What is a liquid? A solid? Properties of liquids and solids depend on Interparticle (Intermolecular) forces: - vaporization/condensation/freezing - equilibrium vapor pressure/volatility - surface tension - boiling point/freezing point We are going to learn about Interparticle or Intermolecular forces first! (Different order than in chapter.)
TYPES OF INTERPARTICLE FORCES - SEE HANDOUT All forces of attraction between atoms, ions, molecules are “Interparticle” forces Includes ionic bonding, covalent bonding, metallic bonding, network covalent bonding, and ion-dipole attraction Important Subcategory is Intermolecular Forces Also called Van Der Waal’s forces Weak to moderate forces of attraction Nota type of bonding Includes three main ones: London Dispersion Forces, Dipole-dipole Attraction and Hydrogen Bonding Force of Attraction
Intermolecular Forces 1. London dispersion forces (LDF) - Small to moderate strength - Depend on size of electron cloud (and so also molar mass) of atom or molecule - Noble gases, diatomic elements, and many other nonpolar compounds
Nonpolar molecules such as H2 can develop instantaneous dipoles and induced dipoles. The attractions between such dipoles, even through they are transitory, create London dispersion forces. (See figures 12.17 & 18)
Intermolecular Forces 2. Dipole-dipole attraction - Moderate strength - Molecules that have polar covalent bonds - Polar molecules d+ and d- attraction Table of Properties of Hydrohalogens H-F H-Cl H-Br H-I DEN 1.4 1.1 0.8 0.4 # e-s 10 18 36 54 BP 291 188 206 238
There are many dipole-dipole interactions possible between randomly arranged ClF molecules. In each interaction, the positive end of one molecule is attracted to the negative end of a neighboring ClF molecule.
Intermolecular Forces 3. Hydrogen-bonding force of attraction (enhanced dipole-dipole) - Strong force, but much less than real bonding - Memory helper: E.T. FONHome: only F-H, O-H and N-H have this type of force - Due to small radius and high EN - See in boiling point data
Depiction of hydrogen bonding among water molecules. The dotted lines are the hydrogen bonds.
Hydrogen Bonding and Water: Water - 80% hydrogen-bonded - very tight arrangement (also high viscosity high density and high specific heat) Ice - crystal is very open, less dense than liquid (4. Dipole - induced dipole between diff types of molecules, O2 in H2O)
Diagrams of hydrogen bonding involving selected simple molecules. The solid lines represent covalent bonds; the dotted lines represent hydrogen bonds.
If there were no hydrogen bonding between water molecules, the boiling point of water would be approximately - 80C.
Notice that molecules with F-H, O-H and N-H have HIGH BPs because of Hydrogen-bonding forces of attraction.
Properties and H-Bonding Table on page 411 in Tro.
PRACTICE IDENTIFYING THE TYPE OF IM FORCE: CH4(g) C6H6(l) Br2(l) HBr(l) IBr(s) CH3OH(l)
Distinguishing Properties of Solids, Liquids, and Gases Property Solid Liquid Gas Vol/Shape Def vol Def vol; indef shape Indef vol/shape & def shape Shape of container Shape of container Density High High, usually < Solid Very low Compressibility Small Small Large Thermal ExpansionVery small Small Moderate Strength Strong Moderate Weak of IM Forces Example Sucrose Water Carbon dioxide
There are six changes of state possible for substances: learn all 6 Fusion
BP, FP, Phase Changes, and DHophase Phase changes: changes of state Learn all six Phase changes are accompanied by heat flow, called Enthalpy of phase change or DHophase Heat of vaporization: liquid to vapor; energy (J) to vaporize 1 mol at constant T & P; DHvap for water is 40.7 kJ/mol; DHcond = -DHvap Heat of fusion: solid to liquid; energy (J) to melt 1 mol at constant T & P; DHvap for water is 6.02 kJ/mol; DHfrz = -DHfus
BP, FP, Phase Changes, and DHophase Sensible heat transfer: temperature is changed but not phase q = m * cp * DT m is mass, cp is specific heat and DT is Tf – Ti Latent heat transfer using DHophase: phase changes but not temperature q = m * DHophase m is mass or moles depending on units See examples 12.1 and 12.2. Try skill builders.
BP, FP, Phase Changes, and DHophase Specific heat: energy required to raise temperature of 1.00 gram of substance by 1.00oC cp for water is 4.184 J/g.oC Find heat required to raise the temperature of 20.0 g of water from 25.0oC to 35.0oC. q = 20.0 g (4.184 J/goC)(35.0-25.0)oC = 837 J
Practice Calculations for Heating Curves Calculate the total heat absorbed when a 15.5 gram cube of ice melts, warms to the BP of water and then vaporizes completely. (DHfus = 6.02 kJ/mol; DHvap = 40.7 kJ/mol) There will be two latent heat transfers and one sensible heat transfer. Latent: 15.5 g (1mol/18.015g)(6.02kJ/mol) = 5.18 kJ Sensible: 15.5 g (4.184J/goC)(100.0oC)(1kJ/103J) = 6.49 kJ Latent: 15.5 g (1mol/18.015g)(40.7kJ/mol) = 35.0 kJ Total = 46.7 kJ
BP, FP, Phase Changes, and DHophase Boiling point: temperature at which the vapor pressure of a liquid is equal to the external pressure above the liquid, usually atmospheric pressure of 1 atm (Normal BP is at 1 atm) Freezing point: temperature at which a liquid changes into a solid at 1 atm
In the evaporation of a liquid in a closed container (a), the liquid level drops for a time (b) and then becomes constant (ceases to drop). At that point a state of equilibrium has been reached in which the rate of evaporation equals the rate of condensation (c).
Equilibrium Vapor Pressure In closed system: at any given temperature, rate of vaporization = rate of condensation At dynamic equilibrium: number of molecules in gas phase and number of molecules in liquid phase stay the same, but processes still happening Vapor pressure taken at equilibrium = the partial pressure Vapor pressure changes with change in temperature Normal boiling point is when vapor pressure is 1.00 atm or 760. Torr
Vapor Pressure of Water at Various Temperatures Plot vapor pressure curves (pressure vs. temperature): curved line like Figure 11.26. Normal boiling point is the temperature at which vapor pressure = atm pressure = 760 torr.
Properties of Liquids Just read about surface tension, viscosity and capillary action
Types of Crystalline Solids There are two types of solids: crystalline and amorphous. We are looking at crystalline. There are FOUR types of crystalline solids: Molecular: solids made of molecules with covalent bonds; molecules held in place by IM forces; low to mod MPs Ionic: solids made of formulas units of ions; held in place by ionic bonding; high MPs Metallic: composite units are atoms; held in place with metallic bonding; range of MP’s but usually higher than molecular solids Network covalent: atoms held together in network covalent bonding; only diamond, SiC, SiO2. (Atomic solids like noble gases do not occur at normal conditions.)