Pirate Chemistry
Effects of Intermolecular Forces
In the last section we learned how to identify what kind of intermolecular forces are present in a
molecule. We also learned how strong those IMFs are. So what? What does this have to do
with anything? How can we apply this knowledge to either make predictions or explain observable properties?
We can do both of those things if we apply the idea of intermolecular forces to a variety of
situations.
Boiling Point
When a substance boils (goes from the liquid state to the gaseous state) the molecules go from
being very close together as a liquid to being separated at a distance as a gas. If the molecules
are attracted to each other, it will be much more difficult to separate them; thus the boiling point
should go up.
To go from a liquid to a gas, the liquid particles have to be heated
until they vibrate fast enough to break away from each other completely and zip around their container bumping into each other and
the walls. If the particles are attracted to each other because of
IMFs, this will require more energy and make the boiling point
increase.
This table shows the boiling points of various alkanes, substances that only contain carbon and
hydrogen. Because the electronegativity value of C is 2.5 and H is 2.1 and therefore the difference is only 0.4, the molecules are always non-polar. All the molecules listed in the table below only contain London Forces. For London Forces, you get an increase in the strength of the
IMF as the molecules get bigger.
Compound
IMF
Formula
Mass Boiling Point
Methane
London
CH4
16
-161oC
Ethane
London
C2H6
30
-88 oC
Propane
London
C3H8
44
-42 oC
Butane
London
C4H10
58
0 oC
You can see that as the mass increases, so
does the boiling point.
All text copyright Chris Smith 2009. All pictures obtained from internet and are copyright of their owners but assumed to be public accessible. If you are the owner of a picture and want it removed, email csmith@d211.org
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This next table shows similarly sized molecules. Thus they have roughly the same amount of
London forces. However, the examples given show wildly different boiling points because the
molecules exhibit different strength of intermolecular forces. Remember that:
Strength of IMFs:
London Forces < Dipole Forces < H-bonds
Molecules with stronger IMFs tend to have higher boiling points because their intermolecular
forces are stronger; the molecules ―stick‖ together stronger and are harder to pull apart.
Compound
IMF
Formula
Butane
London
C4H10
58
0 oC
Propanal
Dipole
C3H6O
58
49 oC
60
97 oC
Propanol
H-Bonds C3H7OH
Mass Boiling Point
All of these molecules have almost identical masses. However, their boiling points
are very different due to the variances in
the types of IMFs present.
H H H H
H C C C C H
H H HH
H H
O
H C C+ C
H H H
Butane, C4H10
Weak London Forces
Propanal, C3H6O
Stronger Dipole Forces
—
H H H
—
H C C C O
+
H H H H
Propanol, C3H7OH
Strongest H-Bonds
Here is another example of this phenomenon. Examine the two structures below. Both have 2
carbon atoms, 8 hydrogen atoms, and 1 oxygen atom. In the first case, ether, the oxygen is in
the middle and is bonded to the carbon atoms. In the second case, ethanol, the oxygen is near
the end and is directly bonded to a hydrogen atom. Ether is polar with Dipole Forces but the
ethanol is polar with H-Bonds. Ether boils at –23 oC while Ethanol boils at 78.5 oC; over 100
o
C more for simply re-arranging the atoms to get the capacity to form H-bonds!
—
H C
H H
O
+
H
C
H H
Ether, H3COCH3
Dipole Forces
Boiling Pt: -23 oC
H H
—
H C C O
+
H H H
Ethanol, C2H5OH
H-bonds
Boiling Pt: 78.5 oC
Boiling Point increases as the strength of the intermolecular forces increases.
Substances with stronger IMFs have higher boiling points.
All text copyright Chris Smith 2009. All pictures obtained from internet and are copyright of their owners but assumed to be public accessible. If you are the owner of a picture and want it removed, email csmith@d211.org
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Evaporation
Similar to the boiling point situation, molecules that have strong intermolecular forces tend to
evaporate slower than those with weak intermolecular forces. Look at the picture below. Two
beakers were filled to the 100 mL mark; one with water and one with cyclohexane. Water is
polar with strong H-bonds while cyclohexane is non-polar with only weak London Forces. The
beakers were allowed to sit, side-by-side for a period of time. Examine the second picture. The
water has hardly evaporated at all while the a considerable amount of the cyclohexane has
evaporated in the same time period. Cyclohexane is held together weaker than water and thus
the molecules evaporate faster.
H
—
H
O
+
H
H
H
CH
C
H
C
H
H
C
Water, H2O
Strong H-bonds
HC
H
C
H
H
H
Cyclohexane, C6H12
Weak London Forces
cyclohexane
water
cyclohexane
water
After some time, the water has evaporated very
little and the cyclohexane has evaporated more
If the particles in a sample have
stronger intermolecular forces, the liquid will evaporate slowly as the molecules are held together by their attractions.
All text copyright Chris Smith 2009. All pictures obtained from internet and are copyright of their owners but assumed to be public accessible. If you are the owner of a picture and want it removed, email csmith@d211.org
http://www.tapintoquality.com/facts/glossary/
Both liquids start at the same level here
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Look at the pictures below for one more example. The first beaker contains ethanol C2H5OH,
the second beaker contains ethylene glycol C2H4(OH)2, and the third beaker contains glycerol
C3H5(OH)3. All three of these substances contain –OH groups which means they all make
strong H-bonds. But ethanol has only 1 site to make these, ethylene glycol has 2, and glycerol
has 3 sites to make H-bonds. Consequently, after a period of time, much of the 1st has evaporated, some of the 2nd has evaporated, and little of the third has evaporated. Glycerol has the
strongest IMFs because it has the most sites for H-bonding and so evaporates the slowest.
H H
H C C +O
H H H
—
Ethanol, C2H5OH
1 H-bond site
—
H H
—
O C C O
+
+
H H H H
Ethylene Glycol, C2H4(OH)2
2 H-bond sites
—
—
H H H
O +C +C C O
+
H H O H H
—
H
Glycerol, C3H5(OH)3
3 H-bond sites
Ethanol, C2H5OH
1 H-bond site
Ethylene Glycol, C2H4(OH)2
2 H-bond sites
Glycerol, C3H5(OH)3
3 H-bond sites
All liquids start at the same level
Ethanol, C2H5OH
1 H-bond site
Evaporate fastest
Ethylene Glycol, C2H4(OH)2
2 H-bond sites
Evaporates slower
Glycerol, C3H5(OH)3
3 H-bond sites
Evaporates slowest
Ethanol has evaporated most after some time
Evaporation rate increases as the strength of the intermolecular forces decreases. Substances will stronger IMFs evaporate slower.
All text copyright Chris Smith 2009. All pictures obtained from internet and are copyright of their owners but assumed to be public accessible. If you are the owner of a picture and want it removed, email csmith@d211.org
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Solubility
Let’s examine two very different substances, water and cyclohexane. We saw them earlier
when we discussed evaporation. Water is very polar with H-bonds while cyclohexane is nonpolar with only London Forces. Let’s mix the two liquids together. Look what happens:
The two liquids don’t mix. There are two distinct layers of liquids in the tube. Even if the tube
is shaken and the two liquids are mixed up, they eventually separate back into their two layers.
Water and cyclohexane shaken together to
mix them up
Two layers beginning to re-form
What’s going on here? Why won’t they mix?
All text copyright Chris Smith 2009. All pictures obtained from internet and are copyright of their owners but assumed to be public accessible. If you are the owner of a picture and want it removed, email csmith@d211.org
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Obviously this must have something to do with their different intermolecular forces. Remember:
H
H
O
+
H
H
H
Water, H2O
Strong H-bonds
CH
C
H
C
H
H
C
HC
H
C
H
http://wpcontent.answers.com/wikipedia/commons/
thumb/6/65/Cyclohexane-3D-space-filling.png/100pxCyclohexane-3D-space-filling.png
—
H
H
Cyclohexane, C6H12
Weak London Forces
The polar water molecules are going to be attracted to each other. There is no reason for them
to want to have any interactions with the cyclohexane. Even if the two were mixed up, the water molecules will ―find‖ each other in the mixture as their negative and positive charges attract
to each other. This will leave the cyclohexane molecules to group together into their own layer
which is why we see the two distinct layers.
Even when forced to be
mixed up, the polar water
molecules will be attracted
to each other and not the
cyclohexane.
The waters will begin to
attract each other because
of their polar sites and begin to group together
Eventually all the water
molecules will be together
in one layer with the cyclohexanes in another layer.
Non-polar
layer
Polar layer
All text copyright Chris Smith 2009. All pictures obtained from internet and are copyright of their owners but assumed to be public accessible. If you are the owner of a picture and want it removed, email csmith@d211.org
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http://www.stevespanglerscience.com/img/cache/bcb9b8db117ee64376aedaf7af3595ca/sevenlayer-251908.jpg
Notice in the last picture, the cyclohexane is on top and the water is on the bottom. Why is
that? Remember our density column?
This picture shows four different liquids in what is
called a density column. The bottom layer is the most
dense and the top layer is the least dense.
Water has a density of 1.0 g/mL while cyclohexane has a density of 0.78 g/mL. Cyclohexane is
less dense so floats on top.
Cyclohexane has a density of 0.78 g/mL. It floats on top.
Water has a density of 1.0 g/mL. It sinks to the bottom.
All text copyright Chris Smith 2009. All pictures obtained from internet and are copyright of their owners but assumed to be public accessible. If you are the owner of a picture and want it removed, email csmith@d211.org
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The basic idea from the previous example is that molecules that are polar do not tend to mix
with those that are non-polar. Polar and non-polar don’t mix. Implied in that statement is another idea, though. If polar and non-polar don’t mix, will polar mix with polar? Will non-polar
mix with non-polar? Let’s do 2 quick experiments.
Experiment 1
Let’s mix two different polar liquids together: water and ethanol.
H H
H C C +O
H H H
Ethanol, C2H5OH
1 H-bond site
—
—
H
O
+
H
Water, H2O
2 H-bond sites
Both of these liquids are polar with H-bonding capabilities. When they are mixed together here
is the result:
Water and ethanol
Both mixed together gives only 1 layer
Notice that there is only one layer here. The polar molecules of the two different liquids are
perfectly happy to be attracted to each other because of their similar properties.
What would happen if two non-polar liquids are mixed?
All text copyright Chris Smith 2009. All pictures obtained from internet and are copyright of their owners but assumed to be public accessible. If you are the owner of a picture and want it removed, email csmith@d211.org
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Experiment 2
Let’s mix two different non-polar liquids together: cyclohexane and toluene
H
H
CH
H
C
H
C
H
H
HC
H
C
C
H
H
H
Cyclohexane, C6H12
Weak London Forces
H
H
H
C
C
C
H
C
C
C
C
H
H
H
H
Toluene, C6H5CH3
Weak London Forces
Both of these liquids are non-polar with only London Forces. When they are mixed together
here is the result:
Cyclohexane and toluene
Both mixed together gives only 1 layer
Notice again that there is only one layer here. The non-polar molecules of the two different liquids are soluble (able to dissolve) in each because of similar properties. We can summarize this
concept in a different way:
Like dissolves like.
Polar substances will dissolve in other polar substances and nonpolar substances will dissolve in other non-polar substances.
All text copyright Chris Smith 2009. All pictures obtained from internet and are copyright of their owners but assumed to be public accessible. If you are the owner of a picture and want it removed, email csmith@d211.org
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Questions
1.
2.
3.
4.
What is the relationship between strength of intermolecular forces and boiling point?
What is the relationship between strength of intermolecular forces and rate of evaporation?
What is the rule that governs whether or not one substance will dissolve in another?
List the following intermolecular forces in order of increasing strength:
H-bonds
London Forces
Dipole Forces
5. Put the following substances in order of increasing boiling points:
C2H6 C5H12 C6H14 CH4 C3H8
6. Which of the following substances will most like have the highest boiling point?
C6H12 C6H11OH
C4H10 C5H8(OH)3
7. Which of the following substances will most likely evaporate the fastest?
C5H12 H2O C4H6O
C2H5OH
8. Which of the following substances will most likely evaporate the slowest?
C2H5OH
C2H6 C2H4(OH)2
9. Which of the following substances would be most likely to dissolve in water?
CH4 CH3OH
C6H6 C6H11O
10. If you wanted to dissolve grease, a non-polar substance, off your hands, which of the following liquids would be best?
Water, H2O Antifreeze, C2H4(OH)2
Gasoline, C8H18
Alcohol, C2H5OH
11. If substance A (density 0.95 g/mL) exhibits H-bonding and substance B
(density 1.23 g/mL) has only London forces and the two were mixed
into the beaker to the right. How many layers would form? Draw
them. Label the layers as A or B.
Temperature (Celcius)
Boiling Point (deg Celcius)
12. Explain why water has such an
unusually high boiling point
relative to the other hydrides in
group VI even though water is
the smallest of the elements in
the group.
150
100
50
0
-50
-100
Boiling Point
(deg Celcius)
H2O
H2S
H2Se
H2Te
100
-60
-40
0
Substance
Boiling Point (deg Celcius)
All text copyright Chris Smith 2009. All pictures obtained from internet and are copyright of their owners but assumed to be public accessible. If you are the owner of a picture and want it removed, email csmith@d211.org