Standard Enthalpies of Formation
A useful way to determine the values of enthalpy changes is in terms of the standard
enthalpies of formation of compounds. This is referred to as the standard enthalpy of
formation, Hf.
standard enthalpy of formation, Hf :
H for the reaction in which 1 mole of pure compound is formed from elements in
their most stable form and in their standard state
From the Hf values for the reactants and products or a reaction, the H for the
reaction can be calculated using Hess’s Law. In other words,
H =
H =
sum of standard enthalpies
of formation of products
[sum of Hf products]
sum of standard enthalpies
of formation of reactants
-
[sum of Hf reactants]
*Note: Hf of an element is zero.
Example:
Find H for the oxidation of NH3 according to the equation
4 NH3 (g) + 5 O2 (g)  4 NO (g) + 6 H2O (g)
from the standard enthalpies of formation given in the Appendix.
NH3 (g)
O2 (g)
NO (g)
H2O (g)
Hf = - 45.9 kJ/mol
Hf = 0
Hf = 90.2 kJ/mol
Hf = - 241.8 kJ/mol
H = [sum of Hf products] - [sum of Hf reactants]
= [ 4 mol (90.2 kJ/mol) + 6 mol (- 241.8 kJ/mol)] – [ 4 mol (- 45.9 kJ/mol) + 0 ]
= 360.8 kJ –1450.8 kJ + 183.6 kJ
= - 906.4 kJ
Practice Problems
(a) Find H for the combustion of methane fuel.
(b) The standard enthalpy of combustion of benzene, C6H6 (l), to carbon dioxide and
liquid water is – 3273 kJ/mol. What is Hf benzene?
Multistep Problems Using Standard Enthalpies of Formation
Example:
When octane burns in an automobile engine, heat is released to the air and the metal in
the car engine, but a significant portion is absorbed by the liquid in the cooling system –
an aqueous solution of ethylene glycol. What mass of octane is completely burned to
cause the heating of 20.0 kg of aqueous ethylene glycol automobile coolant from 10C
to 70C? The specific heat of ethylene glycol is 3.5 J/(gC). Assume water is produced
as a gas and that all the heat flows into the coolant.
Example:
One way to heat water in a home or cottage is to burn propane. If 3.20 g of propane
burns, what temperature change will be observed if all the heat from combustion
transfers into 4.00 kg of water?