Advertisements
Advertisements
Question
Explain how heat absorbed at constant volume is measured using a bomb calorimeter with a neat diagram.
Advertisements
Solution
(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
\[\ce{∆U°_{C(pressure)} = ∆U°_{C(volume)} + ∆n_{{g}}RT}\]
APPEARS IN
RELATED QUESTIONS
The amount of heat exchanged with the surrounding at constant pressure is given by the quantity
C(diamond) → C(graphite), ∆H = –ve, this indicates that
The enthalpies of the formation of Al2O3 and Cr2O3 are – 1596 kJ and – 1134 kJ, respectively. ∆H for the reaction \[\ce{2Al + Cr2O3 -> 2Cr + Al2O3}\] is
If one mole of ammonia and one mole of hydrogen chloride are mixed in a closed container to form ammonium chloride gas, then
Given that \[\ce{C_{(g)} + O2_{(g)} -> CO2_{(g)}}\] ∆H° = – a kJ; \[\ce{2CO_{(g)} + O2_{(g)} -> 2CO2_{(g)}}\] ∆H° = – b kJ; Calculate the ∆H° for the reaction \[\ce{C_{(g)} + 1/2O2_{(g)} -> CO_{(g)}}\]
Predict the feasibility of a reaction when both ΔH and ΔS negative.
Predict the feasibility of a reaction when ΔH decreases but ΔS increases.
What is the unit of calorific value?
In a constant volume calorimeter, 3.5 g of gas with molecular weight 28 was burnt in excess oxygen at 298 K. The temperature of the calorimeter was found to increase from 298 K to 298.45 K due to the combustion process. Given that the calorimeter constant is 2.5 kJ K−1. Calculate the enthalpy of combustion of the gas in kJ mol−1.
A gas mixture of 3.67 lit of ethylene and methane on complete combustion at 25°C and at 1 atm pressure produce 6.11 lit of carbon dioxide. Find out the amount of heat evolved in kJ, during this combustion. (ΔHC(CH4)) = − 890 kJ mol−1 and (ΔHC(C2H4)) = − 1423 kJ mol−1
