Chapter 11 Intermolecular Forces and Liquids and Solids

Properties of Liquids and Solids. Intermolecular Forces

Intermolecular Forces

Dipole Moment

Remember that Fluorine is has the highest value of electronegativity, χ? Due to electronegativity difference, Δχ, b/w two atoms, bond would polarize.


If the Δχ = 0, then the bond is said to be non-polar.
If the Δχ is non-zero, the bond is polar.

  So for molecule N=O, we have
As it is seen, arrow indicates the dipole moment of the bonds between N and O, and the arrow points toward more negative side, indicated with δ-, and the other end is more positive side, δ+. This is so because oxygen is closer to F in the periodic table than nitrogen.

 

 

  Adding all Polarities of bonds molecular dipole moment, μ

The bond dipole moments of O-H bonds (red arrow) in water points toward oxygen atom. When these two bond dipoles are added together, molecular dipole moment (blue arrow) is produced. As in the bond dipole, the direction the arrow is pointing has partially negative charge, and the opposite end has partially positive charge.

 

 

 

 

Example: Draw in polar bonds with arrows to show the bond dipole moments and molecular dipole moment of dimethyl ether (CH3O CH3), tetrafluoromethane (CF4) and HCOCH2CF3.

 

 

  In order to deduce dipole moments of molecule is to know the 3-D structure of the molecule. Then, draw in the bond dipoles (red arrows). We consider C-H and C-C bonds to be non-polar bonds, therefore we don't draw in the arrow for these bonds. After bond dipoles are drawn in, all bond dipole vectors are added, and the resulting vector is the molecular dipole moment (blue arrow ).

For CF4, two of the red bond dipoles form a resultant green dipole in the plane of the computer monitor. For other two bond dipoles, with orange color, form another green resultant pointing in the opposite direction. And, they cancel each other, therefore the moleuclar dipole moment of CF4 is zero.

 

 

 

Bond dipole is a Very Important Concept!! You'd have easier time knowing bond dipole and moleucular dipole moments in organic chemistry, Yes, in organic chemistry!

 

 

Intermolecular Forces

Forces b/w molecules

Forces responsible for condensing gas into liquid and liquid into solid

Forces responsible for making different compounds to have different melting point and boiling point.

Four types:

  • Ion-Dipole Interaction
  • Dipole-Dipole Interaction
  • Dispersion (London force, induced dipole interaction, or instantaneous dipole)
  • Hydrogen Bond

     

     

  • Ion-Dipole Interaction
  • Force exits between ion and molecule with permanent dipole moment. Cation is attracted to the partially negative side of the polar molecule, and anion is attracted to the partially positive end of the polar molecule.

    Water surrounds Na+ ion in a) and F- ion surrounded by water in b). The red sphere in water is the oxygen atom. Notice that the oxygen is pointing toward Na ion, and the hydrogen atoms point toward F ion.

     

     

  • Dipole-Dipole Interaction
  • Interaction b/w two molecules with permanent dipole moments. The partially positive charge side is attracted toward partially negative side, and vise versa.

     

    So, if you have a bunch of N=O molecules, you'd see the packing of molecules as shown on the left diagram in the figure below. On the right diagram negative-negative interactions and positive-positive interactions, both represented by green arrows, are the repulsive interaction. So the structure is unstable. It'd eventually become the same packing as on the left.

     

     

     

     

     

     

    Dipolar vs non-dipolar interactions

    N2NOO2
    Dipole(debye)00.1530
    Mass (g/mol)283032
    Boiling pt(K)7712190

     

     

  • Dispersion Interaction
  • Consider non-polar non-ionic molecule A approaching non-polar non-ionic molecule B (top diagram on left).

    As they are close to each other, B sees the electrons on A, and the electrons on the left-side of B starts to avoid the approaching A (lower diagram on left). This induces polarity on molecule B, which in turn can polarize A as well. This is the source for the attractive force between non-polar non-ionic molecules.

    If the molecule is easy to deform → said to be Polarizable.

    Halogenmp (K)bp (K)
    F253.585.0
    Cl2171.6239.1
    Br2265.9331.9
    I2386.8457.5
    As seen in Table above, both the melting point and boiling point are higher for molecules with higher polarizability, hence having stronger intermolecular force. Heavier the molecule → more polarizable. I

     

     

     

     

     

  • Hydrogen Bonding
  •  

      Sharing of hydrogen between two molecules

  • One must be a Hydrogen donor -- possessing H
  • One must be a Hydrogen acceptor -- possessing at least one pair of lone pair electrons

    Each molecule must act as both the donor and acceptor.

    The picture on the right is a dimer of water. The molecule on left is the H-donor and the one on right is the acceptor.

    The shared hydrogen is co-linear with the lone pair-H-donor.

    The binding energy for water dimer is quite small (3.4 kcal/mol). But, this is the force holding two strands of DNA.

    Comparison of the strength of intermolecular forces

    ForceStrength in kJ/molStrength in kcal/mol
    Ion-Dipole10 - 50 kJ/mol 2 - 12 kcal/mol
    Dipole-Dipole3 - 4 kJ/mol 0.5 - 1 kcal/mol
    Dispersion1 - 10 kJ/mol 0.2 - 2 kcal/mol
    H-bond10 - 40 kJ/mol 2 - 10 kcal/mol

     

     

    Example: What is the most important intermolecular force exit in each of the following liquid? Can you draw two molecules for each and show how they are aligned? 1) Liquid ammonia (NH3, 2) C6H14, and 3) Formaldehyde (COH2)
    1) Liquid ammonia, NH3, has lone pair electrons and H, therefore it can H-bond to each other. The two molecules can interact in the following way.

    2) C6H14 (hexane) is non-polar and elongated molecule. So, it only has dispersion interaction. In order to liquify, two molecules must be aligned sideways as shown below.

    3) Formaldehyde is a dipolar molecule. The dipole-dipole interaction entails lining up the two dipole moments as follows:

     

     

     

    Liquid Properties

    Surface Tension Energy needed to increase surface area (Cohesive energy)

    The red dashed line is the meniscus you read for measurements, like measuring the volume of water with graduated cylinder.

    Mercury side = Convex H2O side = Concave
    Hg Intxn with glass is weak H2O Intxn with glass is strong
    Cohesive Energy of Hg is smaller than intxn b/w Hg and glass Cohesive Energy is larger than intxn b/w H2O and glass

  • Clean waxed car → water beads up -- cohesion (or surface tension) wins
  • Dirty car → water spreads -- cohesion (or surface tension) wins

     

     

     

     

    Crystal Structure

    Two types of solids
  • Crystaline - ordered unit cells (repeating unit cell pattern) -- metal crystal, salt crystals
  • Amorphous - no long-range order -- glass, polymers

     

    Types of Crystaline Solids

    TypesIntermolecular ForcesPropertiesExamples
    IonicIon-ion forceBrittle, hard, high-meltingNaCl, KBr, MgCl2
    MolecularDispersion, dipole-dipole, H-bondSoft, low-melting, non-conductingH2O, Br2, CO2, CH4
    Covalent networkCovalent bondsHard, high-meltingdiamond, SiO2
    MetallicMetallic bondVariable hardness and melting points, conductingNa, Zn, Cu, Fe

     

     

     

    X-Ray Diffraction

    X-ray is used to determine the structure of crystal -- very important tool in Biochemistry -- Protein structures are determined this way. The structure of DNA was discovered by this technique.

    Above diagram shows how the experiment is carried out. X-ray is irradiated onto a crystal. Then the diffraction pattern is photographed. This is the diffraction pattern of one form of DNA. As can be seen, the structural parameters are obtained from the diffraction pattern.

     

     

    The red arrows from left to fall on atoms indicates incident X-ray, and the red arrows going away from atoms are the diffracted (outgoing) X-ray. The incident rays with the wave length, λ, on the 2nd layer must travel extra distances covered by the lengths, C-B and B-D. The diffraction pattern only appears when the extra distances, C-B and B-D are integral multiples of λ

    By using trigonometry, one can show the extra distnaces traveled is:

    CB=BD=dsinθ

    Then, according to Bragg, the distance C-B and B-D is related to integral multiple of λ, thus one obtians the relationship between incident X-ray wave length and the structural parameter, d is given as

    d= nλ 2sinθ

    This is called Bragg equation.

     

     

     

    Crystal Types

    Crystalline solids have repeating unit → Unit Cell.

     

    The repeating pattern of unit cells determine the structure of crystal.

     

    Crystal packing

    Crystal packing shows how the atoms are arranged in a crystal. There are four types:

    Simple Cubic
    Body-Centered Cubic
    Hexagonal Closest-Packing
    Cubic Closest-Packing

    Unit Cells

    Within the packing pattern, one can consider a unit of repeating cells, called unit cell, which contains fewest possible atoms. For example, simple cubic packing pattern can be considered to have a unit cell with the edge length a and contains 1/8 of atoms at each corner, which means there are 1 atom in the unit cell, as shown below. There are two other types of unit cell within the so-called simple cubic unit cell.

    3 types:

    Primitive Cubic
    Body-centered Cubic
    Face-centered Cubic

    In the following diagram, the relationship between the edge length and atomic radius of monoatomic solid that packs in a simple cubic cells.

     

     

     

    Example: What is the density of tungsten (W) in g/cm3 which assumes body-centered cubic packing? The atomic radius of W is 1.39 Å.


    Using the above diagram, we can get the edge length, a, from the atomic radius, r, by using the relationship shown in the diagram above, and it is a= 4r 3 Therefore, a = 4(1.39 Å)/√3 = 3.210 Å. So the volume of the unit cell is (3.210 Å)3.

    We need the mass of W atoms in the unit cell. We can obtain it by counting the number of atoms in the cell and convert them into g, using Avogadro's number.

    There are two W atoms in the cell. Then, ? g W = 2 W ( 1mo l W 6.022× 10 W 23 )( 183.84 g W 1mo l W )=61.0× 10 g W 23 The volume is (3.210 Å)3, but it needs to be converted to cm3. ?c m 3 = ( 3.210 A ) 3 ( 1m 10 10 A ) 3 ( 100cm 1m ) 3 =33.08× 10 24 c m 3 The density, then is, d= m V = 61.0× 10 g W 23 33.08× 10 24 c m 3 =18.4g/c m 3 The measured density is 19.25 g/cm3.

    Allotropes = same formula with different structural forms

    Carbon has 40 allotropes! Graphite (graphene → Nobel Prize for synthesizing), Bucky ball (Nobel for determining structure), Bucky tubes, diamond, etc.

    Phase Changes

    Phase Change

    A few names associated with phase change:
    Fusion (melting) solid → liquidΔ H >0
    Freezing liquid → solidΔ H < 0
    Vaporization liquid → gasΔ H > 0
    Condensation gas → liquidΔ H < 0
    Sublimation solid → gasΔ H > 0
    Deposition gas → solid Δ H < 0

    This is a plot of heating curve of water. Plotted is the temperature of water by adding more heat (energy) to the water. Notice there are two plateaus at melting point and at boiling point.

    In order for you to make 100° liquid H2O to 100° gas H2O (steam), it needs about 40 kJ/mol of energy.

    This is the heat of vaporization, ΔHvap = 40.67 kJ/mol.

    Similarly, you see less than 10 kJ/mol is needed to melt ice into liquid → heat of fusion, ΔHfus = 6.01 kJ/mol

     

     

     

     

     

     

     

    Liquid Vapor Equilibrium

    Vapor Pressure ← Dyanmic equilibrium b/w liquid and gas

     

     

     

      Over the surface of liuquid bromine, you see vapor of gaseous bromine. Liquid and gas establish a so-called dynamic equilibrium.

     

     

    Ratein = Rateout

     

     

    Rate of Br2(g) goes in to the liquid is equal to the Rate of Br2(l) becoming gas.

     

     

    Higher the temperature → Higher the vapor pressure

     

    Higher T → Larger EK → Easier to overcome intermolecular intxns.

     

    T > 0 K, there is always vapor exists over solid or liquid.

     

     

    Clausius-Clapeyron Equation


    If you plot Pvap as a fxn of T, you get a picture on the left

     

     

    But if you plot ln(P) vs. 1/T, instead, the curve becomes linear!

     

      Linear equation is easier to work with!!!

     

      The relationship b/w ln(P) and 1/T is,

     

     

     

      ln( P 2 P 1 )= Δ H vap R ( 1 T 2 1 T 1 ) In this equation, R = 8.314 J/molK is used. It means that for T, we should be using Kelvin unit. For pressure, you can use whatever the unit as long as P1 and P2 have the same unit. Δ Hvap is the heat of vaporization (for water, it is 40.67 kJ/mol as we discussed above).

    Example:Denver is known as Mile High City. If the barometric pressure is measured to be 0.837 atm, what is the normal boiling point of water in Brooklyn (sea level)?

     

    We know water boils at 100°C at sea level at which barometric pressure is 1.00 atm. Let's assign P1 = 1.00 atm and T1 = 100°. So, we are solving for T2. Then,

      R Δ H vap ln( P 2 P 1 )= 1 T 2 1 T 1 1 T 2 = R Δ H vap ln( P 1 P 2 )+ 1 T 1 T 2 = 1 R Δ H vap ln( P 1 P 2 )+ 1 T 1

    Substitute #'s in. T 2 = 1 8.314 J molK 40.67× 10 3 J mol ln( 1.00atm 0.837atm )+ 1 373.15K = 1 3.6374× 10 5 /K+2.6799× 10 3 /K = 1 2.7163× 10 3 /K

    T2 = 368.15K, or 95.0°C.

     

     

    Phase Diagram

    Phase diagram shows phases of substance as a function of temperature and pressure.

     

      Each solid lines represent phase boundary. For example, the line in the diagram with an arrow is a phase boudnary b/w solid and liquid.

     

      We know:

  • At relatively high pressure and low temperature, substance is solid

  • Keeping the pressure relatively high, and increase the temperature to higher, substance becomes liquid

  • If the temperature is further increased liquid becomes gas

     

      So, the left-most (green region) is solid, middle part (pink region) is liquid, and bottom part (yellow region) is gas.

     

     

    Important points and processes on phase diagram

     

      Few important Points on the Phase Diagrams:

  • Normal Melting point = melting point measured at 1 atm
  • Normal Boiling point = boiling point measured at 1 atm
  • Triple Point = all three phases co-exists
  • Critical point = Can't be condensed to liquid by pressurizing further; Can't be vaporized with higher temperature

     

      Supercritical Fluid is used to decaffinate coffee beans.

     

      Important Processes:

  • 1 = Fusion (melting) ⇆ Freezing
  • 2 = Vaporization ⇆ Condensation
  • 3 = Sublimation ⇆ Deposition