of a formation reaction, called the enthalpy of formationor (more loosely) the
heat of formation.As an example,
^12 N 2 (g) + O 2 (g) →NO 2 (g)is the formation reaction for NO 2. As a counterexample,
2NO 2 (g) + ^12 O 2 (g) →N 2 O 5 (g)is notthe formation reaction for N 2 O 5 because the reactants are not all the el-
ements that compose N 2 O 5. For tabulation purposes, most of the fHvalues
are measured with respect to the standard state of the reactants, so they are
usually fH° (with ° to indicate standard state).
Example 2.16
Determine whether the following reactions are formation reactions or not, and
if not, why. Assume that the reactions are occurring under standard conditions.
a.H 2 (g) + ^12 O 2 (g) →H 2 O ()
b.Ca (s) + 2 Cl (g) →CaCl 2 (s)
c.2 Fe (s) + 3S (rhombic) + 4O 3 (g) →Fe 2 (SO 4 ) 3 (s)
d.6C (s) + 6H 2 (g) + 3O 2 (g) →C 6 H 12 O 6 (s) (glucose)Solution
a.Yes, this is the formation reaction for liquid water.
b.No. The “standard form” of chlorine is a diatomic molecule.
c.No. The “standard form” of elemental oxygen is the diatomic molecule. The
O 3 in the formula is the allotrope ozone.
d.Yes, this is the formation reaction for glucose.Notice that, by definition, the enthalpy of formation for elements in their
standard state is exactly zero. This is because, no matter what the absolute
enthalpies of the product and reactant are, they are the same, so the change in
enthalpy for the reaction is zero. For example,
Br 2 ()→Br 2 ()is the formation reaction for elemental bromine. Since there is no change in
the chemical identity over the course of the reaction, the enthalpy change is
zero and we say that fH°0 for elemental bromine. The same situation ex-
ists for all elements in their standard states.
The reason we focus on formation reactions is because it is the changes in
enthalpy for formation reactions that are tabulated and used to determine en-
thalpy changes for chemical processes. This is because any chemical reaction
can be written as an algebraic combination of formation reactions. Hess’s law
therefore dictates how the fH° values are combined.
As an example, let us examine the following chemical reaction:
Fe 2 O 3 (s) + 3SO 3 ()→Fe 2 (SO 4 ) 3 (s) (2.54)
What is the rxnH° for this chemical reaction? We can separate this reaction
into formation reactions for every reactant and product in the process:
Fe 2 O 3 (s)→2Fe (s) + ^32 O 2 (g) (a)
3[SO 3 ()→S (s) + ^32 O 2 (g)] (b)
2Fe (s) + 3S (s) + 6O 2 (g)→Fe 2 (SO 4 ) 3 (s) (c)2.10 Chemical Changes 55