great the applied pressure. The critical pressure (P c ) is the minimum pressure that must be applied to bring about liquefaction at the critical temperature. 31

32 The Critical Phenomenon of SF 6 T < T c T > T c T ~ T c T < T c 32

33 Solid-Liquid Equilibrium H 2 O (s) H 2 O (l) The melting point of a solid or the freezing point of a liquid is the temperature at which the solid and liquid phases coexist in equilibrium 33

34 Molar heat of fusion (ΔH fus ) is the energy required to melt 1 mole of a solid substance at its freezing point. 34

35 Solid-Gas Equilibrium H 2 O (s) H 2 O (g) Molar heat of sublimation (ΔH sub ) is the energy required to sublime 1 mole of a solid. ΔH sub = ΔH fus + ΔH vap ( Hess s Law) 35

Clausius-Clapeyron Equation ln P = - ΔH vap RT + C P = (equilibrium)

36 Molar heat of vaporization (ΔH vap ) is the energy required to vaporize 1 mole of a liquid at its boiling point. Clausius-Clapeyron Equation ln P = - ΔH vap RT + C P = (equilibrium) vapor pressure T = temperature (K) R = gas constant (8.314 J/K mol) Vapor Pressure Versus Temperature 36

37 Clausius-Clapeyron Equation We can derive the Clausius-Clapeyron equation considering the liquid-vapor phase equilbrium. In this condition: ΔG vap = ΔG liq If we induce an infinitesimal perturbation, the system will be driven to a novel equilibrium state: ΔG vap + dg vap = ΔG liq + dg liq As a consequence: dg vap = dg liq 37

38 If we consider the expression of the Gibbs free energy: G = H - TS The differential form: dg = dh d(ts) = dh TdS - SdT but: H = U + PV and, if we introduce this relation into the previous one: dg = d(u + PV) d(ts) = du + PdV + VdP TdS SdT from the first law of thermodynamics: U = q l with q = TdS and l = PdV 38

39 Considering that: du TdS + PdV = 0 The expression is: dg = If: VdP - SdT dg vap = dg liq We have: V vap dp S vap dt = V liq dp - S liq dt and: (V vap V liq )dp = (S vap S liq )dt Considering that V vap >> V liq 39

40 VdP = ΔSdT Because: V = RT/P and ΔS = ΔH/T The equation become: dp = ΔH dt P RT 2 dlnp = ΔH dt RT 2 and the integral form: lnp = - ΔH + C RT 40

41 Alternate Forms of the Clausius-Clapeyron Equation At two temperatures or 41

42 A phase diagram summarizes the conditions at which a substance exists as a solid, liquid, or gas. Phase Diagram of Water 42

43 Effect of Increase in Pressure on the Melting Point of Ice and the Boiling Point of Water 43

44 3-D Structure of Water Water is a Unique Substance Maximum Density 4 C Density of Water Ice is less dense than water 44

45 Phase Diagram of Carbon Dioxide At 1 atm CO 2 (s) CO 2 (g) 45

Intermolecular Forces and Liquids and Solids

Intermolecular Forces and Liquids and Solids Chapter 11 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. A phase is a homogeneous part of the system in contact