111

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

Fall 2014

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.

http://www.youtube.com/watch?v=LBjRDF4XV8Q

"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.

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Dynamic Equilibrium

liquid gasThe 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.

4

Evaporation of Liquid in Sealed Container

Fig. 12.21 p.543 Why must the container be sealed?

55

Dynamic Equilibrium

liquid gasAt 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.

6

Melting Involves Equilibrium(Fig. 12.22 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. 12.22 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.

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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?

endothermic

endothermic

exothermic

exothermic

E

SOLID

LIQUID

GAS

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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

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Vapor Pressure 1 atm

Vp increases with temperature. Why?

Why are the curves different for the different liquids?

Fig. 12.23 p.545

Significance of vp =1atm?

10

Effect of IMF on VPIf 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!

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Vapor PressureSome factors do NOT affect the vp:

1) surface area of the liquid

2) volume of the liquid

3) volume of the container

Fig.12.24 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.

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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.

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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.

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Effect of Atmospheric Pressure on BP

normal atm pressureWater 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

110 oC

Link to check altitude and atmospheric pressure of various cities.

1 psi = 0.06803 atm

http://www.turblex.com/altitude/index.cfm

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How a Pressure Cooker WorksThe 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.

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BPH2S 60C

H2Se 41C

H2Te 2C

Effect of IMF on BP

What is the trend?

Why is there such a trend?

Let us now examine this graphically.

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Bp of H2O, H2S, H2Se, H2Te

Rel Size (H2O, H2S, H2Se, H2Te)

BP

(deg

C)

H2Te

H2Se

H2S

H2O

● ?H2O

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BP

H2O 100CH2S 60CH2Se 41CH2Te 2C

Effect of IMF on BP

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?

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Effect of IMF on BP

Relationship Between Boiling Point & Relative Size of Atom (H Compounds of Group VA)

Relative Size of Atom (NH3, PH3, AsH3)

Bo

ilin

g P

oin

t in

Deg

ree

Cel

siu

s

0 1 2 3 4-150

-130

-110

-90

-70

-50

-30

AsH3

PH3

NH3

● ?NH3

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HI

HBr

HCl

HF

●? HF

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Energy of Changes of StateFusion 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?

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trend??

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

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Heating CurveFig.12.27 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.)

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Cooling Curve

Fig. 12.27 p.549

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 515.0 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.

25Practice Exercises: 12.8 & 12.9 p. 551