Intermolecular Forces and Phase Change
For the following compounds,
4.
5.
6.
7.
1. Write a condensed structural formula
2. Draw an organic line drawing
3. Identify the type of functional group
Cond. Struct. Formula
2-methyl propane
butane
methyl ethyl ether
Line Drawing
CH3–CH–CH3
|
CH3
CH3–CH2–CH2–CH3
CH3–O–CH2–CH3
O
Indicate the type of intermolecular forces in pure liquid
Rank (1-7) from lowest to highest boiling point
Rank (1-7) from lowest to highest heat of vaporization
Rank (1-7) from lowest to highest solubility in water
Functional Grp
Intermolecular F
b.p.
rank
ΔHvap
rank
Solub.
rank
Alkane
Ind. Dipole-Ind.
Dipole
1
1
3
Alkane
Ind Dipole-Ind Dipole
2
2
2
Ether
Ind Dipole-Ind Dipole
+ Dipole-Dipole
3
3
4
Alkane
Ind Dipole-Ind Dipole
5
4
1
Ketone
Ind Dipole-Ind Dipole
+ Dipole-Dipole
4
5
5
Alcohol
Ind Dipole-Ind Dipole
+ H-bonding
6
6
7
Alcohol
Ind Dipole-Ind Dipole
+ H-bonding
7
7
6
hexane
CH3(CH2)4CH3
propanone (acetone)
2-propanol (isopropanol)
2-pentanol
CHEM 131
O
||
CH3–C–CH3
O
OH
|
CH3–CH–CH3
OH
|
CH3CH(CH2)2CH3
OH
OH
IMF’s & Phase Change
Page 1
Note: Electronegativities and Dipole Moments are listed on last page
For problems 1-3, choose from the following compounds:
a) CH3(CH2)3OH
b) (CH3) 3CH
c) HCl
d) CH3CN
1. Which has the strongest London forces? CH3(CH2)3OH, largest MW and favorable shape
2. Which has the strongest hydrogen-bonding? CH3(CH2)3OH, only one w/ H bonding
3. Which has the strongest dipole-dipole forces? CH3CN, highest dipole moment
For problems 4-6, choose from the following compounds:
a) CO
b) BaCl2
c) H2
d) HF
e) Ne
4. Which has the highest boiling point? BaCl2, ionic bonding
5. Which has the lowest boiling point? H2, only dispersion forces and smallest MW
6. Of choices d or e, which has the higher boiling point? HF, similar MW and has H-bonding
7. Consider the following substances: CH3(CH2)2CH3, (CH3) 3CH, H2O, CH3Cl
which has the strongest London forces? CH3(CH2)2CH3 most favorable shape of high MW
which has the strongest hydrogen-bonding? H2O
which has the strongest dipole-dipole forces ? CH3Cl
8. Which of the following substances would have the greatest surface tension at 25°C?
b) CH3Cl
a) CH4
c) CH3CH2CH3
d) CH3OH (H-bonding)
9. Arrange the following in order of lowest to highest boiling point:
i) CH3(CH2)4CH2Cl
__ii___
__iii___
Lowest b.p.
Chem 131
ii) CH3CH2Cl
iii) CH3(CH2) 4CH3
___i__
__iv__
Æ
Highest b.p.
IMF’s & Phase Change
iv) CH3(CH2)4CH2OH
Page 2
10. The vapor pressure above a liquid in a closed container can be increased by
a)
b)
c)
d)
increasing temperature
increasing external pressure
adding a solute
adding more liquid
11. The boiling point of water on top of Mt. Tam is ___________ that at College of Marin.
a)
b)
c)
d)
higher than
lower than
equal to
not related to
12. Increasing the total pressure above a liquid will cause the boiling point of the liquid to
a) increase
b) decrease
c) remain the same
d) depends on the liquid
13. The temperature at which the vapor pressure of a liquid is equal to 1.0 atm is called the
normal boiling point.
Electronegativities: H 2.1, C 2.5, O 3.5, Cl 3.0, F 4.0
Dipole Moments of various compounds:
CH3(CH2)2CH3 0.1 D
H2O 1.85 D
CH3Cl 1.90 D
CH3(CH2)3OH 1.66 D
(CH3)3CH 0.13 D
HCl 1.08 D
CH3CN 3.9 D
Chem 131
IMF’s & Phase Change
Page 3
(a) Identify the types of intermolecular forces present in each substance.
CS2: Dispersion (Ind. Dipole)
CH3CH2OH: Dispersion + H-bonding
C7H16: Dispersion
(b) What is the vapor pressure of ethanol at 60°C? 360 mm Hg
(c) Considering only carbon disulfide and ethanol, which has the stronger intermolecular forces
in the liquid state?
Ethanol, since its vapor pressure is always lower at the same temp.
(d) At what temperature does heptane have a vapor pressure of 500 mm Hg? ~84°C
(e) What are the approximate normal boiling points of each of the three substances?
CS2: 46°C
CH3CH2OH: 78°C
C7H16: 99°C
(f) At a pressure of 400 mmHg and a temperature of 70°C, is each substance a liquid, a gas, or a
mixture of liquid and gas?
CS2: gas
CH3CH2OH: gas
C7H16: liquid
(g) If 1.0 L of ethanol (0.785 g/cm3) is placed in a 30°C room that is 2.0 m x 2.0 m x 3.0 m, how
much of the ethanol will evaporate?
From the graph, we can see that the vapor pressure of ethanol at 30°C is about 80
mmHg. This means that 80 mmHg (i.e., 80 torr) is the maximum partial pressure of
Chem 131
IMF’s & Phase Change
Page 4
ethanol that could be created in the gas phase; at this point, the liquid would stop
evaporating. Therefore, we can use the ideal gas equation to determine how many
moles of ethanol are needed to create this partial pressure in the given sized room, and
compare it to the number of moles of ethanol actually available.
V = 200cm x 200cm x 300cm = 12,000,000cm3 = 1.2 x 107 mL = 1.2 x 104 L
⎛ 1 atm ⎞
⎟⎟ = 0.105 atm
PEtOH = 80 mmHg ⎜⎜
⎝ 760 mmHg ⎠
n EtOH
(
)
0.105 atm 1.2 × 10 4 L
PEtOH V
=
=
= 50.8 mol CH 3CH 2 OH
L ⋅ atm
RT
(303K )
0.08206
mol ⋅ K
Using the molar mass and density of liquid ethanol, we can determine how many mL of
liquid ethanol would contain this many moles:
⎛ 46.07 g
50.8 mol CH 3 CH 2 OH ⎜⎜
⎝ 1 mol
⎞⎛ 1 mL ⎞
⎟⎟⎜⎜
⎟⎟ = 2980 mL = 3.0 L
⎠⎝ 0.785 g ⎠
In other words, we could evaporate up to 3.0 L of liquid ethanol at this temperature
before reaching the saturation point (i.e., the equilibrium vapor pressure).
Therefore, all (1.0 L) of the ethanol will evaporate.
(h) Will the room become warmer or colder as a result of the evaporation? How much energy is
involved in the exchange?
Evaporation of any liquid is an endothermic process; it requires an input of energy.
This energy will be extracted from the surroundings, thereby causing the room to
become colder.
The amount of energy required can be determined from the enthalpy of vaporization of
the material*
⎛ 1000 mL
1.0 L CH 3CH 2 OH ⎜⎜
⎝ 1L
⎞⎛ 0.785 g
⎟⎟⎜⎜
⎠⎝ 1 mL
⎞⎛ 1 mol ⎞⎛ 41.7 kJ
⎟⎟⎜⎜
⎟⎟⎜⎜
⎠⎝ 46.07 g ⎠⎝ 1 mol
⎞
⎟⎟ = 710 kJ
⎠
Note that if there was little other than air in the room, this amount of heat transfer
would actually drop the temperature of the room quite significantly due to the low
specific heat capacity of air (1.0 J g-1 K-1 = 0.0013 kJ L-1 K-1), and would therefore
reduce the amount of ethanol evaporated due to the lower temperature.
*note that the ΔHvap of ethanol is listed on page A-16 of the textbook as 38.6 kJ/mol, but this is at the boiling
point of ethanol (78°C). The ΔHvap at 25C can be determined as 41.7 kJ/mol from the heats of formation for
liquid and gaseous ethanol, which are listed in Appendix L. Alternatively, a value can be determined by
applying the Clausius-Clapeyron equation.
Chem 131
IMF’s & Phase Change
Page 5