The Foundations of Chemistry

EXAMPLE 13-6 Heat of Fusion

Calculate the amount of heat that must be absorbed by 50.0 grams of ice at 12.0°C to convert

it to water at 20.0°C. Refer to Appendix E.

Plan

We must determine the amount of heat absorbed during three steps: (1) warming 50.0 g of ice

from 12.0°C to its melting point, 0.0°C (we use the specific heat of ice, 2.09 J/g °C);

(2) melting the ice with no change in temperature (we use the heat of fusion of ice at 0.0°C,

334 J/g; and (3) warming the resulting liquid from 0.0°C to 20.0°C (we use the specific heat

of water, 4.18 J/g °C).

Solution

1.50.0 g

2

g

.

0

°

9

C

J

[0.0(12.0)]°C1.25 103 J0.125 104 J

2.50.0 g

33

g

4J

 1.67^104 J

3.50.0 g

4

g

.

1

°

8

C

J

(20.00.0)°C4.18 103 J 0.418 104 J

Total amount of heat absorbed 2.21 104 J22.1 kJ

Note that most of the heat was absorbed in step 2, melting the ice.

You should now work Exercise 56.

SUBLIMATION AND THE VAPOR PRESSURE OF SOLIDS

Some solids, such as iodine and carbon dioxide, vaporize at atmospheric pressure without

passing through the liquid state. This process is known as sublimation.Solids exhibit

vapor pressures just as liquids do, but they generally have much lower vapor pressures.

Solids with high vapor pressures sublime easily. The characteristic odor of a common

household solid, para-dichlorobenzene (moth repellent), is due to sublimation. The reverse

process, by which a vapor solidifies without passing through the liquid phase, is called

deposition.

sublimation

solid3:::::::4gas

deposition

Some impure solids can be purified by sublimation and subsequent deposition of the

vapor (as a solid) onto a cooler surface. Purification of iodine by sublimation is illustrated

in Figure 13-16.

13-12

50.0 g H 2 O(
)

at 0.0 C

50.0 g H 2 O(
)

at 20 C

Step 3:

warm the liquid

(temp. change)

50.0 g H 2 O(s)

at 12.0 C

50.0 g H 2 O(s)

at 0.0 C

Step 1:

warm the ice

(temp. change)

Step 2:

melt the ice

(phase change)

Ice is very efficient for cooling because
considerable heat is required to melt a
given mass of it. However, Hvapis
generally much greater than Hfusion,
so evaporative cooling is preferable
when possible.

506 CHAPTER 13: Liquids and Solids

Figure 13-16 Sublimation can be
used to purify volatile solids. The
high vapor pressure of the solid
substance causes it to sublime
when heated. Crystals of purified
substance are formed when the
vapor is deposited to form solid on
the cooler (upper) portion of the
apparatus. Iodine, I 2 , sublimes
readily. I 2 vapor is purple.