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प्रश्न
Explain how heat absorbed at constant volume is measured using a bomb calorimeter with a neat diagram.
उत्तर
(i) For chemical reactions, heal absorbed at constant volume, is measured in a bomb calorimeter.
(ii) Description of the apparatus and procedure: The inner vessel and its cover are made of strong steel. The cover is fitted tightly to the vessel by means of a metal lid and screws. A weighed amount of the substance is taken in a platinum cup connected with electrical wires for striking an arc instantly to kindle combustion. The bomb is then tightly closed and pressurized with excess oxygen. The bomb is lowered in the water, which is placed inside the calorimeter. A stirrer is placed in the bomb to stir the water uniformly. The reaction is started by striking the substance through electrical heating.
`"P"_"ext" = ("Force" ("F"))/("Area" ("A"))`
(iii) During burning, the exothermic heat generated inside the bomb raises the temperature of the surrounding water bath. The temperature change can be measured accurately using a Beckmann thermometer. Since the bomb calorimeter is sealed, its volume does not change, so the heat measurements, in this case, corresponds to the heat of reaction at constant volume.
Bomb calorimeter
(iv) In a bomb calorimeter experiment, a weighed sample of benzoic acid (w) is placed in the bomb which is then filled with excess oxygen and sealed. Ignition is brought about electrically. The rise in temperature (AT) is noted. Water equivalent or calorimetry equivalent of the calorimeter is known from the standard value of enthalpy of combustion of benzoic acid.
(v) ∆HC(C6H5COOH) = −3227 kJ mol−1
ωe = `(∆"H"_"C"("C"_6"H"_5"COOH"))/(∆"T") xx "W"/"M"_2`
(vi) By knowing o value, the enthalpy of combustion of any other substance is determined by adopting a similar procedure and using the substance in place of’ benzoic acid. By this experiment, the enthalpy of combustion at constant volume (AUC°) is known,
∆UC° = ωe. ∆T
(vii) Enthalpy of combustion at constant pressure of the substance is calculated from the equation
\[\ce{∆U°_{C(pressure)} = ∆U°_{C(volume)} + ∆n_{{g}}RT}\]
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