Standard heats of formation of chemical compounds are one of the most useful thermodynamic quantities. It is the change of enthalpy that accompanies the formation of one mole of a substance in its standard state from its constituent elements. This is also often used to calculate the heat of combustion, which is the energy released as heat when one mole of a compound undergoes complete combustion with oxygen under standard conditions. Heat released in a chemical reaction can be determined experimentally by using an adiabatic calorimeter. Calorimetry is the most convenient way to measure the heat of combustion. It studies the heat transfer from a hotter object to a colder one. In a calorimeter, the combustion reaction occurs in a closed container under constant volume. A compound, usually a hydrocarbon, is burned in the presence of excess oxygen forming carbon dioxide and water as products. Important thermodynamic information, such as the enthalpy of combustion, could be obtained by measuring the temperature change. The bomb is immersed in a quantity of water and surrounded by an adiabatic shield that serves as the heat insulator. In this experiment, a
commercial bomb calorimeter (Parr) is used to determine the heat of combustion of an organic compound. The sample is held in a cup that is placed near the ignition wire that is used to start the combustion reaction as shown in Figure 1.
Figure 1: Schematic of a constant volume bomb calorimeter.
Then it is charged with oxygen gas to the pressure of about 25 atm. The assembled bomb is then placed in a bucket of water. The thermometer is placed in the water bath to measure the rise in temperature. Continuous
stirring ensures the even distribution of heat. When current is applied through the wire the sample inside combusts. The release of heat is transferred to the water and measured as a temperature change.
First, the heat capacity of the calorimeter needs to be determined to calculate the...