5 Steps to a 5 AP Chemistry

Substituting the mathematical relationship for q gives:

−(csolid×msolid×ΔTsolid) =cwater×mwater×ΔTwater

This equation can then be solved for the specific heat capacity of the solid.

The constant-volume bomb calorimeter is used to measure the energy changes that

occur during combustion reactions. A weighed sample of the substance being investigated

is placed in the calorimeter, and compressed oxygen is added. The sample is ignited by a

hot wire, and the temperature change of the calorimeter and a known mass of water is

measured. The heat capacity of the calorimeter/water system is sometimes known.

For example, a 1.5886 g sample of glucose (C 6 H 12 O 6 ) was ignited in a bomb calorime-

ter. The temperature increased by 3.682°C. The heat capacity of the calorimeter was

3.562 kJ/°C, and the calorimeter contained 1.000 kg of water. Find the molar heat of reac-

tion (i.e., kJ/mole) for:

C 6 H 12 O 6 (s) +6 O 2 (g) →6 CO 2 (g) +6 H 2 O(l)

Answer:

total heat =13.12 kJ +15.40 kJ = 28.52 kJ

Note: The temperature increased so the reaction was exothermic (−)

→−28.52 kJ

(.

.

1 000

1000

1

4 184 1

1000

kg)

g

kg

J

gC

kJ

J

⎛

⎝

⎜

⎞

⎠

⎟

°

⎛

⎝

⎜

⎞

⎠

⎟

⎛

⎝⎝

⎜

⎞

⎠

⎟(.3 682 C) = 15.40 kJ°

(.

(

(.

3 562

3 682

kJ)

C)

C)=13.12 kJ

°

°

Thermodynamics  125

Electrical

source

+

• 
  

Stirrer

Thermometer

Water bath

Ignition coil

Stirrer Thermometer

Polystrene cups

Water

Sample

(a) Coffee-cup calorimeter

(constant pressure) (b) Bomb calorimeter (constant volume)

Figure 9.1 Two types of calorimeters.