Page 142 - Physical chemistry understanding our chemical world

ENTHALPY 109

In summary, the temperature of a reaction mixture changes because energy is
released or liberated. The temperature of the reaction mixture is only ever constant in
the unlikely event of H O being zero. (This argument requires an adiabatic reaction
r
vessel; see p. 89.)
Some standard enthalpies have special names. We consider below some of the more
important cases.

Are diamonds forever?

Enthalpies of formation

We often hear it said that ‘diamonds are forever’. There was even a James Bond novel
and film with this title. Under most conditions, a diamond will indeed last forever,
or as near ‘for ever’ as makes no difference. But is it an absolute statement of fact?
Diamond is one of the naturally occurring allotropes of carbon,
the other common allotrope being graphite. (Other, less common, Some elements exist in
allotropes include buckminster fullerine.) If we could observe a several different crys-
diamond over an extremely long time scale – in this case, several tallographic forms. The
billions of years – we would observe a slow conversion from bril- differing crystal forms
liant, clear diamond into grey, opaque graphite. The conversion are called allotropes.
occurs because diamond is slightly less stable, thermodynamically,
than graphite.
Heating graphite at the same time as compressing it under enormous pressure
will yield diamond. The energy needed to convert 1 mol of graphite to diamond is
−1
2.4kJ mol . We say the ‘enthalpy of formation’ H f for the diamond is +2.4kJ
mol −1 because graphite is the standard state of carbon.
We define the ‘standard enthalpy of formation’ H O as the
f
enthalpy change involved in forming 1 mol of a compound from The ‘standard enthalpy
its elements, each element existing in its standard form. Both T of formation’ H f O is
and p need to be specified, because both variables influence the the enthalpy change
magnitude of H. Most books and tables cite H O at standard involved in forming
f
O
pressure p and at a temperature of 298 K. Table 3.1 cites a few 1 mol of a compound
or non-stable allotrope
O
representative values of H .
f from its elements, each
It will be immediately clear from Table 3.1 that several val-
element being in its
ues of H f are zero. This value arises from the definition we
standard form, at s.t.p.
chose, above: as H f relates to forming a compound from its con-
stituent elements, it follows that the enthalpy of forming an element
can only be zero, provided it exists in its standard state. Inci-
dentally, it also explains why H f (Br 2 , l) = 0 but H f (Br 2 , g) =
−1
29.5kJ mol , because the stable form of bromine is liquid at s.t.p.
We define the enthalpy
For completeness, we stipulate that the elements must exist in offormationofan ele-
their standard states. This sub-clause is necessary, because whereas ment (in its normal
most elements exist in a single form at s.t.p. (in which case their state) as zero.
enthalpy of formation is zero), some elements, such as carbon