The Foundations of Chemistry

EXAMPLE 13-5 Heat of Fusion

The molar heat of fusion, Hfus, of Na is 2.6 kJ/mol at its melting point, 97.5°C. How much
heat must be absorbed by 5.0 g of solid Na at 97.5°C to melt it?

Plan

Melting takes place at a constant temperature. The molar heat of fusion tells us that every mole
of Na, 23 grams, absorbs 2.6 kJ of heat at 97.5°C during the melting process. We want to know
the amount of heat that 5.0 grams would absorb. We use the appropriate unit factors,
constructed from the atomic weight and Hfus, to find the amount of heat absorbed.

Solution

_?_kJ5.0 g Na0.57 kJ

2.6 kJ



1 mol Na

1 mol Na



23 g Na

13-11 Heat Transfer Involving Solids 505

TABLE 13-7 Some Melting Points and Heats of Fusion

Heat of Fusion

Melting Point

Substance (°C) J/g kJ/mol

methane, CH 4  182 58.6 0.92
ethyl alcohol, CH 3 CH 2 OH  117 109 5.02

water, H 2 O 0 334 6.02
naphthalene, C 10 H 8 80.2 147 18.8

silver nitrate, AgNO 3 209 67.8 11.5
aluminum, Al 658 395 10.6

sodium chloride, NaCl 801 519 30.3

Transitions among the three states of matter. The transitions shown in blue are endothermic
(absorb heat); those shown in red are exothermic (release heat).

Gas

Solid Liquid

Melting

Freezing

Condensation

Evaporation

Deposition

Sublimation