Chemistry - A Molecular Science

LIQUID-VAPOR TRANSITION Once the entire solid has melted, additional h

eating causes the temperature of the liquid to

rise, and the movement of the molecules becomes more chaotic as their thermal energy increases. However, thermal energy is the

average

kinetic energy, and not all molecules in

the liquid have the same kinetic energy. Con

sequently, a small fraction of molecules have

enough kinetic energy to break their intermol

ecular interactions. Those at the surface do so

by escaping into the gas phase and

evaporation

(liquid

→

vapor) begins.

Molecules in the gas phase co

llide with one another, and

intermolecular interactions

can be established when they do. However, if

the molecules have sufficient kinetic energy,

they break the interaction and move on. Occasionally, colliding molecules do not have sufficient kinetic energy to escape their inter

action, and they stick to one another and

condensation

(vapor

→

liquid) begins. As the collision frequency increases, so too does

the rate of condensation. The frequency of

the molecular collisions depends upon the

concentration of the gas, which is proportiona

l to its partial pressure (Section 7.1). Hence,

there is a pressure at any given temperature at which the rates of evaporation and condensation are equal, and the system r

eaches another dynamic equilibrium: liquid

U

gas. The pressure of the gas at which the li

quid and vapor are in equilibrium is called the

vapor pressure (P

o) at that temperature. If the temperature is increased, the fraction of

molecules in the liquid with sufficient kinetic energy to escape into th

e gas also increases,

which increases the rate of evaporation. Ho

wever, as more molecules escape into the

vapor, the rate of condensation begins to

increase as well. Eventually, the two rates

become equal again and equilibrium is re-est

ablished. However, both rates have increased

as has the vapor pressure. Thus, the vapor pressure of a substance increases with its temperature. The vapor pressure of water as a function of temperature is shown in Table 7.2 and Figure 7.15.

Table 7.2

Vapor pressure (P

o) of water at various

temperatures

T (

oC) P

o (torr)

oT(

C) P

o (torr)

0

4.6

50

92.5

5

6.5

55

118.0

10

9.2

60

149.4

15 12.8

65 187.5

20 17.5

70 233.7

25 23.8

75 289.1

30 31.8

80 355.1

35 41.2

85 433.6

40 55.3

90 525.8

45 71.9

95 633.9

Weather reports state the partial pressure of water in the atmosphere as a relative

humidity or dew point. The

relative humidity

is 100% times the ratio of the observed

partial pressure of water in the atmosphere

to the vapor pressure of water at that

temperature. The

dew point

is the temperature at which the atmospheric water would

begin to condense;

i.e

., the temperature at which the vapor pressure of H

O equals its 2

partial pressure in the atmosphere. For exampl

e, consider a day on which the temperature

is 25

oC and the partial pressure of H

O is 12.8 torr. We note from Table 7.2 that the vapor 2

pressure of water is 12.8 torr at 15

oC, so 15

oC is the dew point. The vapor pressure of

Chapter 7 States of Matter and Changes in State

© by

North

Carolina

State

University