Chemistry Reference
In-Depth Information
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Water and steam are very useful for heating buildings. The high specific
heat of water makes it useful for baseboard heating of homes and small
buildings. It has the highest heat capacity of any entry in Table 14.4 except
hydrogen, and it is liquid over a useful range.
The high heat of vaporization and the reasonable normal boiling point
of water make steam useful for steam heating of large buildings. As steam
condenses, it gives off 2260 J of heat per gram. (Carbon dioxide has a
higher heat of sublimation, but its low sublimation point makes it suitable
for refrigeration of ice cream trucks rather than for heating of buildings.)
Sodium is used as a heat transfer agent in cooling nuclear reactors.
It has a relatively high specific heat and an acceptable liquid range for the
purpose. (It melts just below 100°C.)
ITEM OF INTEREST
When we add heat to a substance that is not at a temperature at which a
phase change will occur, its temperature rises. Suppose that the temperature rises
to the temperature of a phase change. Then, the phase change will occur at con-
stant temperature until it is complete, after which the temperature will rise again.
For example, if we add heat to ice at the ice will warm to At that
temperature, melting begins, and the temperature stays at until all the ice
has melted. Then the temperature of the liquid water rises as we add even more
heat. After the temperature of the liquid reaches the boiling point, the addition
of more heat causes it to boil. Further heating increases the temperature of the
vapor. We can draw a graph that shows what happens as we heat a pure sub-
stance. Such a graph is called a
heating curve
(Figure 14.7).
How do we calculate the energy required to raise the temperature of a sub-
stance and cause it to change phase? We do separate calculations for each step
and then sum the results.
20°C,
0°C.
0°C
EXAMPLE 14.13
Calculate the energy required to change a 17.1-g sample of water from
to steam at
87.7°C
121.0°C
and 1.00 atm. Use data from Tables 14.4 and 14.5.
Figure 14.7
Heating Curve
for Water
Warming of vapor
120
100
Boiling
80
60
Warming of liquid
40
Warming
of solid
20
0
Melting
-20
Heat added
