Honors Chemistry
Name: ______________________________________ Date: ________________ Mods: __________
Standard Enthalpy of Formation: In-Class Examples
Standard Enthalpy of Formation (Hfº)
Definition:
The enthalpy change when one mole of a compound is
states (at 298K/25°C and atmospheric pressure).

formed from its elements in their standard
Hfº for the most stable form of any element is zero
o Metals  solids (except for Mercury)
o Diatomics (BrINClHOF)  gases [except for Bromine (l) and Iodine (s)]
o Carbon  graphite is most stable form
1) Write a standard enthalpy of formation equation and give the Hfº for each compound below:
a) C2H5OH (l)
b) NH3 (g)
c) NaHCO3 (s)
Using Enthalpies of Formation (Hfº) to Calculate Enthalpies of Reaction (Hrxnº)
Since enthalpy is a state function, the only thing that matters when determining Hrxn is the initial and
final state of the reaction, not on how you got from one state to another state. In any chemical reaction,
the reactants are the initial state, and the products are the final state. Therefore, given any balanced
chemical reaction and Appendix C in the textbook, the standard enthalpy change for the entire reaction
can be determined using the equation:
Hºrxn = [nHfº(products)] – [nHfº(reactants)]
* n = # of moles of reactants and products (coefficients)
2) Calculate the standard enthalpy change for the combustion of 1 mole of propane, C3H8 (g).
3) Determine the standard enthalpy change for the following reaction:
CH4(g) + 4 Cl2(g)  CCl4(g) + 4 HCl(g)
4) Calculate the standard enthalpy change for the following reaction:
3 Fe2O3(s) + CO(g)  2 Fe3O4(s) + CO2(g)
5) The combustion of thiophene, C4H4S(l), a compound used in the manufacture of pharmaceuticals,
produces gaseous carbon dioxide and sulfur dioxide as well as water vapor. The standard
enthalpy change for the reaction of one mole of C4H4S(l) is -2523 kJ. Use this information and
data from Appendix C to find Hfº for C4H4S(l).
6) Use the balanced equation and information given below to calculate Hfº for SO2Cl2:
SO2Cl2(g) + 2 H2O(l)  2 HCl(g) + H2SO4(aq) Hºrxn= -62 kJ
Enthalpy of Formation & Hess’s Law: Homework
1) Heats of Formation: Write standard formation equations for one mole of the following compounds.
Use Appendix C for the standard enthalpy values.
Compound
Formation Equation
Hfº (kJ/mol)
CO(g)
C(graphite) + ½ O2(g)  CO(g)
–110.5
CH4(g)
Ag2O(s)
NaCl(s)
C6H12O6(s)
MgO(s)
Fe2O3(s)
Ca(OH)2(s)
SO3(g)
2) Use the equation Hº =  Hfº(products) -  Hfº (reactants) and Appendix C in the book to answer the
following:
a. Calculate the standard enthalpy change for the combustion of one mole of ethane, C2H6(g).
b. Calculate the standard enthalpy change for the combustion of one mole of butane, C4H10 (l).
c. What is the standard enthalpy change for the following thermite reaction:
2 Al (s) + Fe2O3 (s)  Al2O3 (s) + 2 Fe (s)
d. Calcium carbide reacts with water to form acetylene gas and calcium hydroxide. From the
following thermochemical equation and the data in Appendix C, calculate the enthalpy of
formation, ΔHfº, for CaC2(s).
CaC2(s) + 2 H2O(l)  C2H2(g) + Ca(OH)2 (s)
ΔHºrxn = -127.2 kJ
e. Solid napthalene (C10H8) combusts to yield CO2(g) and H2O(g) at STP and has a Hºrxn = -5154
kJ/mol. Calculate the enthalpy of formation, ΔHfº, of napthalene.
f.
Liquid methanol (CH3OH) is sometimes used as a fuel in high-performance engines in race cars.
Gasoline is actually a mixture of compounds but, for this problem, assume that gasoline is pure
liquid octane (C8H18). The heat of formation, ΔHfº, of octane is -249.9 kJ/mol. Write the standard
combustion reactions for one mole of methanol and one mole of octane. Then, determine the
standard enthalpy of reaction, Hºrxn, for each combustion reaction. Based on your calculations,
which combustion reaction releases more heat per mole?