Hess’s law type 2
The second way to apply Hess’s law uses standard enthalpies of formation.
- formation reaction
a reaction in which a compound is formed from its elements
C(s) + O2(g) → CO2(g),
- standard enthalpy of formation ()
the molar enthalpy for a formation reaction (expressed per mole of product produced) where the elements are in their standard states and the reaction occurs at SATP (25 ºC and 101.3 kPa)
We can calculate the enthalpy change of a reaction using the individual values of the products and reactants:
What is the enthalpy change for CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(l)?
Begin with the formula given above. Substitute moles and standard enthalpies of formation for each product and reactant:
This evaluates to −890.7 kJ. Notice that the answer is in kJ, not kJ/mol.