# Heat of Formation (Definition, Applications, Solved Problems)

## Heat of Formation

– The heat of formation of a compound is defined as The change in enthalpy that takes place when one mole of the compound is formed from its elements.

– It is denoted by ΔH.

– For example, the heat of formation of ferrous sulphide and acetylene may be expressed as:

– Similarly, the reaction between gaseous hydrogen and gaseous chlorine to form gaseous hydrogen chloride is represented by the equation:

– It may be noted in this case that – 44.0 kcal is not the heat of formation of hydrogen chloride because this amount of heat is evolved when two moles of hydrogen chloride are formed.

– The heat of formation of hydrogen chloride, therefore, would be – 44.0/2 = – 22.0 kcal and the equation can be written as:

## Standard Heat of Formation

– The standard heat of formation of a compound is defined as:

The change in enthalpy that takes place when one mole of a compound is formed from its elements, all substances being in their standard states (298 K and 1 atm pressure).

– The standard heat of formation of some compounds is given in the following table:

– By convention, the standard heat of formation of all elements is assumed to be zero.

## Standard Heat of Reaction (ΔHº) from Standard Heat of Formation (ΔHof)

– We can calculate the heat of reaction under standard conditions from the values of standard heat of formation of various reactants and products.

– The standard heat of reaction is equal to the standard heat of formation of products minus the standard heat of formation of reactants.

– Let us consider a general reaction:

aA + bB → cC + dD

– The standard heat of reaction is given by:

## Solved Problem

Problem (1): Calculate ΔHº for the reaction:

CO2(g) + H2(g) → CO(g) + H2O(g)

given that ΔHof for CO2(g), CO(g) and H2O(g) are – 393.5, – 111.31 and – 241.80 kJ mol–1 respectively.

Solution:

– we have here:

Problem (2): The standard heats of formation of C2H5OH(l), CO2(g), and H2O(l) are – 277.0, –393.5, and –285.5 kJ mol–1 respectively. Calculate the standard heat change for the reaction:

Solution:

– we have here:

– we can read this subject also: Measurement of The Heat of Reaction

Reference: Essentials of Physical Chemistry /Arun Bahl, B.S Bahl and G.D. Tuli / multicolour edition.