Heat of Combustion of Benzoic Acid under Thermodynamic Standard Conditions

Abstract

BENZOIC acid is widely used in thermochemistry as a calorimetric standard substance. This widespread usage as a standard means that benzoic acid is normally combusted in nearly identical bomb calorimeters. As a consequence it has become common practice to quote its heat of combustion under standard bomb conditions (ΔQ_B) as suggested by Washburn¹, rather than under thermodynamic standard conditions (− ΔE°), as this often eliminates the necessity for lengthy thermodynamic conditions. At present the standard bomb conditions for the combustion of benzoic acid are²: (a) The combustion reaction is referred to 25° C. (b) The sample is burned in a bomb of constant volume in pure oxygen at an initial pressure of 30 atm. at 25° C. (c) The number of grammes of sample burned is equal to three times the internal volume of the bomb in litres. (d) The number of grammes of water placed in the bomb before combustion is equal to three times the internal volume of the bomb in litres. For certain other applications, notably when benzoic acid is used as a starter for the combustion of a less easily combusted material, it is necessary to know its heat of combustion under thermodynamic standard conditions. Such a case arose recently at Harwell in a re-determination of the heats of combustion of natural and artificial graphites.