Enthalpy of combustion
Generally combustion reactions occur in oxygen atmosphere (excess oxygen) with evolution of heat. These reactions are exothermic in nature. Enthalpy changes of combustion reactions are used in industrial heating and in rocket fuels and in domestic fuels.
(QWKDOS\ FKDQJH RI FRPEXVWLRQ ûcH, of a substance at a given temperature is defined as the enthalpy change of the reaction accompanying the complete combustion of one mole of the substance in presence of excess oxygen at that temperature. The enthalpy change of combustion of substances in their standard states are known as standard enthalpy change RI FRPEXVWLRQ ûcH). These values are useful to experimentally determine the standard enthalpy change of formation of organic compounds.
Enthalpy changes of combustion of chemical substances are experimentally determined using a bomb calorimeter.
The bomb calorimeter apparatus is shown in Fig.12.3. The inner vessel or the bomb and its cover are made of strong steel. The cover is fitted tightly to the vessel by means of metal lid and screws. A weighed amount of the substance is taken in a platinum cup or boat connected with electrical wires for striking an arc instantly to kindle combustion. The bomb is then tightly closed and pressurised with excess oxygen. The bomb is lowered in water which is placed inside the calorimeter. A stirrer is placed in the space between the wall of the calorimeter and the bomb, so that water can be stirred, uniformly. The reaction is started in the bomb by heating the substance through electrical heating. During burning, the exothermic heat generated inside the bomb raises the temperature of the surrounding water bath. The enthalpy measurements in this case corresponds to the heat of reaction at constant volume. Although the temperature rise is small (only by few degrees), the temperature change can be measured accurately using Beckmann thermometer.
In a typical 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 (∆T) is noted. Water equivalent of the calorimeter is known from the standard value of enthalpy of combustion of benzoic acid.
∆HcoC6H5COOH(s) = -3227 kJ mol-1
∆HcoC6H5COOH x ( w / M2 ) = wc ∆T
(where M2 = mol.wt benzoic acid).
Knowing wc value, the enthalpy of combustion of any other substance is determined adopting the similar procedure and using the substance in place of benzoic acid. By this experiment, the enthalpy of combustion at constant volume ∆HcVol) is known
∆Hco(Vol) = wc ∆T
Enthalpy of combustion at constant pressure of the substance is calculated from the equation,
∆Hco(Pr) = ∆ Hc0(Vol) + ∆ n(g)RT
and ∆n(g) is known from the difference in the number of moles of the products and reactants in the completely balanced equation of combustion of the substance with excess oxygen.