The Foundations of Chemistry

very strongly with one another, are not effectively separated by simple distillation but
require a modification called fractional distillation (Section 14-10).

HEAT TRANSFER INVOLVING LIQUIDS

Heat must be added to a liquid to raise its temperature (Section 1-13). The specific heat
(J/g °C) or molar heat capacity(J/mol °C) of a liquid is the amount of heat that must
be added to the stated mass of liquid to raise its temperature by one degree Celsius. If
heat is added to a liquid under constant pressure, the temperature rises until its boiling
point is reached. Then the temperature remains constant until enough heat has been added
to boil away all the liquid. The molar heat(or enthalpy) of vaporization (
Hvap)of a
liquid is the amount of heat that must be added to one mole of the liquid at its boiling
point to convert it to vapor with no change in temperature. Heats of vaporization can also
be expressed as energy per gram. For example, the heat of vaporization for water at its
boiling point is 40.7 kJ/mol, or 2.26 103 J/g.

2.26 103 J/g

Like many other properties of liquids, heats of vaporization reflect the strengths of
intermolecular forces. Heats of vaporization generally increase as boiling points and inter-
molecular forces increase and as vapor pressures decrease. Table 13-5 illustrates this. The
high heats of vaporization of water, ethylene glycol, and ethyl alcohol are due mainly to
the strong hydrogen-bonding interactions in these liquids (see Section 13-2). The very
high value for water makes it very effective as a coolant and, in the form of steam, as a
source of heat.
Liquids can evaporate even below their boiling points. The water in perspiration is an
effective coolant for our bodies. Each gram of water that evaporates absorbs 2.41 kJ of
heat from the body. We feel even cooler in a breeze because perspiration evaporates faster,
so heat is removed more rapidly.

1 mol



18.0 g

1000 J



kJ

40.7 kJ



mol

_?_J



g

13-9

The specific heat and heat capacity of

a substance change somewhat with its

temperature. For most substances, this

variation is small enough to ignore.

Molar heats of vaporization (also called

molar enthalpiesof vaporization) are

often expressed in kilojoules rather

than joules. The units of heat

of vaporization do notinclude

temperature. This is because boiling

occurs with no change in temperature.

13-9 Heat Transfer Involving Liquids 499

See the Saunders Interactive

General Chemistry CD-ROM,

Screen 13.8, Properties of Liquids (1):

Enthalpy of Vaporization.

The heat of vaporization of water is

higher at 37°C (normal body

temperature) than at 100°C (2.41 kJ/g

compared to 2.26 kJ/g).

TABLE 13-5 Heats of Vaporization, Boiling Points, and Vapor Pressures

of Some Common Liquids

Heat of

Vaporization

at

Vapor Boiling Boiling Point

Pressure Point

Liquid (torr at 20°C) at 1 atm (°C) J/g kJ/mol

water, H 2 O 17.5 100. 2260 40.7
ethyl alcohol, CH 3 CH 2 OH 43.9 78.3 855 39.3

benzene, C 6 H 6 74.6 80.1 395 30.8
diethyl ether, CH 3 CH 2 OCH 2 CH 3 442. 34.6 351 26.0

carbon tetrachloride, CCl 4 85.6 76.8 213 32.8
ethylene glycol, CH 2 OHCH 2 OH 0.1 197.3 984 58.9