1. Intermolecular Forces (IMF). Part II:
Intermolecular Forces (IMF) Part II: Effect of IMF on Changes of State (based on Chap. 12 Sec 4-7 of Jespersen 6th Ed)
Dr. C. Yau
Spring 2014 1 1

2. Changes of State: a Dynamic Equilibrium
"Change of state" refers to a change in physical state, such as a solid to a liquid and a liquid to gas.
"Equilibrium" refers to the state of a reversible reaction when there is no further VISIBLE change in the ratio of amount of reactants to amount of products.

3. Dynamic Equilibrium liquid gas
The term "dynamic equilibrium" stresses the fact that even though the amounts of liquid & gas is no longer changing, there is still something very active going on.
The liquid is still vaporizing into a gas,
and gas is still condensing to the liquid!
No VISIBLE change is observed because the forward & reverse rates are now EQUAL. 3

4. Evaporation of Liquid in Sealed Container
Figure Evaporation of a liquid into a sealed container. (a) The liquid has just begun to evaporate into the container. The rate of evaporation is greater than the rate of condensation. (b) A dynamic equilibrium is reached when the rate of evaporation equals the rate of condensation. In a given time period, the number of molecules entering the vapor equals the number that leave, so there is no net change in the number of gaseous molecules.
Why must the container be sealed?
Fig p.543

5. Dynamic Equilibrium liquid gas
At first the rate of vaporization is fast, but as the molecules in the liquid phase decreases, the rate slows down.
Meanwhile the amt of gas molecules increases, and the rate of condensation increases, until the rates are equal & we say an equilibrium has been reached.
There is no more visible change in the amt of liquid & gas. 5

6. Melting Involves Equilibrium
(Fig a) shows the process of melting, at an equilibrium. As long as no heat is added or removed, at equilibrium, the melting and freezing are at equal rate & the number of particles in the solid becomes constant.
(Fig b) shows the process of sublimation, at an equilibrium. As long as no heat is added or removed, at equilibrium, the sublimation and deposition are at equal rate & the number of particles in the solid becomes constant.
Figure Solid–liquid and solid-vapor equilibria. (a) As long as no heat is added or removed, melting (red arrows) and freezing (black arrows) occur at equal rates and the number of particles in the solid remains constant. (b) Equilibrium is established when molecules evaporate from the solid at the same rate as they condense from the vapor.

7. Energetics in Changes of State
Think about the various changes of state and decide which processes are exothermic, and which ones are endothermic.
solid to liquid
liquid to gas
gas to liquid
liquid to solid
Now put the states in an E diagram. Which state should be on the bottom? E
GAS
endothermic
endothermic
LIQUID
exothermic
exothermic
SOLID 7

8. Vapor Pressure
In changes of states, we are particularly interested in the vapor pressure above the liquid or solid.
It is simply the pressure of the vapor above the liquid or solid, often abbreviated as vp.
At equilibrium, the vp over a liquid is called the equilibrium vp of the liquid.
This vp is dependent on…
(1) Temperature
(2) strength IMF 8

9. Vapor Pressure 1 atm Significance of vp =1atm?
Fig p.545
Vapor Pressure
1 atm
Significance of vp =1atm?
Vp increases with temperature. Why?
Why are the curves different for the different liquids? 9

10. Effect of IMF on VP
If the IMF of a liquid is strong, what do you expect its vp to be like?
high vp or low vp?
Increase of IMF leads to a decrease in vp.
Ether refers to diethyl ether:
CH3CH2OCH2CH3
It is not linear! It is bent at O, like water.
However, unlike water, it does NOT have H-bonding.
Compared with water, ether would have the lower IMF, and therefore higher vp.
Now we have a way to tell whether the IMF is strong or not…just by measuring the vp of the liquid!

11. Vapor Pressure Some factors do NOT affect the vp:
1) surface area of the liquid
2) volume of the liquid
3) volume of the container
Fig p.546 Effect of Volume on VP
a) Equilibrium for smaller volume of container.
b) Volume of container is increased, upsetting the equilibrium.
c) Equilibrium is reached again for the larger volume. VP returns to the original value.

12. Vapor Pressure of Solids
Solids have vapor pressures just like liquids, however, they are generally much lower as the IMF of solids are always stronger than that of the liquids.
As with liquids, the vp decreases with the increase in the strength of the IMF.
NaCl has very strong ionic bonds, and VP is essentially zero at room temperature.

13. Relationship of BP with VP
DEFINITION OF BP (LEARN THIS!)
The boiling point is the temperature at which the vp is equal to the atmospheric pressure.
Bubbles can't form until the vp is at least the pressure of the atmosphere.

14. Effect of Atmospheric Pressure on BP
normal atm pressure
Water boils at 100oC
Denver at 5283 ft (1 mi above sea level) has 0.82 atm
= 0.82 atm x (760 torr/1 atm)
= 623 torr
Water boils at…92 oC
Link to check altitude and atmospheric pressure of various cities.
1 psi = atm
110 oC

15. How a Pressure Cooker Works
The tight seal allows the pressure to build to 15 pounds per square inch (psi) above atmospheric pressure. That raises the boiling point to about 121°C or 257 °F.
Why is that desirable?
A pot roast that normally takes 2-3 hours can be cooked in 30 minutes.

16. Effect of IMF on BP BP H2S 60C H2Se 41C H2Te  2C
What is the trend?
Why is there such a trend?
Let us now examine this graphically.

17. H2O
H2Te
● ?
H2O
H2Se
H2S

18. Effect of IMF on BP BP H2O 100C H2S 60C H2Se 41C H2Te  2C
The increase in BP from H2S to H2Se to H2Te is due to the increase in size of the Group VIA elements (increase in London forces).
H2O is far above what one would expect, due to H-bonding.
What other set of compounds should we examine?

19. Effect of IMF on BP
NH3
AsH3
PH3
● ?
NH3

20. HF HI
HBr ●? HF
HCl

21. Energy of Changes of State
Fusion refers to melting.
molar ΔH fusion = heat transfer when one mole of a solid melts
molar ΔH vap= heat transfer when one mole of a liquid vaporizes
molar ΔH sublimation= heat transfer when one mole of a solid sublimes
molar ΔH condensation= heat transfer when one mole of a gas condenses
Which of these have negative signs?
How do these values change with IMF?

22. trend??
C6H14 has only London forces, why is its Hvap higher than that of NH3?

23. Heating Curve
Fig p.549
When heat is added and the T increases, energy is added into the kinetic energy. (Particles are moving faster and colliding with more force.)
When heat is added but the T stays constant, energy is added into the potential energy. (Intermolecular forces are weakened/broken. Particles are moving further apart.)
Figure Heating and cooling curves. (a) A heating curve observed when heat is added to a substance at a constant rate. The temperatures corresponding to the flat portions of the curve occur at the melting point and boiling point.

24. Fig p.549
Cooling Curve
Figure Heating and cooling curves. (b) A cooling curve observed when heat is removed from a substance at a constant rate. Condensation of vapor to a liquid occurs at the same temperature as the liquid boils. Supercooling is seen here as the temperature of the liquid dips below its freezing point (the same temperature as its melting point). Once a tiny crystal forms, the temperature rises to the freezing point.

25. Example 12.2 p.550 Liquid sodium metal is used as heat transfer material to cool nuclear reactors. How much heat is required to heat 75.0 g of sodium from 25.0 oC to oC? The melting point of sodium is 97.8 oC; the specific heat of solid sodium is 1.23 J/g oC; the molar heat of fusion of sodium is 2.60 kJ/mol; and the specific heat of liquid sodium is 1.38 J/g oC. We will assume that the specific heats do not change with temperature.
Practice Exercises: 12.8 & 12.9 p. 551