The Foundations of Chemistry

Standard molar enthalpy of formation is often called standard molar heat of forma-
tionor, more simply, heat of formation.The superscript zero in
Hf^0 signifies standard
pressure, 1 atmosphere. Negative values for
Hf^0 describe exothermic formation reactions,
whereas positive values for
Hf^0 describe endothermic formation reactions.
The enthalpy change for a balanced equation that gives a compound from its elements
does not necessarily give a molar enthalpy of formation for the compound. Consider the
following exothermic reaction at standard conditions.

H 2 (g)Br 2 (
)88n2HBr(g) 
H^0 rxn72.8 kJ/mol rxn

We see that twomoles of HBr(g) are formed in the reaction as written. Half as much
energy, 36.4 kJ, is liberated when one moleof HBr(g) is produced from its constituent
elements in their standard states. For HBr(g),
Hf^0 36.4 kJ/mol. This can be shown
by dividing all coefficients in the balanced equation by 2.

^12 H 2 (g)^12 Br 2 (
)88nHBr(g) 
H^0 rxn36.4 kJ/mol rxn

H^0 f HBr(g)36.4 kJ/mol HBr(g)

Standard heats of formation of some common substances are tabulated in Table 15-1.
Appendix K contains a larger listing.
When referring to a thermodynamic quantity for a substance,we often omit the descrip-
tion of the substance from the units. Units for tabulated
H^0 fvalues are given as “kJ/mol”;
we must interpret this as “per mole of the substance in the specified state.” For instance,

for HBr(g) the tabulated
Hf^0 value of 36.4 kJ/mol should be interpreted as.

EXAMPLE 15-6 Interpretation of Hf^0

The standard molar enthalpy of formation of ethanol, C 2 H 5 OH(
), is 277.7 kJ/mol. Write
the thermochemical equation for the reaction for which
H^0 rxn277.7 kJ/mol rxn.

36.4 kJ



mol HBr(g)

The coefficients ^12 preceding H 2 (g) and

Br 2 (
) do notimply half a molecule of

each. In thermochemical equations, the

coefficients always refer to the number

of molesunder consideration.

15-7 Standard Molar Enthalpies of Formation, Hf^0603

TABLE 15-1 Selected Standard Molar Enthalpies of Formation at 298 K

Substance 
Hf^0 (kJ/mol) Substance 
H^0 f(kJ/mol)

Br 2 (
) 0 HgS(s) red 58.2

Br 2 (g) 30.91 H 2 (g) 0
C(diamond) 1.897 HBr(g) 36.4

C(graphite) 0 H 2 O(
) 285.8
CH 4 (g) 74.81 H 2 O(g) 241.8

C 2 H 4 (g) 52.26 NO(g) 90.25
C 6 H 6 (
) 49.03 Na(s) 0

C 2 H 5 OH(
) 277.7 NaCl(s) 411.0
CO(g) 110.5 O 2 (g) 0

CO 2 (g) 393.5 SO 2 (g) 296.8
CaO(s) 635.5 SiH 4 (g) 34.0

CaCO 3 (s) 1207.0 SiCl 4 (g) 657.0
Cl 2 (g) 0 SiO 2 (s) 910.9

The 
H^0 fvalues of Br 2 (g) and

C(diamond) are not equal to 0at 298 K.

The standard states of these elements

are Br 2 (
) and C(graphite),

respectively.