The Foundations of Chemistry

EXAMPLE 15-1 Heat Capacity of a Calorimeter

We add 3.358 kJ of heat to a calorimeter that contains 50.00 g of water. The temperature of
the water and the calorimeter, originally at 22.34°C, increases to 36.74°C. Calculate the heat
capacity of the calorimeter in J/°C. The specific heat of water is 4.184 J/g°C.

Plan

We first calculate the amount of heat gained by the water in the calorimeter. The rest of the
heat must have been gained by the calorimeter, so we can determine the heat capacity of the
calorimeter.

Solution

50.00 g H 2 O(
) at 22.34°C 88n 50.00 g H 2 O(
) at 36.74°C

The temperature change is (36.7422.34)°C14.40°C.

__?J50.00 g

4

g

.1



8

°

4

C

J

14.40°C3.012 103 J

The total amount of heat added was 3.358 kJ or 3.358 103 J. The difference between these
heat values is the amount of heat absorbed by the calorimeter.

__?J3.358 103 J3.012 103 J0.346 103 J, or 346 J absorbed by calorimeter

To obtain the heat capacity of the calorimeter, we divide the amount of heat absorbed by the
calorimeter, 346 J, by its temperature change.

__?

°

J

C



14

3

.

4

4

6

0°

J

C

 24.0 J/°C

The calorimeter absorbs 24.0 J of heat for each degree Celsius increase in its temperature.

You should now work Exercise 58.

Figure 15-3 A coffee-cup calorimeter. The

stirring rod is moved up and down to ensure

thorough mixing and uniform heating of the

solution during reaction. The polystyrene walls

and top provide insulation so that very little heat

escapes. This kind of calorimeter measures qp,

the heat transfer due to a reaction occurring at

constant pressure.

15-4 Calorimetry 597

Thermometer

Polystyrene

cover

Polystyrene

cups

Glass

stirring rod

Water

Beaker

One way to add heat is to use an

electric heater.