Heat of Formation - QS Study

Heat of Formation

“The standard heat of formation of a compound is defined as the change in hear content, ∆H, when one mole of the substance is formed from its elements in their most stable form in their standard stares”.

In calculating heats of formation the enthalpies of the elements in their standard states are arbitrarily taken as zero. For example, in the reaction

C (graphite) + O2 (g) → CO2 (g); ∆H0 = – 393.5 kJ

it is implied that the heat of formation of carbon dioxide is – 393.5 kJ, because 393.5 kJ of heat is evolved when one mole of carbon dioxide gas is formed at 1 atmosphere pressure and 250 C from carbon (graphite) and oxygen under the same conditions. As the heat of formation of a compound is the difference between the heat content of the compound and that of its elemeans and since the heat of its element (graphite and oxygen in the above example) is taken as zero by convention, it follows that the heat of formation of the compound is equal to its heat content.

The standard heats of formation of many organic and inorganic compounds have been compiled. A few of them are given in Table.

Table: Heat of formation of some compounds at 250 C in kJ mol-1

The heats of formation data are very useful in calculating enthalpy changes of reactions which are difficult to measure directly. The change in enthalpy ∆H of a given reaction may be obtained by subtracting the heats of formation, ∆H0F, of the reactant from those of the products.

∆H0reaction = ∑ (∆H0F)products – ∑ (∆H0F)reactants

In using this formula the number of moles of each of the reactants and products in the balanced equation for the reaction has to be considered.