Intermolecular Forces and Liquids and Solids Chapter 11

Depends on the intermolecular distance A phase is a homogeneous part of the system in contact with other parts of the system but separated from them by a well-defined boundary. 2 Phases Solid phase - ice Liquid phase - water Depends on the intermolecular distance

Intermolecular Forces Intermolecular forces are attractive forces between molecules. Intramolecular forces hold atoms together in a molecule. Intermolecular vs Intramolecular 41 kJ to vaporize 1 mole of water (inter) 930 kJ to break all O-H bonds in 1 mole of water (intra) “Measure” of intermolecular force boiling point melting point DHvap DHfus DHsub Generally, intermolecular forces are much weaker than intramolecular forces. Chemical bonding and physical properties!

Intermolecular Forces Dipole-Dipole Forces Attractive forces between polar molecules Orientation of Polar Molecules in a Solid So higher dipole, larger attractive force.

Intermolecular Forces Ion-Dipole Forces Attractive forces between an ion and a polar molecule Ion-Dipole Interaction Small r, larger F

Interaction Between Water and Cations Hydration(水合) in solution

Intermolecular Forces Dispersion Forces 分散力(also be called as London Force) Attractive forces that arise as a result of temporary dipoles induced in atoms or molecules ion-induced dipole interaction dipole-induced dipole interaction

Dispersion Forces 極化率 The polarizability: the ability to distortion the electron distribution of a molecular or atoms. Normally, large amount of electrons or low-density electron cloud will show higher polarization Dispersion force is the transition of the dipole-induced attraction So He or N2 gas/non-polarized maculars can be condensed in low-temperature.

Induced Dipoles Interacting With Each Other Transition state

Intermolecular Forces Dispersion Forces Continued Polarizability is the ease with which the electron distribution in the atom or molecule can be distorted. Polarizability increases with: greater number of electrons more diffuse electron cloud Dispersion forces usually increase with molar mass. Some dispersion force larger than polarized dipole-dipole force! e.g. CCl4 (76.5o)shows higher boiling point than CH3F(-78.4o)

What type(s) of intermolecular forces exist between each of the following molecules? HBr HBr is a polar molecule: dipole-dipole forces. There are also dispersion forces between HBr molecules. CH4 CH4 is nonpolar: dispersion forces. S O SO2 SO2 is a polar molecule: dipole-dipole forces. There are also dispersion forces between SO2 molecules.

Intermolecular Forces Hydrogen Bond The hydrogen bond is a special dipole-dipole interaction between they hydrogen atom in a polar N-H, O-H, or F-H bond and an electronegative O, N, or F atom. A H … B or A & B are N, O, or F

Why is the hydrogen bond considered a “special” dipole-dipole interaction? Decreasing molar mass Decreasing boiling point

Hydrogen Bond HCOOH and water

The average hydrogen bond is ~ 40KJ/mol. The strength of hydrogen bond depends on the electronegativity Of atoms. e.g. HF shows higher strength than H2O. However, H2O shows Higher boiling point than HF? Each H2O have 4 H-bond.

Strong intermolecular forces 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

H2O show very strong H-bond, therefore shows higher surface tension than other liquid

Polarized molecular and non-polarized molecular

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

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

Similar to H2O, it shows high H-bond Viscosity CH2-OH 甘油 Similar to H2O, it shows high H-bond

Ice is less dense than water 3-D Structure of Water Water is a Unique Substance 2 共價鍵 2氫鍵 Maximum Density 4°C Density of Water Ice is less dense than water

ice benzene

Unit cells in 3 dimensions 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 Molecules Ions Unit Cell Unit cells in 3 dimensions

Seven Basic Unit Cells

Three Types of Cubic Unit Cells

Arrangement of Identical Spheres in a Simple Cubic Cell

Arrangement of Identical Spheres in a Body-Centered Cubic Cell

A Corner Atom, a Edge-Centered Atom and a Face-Centered Atom Shared by 2 unit cells Shared by 4 unit cells Shared by 8 unit cells

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

Relation Between Edge Length and Atomic Radius

When silver crystallizes, it forms face-centered cubic cells When silver crystallizes, it forms face-centered cubic cells. The unit cell edge length is 409 pm. Calculate the density of silver. d = m V V = a3 = (409 pm)3 = 6.83 x 10-23 cm3 4 atoms/unit cell in a face-centered cubic cell 107.9 g mole Ag x 1 mole Ag 6.022 x 1023 atoms x m = 4 Ag atoms = 7.17 x 10-22 g d = m V 7.17 x 10-22 g 6.83 x 10-23 cm3 = = 10.5 g/cm3

Closest Packing

Closest Packing

HCP and CCP AB stacking ABC stacking

X-Ray Diffraction (XRD) X-光繞射光譜

Applications: MoS2 crystalline

Applications: Graphite oxide

XRD

The crystal types: Ionic crystals Covalent Crystals Molecular Crystals Metallic Crystals

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 CaF2

EX: KH2PO4

NaCl(FCC)

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

From graphite to graphene Slice down to Atomic layer

What is graphene? Imagine a piece of paper but a million times thinner. This is how thick graphene is. The thinnest while strongest material in the world! (0.77 mg/1m2 Graphene, can sustain 4 kg ) Imagine a material more conducting than silver. This is how conductive graphene is. Image from Nobel committee From a recently interview with A. K. Geim.

Electrical properties of Graphene Graphene resistivity: 1.0x10-8 Ohm m Also, it’s current density is > 1.0X108 A/cm From wikipedia

Mechanical properties Graphene : ~1000 GPa From wikipedia

Thermal conductivity Graphene : ~5300 W/mK Movie From wikipedia

Optical properties Graphene : ~1000 GPa Science (2008)

Carrier mobility in graphene : 1/300 of light speed Low energy is like a massless 2D Dirac model Zero band gap semiconductor. Carrier shows Linear dispersion around Femi- level. The electronic behavior can be described by QM ‘s Dirac equation. 4. In condense physic : The unique Massless Dirac fermions K K’ M 石墨烯 更为奇特之处是它具有独特的电子结构和电学性质. 石墨烯的价带(π 电子)和导带(π*电子)相交于费米能 级处(K 和K′点), 是能隙为零的半导体, Ballistic transport on submicron to micron distances. IBM already demonstrated 100 GHz high frequency device. Dispersion: http://www.ece.mcgill.ca/~ts7kop/images/graphene_lcao.jpg Dr. Thomas Szkopek, McGill University

Energy dispersion

Density of state of Graphene

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

EX: S8

Cross Section of a Metallic Crystal 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 (high density) Cross Section of a Metallic Crystal nucleus & inner shell e- mobile “sea” of e-

Amorphous solid: Glass

Phase Changes Greatest Order Least

The equilibrium vapor pressure is the vapor pressure measured when a dynamic equilibrium exists between condensation and evaporation H2O (l) H2O (g) Rate of condensation evaporation = Dynamic Equilibrium

Measurement of Vapor Pressure Before Evaporation At Equilibrium

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

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

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.

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

The Critical Phenomenon of SF6 T < Tc T > Tc T ~ Tc T < Tc

Solid-Liquid Equilibrium H2O (s) H2O (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

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

Solid-Gas Equilibrium H2O (s) H2O (g) Molar heat of sublimation (DHsub) is the energy required to sublime 1 mole of a solid. DHsub = DHfus + DHvap ( Hess’s Law)

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

Phase Diagram of Carbon Dioxide At 1 atm CO2 (s) CO2 (g)

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

CH11 Homework 12.98 Given the phase diagram of carbon shown here, answer these questions: (a) How many triple points are there and what are the phases that can coexist at each triple point? (b) Which has a higher density, graphite or diamond? (c) Synthetic diamond can be made from graphite. Using the phase diagram, how would you go about making diamond? 12.20 Explain the difference in the melting points of these compounds:

12. 80 The vapor pressure of benzene, C6H6, is 40. 1 mmHg at 7. 6°C 12.80 The vapor pressure of benzene, C6H6, is 40.1 mmHg at 7.6°C. What is its vapor pressure at 60.6°C? The molar heat of vaporization of benzene is 31.0 kJ/mol.