What determines the phase of a substance? Temperature Pressure Interparticle Forces of Attraction

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1 Liquids and Solids

2 What determines the phase of a substance? Temperature Pressure Interparticle Forces of Attraction

3 Types of Interparticle Forces Ionic Bonding Occurs between cations and anions Metallic Bonding Occurs between metal atoms (characterized by delocalized sharing of valence electrons) Covalent Bonding Occurs between nonmetal atoms (localized sharing of valence electrons) forming molecules

4 Types of Intermolecular Forces The forces holding molecules together are usually called van der Waals forces or intermolecular forces they are not as strong as the ionic, metallic, and covalent bonds They come in 2 main types: dipole-dipole and dispersion forces

5 Dipole-Dipole Force Occurs when a polar molecule attracts another polar molecule Hydrogen bonding A special type of dipole attraction that is very strong and occurs when the molecules contain Hydrogen at one end with a Nitrogen, Oxygen, or Fluorine at the other end.

6 So what is a polar molecule? Dipoles are polar molecules. Polarity is caused by unequal sharing of electrons in molecules resulting in a slight positive end and negative end on the molecule The shape of the molecule and the difference in electronegativity must be considered in determining polarity.

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9 Dispersion Forces Dispersion forces are thought to act in all molecules but would be the only force available to hold nonpolar molecules together in the liquid or solid phase. These are caused by unequal electron distribution in the electron cloud creating a temporary dipole effect which then acts on neighboring molecules inducing them to become temporary dipoles.

10 Dispersion cont They are considered the weakest of the forces of attraction, but get stronger the more electrons there are in the molecule. Larger molecules are quite polarizable because they have more electrons

11 Interparticle forces also act between different types of particles forming solutions

12 Liquids Fluid and lack appreciable compressibility Individual particles do not have fixed positions in the liquid but the forces of attraction limit the range of motion so particles remain closely packed.

13 Liquid Properties: Density For most substances, the liquid phase is less dense than that of the solid phase. Water is an exception where liquid phase water at 4 C has the greatest density for the substance at 1 g/ml.

14 Liquid Properties - continued Surface tension is the energy required to increase the surface area of a liquid. Capillary action is the movement of a liquid up or into a narrow tube due to adhesive and cohesive forces. Viscosity increases with decreased temperature, stronger forces between particles, and the interlocking of the particles.

15 Solids Constant shape & volume Lack Compressibility and Fluidity Definite Melting Point All true solids have a crystal lattice structure: A regular, repeating 3-D pattern of particles connected by forces of attraction

16 Definite Melting Point When enough energy is added, the lattice falls apart and the solid melts. The temperature where this change occurs is called the melting point or freezing point. Youtube clip showing use of a melting point apparatus: atch?v=a9ejdwddf6c

17 Solids can be classified based on the Type of forces holding the particles together Type of particles found in the solid material

18 Class Particle Force of attraction Metallic Metal atoms Metallic bond Examples Cu, Ag, Au, alloys Ionic cations & anions Ionic bond NaCl, CaO, CuSO 4 Network Nonmetal atoms Covalent bond Diamond, quartz Polar Molecular Polar molecules Dipole to dipole force Ice, sugar Nonpolar Molecular Nonpolar Molecules Dispersion forces I 2, S 8, dry ice (CO 2 ) Atomic Nonmetallic atoms Dispersion forces Grp 18

19 Crystal Structures Lattice: a 3D system of points designating the positions of the particles in the substance (ions, atoms, molecules ) Unit Cell: smallest repeating unit of a crystal lattice structure Cubic Unit Cell: perfect cube

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22 Simple Cubic Unit Cell Contains a total of 1 atom mass within its structure Side=2r

23 Body-centered cubic

24 Face-centered cubic

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27 Calculate the density of platinum metal if it has a face centered cubic unit cell and a crystallographic radius of 135 pm.

28 Calculate the crystallographic radius of Vanadium atoms if the metal has a density of 6.11 g/cm 3 and is known to have a body centered cubic unit cell.

29 Use the following crystallographic information for Gold, determine the type of cubic unit cell. Unit cell edge length 4.08 x 10-8 cm Density = 19.3 g/cm 3

30 Phase Changes

31 ENDOTHERMIC CHANGES Melting: Solid to liquid change The melting point is the temperature at which the forces holding the crystal lattice break Vaporization: Liquid to gas phase change Evaporation occurs at the surface of the liquid over all temperatures where liquid exists Boiling occurs at a specific temperature (boiling point) and is a liquid to gas change from within the body of the liquid (bubbles of gas form) Sublimation: Solid to gas phase change Dry ice (solid CO2) is a good example but many substances can skip the liquid phase depending on the conditions

32 Exothermic Changes Freezing: Liquid to solid phase change The freezing point (same as melting point) is the temperature where the forces reform and stabilize the crystal lattice Condensation: Gas to liquid phase change Occurs over all temperatures where the liquid exists often caused by the vapor contacting a colder surface (dew) Deposition: Gas to solid phase change A good example is the formation of frost or snowflakes where water vapor skips the liquid phase and changes from gas to solid directly

33 Temperature-time graph

34 Calorimetry applications How much thermal energy is needed to change the temperature of 50.0 g of ice at C to 50.0 g of steam at C? The specific heat of ice = 2.11 J/g-C, water = 4.18 J/g-C, steam = 2.08 J/g-C The enthalpy of vaporization is 2260 J/g at C and the enthalpy of fusion is 334 J/g at 0.0 C.

35 PRACTICE What would happen to a 25.0 gram sample water at 30.0 C if 3.5 kj of thermal energy is removed from the sample?

36 Vapor pressure curve

37 CLAUSIUS-CLAPEYRON equation Useful in determining the heat of vaporization for a sample. ln P 1 P 2 = H vap R 1 T 2 1 T 1 The pressures must be the same units, heat of vaporization is in J/mol, R is 8.31 J/K-mol, and temperature is in Kelvin units.

38 What is the boiling point of water at 450 torr? (heat of vaporization is 40.7 kj/mol)

39 Phase Diagram

40 Phase diagram for water