Chemistry, Third edition

STANDARD ENTHALPY OF FORMATION 227

●If an element exists in more than one allotrope, H^ —frefers to the compound pro-

ducing reaction in which that element is present in its reference state.

Think of water. According to the definition, the standard enthalpy of formation

of water is the standard enthalpy change involved when pure hydrogen and pure

oxygen react to give one mole of pure water – all at a pressure of 1 atm. At our

chosen temperature of 25 °C, both hydrogen and oxygen will be gases and water will

be a liquid. The reaction will then be represented as

H 2 (g)^1 ⁄ 2 O 2 (g) H 2 O(l) H—^ f 285.83 kJ mol^1

The standard enthalpy change represented by the equation

2H 2 (g)O 2 (g) 2H 2 O(l) H—^ f 571.66 kJ mol^1

does not equal the standard enthalpy change of formation, since the equation indi-

cates that two moles (not one mole) of water are made.

AlthoughH^ —f(H 2 O) may be measured experimentally, by studying the reaction

of H 2 and O 2 in the laboratory, for many compounds direct measurement of

H^ —f is impossible. For example, consider the thermochemical equation showing

the formation of methanoic (formic) acid:

H 2 (g)O 2 (g)C(s) HCO 2 H(l) H^ —f(298 K)  402.1 kJ mol^1

If we heat hydrogen, oxygen and graphite together, so little methanoic acid is pro-

duced that it is impossible to directly measure its enthalpy change of formation. For

this reason, the enthalpy change of formation is calculated indirectly, using Hess’s law.

Examples of thermochemical equations where the standard enthalpy changes are

equal to the standard enthalpy of formation of compounds are

1.ammonia

(^1) ⁄
2 N 2 (g)
(^3) ⁄
2 H 2 (g) NH 3 (g) H
—
f 46.1 kJ mol
 1
2.benzoic acid
7C(s)3H 2 (g)O 2 (g) C 6 H 5 CO 2 H(s) H—^ f 385.1 kJ mol^1
3.hydrogen iodide
(^1) ⁄
2 H 2 (g)
(^1) ⁄
2 I 2 (s) HI(g) H
—
f26.48 kJ mol
 1
Standard enthalpy of formation of elements
The standard enthalpy of formation of diamond refers to the process
C(s) (graphite) C(s) (diamond) H—^ f1.895 kJ mol^1
Not surprisingly, the standard enthalpy of formation of an element in its refer-
ence stateis zero, since the reaction involves no change:
C(s) (graphite) C(s) (graphite) H—^ f0.000 kJ mol^1
This might appear trivial, but it will be important to keep this in mind when we use
H^ —f values in calculations later.