Liquids, Solids and Phase Changes

Chapter 10 Liquids, Solids and Phase Changes Chapter 10 1 KMT of Liquids and Solids Gas molecules have little or no interactions. Molecules in the Liquid or solid state have significant interactions. Liquids and solids have well-defined volume. Liquid molecules flow, while solids are held rigid. Chapter 10 2 1

Properties of Liquids 1. Liquids have a variable shape, but a fixed volume. 2. Liquids usually flow readily. However, not all liquids flow at the same rate. 3. Liquids do not compress or expand significantly The volume of a liquid varies very little as the temperature and pressure change. 4. Liquids have a high density compared to gases. Liquids are about 1000 times more dense than gases. 5. Liquids that are soluble mix homogeneously. Liquids diffuse more slowly than gases but eventually will form a homogeneous mixture. Chapter 10 3 Properties of Solids 1. Solids have a fixed shape and volume. Unlike liquids, solids are rigid. 2. Solids are either crystalline or non-crystalline. Crystalline solids contain particles in a regular, repeating pattern. 3. Solids do not compress or expand to any degree Assuming no change in physical state, temperature and pressure have a negligible effect on the volume of a solid. 4. Solids have a slightly higher density than their corresponding liquid One important exception is water; ice is less dense than liquid water. 5. Solids do not mix by diffusion The particles are not free to diffuse in a solid heterogeneous mixture. Chapter 10 4 2

Intermolecular Force Concept An intermolecular force is an attraction between molecules Intramolecular bonds occur between atoms within a molecule. Intermolecular forces are much weaker than intramolecular bonds These forces are due to dipole moments within the molecules There are three main intermolecular forces: Dipole forces Hydrogen bonds Dispersion forces Chapter 10 5 Dipole Moments Polar covalent bonds form between atoms of different electronegativity. This is described as a bond dipole. The figure with the electronegativity values in in CHP 7 (pg. 248) Chapter 10 6 3

Rules for Determining the Polarity of Molecules Rule 1: If the central atom has an odd number of lone pairs, the molecule is polar One exception is a Linear Molecule (MG) that has a Trigonal Bipyramidal EPA Rule 2: If there are no lone pairs and the central atom is bound to only one type of atom (for example, CH 4 ) then the molecule is non-polar. Rule 3: If there are no lone pairs on the central atom, but it is surrounded by more than one type of atom (for example, CH 3 Cl), you must look at the shape of the molecule. Linear, Trigonal planar & Tetrahedral = Polar Trigonal Bipyramidal & Octahedral = Look at structure Chapter 10 7 Dipole Moments Dipole Moment (µ): The measure of net molecular polarity or charge separation. µ = Q r r = distance between charges δ+ = Q, δ = Q Dipole moments are expressed in debyes (D) 1 D = 3.336 x 10 30 C m A proton and electron separated by 100 pm have µ = 4.80 D (This is the dipole moment for a fully IONIC bond!) Chapter 10 8 4

Dipole Moments Chapter 10 9 Dipole Moments Which of the following compounds will have a dipole moment? Show the direction of each. SO 2 NH 3 CF 4 TeH 4 PF 6 XeOF 4 AlCl 3 BF 4 SiCl 4 ICl 4 Chapter 10 10 5

Dipole Forces Two types of Dipole Forces: Dipole-Dipole Interactions Ion-Dipole Interactions Chapter 10 11 Dipole Forces and the Boiling Point For polar molecules, the dipole-dipole attractions influence temperatures at which state changes occurs In particular, the boiling point of the liquid As the temperature of a substance is increased, what do you think happens to the molecules? Eventually, the molecules will gain enough Kinetic energy to override the intermolecular forces and escape their state and move into gaseous state. The strength of the IMFs is directly related to the temperature at which this occurs. HOW? IMFs Boiling Point Chapter 10 12 6

London Dispersion Forces London Dispersion Forces are dipole attractions that result from the formation of instantaneous, temporary dipoles in non-polar molecules due to electron motion. In a molecule, electrons are constantly orbiting the nucleus and a region may become temporarily electron poor and slightly positive while another region becomes slightly negative. This creates a temporary dipole and two molecules with temporary dipoles are attracted to each other. Chapter 10 13 Hydrogen Bonds Hydrogen bonds are present when a molecule has an N-H, O-H, or F-H bond. Chapter 10 14 7

Hydrogen Bonds Hydrogen bonds are particularly important to your DNA and protein structure and in water Chapter 10 15 How to Determine What Type of Intermolecular Forces are Present When trying to determine what type of intermolecular forces are involved in the attraction between two molecules ask yourself the following questions: 1) Is there any potential for Hydrogen bonding between the two molecules? o In other words, look for O-H, N-H, or F-H groups 2) Is one or both of the molecules polar (or an ion)? o If both, then usually dipole-dipole (or ion-dipole) interactions o If one is polar and the other is nonpolar, then should be dipole-induced dipole interaction 3) If both molecules are nonpolar, look and see if one molecule is charged. o If yes, then should be an induced dipole interaction o If no, then London dispersion force interaction Chapter 10 16 8

Intermolecular Forces Chapter 10 17 Heat is necessary to raise the temperature and change the physical state of a substance. Specific heat is the amount of heat required to raise 1.00 g of a substance 1 C. Phase Changes Liquid water is the reference and its specific heat is 4.184 J/(g C) The specific heats of ice and steam are about half that of liquid water. Chapter 10 18 9

Heating and Cooling Curves We can graph the amount of energy required to change the temperature and physical state of a substance. H vap H con H fus H sol Chapter 10 19 Energy from Heating Curves We can use the energy curves and Enthalpy values for a molecule to calculate how much energy is required to change the temperature and/or state of a sample. These problems can be broken into two types of calculations: 1: The amount of energy required to raise the temperature: heat = (Specific Heat) ( T) (m) 2: The amount of energy required to change the state: heat = (H xxx ) (m) Chapter 10 20 10

Energy Calculation Problem Calculate the amount of energy (kj) needed to heat 346 g of liquid water from 0 C to 182 C. Assume that the specific heat of water is 4.184 J/g C over the entire liquid range and that the specific heat of steam is 1.99 J/g C. The molar heats of fusion and sublimation of molecular iodine are 15.27 kj/mol and 62.30 kj/mol, respectively. Estimate the molar heat of vaporization. Chapter 10 21 Physical Properties of Liquids There are four physical properties of liquids that we can relate to their intermolecular attractions : Vapor Pressure Boiling Point Viscosity Surface tension Chapter 10 22 11

Vapor Pressure At the surface of a liquid, some molecules gain enough energy to escape the intermolecular attractions of neighboring molecules and enter the vapor state. This is evaporation. The reverse process is condensation. When the rates of evaporation and condensation are equal, the pressure exerted by the gas molecules above a liquid is called the vapor pressure. Chapter 10 23 Vapor Pressure Trend The stronger the intermolecular forces between the molecules in the liquid, the less molecules escape into the gas phase. Intermolecular Forces are indirectly proportional to the Vapor Pressure As the attractive force between molecules increases, vapor pressure decreases. Chapter 10 24 12

Vapor Pressure vs. Temperature As the temperature increases, the vapor pressure of a liquid increases. They are directly proportional Again, the stronger the intermolecular attractions, the lower the vapor pressure at a given temperature. Chapter 10 25 Vapor Pressure Clausius Clapeyron Equation: Provides a link between vapor pressure (P), temperature (T), and molar heat of vaporization ( H vap ). ln H = R 1 T vap Pvap + y = m x + b By taking measurements at two temps, we get: P ln P H = R 1 T T 1 1 vap 1 2 2 C Chapter 10 26 13

Vapor Pressure Ethyl ether is a volatile, highly flammable organic liquid that is used mainly as a solvent. The vapor pressure of ethyl ether is 401 mm Hg at 18 C. Calculate its vapor pressure at 32 C. The vapor pressure of ethanol is 100 mm Hg at 34.9 C. What is its vapor pressure at 63.5 C? ( H vap for ethanol is 39.3 kj/mol) Chapter 10 27 Boiling Point The normal boiling point of a substance is the temperature where the vapor pressure is equal to 1 atm. The stronger the intermolecular attractions, the higher the boiling point of the liquid. Intermolecular Forces Boiling Point Vapor Pressure 1 Boiling Point Chapter 10 28 14

Viscosity The viscosity of a liquid is a liquid s resistance to flow. Viscosity is the result of an attraction between molecules. The stronger the intermolecular forces, the higher the viscosity. Chapter 10 29 Surface Tension The attraction between molecules at the surface of a liquid is called surface tension. The stronger the intermolecular attractions, the stronger the surface tension of a liquid. Chapter 10 30 15

The Solid State Solids are divided into two categories: Crystalline: Rigid and long-range order Amorphorous: Lacks well-defined arrangement Chapter 10 31 Crystalline Solids Crystalline solids can be classified in two ways: The arrangement of the particles (Crystal Lattice) The types of forces that hold the particles together Structure of a crystalline solid is based on the unit cell, a basic repeating structural unit. Cell Types of crystal lattices based on a cube include: Simple Cubic (SC) Body-centered Cubic (BCC) Face-centered Cubic (FCC) Hexagonal or Cubic Close Packing There are many others! The number of particles touching each atom (the coordination number) and the number of atoms in each cell depends on the cell type Chapter 10 32 16

Crystalline Solids To determine the number of atoms in a unit cell, remember the following: An atom is considered a corner, edge, face or center. Corner = 1/8 in unit cell Edge = ¼ in unit cell Face = ½ in unit cell Center = 1 in unit cell The number of particles touching each atom (the coordination number) and the number of atoms in each cell depends on the cell type Chapter 10 33 Crystalline Solids Simple Cube and Body-Centered Cube: Chapter 10 34 17

Crystalline Solids Simple Cube and Body-Centered Cube: Coordination # # atoms in cell SC 6 1 BCC 8 Chapter 10 35 2 Crystalline Solids Hexagonal Close Packing Arrangements (a-b-a-b):( Chapter 10 36 18

Crystalline Solids Cubic Close Packing Arrangements (a-b-c-a-b-c):( Chapter 10 37 Crystalline Solids Face-Centered Cube: FCC Coordination # # atoms in cell 12 Chapter 10 38 4 19

Crystalline Solids Chapter 10 39 Density of Crystalline Solids Silver metal crystallizes in a FCC arrangement with the edge of the unit cell having a length of d = 407 pm. What is the radius (in picometers) of a silver atom? Nickel has a FCC arrangement with the edge of the unit cell having a length of d = 352.4 pm. What is the density of the Nickel in g/cm 3? Chapter 10 40 20

Density of Crystalline Solids Sodium has a density of 0.971 g/cm 3 and crystallizes with a body-centered cubic unit cell. What is the radius of a sodium atom, and what is the edge length of the cell (in pm)? Chapter 10 41 Ionic Solids A crystalline ionic solid is composed of cations and anions arranged in a regular, repeating pattern. Usually a Face-centered cubic arrangement but not always The face positions are occupied by one of the ions, with the other ion filling in the holes as needed. The ratio of cations to anions in a unit cell must be consistent with the ionic formula for the compound Ionic solids are very hard and brittle with high melting points They are also good conductors of electricity but only in their aqueous or liquid state Chapter 10 42 21

Ionic Solids Chapter 10 43 A crystalline molecular solid has molecules arranged in a particular conformation. A separate water molecule is found at each lattice point Molecular Solids The solids are held together by intermolecular forces These compounds are fragile (compared to ionic compounds) with melting points dependent on the intermolecular forces holding them together. They do not conduct electricity Chapter 10 44 22

Covalent Network Solids A crystalline covalent network solid has molecules arranged in a particular conformation. These molecules are held together by covalent bonds They are generally hard and high melting They do not conduct electricity Chapter 10 45 Metallic Crystals A crystalline metallic solid is composed of metal atoms arranged in a definite pattern. BCC, FCC or Hex Arrangement A metallic crystal is made up of positive metal ions surrounded by a sea of valance electrons. Metallic solids are good conductors of electricity because electrons are free to move about the crystal. Most metals are malleable and ductile as well. Chapter 10 46 23

Phase Diagrams A Phase Diagram is a graphical display of the temperatures and pressures at which two phases of a substance are in equilibrium. Triple Point: The only condition under which all three phases can be in equilibrium with one another. Critical Temperature (T c ): The temperature above which the gas phase cannot be made to liquefy at any pressure. Critical Pressure (P c ): The minimum pressure required to liquefy a gas at its critical temp. Chapter 10 47 Phase Diagrams Water Chapter 10 48 24

Phase Diagrams Approximately, what is the normal boiling point and what is the normal melting point of the substance? What is the physical state when: A) T = 150 K, P = 0.5 atm B) T = 325 K, P = 0.9 atm C) T = 450 K, P = 165 atm Chapter 10 49 25