Transcript Implication of Intermolecular Forces

Implication of
Intermolecular Forces
Here’s some…
Physical properties that show “intermolecular
forces”:
1. Boiling point
2. Melting point
3. Surface tension
4. Viscosity
5. Capillary action
6. Evaporation
Surface Tension
You see it all the time – beading of water on a surface.
Why are liquids liquid?
Because the molecules like each other a little! (If they
liked each other a LOT, they’d be solids. If they
BARELY liked each other, they’d be gases.)
SURFACE tension
What’s the difference between the surface of a
liquid and the rest of the liquid?
It’s lonely at the top!
It’s a lazy world…
The world wants to be in the lowest energy state
possible.
Surface molecules are higher in energy than “bulk”
molecules because their “friends” stabilize them.
Liquids attempt to minimize the # of surface
molecules.
The lowest surface/volume solid object is a sphere!
Water bugs LOVE surface
tension
It’s how they walk on water!
It’s also how a paperclip floats.
Why do most things float rather than sink?
DENSITY!
But a paper clip(or water bug) have a higher density
than water yet they float. Why?
Surface Tension!
Surface Tension is Energy
The “surface tension” of a liquid is the energy required
to increase the surface area, so it has UNITS!
UNITS! UNITS! of J/m2.
In other words, water has a surface tension of 0.0728
J/m2 which means it takes 0.0728 J to increase the
surface area by 1 square meter.
If you put a paperclip (or a bug) onto the surface of the
water, sinking increases the surface area!
Too much surface!
The liquid resists the penetration because of
the increase in surface. If this resistance is
stronger than the down force – it floats!
Viscosity
Another implication of intermolecular forces can
be seen in the “viscosity” of a liquid.
Viscosity is “pourability” of a liquid, termed
“resistance to flow”. For example think of
maple syrup vs. water. Or motor oil vs.
water.
Viscosity is about the
attractiveness of your neighbors
If you are surrounded by VERY attractive
neighbors, you don’t want to leave!
Capillary Action
The source of your meniscus!!
If you love the straw more than yourself!
Phase Changes
When I want to talk to you in common terms, I
very sloppily use the term “strong” or “weak”,
“like a LOT”, “like a little”, etc.
But what is “strong” attraction or “weak”
attraction?
The answer is: it DEPENDS!
It’s all relative.
I’m at a party. I see a tall Brazilian lingerie
model. Do I have a STRONG attraction for
her?
1.
2.
3.
Do I want to hook up for the night?
Take her on vacation for a week?
Marry her?
Molecules feel the same way!
I made this drawing – we’re
going to KEEP using it! 
The force of attraction isn’t the only factor.
The molecules are MOVING – or at least trying to!
Turn up the heat!
Raising Temp, increases KE.
Increasing KE, makes the molecules move faster.
Their attraction to each other stays the same.
At some point, flying away is preferable than staying put (including
many Brazilian lingerie models!)
How do you know the phase
has changed?
We tend to think that we can “see” the
difference. But there must be some
“doorway” between “solid” and “liquid” – the
exact location of that doorway is determined
by the kinetic energy of the molecules
compared to the force (energy) of attraction
among the molecules.
3 kinds of phase changes
1.
Melting – solid becomes liquid
“Fusion” is the reverse process.
2.
Vaporization – liquid becomes gas
“condensation” is the reverse process.
3.
Sublimation – solid becomes gas (no middle
man!)
“deposition” is the reverse process.
How do you go from a liquid to
a solid?
Ice @ -10ºC
How do you make water?
Get to the melting point (0 C). How?
Q=mcT
Once you are at the melting point, does it just melt?
Of course not – or I wouldn’t ask the question!
How do we start?
With a balanced equation, of course!
H2O (s)  H2O (l)
Barely a chemical reaction (no bonds get
broken) but still a change…
At the melting point
The molecules have enough energy to match the force
of attraction between them. They are ready to go!
But they still need to be pulled apart.
Organization
The ice molecules are in a rigid lattice.
Water molecules are loosely flowing around each
other.
It takes energy to set you free!
Imagine if I handcuff you all together and put you in my
dungeon. Even if I unlock all the handcuffs, you still
need to get up and walk away!
Our old pal, H!
H2O (s)  H2O (l)
It’s a reaction, there’s an enthalpy change!!!
Hfus = (+)
This is the amount of energy it takes to separate the
molecules once they have enough energy.
Works for vaporization also
H2O (l)  H2O (g)
It’s a reaction, there’s an enthalpy change!!!
Hvap = (+)
Melting curve!
This is why your entire driveway doesn’t melt at 0!
steam
Temp 100 C
water
Phase change
0C
ice
Heat added (J)
Phase change
Clicker question
I have 10 g of ice at -10 C. If ice has a specific
heat of 2.09 J/gC and a Hfus = 6.02 kJ/mol,
how much heat must be added to completely
melt the ice?
A. 209 J
B. 3552 J
C. 6020 J
D. 60,229 J
E. I love to hate you.
Clicker question
I have 10 g of ice at -10 C. If ice has a specific heat of
2.09 J/gC and a Hfus = 6.02 kJ/mol, how much
heat must be added ?
1st, I heat up the ice:
Q = mcT = 10 g (2.09 J/gC)(10C) = 209 J
Then I melt it:
Q = n Hfus
10 g H2O * (1 mol/18.01 g) =0.5552 mol
Q = 0.5552 mol * 6020 J/mol = 3343 J
Total heat required: 209 J + 3343 J = 3552 J
Vaporization works the same
Once you are at the boiling point, you need to
add heat to make the phase change.
When you are going the opposite direction
(freezing or condensing) the H is just the
opposite sign:
Hfus = - Hfreeze
Hvap = - Hcondense
Vaporization is more
interesting!
There’s a gas!
Gases have pressure!
Gases are more interesting!
What is the boiling point?
We could start with the melting point: what is it?
It’s the temperature at which the solid will
spontaneously turn to liquid.
It’s the temperature at which the solid and liquid
are at equilibrium.
What is the boiling point?
Could we say the same thing?
It’s the temperature at which the liquid will
spontaneously turn to gas.
It’s the temperature at which the gas and liquid
are at equilibrium.
Well…sort of…
Vapors…what about ‘em?!?!
Give her a squeeze:
Liquid  gas
Changing the pressure should move the boiling
point!
Why does water evaporate at
room temperature?
Liquid  gas
This equilibrium actually exists at multiple
temperatures. And some molecules have enough
energy to escape!
# mol
Kinetic Energy
Vapor Pressure
Liquid  gas
Since this equilibrium exists at multiple temperatures,
there is always a little gas.
If there is a little gas, there is a little pressure due to
the gas.
Vapor pressure is the pressure exerted by the gas
molecules in equilibrium with the liquid!
A whole new definition of
boiling!
Boiling point can now be defined in terms of the
vapor pressure:
Boiling is the point at which the vapor pressure
EQUALS atmospheric pressure
Vapor Pressure is predictable
Clausius-Clapeyron equation
Pvap = e-Hvap/RT
Look familiar!
k = Ae-Ea/RT - Arrhenius equation – it’s general
for all activated processes!
What’s old C-C good for?
Hvap (water) = 40.7 kJmol
(water) = who cares!!!
ln P2 = - Hvap (1 – 1)
P1
R
T 2 T1
What does this tell us!!!
(Boiling point of water at any pressure!)
Clicker Question
What’s the boiling point of water of water at the
top of Mt. Everest where the atmospheric
pressure is only 0.32 atm?
A. 70 C
B. 130 C
C. 110 C
D. 90 C
E. Loving the weather.
What’s old C-C good for?
Hvap (water) = 40.7 kJmol
 (water) = who cares!!!
P2 = 0.32 atm
T2 = ?
R = 8.314 J/mol K
ln P2 = - Hvap (1 – 1)
P1
R
T 2 T1
Do I know anything else!
(Water boils at 100 C at 1 atm pressure)
What’s old C-C good for?
Hvap (water) = 40.7 kJmol
P2 = 0.32 atm
T2 = ?
R = 8.314 J/mol K
ln 0.32 = - 40,700 J/mol (1 – 1
)
1
8.314 J/mol K T2 373 K
-1.139 = -4895 (1/T2 – 2.681x10-3)
2.3269x10-4 = 1/T2 – 2.681x10-3
2.914x10-3 = 1/T2
343 K = T2 (or 70 C)