Barrons SAT Subject Test Chemistry, 13th Edition

Figure 28. Hydrogen Peroxide

Water Calorimetry Problems

A calorimeter is a container well insulated from outside sources of heat or cold
so that most of its heat is contained in the vessel. If a very hot object is placed in
a calorimeter containing some ice crystals, we can find the final temperature of
the mixture mathematically and check it experimentally. To do this, however,
certain behaviors must be understood. Ice changing to water and then to steam
does not represent a continuous and constant change of temperature as time
progresses. In fact, the chart would look as shown in Figure 29.

TIP

Water’s heat of vaporization = 40.79 kJ/mol

Figure 29. Changing Ice to Steam

TIP

Water’s heat of fusion = 6.01 kJ/mol

From this graph, you see that heat is being used at 0°C and 100°C to change
the state of water, but not its temperature. One gram of ice at 0°C needs 80

calories or 3.34 × 10^2 joules to change to water at 0°C. This is called its heat of
fusion. Likewise, energy is being used at 100°C to change water to steam, not to
change the temperature. One gram of water at 100°C absorbs 540 calories or 2.26

× 10^3 joules of heat to change to 1 gram of steam at 100°C. This is called its heat
of vaporization. This energy absorbed at the plateaus in the curve is being used