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Attractive Forces

VIEWS: 196 PAGES: 58

									Attractive Forces

       2011
  337: Chapter 13
  334 p. 385-392
                                    Outline
• Overview
    – Inter- vs. Intra-molecular Forces
    – Inter-molecular forces and Properties
• Types of inter-molecular forces (Van der Waals)
    – Hydrogen
    – Dipole
    – London dispersion forces
• Properties of Compounds
    –   Boiling and Melting point
    –   Viscosity
    –   Vapor Pressure
    –   Surface Tension
• Summary
• Bellringer
• Sheet
• Goal: Describe three inter molecular forces

• What is the difference between intra
  molecular forces and inter molecular forces?
         Intra vs Inter Molecular
                  Forces

     Intra molecular forces (chemical bonds)
       within molecules.




   Inter–molecular Forces – attractive forces between molecules.
                                    Outline
A. Inter vs Intra Molecular Forces
    – Inter- vs. Intra-molecular Forces
    – Inter-molecular forces and Properties
B. Types of inter-molecular forces (Van der Waals)
    1. Hydrogen
    2. Dipole
    3. London dispersion forces
C. Properties of Compounds
    –   Boiling and Melting point
    –   Viscosity
    –   Vapor Pressure
    –   Surface Tension
• Summary
• What are the three types of attractive forces.
• Rank their strengths.
      1. Hydrogen Bonding


• Present between molecules where H is
  bonded to N O or F

• Not a real bond

• Strongest IMF
  (explains why water is a liquid)
     First Intermolecular Force

1.    Hydrogen Bonding
  2.   Dipole-Dipole Forces

• Present between polar molecules
            +
       -




                                  View animation online.
                 C. Johannesson
         3. London Dispersion Forces

   Present in all molecules

   Caused by momentary
    induced dipoles.

   The only attractive force in
    nonpolar molecules.

   Increases with # electrons
    (also with increased mass)
View animation online.
                           C. Johannesson
Draw Hydrogen “Bonds”
Draw Hydrogen “Bonds”
                  Bell ringer:
      Inter- vs. Intra-molecular Forces
• Draw three water molecules and label the covalent
  bonds and the hydrogen bonds.
 Inter- vs. Intra-molecular Forces
• Draw three water molecules and label the
  covalent bonds and the hydrogen bonds.
Evidence of Hydrogen Bonding
H2O                  CH3OH




              +              =

      50 ml       50 ml =        95 ml
Intermolecular Forces Flowchart
                                                          ion – ion forces
 Is the substance ionic?1   YES
                                                            predominate

           NO

                                                     Hydrogen bonding forces
 Can it Hydrogen bond?2     YES
                                                          predominate

           NO

                                                       dipole – dipole forces
       Is it polar?         YES
                                                           predominate


           NO

London dispersion forces
     predominate

                1 Ionic substances contain metal-to-nonmetal bonds.
                2 Hydro`gen bonding substances contain H–O, H–N, or H–F bonds.
      C. Determining IMF

 NCl3
  • polar = London dispersion force,
                and dipole-dipole
 CH4
  • nonpolar = London dispersion force
 HF
  • H-F bond = London dispersion force, dipole-
    dipole, and hydrogen bonding
 NaCl
  • Na-Cl bond = ionic force,
                   C. Johannesson
    Bellringer – identifying IMF

 CO2
   • nonpolar = London dispersion force (gas)
 LiCl
   • ionic attraction (solid)
 PH3
   • polar = London dispersion force,
               and dipole-dipole
 SCl2
   • polar = London dispersion force,
               and dipole-dipole
   CH3 NH2
    • N-H bond = London dispersion force, dipole-dipole, and
                 hydrogen bonding
                Wksheet
• Draw 3 CO2 molecules, PH3 and NH3 molecules
  and identify the attractive forces between
  them.
                     Goal
• What two factors influence the boiling point
  of a substance?
  The strength of the inter molecular force?
Identify whether each substance is a solid, liquid or
gas at room temperature. (22oC)




  Substance     Melting      Boiling                 Phase?
               Point (oC)   Point (oC)      (Solid, Liquid or Gas?)

      A   (22oC)     75          130        Below mp Solid
      B            -220           10 (22oC) Above bpgas
      C             -10 (22oC)    45       Above mp & Below bpliquid
      D              6 (22oC)    150      Above mp & Below bpliquid
  Identify the phase of each halogens at room
  temperature. (22oC) and use intermolecular
  forces to explain the observed trend.

   Substance     Melting     Boiling           Phase?
                Point (oC) Point (oC) (Solid, Liquid or Gas?)
      F2           -220       -188 (22oC) Above bp gas
      Cl2          -101        -3 (22oC) Above bpgas
      Br2           -7 (22oC) 59      Above mp & below bpliquid
       I2   (22oC) 114        185     Below bpliquid


More electrons  stronger London Dispersion Force
….so requires more energy to break the attraction
and to separate molecules.
         3. London Dispersion Forces

 Present        in all molecules

 Caused   by a momentary
  induced dipoles.
 The only attractive force in
  nonpolar molecules.
 Increases with # electrons
  (also with increased mass)
   • So iodine is a solid at RT
View animation online.
                            C. Johannesson
Identify the phase of each substance at room
temperature. (22oC) and use intermolecular
forces to explain the observed trend.
 Substance   Melting    Boiling                 Phase?
              Point    Point (oC)      (Solid, Liquid or Gas?)
               (oC)
   CH4        -183       -161       (22oC) Below bp gas
    CF4       -184       -128       (22oC)   Above bpgas
   CCl4        -23 (22oC) 77         Above mp & below bpliquid
   CBr4 (22oC) 90         190         Below mpsolid

More electrons 
      stronger London Dispersion Force
      ….so requires more energy to break the
      attraction and to separate molecules.
     Compare the boiling points of
      hydrocarbons and alcohols
• Add chart
       Bell ringer: Predict
Which has the higher boiling point:
          H2O and H2S




H2O =   100 oC        H2S =   -61oC
                   Boiling Point
• Effect on the boiling point of liquids

  – H2O vs. H2S

  – NH3 and PH3

  – HF and HCl
                     Goal
• What two factors influence the boiling point
  of a substance?
  The strength of the inter molecular force?
             Which Hydrogens are more
EN H = 2.2
             “electropositive” and why?
EN N = 3.0                           Dipole-Dipole
EN P = 2.1                           attractions do exist
                                     between these polar
                                     molecules


                                      END = 0.1




                              0.8
                        END = ____
More electrons 
           stronger London Dispersion Force




More polar the bond  stronger attractive force between molecules
….so requires more energy to break the attraction and to separate
molecules.
                         Goal
  • What two factors influence the boiling point
    of a substance?
    The strength of the inter molecular force?

• More electrons  stronger London Dispersion Force (so
  requires more energy to break the attraction and to
  separate molecules
• The larger the difference in electronegativity;
• The more polar the bond is  stronger attractive force
• Ionic > H-bonding > LDF & dipole – dipole
             BUT LDF can be greater than dipole -dipole
                                    Outline
• Overview
    – Inter- vs. Intra-molecular Forces
    – Inter-molecular forces and Properties
• Types of inter-molecular forces (Van der Waals)
    – Hydrogen
    – Dipole
    – London dispersion forces
• Properties of Compounds
    –   Viscosity
    –   Boiling and Melting point
    –   Vapor Pressure
    –   Surface Tension
             Bell ringer:
   Predict which has the higher bp?
• CH3Cl     CH4

• CH3OH C2H8


• CH3OCH3          CH2NH2

• CBr4      CCl4       CI4
How do attractive forces influence the
 physical properties of substances?
• Viscosity -

  – Maple syrup
  – Molasses


- A measure of a liquid’s resistance to flow.
- Molecules with strong attractive forces have
  high viscosities.
Demo–which substance is more viscous?
• Isopropyl Alcohol

• Ethylene glycol (antifreeze)

• Glycerin

More hydrogen bonding  stronger attractive forces  higher viscosity



       How does viscosity change with temperature?
                 Viscosity


(a)CCl4 - small, symmetrical molecules
           fairly weak intermolecular forces
           low viscosity.

(b) Octadcane, C18H38 - long molecules
          fairly strong intermolecular forces.
          has a relatively high viscosity.
           London Dispersion Forces
Increases as the number of electrons increase
  (so nonpolar substances can be liquids or solids).
           Intermolecular Forces
Also affected by molecular shape
      coronene – very flat structure (high LDF)
                                    Outline
• Overview
    – Inter- vs. Intra-molecular Forces
    – Inter-molecular forces and Properties
• Types of inter-molecular forces (Van der Waals)
    – Hydrogen
    – Dipole
    – London dispersion forces
• Properties of Compounds
    –   Boiling and Melting point
    –   Viscosity
    –   Vapor Pressure
    –   Surface Tension
• Summary
      Define Vapor Pressure (Demo)
                       Condensation g l




                       Vaporization l  g

 Vapor Pressure – the pressure exerted by vapor
                  when the rate of condensation
                  equals the rate of evaporation.
• Which liquid has higher vapor pressure?
                Acetone has higher vapor pressure
 • Which liquid (A or B) has stronger attractive
   forces?     Water has higher attractive forces
                       Goal
• Explain how vapor pressure is related to
  attractive force.
  – Explain whether molecules with high or low
    attractive forces have high vapor pressures.

• Molecules with strong attractive forces
  evaporate less – so the vapor pressure is low.
• Molecules with low attractive forces
  evaporate easily – so the vapor pressure is
  higher.
How does temperature affect vapor
           pressure?
           Vapor Pressure

pressure of vapor above a liquid at equilibrium
depends on temperature & strength of IMF




                                        v.p.
                                               temp

      temp      v.p.                       IMF v.p.
                       C. Johannesson
Vapor Pressure Of Water




        Vapor pressure increases as temper-
       ature increases. For all liquids. Why?
                                                              Vapor
                                CH3OH
                                                             Pressure
Vapor pressure, mmHg


                       CS2
                                   CH3CH2OH                   Curves
                                                 H2O       Normal boiling
                                                            point: temp at
                                                          which Pvapor = 1 atm

                                                         Which of these liquids
                                                         must have the weakest
                                                           forces of attraction
                                               C6H5NH2   between its molecules?
                                                           How can you tell?

                             Temperature, oC
                 AF Polls
• http://www.polleverywhere.com/free_text_p
  olls/MjIxNzI5MDI
Compare heat of vaporization and
       attractive forces
         States of Matter Compared




       Gas                    Liquid                    Solid
Intermolecular forces   Intermolecular forces   Intermolecular forces
    are negligible.        are significant.      are very important.
              Vaporization
• Vaporization is the conversion of a liquid to
  a gas.
• The heat of vaporization (DHvap) is the
  quantity of heat that must be absorbed to
  vaporize a given amount of liquid at a
  constant temperature.
  Some Enthalpies of Vaporization




Explain why water has a higher Hvap than CS2.
Water has stronger attractive forces than CS2
and thus requires more energy to change
phases from a liquid to a gas.
     Water – unique properties
• Video:
  http://www.learner.org/vod/vod_window.htm
  l?pid=804

• Poll:
  http://www.polleverywhere.com/free_text_p
  olls/MzI3NDI3MzI1
                Water’s Unique Properties




•   Holt Chapter 11 p.389
•   http://www.its.caltech.edu/~atomic/snowcrystals/
                           Freezing water




The pictures on this page are provided courtesy of the MathMol project at the NYU/ACF
Scientific Visualization Laboratory.
             B. Liquid Properties
• Surface Tension
   – attractive force between particles in a liquid that
     minimizes surface area




                           C. Johannesson
Surface Tension




      Molecules within the body of a liquid are
      attracted equally in all directions. Those at
      the surface, however, are pulled downward
      and sideways, but not upward.
http://www.chem1.com/acad/sci/aboutwater.html
                  Cohesion
            http://en.wikipedia.org/wiki/Cohesion_(chemistry)




           Cohesion – how much the molecules stick to
           each other.

           Adhesion – how well the molecules adhere
           stick to something else.

           Capillary action – the ability of water to be
           drawn up a thin tube.


http://upload.wikimedia.org/wikipedia/commons/thumb/6/64/Capillarity.svg/180px-Capillarity.svg.png
             B. Liquid Properties
• Capillary Action
   – attractive force between the surface of a liquid and the
     surface of a solid




                                     water       mercury
                           C. Johannesson

								
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