Chemistry - A Molecular Science

Chapter 7 States of Matter and Changes in State

more like a liquid. This unusual material is called a

supercritical fluid

. The unusual

properties of supercritical fluids make them ex

cellent solvents. For example, supercritical

CO

is used as a solvent in the dry cleaning i 2

ndustry because it is gentle on material and is

easily removed by reducing the pressure to convert the CO

back to a gas. 2

Example 7.7 a) Which substance has the higher boiling point, CH

or CCl 4

? 4

Dispersion forces are the only forces at

work in these molecules because neither

molecule is polar and neither can exhibit hy

drogen bonding. Dispersion forces increase

with molecular size or mass. Thus, CCl

(M 4

= 154 g/mol) boils at m

a higher temperature

than CH

(M 4

= 16 g/mol). The observed boiling points are: CClm

= +77 4

oC; CH

= -164 4

oC.

b) Which compound has the higher boiling point, PF

or Kr? 3

PF

(^3)
The formula weights are 88 and 84, respectively, so the dispersion forces are expected to be very similar. The difference between these substances is that PF
is a polar molecule 3
(see structure in margin), so it exhibits dipole-dipole forces as well. Consequently, the boiling point of PF
is higher. The boiling points are: PF 3
= -102 3
oC; Kr = -152
oC.
c) Which has the higher vapor pressure at 298K, CH
-CH 3
-OH or H 2
C-O-CH 3
? 3
Both compounds are polar and have identical molar masses, but CH
-CH 3
-OH has an O- 2
H bond. Thus, CH
-CH 3
-OH molecules can hydrogen bond to one another, while H 2
C-O- 3
CH
molecules cannot. At room temperature, far fewer CH 3
-CH 3
-OH molecules are able to 2
escape into the vapor state, so its vapor
pressure is much lower than that of H
C-O-CH 3

. 3

d) Which substance has the higher normal boiling point, CH

-NH 3

or CH 2

-F? 3

Hydrogen bonding in CH

-NH 3

raises its boiling point above that of CH 2

-F, which does not 3

form hydrogen bonds. The observed boiling points are: CH

NH 3

= -6 2

oC and CH

-F = -78 3

oC. Note that CH

F has both hydrogen and fluorine atoms but, because they are not 3

bound to one another, there is no hydrogen bonding.

SOLID-VAPOR TRANSITION Under certain conditions, a solid can be

in equilibrium with its vapor: solid

U

vapor. The

solid-vapor equilibrium is also dynamic and occurs when the rate of

sublimation

(solid

→

vapor) equals the rate of

deposition

(vapor

→

solid). Any solid, such as garlic, that has an

odor is subliming because gas molecules are th

e ones that interact with the olfactory

receptors in the nose to produce the sense of smell. Carbon dioxide cannot be a liquid at normal conditions, so solid CO

(dry ice) sublimes, but it does not melt at normal 2

© by

North

Carolina

State

University