CHEM 200/202 Professor Byron W. Purse Office: virtual only this semester - SDSU Chem 200/202

CHEM 200/202
          Professor Byron W. Purse
                bpurse@sdsu.edu
        Office: virtual only this semester

    All course-related emails are to be sent to:
             chem200@sdsu.edu

   My office hours will be held on Zoom on
Monday from 3:00 to 5:00 PM. Appointments can
 be made for special issues, but not one-on-one
                    tutoring.

PROBLEM
 Small metal cylinders, filled with 16.0 g of compressed CO2(g)
are often used by bicyclists to inflate flat tires. If such a cylinder
has a volume of 25.0 mL, and can fill a 1.25 L bicycle inner tube
  to a pressure of 7.05 atm at 22.4°C, what is the pressure of
   CO2(g) inside a sealed cylinder at the same temperature?

QUESTION
 Pressure is a measure of force per unit area. What
does this imply regarding the velocity of gas particles?
      Answer
      A •The velocity of gas particles is independent of the
        mass of the particle.
      B •The velocity of gas particles is directly proportional to
        the mass of the particles.
      C •The velocity of gas particles is inversely proportional
        to the mass of the particles.
      D •All of the above statements are true.
      E •None of the above statements are true.

QUESTION
A sample of an ideal gas is heated in a steel container from
 25°C to 100°C. Which quantity will remain unchanged?

     Answer:
       A The average kinetic energy of the gas
          particles.
        B The collision frequency.
       C The density.
       D The pressure of the gas.

QUESTION
      At very high pressures (~1000 atm) the measured
      pressure exerted by a real gas is greater than that
           predicted by an ideal gas. Why is that?
Answer:
  A
   B •Because it is difficult to measure high pressures accurately.
  C •Because real gases will condense to form liquids at that pressure.
     •Because gas phase collisions prevent the molecules from colliding with the
  D container walls.
     •Because of the attractive intermolecular forces between the gas molecules.
   E •Because the volume occupied by the gas molecules becomes significant.

CHAPTER 10.1
INTERMOLECULAR FORCES

LECTURE OBJECTIVES
•   Chapter 10.1

    •   Identify, describe, and compare the different intermolecular forces

•   Chapter 10.2

    •   Distinguish between adhesive and cohesive forces.

    •   Define viscosity, surface tension, and capillary rise.

    •   Describe the roles of intermolecular forces in the above
        properties/phenomena.

VARIATIONS IN COVALENT INTERMOLECULAR BONDS

ION-DIPOLE
INTERACTIONS
•The attraction between the
ions and the dipole of the
water is greater than the
attraction between
oppositely charged ions.
•This allows ionic substances
to be dissolved in polar
solvents.

THE HYDROGEN
        BOND
• Hydrogen  bonds are a dipole-
 dipole intermolecular force.

• Hydrogen   bonds can occur when
 a H atom in a molecule is bound
 to a small, highly electronegative
 atom, with lone pairs of electrons.

• The H from one of these
 molecules is attracted to the lone
 pairs of the electronegative atom
 on another molecule.

• Typically
         formed with H bond to
 N, O or F.

HYDROGEN BONDS IN WATER

                    Hydrogen
                   bond donor
                    Hydrogen
                  bond acceptor

QUESTION
Which of the following compounds forms
    intermolecular hydrogen bonds?
                   Answer:
                CH4 A
           CH3CH2SH B
          CH3CH2OH C
                PH3 D

HYDROGEN BONDING &
          BOILING POINT
• Boiling
        points increase
 with the mass of the
 substance.

• Without  hydrogen
 bonding water would
 boil at about -80°C.

• StrongH bonding
 necessitates more
 energy to separate the
 molecules in order to
 form a gas .

HYDROGEN BONDING IN BIOLOGY
• Hydrogen bonding is crucial in the function most biologically
 important molecules.

• Thedouble-chain structure of DNA is a result of hydrogen
 bonding.

POLAR MOLECULES & DIPOLE-
         DIPOLE FORCES
  Molecules that are polar, but do not necessarily form
hydrogen bonds (e.g. CH2Cl2), are able to influence each
          other when in liquid or solid form.

        Liquid                        Solid

DIPOLE MOMENT & BOILING POINT

POLARIZABILITY & CHARGE-INDUCED
            DIPOLE FORCES
• Polarizability   increases down a group.             Distortion of an
    • The larger electron clouds, further from the     electron cloud.
      nucleus are easier to polarize.

• Polarizability   decreases left to right across a
                                                                O2(g)
 period.

    • Increased     Zeff results in smaller electron
      clouds.

• Cations   are less polarizable than their parent                 O2(g)
 atom.
                                                       Na+(g)
• Anions    are more polarizable than their parent
 atom.

DISPERSION FORCES AMONG
  NONPOLAR MOLECULES
• Nonpolar compounds can
 gain temporary polarity as
 their electrons move.

• This polarity can influence
 the polarity of other
 compounds when they
 are close to each other.

• Dispersion (London)
 forces are important in
 the attraction of all
 substances.

DISPERSION FORCES & BOILING POINT
  The structure of a molecule will influence the
  forces that act upon it.
  Two different forms of C5H12 have different
  extents of dispersion forces, thus different
  boiling points.

MOLECULAR MASS & BOILING POINT

• Increased molecular mass correlates
 with an increase in the number of
 electrons.

•A larger number of electrons allows
 for greater polarizability.

• Heavier atoms, or molecules with
 more atoms, have greater dispersion
 forces and higher boiling points.

SUMMARY OF
INTERMOLECULAR FORCES

NON-COVALENT INTERACTIONS IN
               PROTEINS
Ionic bonding   Hydrogen bonding   Dispersion forces

QUESTION
   Rank the compounds below in order of
 increasing melting points (1=highest). Also
determine the dominant intermolecular force
    for each compound in a pure solution.

                   Rank       Intermolecular
  Compound
                (1=highest)        force

     Na2O            1            Ionic

     C3H8            4          Dispersion

                                Hydrogen
    C2H5OH           3
                                 bonding

      NaI            2            Ionic

PROPERTIES OF THE LIQUID
         STATE
•   Surface tension

•   Capillarity

•   Viscosity

THE MOLECULAR
     BASIS OF SURFACE
         TENSION
•   For a liquid, the surface molecules
    interact with fewer molecules than
    those “inside” the liquid.

•   Surface molecules have net a
    downward attraction due to the
    intermolecular forces.

•   Pushing into the liquid increases the
    total surface area, requiring energy to
    disrupt more of the attractive forces.

SURFACE TENSION &
INTERMOLECULAR FORCES
                           Surface tension
 Substance        Formula       2          Major force(s)
                           (J/m ) at 20°C
                                      –2   dipole-dipole:
Diethyl ether   (CH3CH2)2O    1.7×10
                                             dispersion
  Ethanol       CH3CH2OH        2.3×10–2      H bonding
                                              H bonding;
  Butanol       CH3(CH2)3OH     2.5×10–2
                                              dispersion
   Water           H2O          7.3×10–2      H bonding
                                               Metallic
  Mercury           Hg          48×10–2
                                               bonding

CAPILLARY ACTION
• The meniscus (top layer) of water water in
 a thin tube is curved downward, whereas
 the meniscus for mercury is curved
 upwards.

• Water can hydrogen bond (adhesive
 force) with the glass (SiO2) and forming
 these bonds pulls some water up the glass
 walls.

• Hg forms metallic bonds which are much
 stronger than any dispersive forces with
 SiO2. The Hg pulls away from the glass
 surface and tries to form as many metallic
 bonds (cohesive force) as possible.

VISCOSITY
• Theviscosity of a solution is related to how easily the
 molecules in the liquid can move past each other.

• Higher temperatures increase molecular velocity, making it
 easier to overcome the attractive forces.

• The  shape of the molecules also influence the viscosity: small
 spherical molecules do not interact strongly with each other
 (e.g. CCl4), larger molecules interact more strongly with
 each other, restricting their movement.

VISCOSITY OF A POLYMER IN SOLUTION

QUESTION
 Which of the following liquids would you
expect to have the lowest surface tension?
           Answer:
             A Pb
              B CH3OCH3
             C HOCH2CH2OH
             D H2O
              E CH3CH2OH