Calorimetric bomb

See also: Bomb

A bends calorimetric is an equipment making it possible to measure the release of heat during a reaction carried out to constant volume. Under these conditions concerned heat is equal to the variation of the Fonction of state energy interns U of the reactional system: ΔU = QV . Heat not depending more on the manner of proceeding, one can then determine it.

One can thus determine the calorific value (in J/G) of a substance.

The calorimetric bomb was developed by Pierre-Eugene Marcellin Berthelot in 1879 whereas he studied the phenomena of combustion to constant volume.

Principle

To measure the Calorific value of a substance, it is necessary to determine the heat released by this substance during a reaction of Combustion supplements, fast and single. Complete to avoid the corrections difficult to calculate if part of the substance were not consumed, rapid to avoid the inevitable heat losses even if the system is isolated and single so that secondary reactions do not come to disturb measurement.

The principle of operation of the calorimetric bomb meets these three conditions. It is about a cylinder in Acier, good driver of the Chaleur and very resistant to the pressure which can reach instantaneously after the reaction, several hundreds of bar. One has with the bottom this cylinder a crucible in which will be deposited the substance to be studied. The cylinder is then filled with combustive (Dioxygène with a pressure from 20 to 30 bar to be sure that it will be in excess in order to ensure a complete combustion). The firing device consists of a wire in contact with the substance, which will be crossed by a current of strong intensity to start combustion. The bomb is placed in a Calorimètre including/understanding a great quantity of distilled water, provided with a device of agitation and a thermometer of precision. The quantity of water is important so that the rise in temperature does not exceed a few degrees.

After having measured the initial temperature, one starts the firing then one measures the temperature of the end of combustion when the thermal balance of the calorimeter is reached (moment when the temperature curve goes down again). Knowing the Heat capacity calorimeter, it is then easy to determine the quantity of heat produced by combustion by correcting it of the heat brought by the fusing.

Precaution for use

A calorimetric bomb must be calibrated before each measurement. The calibration is carried out by carrying out the combustion of a body whose calorific value is known (generally of the Benzoic acid , calorific value = 26.453 J.g-1). This calibration will make it possible to determine the heat capacity of the calorimeter.

A calorimetric bomb can contain gases with strong pressures. It is thus necessary to carry out many quality controls to avoid any risk of escape, even of explosion.

Biological application

The oxidation of the Nutriment S proceeds in the human body according to the same principle to obtain the energy necessary to the life. However the process is slower and less intense, indeed the cells could not resist a brutal increase in temperature. One can establish the energy equivalents of various food. In the organization, the oxidation of proteins is not also complete and it leads to the formation of Urée, it is then necessary to withdraw the energy value of produced urea.
  • Lipid: 38 kJ/G (that is to say approximately 9 kcal /g)
  • Glucid: 17 kJ/G (that is to say approximately 4 kcal /g)
  • Protein: 17 kJ/G (that is to say approximately 4 kcal /g)

See too

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