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_f.

– 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 (ΔH^o_f)

– 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

– Now we will discuss some solved problems about this subject.

Problem (1): Calculate ΔHº for the reaction:

CO₂(g) + H₂(g) → CO(g) + H₂O(g)

given that ΔH^o_f for CO_2(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: