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 are 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

Problem (1): Calculate ΔHº for the reaction:

CO₂(g) + H₂(g) → CO(g) + H₂O(g)

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

Solution

Here we have:

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: