Enthalpy of Formation
Consider the complete combustion reaction between methane and oxygen:
CH4(g) + 2 O2(g) --> CO2(g) + 2 H2O(l)
ΔH1
Another way to look at this reaction is to break apart the compound
into elements in their most stable form and then reform the elements into
the compounds.
1) CH4(g) + 2 O2(g)  C(s) + 2 O2(g) + 2 H2(g)
ΔH2
2) C(s) + 2 O2(q) + 2 H2(g) CO2(g) + 2 H2O(l)
ΔH3
CH4(g) + 2 O2(g)  CO2(g) + 2 H2O(l)
The enthalpy of the reaction:
ΔH3 + ΔH2
ΔH1 = ΔH2 + ΔH3
This concept of breaking compounds into elements and reforming
products leads to the concept of an Enthalpy of Formation. This is defined
as the enthalpy change for 1.0 mole of a substance formed by its elements in
their most stable form and in their standard state (1.0 atm and 25°C). The
enthalpy of formation of an element in their most stable form is 0 kJ.
ΔH°rxn= ΣΔHformation, products – ΣΔHformation, reactants
This equation follows from the previous calculation.
Exercise 1 - Give the equation for the enthalpy of formation of carbon
dioxide, propane and ammonium chloride.
Exercise 2 - Give the equation for the enthalpy of reaction for the
combustion of one mole of propane.