Intermolecular Forces

Report
Intermolecular Forces
HONORS CHEMISTRY
JANUARY 2011
Corresponds to “Short Notes on Intermolecular Forces Worksheet”
Classifying the molecule
 First classify the molecule:
 Ionic (M+NM) or Covalent (NM +NM)
 Polar or NonPolar for Covalent Molecules
 Then determine the force of attraction:
 Electrostatic Forces – ionic compounds
 Hydrogen Bonding – H bonded to O, N, or F
 Dipole-Dipole Forces – Polar covalent molecules
 London Dispersion Forces (LDF) – NonPolar covalent
molecules
Question 1: What is the force of attraction
between molecules of the following substances?
a) PCl3 – polar covalent compound; therefore dipole-dipole forces
b) NiCl2 – ionic compound - Ni2+ and Cl-; therefore electrostatic forces
c) I2 – nonpolar covalent compound; LDF
d) HF – covalent compound; H is bound to F therefore Hydrogen
e)
f)
g)
h)
Bonding
HCl – covalent compound; H is bonded to Cl therefore dipole-dipole
forces
CH2O – covalent compound with both H’s and O bonded to Carbon.
NOT Hydrogen bonding because C is bonded to O. However, it does
have dipole-dipole forces
H3PO4 – covalent molecule where the H is bonded to the O; therefore
Hydrogen Bonding
BF3 – covalent compound, trigonal planar with the molecules same on
the all side making the molecule nonpolar; therefore LDF
Ranking the Strength of Intermolecular Forces
 In general:
strongest
weakest
Electrostaic
H bonding
dipole-dipole forces
LDF
2. Rank the strength of the following compounds
from strongest to weakest: PCl3, NiCl2, I2, HF
NiCl2 > HF > PCl3 > I2




NiCl2 is an ionic compound with electrostatic forces; therefore
it is the strongest
HF – hydrogen bonds (2nd strongest)
PCl3 – dipole-dipole forces
I2 – LDF (weakest)
Compounds can be ranked within the category of
Electrostatic Forces:
 Compounds made of ions with higher charges will
be STRONGER than ions with lower charges.
 Smaller ions are STRONGER than larger ions.

Ions become larger as you go down the group
3. Rank the following ionic compounds according to the strength of their
electrostatic forces of attraction: NiCl2, Fe2O3, MgO, KI, BeF2, CrCl3
Fe2O3 > MgO > CrCl3 > BeF2 >NiCl2 > KI
 First, look at the charges that make up the compounds.






Fe3+, O2Mg2+, O2Cr3+, ClBe2+, FNi2+, ClK+, I-
 Then notice that MgO is greater than CrCl3 even though +3 > +2.
 Also, notice that NiCl2 and BeF2 have the same charges; therefore you look at
the size. Be and F are in the second period which makes them smaller than Mg
and O (3rd and 4th period). The smaller ions have stronger forces.
Compounds can be ranked within the category of
London Dispersion Forces:
 These forces are due to a temporary dipole; therefore
the molecules with bigger electron clouds will more
easily and more greatly perturbed.
 The number of electrons is related to the number of
protons, which is related to molar mass.
 Nonpolar compounds with higher molar masses will
have STRONGER London Dispersion Forces.
4. Rank the following from strongest to weakest
intermolecular forces: Cl2, CH4, BF3, SCl2, CO2
SCl2 > Cl2 > BF3 > CO2 > CH4
 All of the molecules are nonpolar except SCl2 which
has dipole-dipole forces of attraction.
 Rank the others in order of Molar Mass.




Cl2  70.91 g/mol
BF3  67.81 g/mol
CO2  44.01 g/mol
CH4  16.05 g/mol
Ranking Physical Properties
 Predictions about various physical properties can be
made based on the strength of the intermolecular
forces.





Solubility
Melting Point
Boiling Point
Vapor Pressure of Pure Substances
Viscosity and Surface Tension
Solubility
 Forces between the solvent and the solute must
overcome the solute-solute and solvent-solvent
forces. Most effective if forces are similar

“Like dissolves like.”
Example: Polar molecules can dissolve into polar solvent.
 Example: Nonpolar molecules can dissolve into nonpolar solvent.

 Determine solubility of ionic compounds from the
solubility rules in book/reference sheet.
 Water is most common solvent. It is polar.
5. Which of the following compounds are likely to
dissolve in water?
a) SCl2 – polar molecule; therefore YES dissolves in water
b) O2 – nonpolar molecule; NOT soluble in water
c) NaCl – ionic compound. It has low charges (Na+ and Cl-)
and according to the solubility rules “group I cations are
always soluble”; therefore YES soluble in water.
d) CO2 – nonpolar; NOT soluble
e) PH3 – polar molecule; YES dissolves in water
6. Which of the following will dissolve in C6H14
(hexane)?
Hexane is a nonpolar solvent; therefore, nonpolar
molecules will dissolve in this solvent
a) SCl2 – polar molecule; NOT soluble in hexane
b) O2 – nonpolar molecule; YES soluble in hexane
c) NaCl – ionic compound; NOT soluble in hexane
d) CO2 – nonpolar; YES soluble in hexane
e) PH3 – polar molecule; NOT soluble in hexane
Melting Point
 For a substance to melt, kinetic forces (increased by
increasing temperature) must overcome the IMF’s
keeping the particles in a fixed position.
 The stronger the intermolecular force, the higher the
melting point.
 Remember that a freezing point has the same value
as a melting point and can be determined exactly the
same way.
7. Rank the following substances from highest melting point
to lowest melting point: KNO3, H2O, N2O, NaCl, F2, Cl2
NaCl > KNO3 > H2O > N2O > Cl2 > F2
 Determine the type of IMF in each molecule then use that information
to arrange from high to low melting points:




Electrostatic forces in the salts: KNO3 and NaCl. Smaller ions have
stronger forces of attraction, so NaCl is smaller and therefore has a
higher melting point.
LDF in F2 and Cl2  lowest melting points. LDF’s stronger with larger
molecule; therefore, Cl2 has a higher melting point than F2.
Water hydrogen bonds. That makes it weaker than the salts but stronger
than the halogens.
N2O is a polar molecule with dipole-dipole forces.
Boiling Point
 Boiling points are always higher than melting point
for any one substance.
 The method for ranking different substances is the
same as that for melting points: the stronger the
IMF forces require higher temperature to escape the
attraction and become a gas (boil).
 Ranking assumes constant atmospheric pressure.
Lower atmospheric pressure decreases the b.p. of
any substance.
Boiling Points & Bond Types
8. Rank the following substance from highest boiling point to
lowest boiling point: CaS, HNO3, KBr, SO2, XeF4
CaS> KBr > HNO3 > SO2 > XeF4
 Determine the IMF’s to rank b.p.:
 CaS and KBr are ionic compounds  electrostatic forces. CaS will
have stronger forces because they have a higher charge of 2 than KBr
with a charge of 1.
 HNO3 will hydrogen bond (H – N)
 SO2 is a polar covalent molecule; therefore, dipole-dipole bonds.
 XeF4 is a nonpolar covalent molecule; therefore, LDF.
Vapor Pressure
 Vapor pressure is determine by the number of
molecules that can escape from the surface of a
liquid.


More molecules can escape if the force of attraction is weak or
if the force pulling them away (KE measured by temp) is high.
Highest vapor pressure has the weakest IMF’s
Temperature vs Vapor Pressure Curve
9. Rank the following substances from highest vapor pressure
to lowest vapor pressure: Br2, NH3, Ar, PCl3, PCl5
Ar > Br2 > PCl5 > PCl3 > NH3
 First look at type of IMF:

NH3 hydrogen bonds
Br2 and Ar have LDF because Br2 is nonpolar and Ar has no bonds.
PCl3 is polar.

PCl5 is nonpolar.


 Look at molar mass of nonpolar molecules:

PCl5 (208.5 g/mol); Br2 (159.8 g/mol); and Ar (39.95 g/mol)
 Rank – highest vapor pressure has weakest IMF’s
Viscosity and Surface Tension
 Depends on both temperature and IMF’s of the
substance.


Increase the temperature decreases both viscosity and surface
tension.
At any give temperature, the substance with stronger IMF’s
has greater viscosity and higher surface tension.
10. Rank the following substance from highest vapor pressure
to lowest vapor pressure: Br2, H2O, CCl4, PH3
H2O >PH3 > Br2, > CCl4
 Liquids with stronger IMF’s have higher viscosity.
11. Rank the following substance from highest vapor pressure
to lowest vapor pressure: Br2, H2O, CCl4, PH3
H2O >PH3 > Br2, > CCl4
 Liquids with stronger IMF’s have higher surface
tensions.

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