Honors Chemistry
Name: ______________________________________ Date: ________________ Mods: __________
Standard Enthalpy of Formation: In-Class Examples
Standard Enthalpy of Formation (Hfº) - the enthalpy change when one mole of a compound is
formed from its elements in their standard states (at 298K/25°C and atmospheric pressure).

Hfº for the most stable form of any element is zero
o Metals  solids (except for Mercury)
o Diatomics (BrINClHOF)  gases (except for Bromine and Iodine)
o Carbon  graphite is most stable form
1) Write a formation equation and give the value of Hfº for each compound below:
a) NH3
b) NaHCO3
c) C2H5OH
Using Enthalpies of Formation (Hfº) to Calculate Enthalpies of Reaction (Hrxnº)
The enthalpy of the reactants, Hreactants, and the enthalpy of the products, Hproducts, depends on the
bonding of the reactants and products, nothing else. Therefore, Hrxn only depends on the initial and
final state of the reaction, not on how you got from one state to another state. This means that Hrxn is
considered to be a “state function”.
If we have a balanced chemical equation, we can use the sum of the standard enthalpies of formation
of the products minus the sum of the standard enthalpies of formation of the reactants to find the
standard enthalpy change for the entire reaction using the following equation:
Hº =  Hfº(products) -  Hfº (reactants) and Appendix C in the book
2) Calculate the standard enthalpy change for the combustion of 1 mole of benzene, C6H6 (l).
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 carbon dioxide and sulfur dioxide gases and liquid water. 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) Hrxnº= -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 kJ/mol
CH4(g)
Ag2O(s)
NaCl(s)
C6H12O6(s)
MgO(g)
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 + 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)
ΔHº = -127.2 kJ
e. Solid napthalene (C10H8) combusts to yield CO2(g) and H2O(l) at STP and has a Hº = -5154
kJ/mol. Calculate the enthalpy of formation, ΔHfº, of napthalene.
f.
Liquid methanol (CH3OH) is often used as a fuel in high-performance engines in race cars.
Compare the standard enthalpy of combustion per mole of methanol with that per mole of
gasoline. Gasoline is actually a mixture of compounds, but assume for this problem that
gasoline is pure liquid octane (C8H18). The heat of formation, ΔHfº, of octane is -249.9 kJ/mol.