THOMPSON’S CALORIMETER

Objective :

·         To determine calorific value of a given solid fuel.

 Apparatus & Substances:

·         Coal     

·         Potassium Nitrate    ( KNO ₃ )              

·         Potassium Chlorate ( KClO ₃ )                        

·         Nitrate Paper                                         

·         Thompson Calorimeter

·         Furnace tube

·         Thermometer (Digital)

·         Stop Watch

·         Measuring Cylinder

·         Water   2000 ml 

 Procedure 

•   Finely powder the coal provided and mix with KNO ₃ and KClO₃ thoroughly. 
•   Pack the mixture into the furnace tube along with a piece of Nitrate paper. 
• Place 2000 ml of water and the thermometer in the measuring cylinder and note the temperature of the water 
• Ignite Nitrate paper-fuse and fix the cover with the stopcock closed and quickly lower into the 2000 ml of water. At the same instance start the stopwatch 
• Take the thermometer readings every half a minute until the temperature drops by several degrees. 
• When the bubbles stop indicating the stopping of the combustion, open the stopcock and clear the tube using the metal wire provided. Now stire the water by moving the calorimeter up and down whilst temperature readings are taken.

Discussion

1.         Minimizing the heat loss.

1)  Radiation           :- Can be minimized by polishing the surface of the calorimeter.

2) Conduction         :- By insulating the calorimeter using an insulating material, conduction can be minimized.

3)   Convection         :- Covering the whole apparatus will minimize the convection loss.

Another method of minimizing the heat losses is by starting the practical at a little lower temperature than the surrounding and finishing the practical at a higher temperature than the surrounding by same amount. This will equate the amount of heat out with the amount of heat in during the practical.

2.         Dickinson method of correcting for radiation losses.

By using the Dickinson method, the maximum theoretical temperature can be found as follows.

Tmax = T1 + (T1 ¬+ T2)

3.         Heat reactions of Nitrate and Chlorate.

Burning of nitrates and chlorates both release O2 which is essential for burning of coal. But when KNO3 burns it release heat to the system (Exothermic) and when KClO3 burns it absorb heat from the surrounding (Endothermic). The ratio of nitrate and chlorate is exactly balanced such that the heat rejected from the exothermic reaction is equals to endothermic reaction. Therefore the heat releases of secondary reactions will not affect to the practical.

 

4.         Calorific value of coal.

The calorific value of coal depends on the place of the mine. Coal mined from different parts of world has different calorific values. This is because of the impurities and other slight variations. An average value can be given as 30500 – 30800 kJ/kg at 150C.

But the practical value we obtained was little less than 20000 kJ/kg. The difference may due to:

•             Coal may not be burnt 100%.

•             There are heat losses during the practical.

•             While measuring chemicals and time, errors might be occurred.

•             Chemicals used may be contaminated.

                                   5.  Other methods available for finding the calorific value of solid fuels.

·         Bomb Calorimeter.

·         Marcet’s Boiler.

·         Separation & Throttling.

·         Boy’s Calorimeter.

6.

1.       Finding the calorific value of a sample of firewood.

·         Grind the given sample as and dry it.

·         Mix the sample with nitrate and chlorate as did in the practical.

·         Rest of the procedure is same as before.

·         Calculations can be done as follows.

Energy released by m g of wood               = Q kJ

Mass of water                                                   = m_w

Mass of calorimeter                                        = m_c

Specific heat of water                                    = S_w

Specific heat of calorimeter                         = S_c

                              Q                                                               = (m_w * S_w + m_c * S_c) * ∆θ            

Calorific value of wood                  = (Q / m) * 1000 kJ/kg

                                                            = (m_w * S_w + m_c * S_c) * ∆θ * 1000 /m kJ/kg