Enthalpies of Formation

In a formation equation one mole of product is formed from the standard or elemental forms of each element. 

Formation Equation

Enthalpy of a Reaction

The enthalpy value associated with an element or compound can be used to find the enthalpy of a reaction. 

Example: The oxidation of ammonia is given by the following reaction:

4 NH3 (g) +  5 O2 (g) →  4 NO (g)  + 6 H2O (g)

Calculate the Horxn if the Hof value for NH3 , NO and H2O are -45.9 kJ/mol, 90.3 kJ/mol and -241.8 kJ/mol respectively.


The enthalpy of a reaction can be determined if we are given the enthalpy of formation value for each compound. 

Problem: Ibuprofen is used as an anti-inflammatory agent used to deal with pain and bring down fevers. If it has a molecular formula of C13H18O2 determine the balanced chemical equation that would give you directly the enthalpy of formation for ibuprofen.


Example: Use the following bond strength values (kJ/mol):

C–H  412          C–O  360          C=O  743           

C–C  348          H–H  436          C=C  611

C≡C  837         C≡O  1072        O–H  464

O=O 498

Calculate the enthalpy of the reaction shown in the formula below: 


When given bond energies then calculating the enthalpy of the reaction requires a different equation.