Atmospheric Pressure, Boiling Points, and Phase Diagrams, Study notes of Chemistry

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What is standard atmospheric pressure?

1. The air pressure at sea level.
2. 760 mm Hg
3. 76.0 mm Hg
4. 1 atm

1

5. 0.1 atm
6. The air pressure in Amherst, MA.
7. The air pressure in Lander, WY.
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Which letter represents the normal boiling

point of water?

A B C

2

D

Diethyl ether has a vapor pressure of 760 mm

Hg at 34 6 °C and the enthalpy of vaporizationHg at 34.6 C and the enthalpy of vaporization

of 28.1 kJ/mol. What is the a vapor pressure of

at 25.0 °C.?

a) 120 mm Hg

b) H

3

b) 197 mm Hg

c) 346 mm Hg

d) 534 mm Hg

e) 4790 mm Hg

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Clausius-Clapeyron equation

⎛ - ΔH

⎜

⎞

⎟

lnP =

• ΔH (^) vap RT

⎛

⎝

⎜

⎞

⎠

⎟ +^ c

lnP 2 =

ΔH (^) vap ⎝ RT 2

⎜

⎠

⎟ +^ c

− lnP 1 =

• ΔH (^) vap RT 1

⎛

⎝

⎜

⎞

⎠

⎟ +^ c

lnP - lnP =

⎛-ΔH (^) vap ⎜

⎞

⎟ −^

⎛-ΔH (^) vap ⎜

⎞

⎟ ⇒^ ln^

⎛P 2

⎝

⎜

⎞

⎠

⎟ =^

⎛-ΔH (^) vap ⎝

⎜

⎞

⎠

⎟

−

⎛ 1

⎝

⎜

⎞

⎠

⎟

UMass Amherst Biochemistry Teaching Initiative

lnP 2 - lnP 1 = ⎝ RT 2

⎜

⎠

⎟ −^

⎝ RT 1

⎜

⎠

⎟ ⇒^ ln^ P ⎝ 1

⎜

⎠

⎟

⎝^ ⎜^ R ⎠⎟⎝^ T 2 T 1

⎜

⎠

⎟

ln P^2 P 1

⎛

⎝

⎜

⎞

⎠

⎟ =^

ΔH (^) vap R

⎛

⎝

⎜

⎞

⎠

⎟

T 1

− 1

T 2

⎛

⎝

⎜

⎞

⎠

⎟

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Why did the lighter have a small flame in Hasbrouck vs. a large flameg at Thai Blue Ginger?

A. The elevation of Hasbrouck is higher so the pressure was higher B. The elevation of Hasbrouck is higher so the pressure was lower C Th li ht ld i H b k th

6

C. The lighter was colder in Hasbrouck so the vapor pressure of the butane was lower D. The lighter was warmer in Hasbrouck so the vapor pressure of the butane was lower E. The lighter was warmer at Thai Blue Ginger so the vapor pressure of the butane was lower- (^) 6 6 6 662 ... 

Phase Diagrams

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Why learn about phase diagrams?

Phase Equilibria — Water

Solid-liquid Gas-liquid

9

Gas-Solid

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Triple Point

— Water

10

At the TRIPLE POINT all

three phases are in

equilibrium.

Phase Diagrams—Important Points for Water

T(T(˚˚C)C) P(mmHg)P(mmHg)

11

Normal boil pointNormal boil point Normal freeze pointNormal freeze point Triple pointTriple point

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Critical T and P

As P and T increase, you finally reach theAs P and T increase, you finally reach the

CRITICAL TCRITICAL T dd PP

.

Pcritical

ure

Note that line goes straight up

Above critical TAbove critical T

no liquid existsno liquid exists

Above critical TAbove critical T

no liquid existsno liquid exists

CRITICAL TCRITICAL T andand PP

12

LIQUID

High Pressu GAS

High Temperature

Tcritical

nono liquid exists liquid exists

no matter howno matter how

high thehigh the

pressure.pressure.

no liquid existsno liquid exists

no matter howno matter how

high thehigh the

pressure.pressure.

Critical T and P

COMPOUNDCOMPOUND TTcc(( oo^ CC)) PP (^) cc((atmatm)) HH 22 OO COCO 22 CHCH 44 FreonFreon--12 12 (CCl(CCl 22 FF2 2 ))

13

What do you predictWhat do you predict TT cc and Pand P cc dependdepend on?on?

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CO 2 Phase Diagram

CO 2 critical point

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For CO 2 what phase changes occur

betweenbetween -100 C and 100 C at 1 atm? 100°C and 100°C at 1 atm?

A. Solid to liquid B. Solid to liquid to gas C. Solid to gas

16

C. Solid to gas D. Liquid to gas E. liquid to supercritical fluid

For CO 2 what phase changes occur

betweenbetween -100 C and 100 C at 12 atm? 100°C and 100°C at 12 atm?

A. Solid to liquid

B. Solid to liquid to gas

C. Solid to gas

17

C. Solid to gas

D. Liquid to gas

E. liquid to supercritical

fluid

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Solid-Liquid Equilibria

In any system, if you increase P theIn any system, if you increase P the DENSITYDENSITY

will go up.will go up.gg pp

ThereforeTherefore —— as P goes up, equilibrium favorsas P goes up, equilibrium favors

phase with the larger density (orphase with the larger density (or SMALLERSMALLER

volume/gram).volume/gram).

Liquid HLiquid H 22 OO Solid HSolid H 22 OO DensityDensity 1 g/cm1 g/cm 33 0.917 g/cm0.917 g/cm 33 cmcm 33 /gram/gram 11 1 091 09

18

cmcm 33 /gram/gram 11 1.091.

ICE LIQUID H2O

favored at low P

favored at high P

ICE LIQUID H2O

favored at low P

favored at high P

Solid-Liquid Equilibria

ICE LIQUID H2O

favored at low P

favored at high P

ICE LIQUID H2O

favored at low P

favored at high P Raising the pressure atRaising the pressure at constant T causes waterconstant T causes water to melt.to melt. The NEGATIVE SLOPE ofThe NEGATIVE SLOPE of the S/L line is unique tothe S/L line is unique to HH 22 OO Almost everythingAlmost everything

Solid H2O

Liquid H2O

P

760

Normal freezing point

19

HH 22 O.O. Almost everythingAlmost everything else has positive slope.else has positive slope. T

760 mmHg 0 ÞC

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Solid-Vapor Equilibria

At P < 4.58 mmHg and T < 0.0098At P < 4.58 mmHg and T < 0.0098 ˚˚CC solid Hsolid H O can go directly to vaporsolid Hsolid H 22 O can go directly to vapor.O can go directly to vaporO can go directly to vapor. ThisThisThisThis

process is calledprocess is called SUBLIMATIONSUBLIMATION

Can you think of any machine that usesCan you think of any machine that uses

21

sublimation?sublimation?

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The vapor pressure of liquid ethane thiol , C 2 H 5 SH , is 100 mm Hg at 260 K. A sample of C 2 H 5 SH is placed in a closed, evacuated 521 mL container at a temperature of 260 K. It is found that all of the C 22 H 55 SH is in the vapor phase andp p that the pressure is 57.0 mm Hg. If the volume of the container is reduced to 390 mL at constant temperature, which of the following statements are correct? Choose all that apply.

1. The pressure in the container will be 76.0 mm Hg. 2 N d i ill

22

2. No condensation will occur.
3. Liquid ethane thiol will be present.
4. Only ethane thiol vapor will be present.
5. The pressure in the container will be 100 mm Hg.

(P 1 V 1 )/T 1 =(P 2 V 2 )/T 2 <-- see page 554 for review.