Lecture 22

Calorimetry

A. Types of Calorimeters

Calorimeters are devices for measuring the amount of heat generated by a chemical reaction or other process (such as a phase change). There are two main types:

• The bomb calorimeter that measures heat changes at constant volume. Since the heat change is measured at constant volume, it is the internal energy change that is being determined.
  
• Calorimeters, such as the ice calorimeter and coffee-cup calorimeter, that measure heat changes at constant pressure. Since the heat change is measured at constant pressure, it is the enthalpy change that is being determined.

B. Bomb Calorimeter

Figure 6.17 in Kotz and Treichel shows a schematic view of a bomb calorimeter. It is called a "bomb" because, when a combustible sample is burned in pure oxygen, the reaction is so rapid that it is essentially an explosion.

The components of a bomb calorimeter are:

• a steel sample chamber which contains the combustible sample and the oxygen
• a container of water (insulated) to absorb the heat given off by the combustion
• a thermometer to measure the temperature change of the water

The experimental procedure is simple:

• weigh the sample and place it in the combustion chamber
• weigh the calorimeter
• add water to the calorimeter and weigh again to get the weight of the water
• assemble the apparatus and measure the beginning temperature of the water
• ignite the sample by means of a spark
• measure the temperature of the water when thermal equilibrium has been re-established.

Now, we know the mass of the calorimeter, the mass of the water, the mass of the sample, and the temperature change that took place when the sample burned. From these data, the specific heat of water, and the heat capacity of the calorimeter, we can calculate the heat of combustion of the sample.

Heat evolved by the reaction = heat gained by the calorimeter and the water

 

C. Example: Determination of the Heat of Combustion of Caffeine

See sample question 9 in Assignment 8.

 

D. The Ice Calorimeter

Experiment 7 in your lab involves the use of the an "ice calorimeter" to measure the heat produced when magnesium metal reacts with an acid. The heat of reaction is measured by determining how much ice melts.

q_reaction + q_ice = 0

q_ice = (mass of ice melted) x (heat of fusion of ice (333 J/g))

The mass of ice melted is determined indirectly in this experiment: it is based on the change in volume that takes place when ice melts. Ice has a density of 0.917 g/mL at 0^o C, whereas water has a density of 1.000 g/mL at 0^o C. Thus equal masses of ice and water have quite different volumes:

Volume occupied by 100.0 g ice: 100.0 g x 1 mL / 0.917 g = 109.0 mL

Volume occupied by 100.0 g water = 100.0 mL

The large volume change can be used to determine the mass of ice melted, as follows. When a given mass of ice melts to form the same mass of water, the volume change is given by

Now since w_ice = w_water = w,

Solving for w, the weight (mass) of ice melted by the reaction, we obtain

Thus, the mass of ice that melted is easily determined from the volume change that you will measure in the lab, along with the densities of water and of ice at 0^o C. The calculation for the enthalpy of reaction proceeds as in Question 8 on Assignment 8.

 

This page is http://chemiris.labs.brocku.ca/~chemweb/courses/chem180/CHEM1P80_Lecture_22.html
Created October 31, 2000 by M. F. Richardson
© Brock University, 2000