In a formation equation one mole of product is formed from the standard or elemental forms of each element.
Concept #1: Understanding a Formation Equation
Concept #2: Understanding Heats of Formation
The enthalpy value associated with an element or compound can be used to find the enthalpy of a reaction.
Example #1: 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.
Practice: 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 #2: 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
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.