5 Steps to a 5 AP Chemistry 2019

124 ❯ STEP 4. Review the Knowledge You Need to Score High

Substituting the mathematical relationship for q gives:

• (csolid × msolid × DTsolid) = cwater × mwater × DTwater

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 calorim-

eter. 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 O(l)

Answer:

°

°

(3.562kJ)

(C)

(3.682C)=13.12kJ







 °

















(1.000 kg) °

1,000 g

1 kg

4.184 J

gC

1 kJ

1,000 J

(3.682C)=15.40 kJ

total heat = 13.12 kJ + 15.40 kJ = 28.52 kJ

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

→ - 28.52 kJ

Electrical

source

+

• 
  

Stirrer

Thermometer

Water bath

Ignition coil

Stirrer Thermometer

Polystyrene cups

Water

Sample

(a) Coffee-cup calorimeter

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

Figure 9.1 Two types of calorimeters.