We will also use enthalpies of formation (
\Delta H ^\circ _f
) to calculate heat of reaction values. The enthalpy of formation (also
called heat of formation) for a compound is the heat that is given off (or
absorbed) when
elements
combine to form
one mole
of a compound. These values allow us to compare
energy changes on a common scale using elements in their most stable form as a
reference. Enthalpies of formation (
\Delta H ^\circ _f
) are measured at standard state (P = 1 atm and T = 25°C) with each reactant
in its most stable form under those conditions. The “°” denotes a value at
standard state. Note that the temperature associated with standard state is
different from the temperature associated STP that we introduced when we
discussed gases.
Shown below is an example in which carbon (as graphite, carbon’s most stable
elemental form at standard state) reacts with diatomic oxygen gas (oxygen’s
most stable elemental form at standard state) to form one mole of carbon
dioxide gas. The value of
\Delta H ^\circ _{rxn}
for this reaction is
-393.5 kJ. Since this reaction produces one mole of carbon dioxide from its
component elements in their most stable states,
\Delta H ^\circ _{rxn}
for this reaction
equals
\Delta H ^\circ _{f}
for
CO_2
(g).
C_{(graphite)} + O_{2(g)} \longrightarrow CO_{2(g)} \Delta H ^\circ _{rxn}
= -393.5 \text{ kJ}
It is worth noting that ΔH°f for any element in its most stable state is equal
to zero because the reactants and products would be the same any such a
reaction.
In order to use
\Delta H ^\circ _f
values
and in order to recognize when
\Delta H ^\circ _f
=
\Delta H ^\circ _{rxn}
we need to be able to
determine whether an element is in its most stable form at standard state. All
metals and metalloids are solids at standard state except for mercury (Hg),
which is a liquid. Noble gases (group 8) are all monatomic gases. Several
elements exist as diatomic species at standard state, these include group 7
(
F_2, Cl_2, Br_2, \text{and }I_2
) plus hydrogen (
H_2
), nitrogen (
N_2
), and oxygen (
O_2
). All of these diatomic species are gases at standard state except for
Br_2
(liquid) and
I_2
(solid). All other nonmetals are solids.
It is worth noting that for carbon the most stable form is graphite (versus
diamond). Graphite should be familiar, as it is the substance used to make
pencil lead.