Chapter 11 Intermolecular Forces, Liquids and Solids CHEMISTRY The Central Science 9th Edition Chapter 11 Intermolecular Forces, Liquids and Solids

11.1: A Molecular Comparison of Liquids and Solids

Text, P. 409 The forces holding solids and liquids together are called intermolecular forces

11.2: Intermolecular Forces The covalent bond holding a molecule together is an intramolecular force The attraction between molecules is an intermolecular force Much weaker than intramolecular forces Melting or boiling: the intermolecular forces are broken (not the covalent bonds)

The stronger the attractive forces, the higher the boiling point of the liquid and the melting point of a solid Text, P. 409 (low boiling point)

Strongest of all intermolecular forces Found only in mixtures Ion-Dipole Forces Interaction between an ion and a dipole (a polar molecule such as water) Strongest of all intermolecular forces Found only in mixtures Text, P. 410

Between neutral polar molecules Dipole-Dipole Forces Between neutral polar molecules Oppositely charged ends of molecules attract Weaker than ion-dipole forces Dipole-dipole forces increase with increasing polarity Strength of attractive forces is inversely related to molecular volume Text, P. 410

London Dispersion Forces Weakest of all intermolecular forces Two adjacent neutral, nonpolar molecules The nucleus of one attracts the electrons of the other Electron clouds are distorted Instantaneous dipole Strength of forces is directly related to molecular weight London dispersion forces exist between all molecules

London dispersion forces depend on the shape of the molecule The greater the surface area available for contact, the greater the dispersion forces Text, P. 412

Hydrogen Bonding Special case of dipole-dipole forces H-bonding requires H bonded to an electronegative element (most important for compounds of F, O, and N)

Hydrogen Bonding Text, P. 413 Boiling point increases with increasing molecular weight. The exception is water (H bonding)

Hydrogen Bonding Text, P. 414

Text, P. 415 Solids are usually more closely packed than liquids (solids are more dense than liquids) Ice is ordered with an open structure to optimize H-bonding (ice is less dense than water)

Text, P. 417

Sample Problems # 7, 9, 11, 13, 15, 17, 19

11.3: Some Properties of Liquids Viscosity Viscosity is the resistance of a liquid to flow Molecules slide over each other The stronger the intermolecular forces, the higher the viscosity Viscosity increases with an increase in molecular weight

Surface Tension Surface molecules are only attracted inwards towards the bulk molecules Molecules within the liquid are all equally attracted to each other

Surface tension is the amount of energy required to increase the surface area of a liquid Cohesive forces bind molecules to each other (Hg) Adhesive forces bind molecules to a surface (H2O) If adhesive forces > cohesive forces, the meniscus is U-shaped (water in a glass) If cohesive forces > adhesive forces, the meniscus is curved downwards (Hg in a barometer)

11.4: Phase Changes Text, P. 420 (Exothermic) (Endothermic)

Generally heat of fusion (melting) is less than heat of vaporization (evaporation) It takes more energy to completely separate molecules than to partially separate them Text, P. 420

Plot of temperature change versus heat added is a heating curve Heating Curves Plot of temperature change versus heat added is a heating curve During a phase change, adding heat causes no temperature change (equilibrium is established) These points are used to calculate Hfus and Hvap Remember: Q = m·Cp·ΔT

Added heat increases the temperature of a consistent state of matter Text, P. 421 Added heat increases the temperature of a consistent state of matter Energy used for changing molecular motion, no T change

Critical Temperature and Pressure Gases are liquefied by increasing pressure at some temperature Critical temperature: the maximum temperature for liquefaction of a gas using pressure A high C.T. means strong intermolecular forces Critical pressure: pressure required for liquefaction

Examples: # 31, 33, WDP # 48 Other WDP examples: # 44, 46, 50 and 51

11.5: Vapor Pressure Explaining Vapor Pressure on the Molecular Level Some of the molecules on the surface of a liquid have enough energy to escape to the gas phase After some time the pressure of the gas will be constant at the vapor pressure (equilibrium is established)

Dynamic Equilibrium: the point when as many molecules escape the surface as strike the surface Vapor pressure is the pressure exerted when the liquid and vapor are in dynamic equilibrium Volatility, Vapor Pressure, and Temperature If equilibrium is never established then the liquid evaporates Volatile substances (high VP) evaporate rapidly The higher the T, the higher the average KE, the faster the liquid evaporates (hot water evaporates faster than cold water)

Vapor pressure increases nonlinearly with increasing temperature Text, P. 426

T is the Kelvin temperature R is the gas constant, When Temperature changes from T1 to T2, Vapor Pressure changes from P1 to P2 These changes are related to ΔH by the equation, Where T is the Kelvin temperature R is the gas constant, ΔHvap is the molar heat of vaporization

This comes from the graph of P vs. inverse of T Straight line Negative slope Equation: C is a constant Use the Clausius-Clapeyron Equation to Predict the vapor pressure at a specified temperature Determine the T at which a liquid has a specified VP Calculate enthalpy of vaporization from measurements of VP’s at different temperatures

Vapor Pressure and Boiling Point Liquids boil when the external pressure equals the vapor pressure Normal BP: BP of a liquid at 1 atmosphere Temperature of boiling point increases as pressure increases

The vapor pressure of water is 1 The vapor pressure of water is 1.0 atm at 373 K, and the enthalpy of vaporization is 40.7 kJ mol-1. Estimate the vapor pressure at temperature 363 and 383 K respectively. Solution where R (= 8.3145 J mol-1 K-1) Using the Clausius-Clapeyron equation, we have: P363 = 1.0 exp (- (40700/8.3145)(1/363 - 1/373) = 0.697 atm P383 = 1.0 exp (- (40700/8.3145)(1/383 - 1/373) = 1.409 atm Note that the increase in vapor pressure from 363 K to 373 K is 0.303 atm, but the increase from 373 to 383 K is 0.409 atm. The increase in vapor pressure is not a linear process.

Sample problems: # 45, WDP # 35 Other WDP examples: # 36 & 37

11.6: Phase Diagrams Phase diagram: plot of pressure vs. Temperature summarizing all equilibria between phases Given a temperature and pressure, phase diagrams tell us which phase will exist

Vapor Pressure curve of the liquid (increase P, increase T) Text, P. 428 Melting point curve: Increased P favors solid phase; Higher T needed to melt the solid at higher P Beyond this point, liquid and gas phases are indistinguishable Vapor Pressure curve of the liquid (increase P, increase T) Stable at low T and high P Stable at low P and high T Triple Point: all 3 phases in equilibrium

The Phase Diagrams of H2O and CO2 Text, P. 429 Line slopes to the left: ice is less dense than water (why?) MP decreases with increased P Text, P. 429 The Phase Diagrams of H2O and CO2

Sample Problems: #49, 51

11.7: Structures of Solids Unit Cells Crystalline solid: well-ordered, definite arrangements of molecules, atoms or ions The smallest repeating unit in a crystal is a unit cell It has all the symmetry of the entire crystal Three-dimensional stacking of unit cells is the crystal lattice Close-packed structure

Unit Cells Text, P. 431

Unit Cells Text, P. 432 Primitive cubic: atoms at the corners of a simple cube each atom shared by 8 unit cells

Unit Cells Body-centered cubic (bcc): atoms at the corners of a cube plus one in the center of the body of the cube corner atoms shared by 8 unit cells center atom completely enclosed in 1 unit cell Text, P. 432

Unit Cells Face-centered cubic (fcc): atoms at the corners of a cube plus one atom in the center of each face of the cube corner atoms shared by 8 unit cells face atoms shared by 2 unit cells Text, P. 432

Unit Cells 1 atom per cell 2 atoms per cell 4 atoms per cell Text, P. 432

The Crystal Structure of Sodium Chloride Text, P. 433 Two equivalent ways of defining unit cell: Cl- (larger) ions at the corners of the cell, or Na+ (smaller) ions at the corners of the cell

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Text, P. 435 11.8: Bonding in Solids

Covalent-Network Solids Text, P. 437

Ionic Solids The structure adopted depends on the charges and sizes of the ions Text, P. 438

Various arrangements are possible Metallic Solids Various arrangements are possible The bonding is too strong for London dispersion and there are not enough electrons for covalent bonds The metal nuclei float in a sea of electrons Metals conduct because the electrons are delocalized and are mobile Close-packed structure Text, P. 440

Amorphous solids (rubber, glass) have no orderly structure IMFs vary in strength throughout the sample No specific melting point Sample Problems # 53, 69, 71, 73, 75

End of Chapter 11 Intermolecular Forces, Liquids and Solids