Bomb calorimeter is normally used for the determination of calorific value of solid fuels. However, it can also be used for liquid fuels. The combustion of the fuel takes place at constant volume in a tightly closed vessel as shown in Figure 2.1. The higher calorific value of the fuel is determined at constant volume.
Figure 2.1 Experimental Set-up of Bomb Calorimeter
The bomb calorimeter consists of strong stainless steel shell which is known as bomb. The capacity of the bomb is 650 cc and it can withstand pressure up to 200 atm. Water is filled in the bomb to a specific level to act as water seal. In the top cover of the bomb, oxygen connection, and product release valve are arranged. Bottom cover of the bomb supports an upright, one of them carrying a ring to support crucible made of silica and quartz. The uprights are provided through the bottom with two insulating firing plugs through which the leads from the main supply are taken via a rheostat. During the test, the bomb is placed in copper calorimeter containing 2,500 CC of water that is agitated by a stirrer run by a motor.
The calorimeter is surrounded by an outer walled vessel with water jacket, and air space between these two containers reduces the radiation loss. The thermometer used to read up to 1/100°C at total range is 5°C.
The fuel is placed in a crucible. The bomb is then connected to oxygen cylinder and pressure is adjusted to about 25–30 atm. Water in the container is continuously stirred and temperature is noted up to steady state. Now stirring is stopped. The fuse wire ignites the fuel in presence of oxygen. The temperature of water starts rising. At certain interval (10 s) temperature is noted up to maximum temperature and after each ½ minute decreasing temperature.
True temperature rise, 
+ Recorded temperature rise
where rc is maximum temperature reached.
Let C is calorific value of the fuel burnt
C1 is calorific value of wire burnt
W is mass of water contained in calorimeter
w is water equivalent of calorimeter
x is quantity of fuel burnt
xw is quantity of wire burnt
θ1 is steady temperature before combustion
θ2 is max temperature after combustion
t is time elapsed for reaching maximum temperature
Example 2.1: A 1 g sample of fuel is burned in a bomb calorimeter containing 1.2 kg of water at an initial temperature of 25°C. After the reaction, the final temperature of the water is 33.2°C. The heat capacity of the calorimeter is 837 J/°C. The specific heat of water is 4.184 J/g °C. Calculate the heat of combustion of the fuel in kJ/mol.
Solution:
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