Intermolecular Forces and Liquids and Solids Chapter 11

Intermolecular Forces and Liquids and Solids Chapter 11 Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. Acknowledgement Thanks to The Mc. Graw-Hill Companies, Inc. for allowing usage in the classroom.

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 11. 1

Kinetic-Molecular Description Intermolecular forces are attractive forces between molecules Intramolecular forces hold atoms together in a molecule. (covalent bond) Intermolecular vs Intramolecular • 41 k. J to vaporize 1 mole of water (inter) • 930 k. J to break all O-H bonds in 1 mole of water (intra) “Measure” of intermolecular force Generally, boiling point intermolecular melting point forces are much weaker than DHvap intramolecular DHfus forces. (only about DHsub 15% as strong) 11. 2

Types of Intermolecular Forces (IMF) • Should actually be called Interparticulate Forces (molecules, ions, and/or atoms) • Ion - ion forces • Ion-dipole forces • Dipole-dipole forces • Dispersion Forces • Hydrogen Bonds IMF determines the boiling point (b. p. ), melting point(m. p. ), vapor pressure, solubility, viscosity, surface tension, etc. Greater IMF greater b. p. , m. p. , solubility, surface tension , and viscosity; lower vapor pressure. Ion - ion forces: (lattice energy-ionic compound) • Remember Chapter 9 • Force depends on the charge on the ions and the distance separating the ions • (STRONG FORCES) E = k Q+Qr

Intermolecular Forces 1. Ion-Dipole Forces Attractive forces between an ion and a polar molecule Example: ions in solution Ion-Dipole Interaction 11. 2

The strength of the interaction depends on the charge and size of the ion and on the magnitude of the dipole moment and size of the molecule. Water molecules Higher charge, smaller size (that is, higher charge density) strong interaction 11. 2

Intermolecular Forces 2. Dipole-Dipole Forces (permanent dipole moment) Attractive forces between polar molecules Orientation of Polar Molecules in a Solid 11. 2

Intermolecular Forces 3. Temporary dipoles (Dispersion forces) What attractive force occurs in nonpolar substances? Attractive forces that arise as a result of temporary dipoles induced in atoms or molecules; based on molar mass; the weakest intermolecular force. – Ion induced – Dipole induced – Instantaneous dipole ion-induced dipole interaction The electron distribution of atom is distorted by the force exerted by the ions or polar molecules. dipole-induced dipole interaction 11. 2

The likelihood of a dipole moment being induced depends not only on the charge of the ion or the strength of the ddipole but also on the polarizability of the atom or molecules. 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 (more electrons), or size of the atom. Melting point increases as the number of electrons in the molecule increase. 11. 2

London Dispersion Force (Dispersion force) Polarizability allows gases containing atoms or nonpolar molecules (e. g. He, N 2) to condense. At any instant it is likely that the atom has a dipole moment created by a specific positions of electrons. This dipole moment is called instantaneous dipole. Induced dipoles interacting with each other. This type of interaction produces dispersion forces, which arise as a result of temporary dipoles induced in atoms or molecules, is responsible for the condensation of nonpolar gases. **Dispersion force exists between all species as long as they have mass.

What type(s) of intermolecular forces exist between each of the following molecules? HBr is a polar molecule: dipole-dipole forces. There also dispersion forces between HBr molecules. CH 4 is nonpolar: dispersion forces. SO 2 O SO 2 is a polar molecule: dipole-dipole forces. There also dispersion forces between SO 2 molecules. 11. 2

Intermolecular Forces 4. Hydrogen Bond The hydrogen bond is a special dipole-dipole interaction between the hydrogen atom in a polar N-H, O-H, or F-H bond an electronegative O, N, or F atom. A H…B or A H…A A & B are N, O, or F 11. 2

Hydrogen Bonds: • Strength of H bonds: up to 40 k. J/mol • Lots of H bonds = strong • compare with strength of typical covalent bonds: 250 k. J/mole)

Why is the hydrogen bond considered a “special” dipole-dipole interaction? Why water has a high boiling point? Decreasing molar mass Decreasing boiling point 11. 2

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 The intermolecular attraction tend to pull molecules into liquids and cause the surface to tighten like a 11. 3 plastic film.

Properties of Liquids Viscosity is a measure of a fluid’s resistance to flow. Strong intermolecular forces High viscosity CH 2 -OH 11. 3

Solid---crystal structure A crystalline solid possesses rigid and long-range order. In a crystalline solid, atoms, molecules or ions occupy specific (predictable) positions. (e. g. Na. Cl) An amorphous solid does not possess a well-defined arrangement and long-range molecular order. (e. g. glass) Structures of crystalline solids: A unit cell is the basic repeating structural unit of a crystalline solid. At lattice points: lattice point Unit Cell Unit cells in 3 dimensions • Atoms • Molecules • Ions 11. 4

Figure 11. 15

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Corner atom Shared by 8 unit cells edge atom Shared by 4 unit cells face centered atom Shared by 2 unit cells 11. 4

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) 11. 4

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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 = a 3 = (409 pm)3 = 6. 83 x 10 -23 cm 3 4 atoms/unit cell in a face-centered cubic cell 1 mole Ag 107. 9 g -22 g x m = 4 Ag atoms x = 7. 17 x 10 mole Ag 6. 022 x 1023 atoms 7. 17 x 10 -22 g m 3 = = 10. 5 g/cm d = V 6. 83 x 10 -23 cm 3 11. 4

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 Cs. Cl Zn. S Ca. F 2 11. 6

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 sp 2 sp 3 diamond allotrope graphite 11. 6

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 11. 6

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 Cross Section of a Metallic Crystal nucleus & inner shell emobile “sea” of e- 11. 6

amorphous solid An amorphous solid does not possess a well-defined arrangement and long-range molecular order. A glass is an optically transparent fusion product of inorganic materials that has cooled to a rigid state without crystallizing Crystalline quartz (Si. O 2) Non-crystalline quartz glass 11. 7

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. 11. 8

On top of a mountain • Water boils at a lower temp. on top of a mountain than at sea level. • Why? because the external pressure (atmospheric pressure) is less on top of a mountain. In an autoclave • An autoclave is used to sterilize medical instruments • The pressure is often 2 atmospheres. • Will water inside the autoclave boil at 100°C?

Molar heat of vaporization (DHvap) is the energy required to vaporize 1 mole of a liquid at its boiling point. Clausius-Clapeyron Equation ln P = - DHvap RT + C P = (equilibrium) vapor pressure T = temperature (K) R = gas constant (8. 314 J/K • mol) Vapor Pressure Versus Temperature 11. 8

Molar heat of fusion (DHfus) is the energy required to melt 1 mole of a solid substance at its freezing point. 11. 8

Heating Curve • Heat of fusion = energy needed to convert a solid to a liquid • Heat of vaporization = energy needed to convert a liquid to a gas • Be able to draw a heating curve Energy is needed to • heat a solid • heat a liquid • heat a gas • convert a solid to a liquid • convert a liquid to a gas • convert a solid to a gas 11. 8

phase diagram A phase diagram summarizes the conditions at which a substance exists as a solid, liquid, or gas. Phase Diagram of Water Triple point All three phases are in equilibrium Solid line: Two phases are in equilibrium 11. 9