Calorimetry is the science of measuring the heat, q, evolved or absorbed for a chemical or physical process. If this process occurs at a constant pressure, the heat measured directly is qp. If you experimented with a "coffee cup calorimeter" in general chemistry, you were measuring qp. qp is the enthalpy change, DH. However, when the process occurs under constant-volume conditions then the measured heat is qv. qv is the internal energy change, DU. In the bomb calorimeter experiment, the volume is held constant during the reaction.

In 1881, Berthelot devised a closed container he called a bomb, based on the fact that many substances, including hydrocarbons, will react easily with oxygen. A general hydrocarbon combustion reaction can be written as follows, where the hydrocarbon reacts with oxygen to produce only water and carbon dioxide:

CxHyOz + O2(g) -----> CO2(g) + H2O(g)

The balanced version of this reaction would be: CxHyOz + (X+Y/4-Z/2) O2(g) -----> X CO2(g) + (Y/2) H2O(g)

The bomb calorimeter used today works by burning a hydrocarbon in high pressure oxygen in a stainless steel container, maintaining a constant volume. The stainless steel container is surrounded by water and the heat evolved by the reaction is absorbed by the surrounding water. By measuring the temperature change of the water, qv can be determined.

In a bomb calorimeter the heat measured for the sample burned is qv, which is the change in internal energy from initial to final states. The enthalpy change, D H, for this process is related to te internal energy, and qv, as follows:

 

D Ho = D U + D (pV)
(Eq. 1)
  
  D (pV) = D nRT
(Eq. 2)
  

where Dn is the change in the number of moles of all gases in the reaction system.

When you carry out your calculations, you will use the general relation between q and heat capacity:

  qv = (Cv) (D T)
(Eq. 3)
  
  where Cv will be the heat capacity of the calorimeter itself.  

The calorimeter heat capacity will need to be determined first with a substanc of known D Hc. After Cv for the calorimeter is determined (and it will be refered to as Ccal) then Eq. 3 can then be used to solve for the unknown heat of combustion of a sample. Therefore, the solution scheme will be as follows:

  First solve for heat capacity of the calorimeter: Ccal = qv,known / D T  
  Then use that value to find q for the samples: qv,unknown = (Ccal) (D T)  

 

In the oxygen bomb calorimeter there are three main parts (see apparatus):

  • A bomb, which houses the sample and oxygen for the combustion reaction.
  • The steel bucket, which holds a measured amount of water, thermometer and the bomb.
  • The outer jacket, which thermally insulates the entire apparatus.

There are two basic methods for measuring the heat of reaction of a sample: isothermal and adiabatic calorimetry. You will be using an adiabatic calorimeter. Details about the difference between the two types of calorimeters are given in Sime, pg. 421. No matter which calorimeter you use, you must calibrate the apparatus first. By measuring the temperature rise from the combustion of a known substance, one can determine the heat capacity of the apparatus. In this experiment you will use benzoic acid (C6H6O2) for your calibration of the apparatus.