Intermolecular Forces and Liquids and Solids Chapter 11 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. Phases Solid phase ice Liquid phase water 1 Intermolecular Forces Intermolecular forces are attractive forces between molecules. Intramolecular forces hold atoms together in a molecule.(chem. Bonds) Intermolecular vs Intramolecular 1 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. Measure of intermolecular force boiling point melting point H vap H fus H sub Intermolecular Forces Dipole Dipole Forces Attractive forces between polar molecules Orientation of Polar Molecules in a Solid 1
Intermolecular Forces Ion Dipole Forces Attractive forces between an ion and a polar molecule Ion Dipole Interaction Dispersion Forces Intermolecular Forces Attractive forces that arise as a result of temporary dipoles induced in NONPOLAR atoms or molecules Dispersion Forces Continued Intermolecular Forces 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 ion induced dipole interaction dipole induced dipole interaction Dispersion forces usually increase with molar mass. Disp. Forces exist in all species: Polar, Nonpolar, etc.
Why is the hydrogen bond considered a special dipole dipole interaction? Which of the following can form H bonds with H O? Decreasing molar mass Decreasing boiling point CH, F, H S, CH 3 OH 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. Properties of Liquids Cohesion is the intermolecular attraction between like molecules Adhesion is an attraction between unlike molecules Strong intermolecular forces Adhesion High surface tension Cohesion 3 3 Properties of Liquids Viscosity is a measure of a fluid s resistance to flow. Strong intermolecular forces Water is a Unique Substance Maximum Density 0 C Density of Water High viscosity Ice is less dense than water 3 3
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. lattic e point At lattice points: Atoms Molecules Ions Unit Cell Unit cells in 3 dimensions Shared by unit cells Shared by unit cells 5
1 atom/unit cell ( x 1/ = 1) atoms/unit cell ( x 1/ + 1 = ) atoms/unit cell ( x 1/ + 6 x 1/ = ) 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 6 6
Types of Crystals Molecular Crystals Lattice points occupied by molecules Held together by intermolecular forces (weak) Soft, low melting point Poor conductor of heat and electricity Ex: H O (ice), C 1 H O 11 (sucrose) 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 Types of Crystals nucleus & inner shell e Cross Section of a Metallic Crystal mobile sea of e 6 6 7
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 ) Non crystalline quartz glass 7 Before Evaporation At Equilibrium
Clausius Clapeyron Equation ln P 1 = H vap ( 1 1 ) P R T T 1 Problem: The vapor pressure of diethyl ether is 01 mm Hg at 1 C. Calculate its vapor pressure at 3 C. See next slide for H vap 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 (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. What is the relationship between T c and intermolecular forces? 9
Molar heat of fusion ( H fus ) is the energy required to melt 1 mole of a solid substance. H O (s) H 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 Melting Freezing Compare to H vap in Table 6 Calculate the amount of energy needed to heat 36 g of liquid water from 0 C to 1 C. Assume that the specific heat of water is.1 J/g x C over the entire liquid range and the specific heat of steam is 1.99 J/g x C. 10
H O (s) H O (g) Molar heat of sublimation ( H sub ) is the energy required to sublime 1 mole of a solid. Sublimation Deposition A phase diagram summarizes the conditions at which a substance exists as a solid, liquid, or gas. Phase Diagram of Water H sub = H fus + H vap ( Hess s Law) 9 Solid CO will not melt at 1 atm. 9 11