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Chapter 10 Liquids and Solids

Chapter 10 Liquids and Solids. Topics. 10.5 Section is self study. Intermolecular forces Dipole-dipole forces Hydrogen bonding London Forces The liquid state Surface tension Capillary action Viscosity An introduction to structures and types of solids

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Chapter 10 Liquids and Solids

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  1. Chapter 10 Liquids and Solids

  2. Topics 10.5 Section is self study • Intermolecular forces • Dipole-dipole forces • Hydrogen bonding • London Forces • The liquid state • Surface tension • Capillary action • Viscosity • An introduction to structures and types of solids • X-ray analysis of solids • Types of crystalline solids • Structure and bonding in metals • Bonding metals for metals • Meta alloys • Molecular solids • Ionic solids • Vapor pressure and changes of state • Phase diagrams

  3. 10.1 Intermolecular Forces Intra- vs. Inter-molecular forces • intramolecular forces • inside molecules (bonding) • hold atoms together into molecule • intermolecular forces • These are what hold the molecules together in the condensed states. • Forces between molecules • They get weaker as phase changes from S – L – G • When a substance changes state, molecule stays together but intermolecular forces are weakened

  4. Intermolecular Forces Gases– fill container, random rapid motion, never coming to rest or clumping together • Motion is mainly translational Liquids – fixed volume, flow and assume shape of container, only slightly compressible, stronger forces hold molecules together • Motion is mainly translational Solids– fixed volume, definite shape, generally less compressible than liquids, forces hold particles in a fixed shape • Motion is mainly vibrational

  5. Intermolecular Forces Intermolecular forces are attractive forces between molecules Intramolecular forces hold atoms together in a molecule. • 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) “Measure” of intermolecular force boiling point melting point DHvap DHfus DHsub Generally, intermolecular forces are much weaker than intramolecular forces.

  6. Dipole – Dipole Foces • Molecules that line up in the presence of a electric field are dipoles. • The opposite ends of the dipole can attract each other so the molecules stay close together. • 1% as strong as covalent bonds • Weaker the covalent bonds with greater distance. • Small role in gases. • Molecules with these forces possess higher melting points and boiling points than nonpolar molecules of comparable molar mass

  7. The strengths of intermolecular forces are generally weaker than either ionic or covalent bonds. Polar molecules have dipole-dipole attractions for one another. 16 kJ/mol (to separate molecules) + - + - 431 kJ/mol (to break bond)

  8. + S .. : : O O Types of intermolecular forces(between neutral molecules that posses dipole moment): Dipole-dipole forces: (polar molecules) .. + S .. : : O dipole-dipole attraction O : .. - - .. : .. - - What effect does this attraction have on the boiling point?

  9. HI -30 HBr -60 HCl -85 BP (oC) Effect of polarity on boiling points • Effect of polarity is usually small enough to be obscured • by differences in molar mass BP increase although polarity decreases

  10. Hydrogen Bonds • A hydrogen bond is an intermolecular force in which a • hydrogen atom covalently bonded to a nonmetal • atom in one molecule is simultaneously attracted to a • nonmetal atom of a neighboring molecule • The strongest hydrogen bonds are formed if the nonmetal • atoms are small and highly electronegative – e.g., N, O, F • very strong type of dipole-dipole attraction • because bond is so polar • because atoms are so small

  11. Hydrogen bond • Cl(HCl) and S(H2S) do not form hydrogen bonding although they have electronegativity similar to N, why? • They are of bigger size to approach the hydrogen atom • Hydrogen bond is 5-10% as strong as the covalent bond

  12. Hydrogen bonding is a weak to moderate attractive force that exists between a hydrogen atom covalently bonded to a very small and highly electronegative atom and a lone pair of electrons on another small, electronegative atom (F, O, or N).

  13. Hydrogen bonding: It is very strong dipole-dipole interaction(bonds involving H-F, H-O, and H-N are most important cases). +H-F- --- +H-F- Hydrogen bonding

  14. Hydrogen bonding between water molecules

  15. Water d+ d- d+

  16. Hydrogen Bonding • Bonding between hydrogen and more electronegative • neighboring atoms such as oxygen and nitrogen Hydrogen bonding between ammonia and water

  17. or … … H H B A A A Examples of hydrogen bond 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. IT IS NOT A BOND. A & B are N, O, or F

  18. Hydrogen Bonding Effects • Solid water is less dense than liquid • water due to hydrogen bonding • Hydrogen bonding is also the reason • for the unusually high boiling point of • water

  19. Boiling Points for Some Non Polar Molecules The larger the molecule the larger the Van der Waals attraction due to more electrons in the molecule. The stronger the attraction, the higher the boiling point.

  20. H2O HF H2Te H2Se NH3 SbH3 H2S HI AsH3 HCl HBr PH3 SnH4 GeH4 SiH4 CH4 100 0ºC Boiling Points -100 Molar mass 200

  21. Hydrogen Bonding in other molecules Many organic acids can form dimers due to hydrogen bonding Certain organic molecules can also form an intramolecular hydrogen bond

  22. Ethanol shows hydrogen bonding Do these compounds show hydrogen bonding?

  23. Do these compounds show hydrogen bonding?

  24. Hydrogen bonding and solubility • Some compounds containing O, N & F show high solubilities in certain hydrogen containing solvents. • NH3 & CH3OH dissolves in H2O through the formation of H-bonds

  25. London Dispersion Forces • Non - polar molecules also exert forces on each other. • Otherwise, no solids or liquids. • Electrons are not evenly distributed at every instant in time. • Have an instantaneous dipole. • Induces a dipole in the atom next to it. • Induced dipole- induced dipole interaction.

  26. London Dispersion Forces The temporary separations of charge that lead to the London force attractions are what attract one nonpolar molecule to its neighbors. London forces increase with the size of the molecules. Fritz London 1900-1954

  27. London Dispersion Forces • They exist in every molecular compound • They are significant only for nonpolar molecules and noble gas atoms • They are weak, short-lived • Caused by formation of temporary dipole moments

  28. Instantaneous polarization causes instantaneous dipole

  29. attraction + - + - “Electrons are shifted to overload one side of an atom or molecule”.

  30. London Dispersion Forces • Relatively weakforces that exist among noble gas atoms and nonpolar molecules. (Ar, C8H18) • Caused by instantaneous dipole, in which electron distribution becomes asymmetrical. • The ease with which electron “cloud” of an atom can be distorted is called polarizability.

  31. Polarizability: the ease with which an atom or molecule can be distorted to have an instantaneous dipole. “squashiness” In general big molecules are more easily polarized than little ones.

  32. Intermolecular Forces Polarizability 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 Dispersion forces usually increase with molar mass.

  33. London Dispersion Forces • Weak, short lived. • Lasts longer at low temperature. • Eventually long enough to make liquids. • More electrons, more polarizable. • Bigger molecules, higher melting and boiling points. • Much, much weaker than other forces. • Also called Van der Waal’s forces.

  34. RelativeMagnitudes of Forces The types of bonding forces vary in their strength as measured by average bond energy. Strongest Weakest Covalent bonds (400 kcal/mol) Hydrogen bonding (12-16 kcal/mol) Dipole-dipole interactions (2-0.5 kcal/mol) Londonforces (less than 1 kcal/mol)

  35. Which one(s) of the above are most polarizable? Hint: look at the relative sizes.

  36. London Forces in Hydrocarbons

  37. Practice • which has highest boiling pt? • HF, HCl, or HBr? • Identify the most important intermolecular forces : • BaSO4 • H2S • Xe • C2H6 • P4 • H2O • CsI ionic dipole-dipole H-bonding London Dispersion

  38. CO2 or OCS CO2: nonpolar so only LD OCS: polar so dipole-dipole PF3 or PF5 PF3: polar so dipole-dipole PF5: nonpolar so only LD SF2 or SF6 SF2: polar so dipole-dipole SF6: nonpolar so only LD SO3 or SO2 SO3: nonpolar so LD only SO2: polar so dipol-dipole Which has stronger intermolecuar forces?

  39. O O S 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. CH4 CH4 is nonpolar: dispersion forces. SO2 SO2 is a polar molecule: dipole-dipole forces. There are also dispersion forces between SO2 molecules. 11.2

  40. 10.2 The Liquid state Properties of Liquids • Low compressibility • Lack of rigidity • High density compared to gases • Beading (beads up as droplets) • Surface tension • Capillary action • Viscosity • Stronger intermolecular forces cause each of these to increase.

  41. Surface tension • The resistance to an increase in its surface area • Polar molecules and liquid metals show high surface tension

  42. Surface tension • Molecules at the the top are only pulled inside. • Molecules in the middle are attracted in all directions. • Minimizes surface area.

  43. O H H O H H Surface Tension d- • One water molecule can hydrogen bond to another because of this electrostatic attraction. • Also, hydrogen bonding occurs with other molecules surrounding them on all sides. d+ d+ d- d+ d+

  44. Surface Tension • A water molecule in the middle of a solution is pulled in all directions.

  45. Surface Tension • This is Not true for molecules at the surface. • Molecules at the surface are only pulled down and to each side. • This holds the molecules at the surface together tightly. • This causes surface tension.

  46. Surface tension • All liquids have surface tension • water is just higher than most others • How can we decrease surface tension? • Use a surfactant - surface active agent • Also called a wetting agent, like detergent or soap • Interferes with hydrogen bonding

  47. Beading • Water drops are rounded, because all molecules on the edge are pulled to the middle- not outward to the air!

  48. Adhesive forcesare intermolecular forces between unlike molecules Cohesive forces are intermolecular forces between like molecules

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