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
11.2 Intermolecular forces are attractive forces between molecules Intramolecular forces hold atoms together in a molecule. (covalent bond) 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) Generally, intermolecular forces are much weaker than intramolecular forces.(only about 15% as strong) “Measure” of intermolecular force boiling point melting point H vap H fus H sub Kinetic-Molecular Description
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 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+Q-Q+Q- r
Intermolecular Forces 1. Ion-Dipole Forces Attractive forces between an ion and a polar molecule Example: ions in solution 11.2 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. Higher charge, smaller size strong interaction Water molecules
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 11.2 ion-induced dipole interaction dipole-induced dipole interaction – Ion induced – Dipole induced – Instantaneous dipole The electron distribution of atom is distorted by the force exerted by the ions or polar molecules.
11.2 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 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. Melting point increases as the number of electrons in the molecule increase. Dispersion forces usually increase with molar mass (more electrons), or size of the atom.
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. Polarizability allows gases containing atoms or nonpolar molecules (e.g.He,N2) 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. London Dispersion Force (Dispersion force)
S O O 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. CH 4 CH 4 is nonpolar: dispersion forces. SO 2 SO 2 is a polar molecule: dipole-dipole forces. There are also dispersion forces between SO 2 molecules. 11.2
Intermolecular Forces 4. Hydrogen Bond 11.2 The hydrogen bond is a special dipole-dipole interaction between the hydrogen atom in a polar N-H, O-H, or F-H bond and an electronegative O, N, or F atom. A H … B A H … A or A & B are N, O, or F
Hydrogen Bonds: Strength of H bonds: up to 40 kJ/mol Lots of H bonds = strong compare with strength of typical covalent bonds: 250 kJ/mole)
Why is the hydrogen bond considered a “special” dipole-dipole interaction? 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 11.3 The intermolecular attraction tend to pull molecules into liquids and cause the surface to tighten like a plastic film.
Properties of Liquids Viscosity is a measure of a fluid’s resistance to flow. 11.3 Strong intermolecular forces High viscosity CH2-OH CH-OH CH2-OH
A crystalline solid possesses rigid and long-range order. In a crystalline solid, atoms, molecules or ions occupy specific (predictable) positions. (e.g. NaCl) An amorphous solid does not possess a well-defined arrangement and long-range molecular order. (e.g. glass) A unit cell is the basic repeating structural unit of a crystalline solid. Unit Cell lattice point Unit cells in 3 dimensions 11.4 At lattice points: Atoms Molecules Ions Solid---crystal structure Structures of crystalline solids:
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 m = 4 Ag atoms 107.9 g mole Ag x 1 mole Ag 6.022 x 10 23 atoms x = 7.17 x 10 -22 g d = m V 7.17 x 10 -22 g 6.83 x 10 -23 cm 3 = = 10.5 g/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 CsClZnSCaF 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 11.6 diamond graphite carbon atoms allotrope sp3 sp2
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 11.6 Cross Section of a Metallic Crystal nucleus & inner shell e - mobile “sea” of e -
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 (SiO 2 ) Non-crystalline quartz glass 11.7 amorphous solid
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 ( H vap ) is the energy required to vaporize 1 mole of a liquid at its boiling point. ln P = - H vap RT + C Clausius-Clapeyron Equation P = (equilibrium) vapor pressure T = temperature (K) R = gas constant (8.314 J/K mol) 11.8 Vapor Pressure Versus Temperature
Molar heat of fusion ( H fus ) 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
A phase diagram summarizes the conditions at which a substance exists as a solid, liquid, or gas. Phase Diagram of Water 11.9 phase diagram Triple point All three phases are in equilibrium Solid line: Two phases are in equilibrium