The Foundations of Chemistry

at the melting point until all of the substance has melted. After melting is complete, the

continued addition of heat results in an increase in the temperature of the liquid, until

the boiling point is reached. This is illustrated graphically in the first three segments of

the heating curve in Figure 13-15.

The molar heat(or enthalpy) of fusion (
Hfus;kJ/mol) is the amount of heat required

to melt one mole of a solid at its melting point. Heats of fusion can also be expressed on

a per gram basis. The heat of fusion depends on the intermolecular forces of attraction in

the solid state. These forces “hold the molecules together” as a solid. Heats of fusion are

usuallyhigher for substances with higher melting points. Values for some common

compounds are shown in Table 13-7. Appendix E has more values.

The heat(or enthalpy) of solidificationof a liquid is equal in magnitude to the heat

of fusion. It represents removal of a sufficient amount of heat from a given amount (1 mol

or 1 g) of liquid to solidify the liquid at its freezing point. For water,

6.02 kJ/mol

or 334 J/g absorbed

ice 3::::::::::::::4water (at 0°C)

6.02 kJ/mol

or 334 J/g released

Melting is always endothermic. The
term “fusion” means “melting.”

Heat added

B

TemperatureA

Gas is

cooled

EXOTHERMIC

PROCESSES

(LIBERATE HEAT)

ENDOTHERMIC

PROCESSES

(ABSORB HEAT)

Liquid is

cooled

Solid is

cooled

Freezing

Condensing

Gas is

warmed

Solid is

warmed

Melting

Boiling

Liquid is

warmed

Figure 13-15 A typical heating curve at constant pressure. When heat energy is added to

a solid below its melting point, the temperature of the solid rises until its melting point is

reached (point A). In this region of the plot, the slope is rather steep because of the low

specific heats of solids [e.g., 2.09 J/g °C for H 2 O(s)]. If the solid is heated at its melting

point (A), its temperature remains constant until the solid has melted, because the melting

process requires energy. The length of this horizontal line is proportional to the heat of

fusion of the substance—the higher the heat of fusion, the longer the line. When all of the

solid has melted, heating the liquid raises its temperature until its boiling point is reached

(point B). The slope of this line is less steep than that for warming the solid, because the

specific heat of the liquid phase [e.g., 4.18 J/g °C for H 2 O(
)] is usually greater than that of

the corresponding solid. If heat is added to the liquid at its boiling point (B), the added heat

energy is absorbed as the liquid boils. This horizontal line is longer than the previous one,

because the heat of vaporization of a substance is always higher than its heat of fusion.

When all of the liquid has been converted to a gas (vapor), the addition of more heat

raises the temperature of the gas. This segment of the plot has a steep slope because of

the relatively low specific heat of the gas phase [e.g., 2.03 J/g °C for H 2 O(g)]. Each step

in the process can be reversed by removing the same amount of heat.