Page 144 - Physical chemistry understanding our chemical world

ENTHALPY 111

Table 3.1 (continued)

O
Compound H /kJ mol −1
f
carbon dioxide (CO 2 ,g) −393.0
copper oxide (CuO, s) −157.3
hydrogen oxide (H 2 O, l) −285.8
hydrogen oxide (H 2 O, g) −241.8
hydrogen fluoride (HF, g) −271.1
hydrogen chloride (HCl, g) −92.3
nitrogen hydride (NH 3 ,g) −46.1
nitrogen hydride (NH 3 ,aq) −80.3
nitrogen monoxide (NO, g) 90.3
nitrogen dioxide (NO 2 , g) 33.2
phosphine (PH 3 ,g) 5.4
silicon dioxide (SiO 2 ,s) −910.9
sodium hydroxide (NaOH, s) −425.6
sulphur dioxide (SO 2 ,g) −296.8
sulphur trioxide (SO 3 ,g) −395.7
sulphuric acid (H 2 SO 4 , l) −909.3

(above), sulphur or phosphorus, have allotropes. The enthalpy of formation for the
stable allotrope is always zero, but the value of H f for the non-stable allotropes will
not be. In fact, the value of H f for the non-stable allotrope is cited with respect to
the stable allotrope. As an example, H f for white phosphorus is zero by definition
(it is the stable allotrope at s.t.p.), but the value of H f for forming red phosphorus
−1
from white phosphorus is 15.9kJ mol .
If the value of H f is determined within these three constraints of standard T ,
standard p and standard allotropic form, we call the enthalpy a standard enthalpy,
O
O
which we indicate using the plimsoll symbol ‘ ’as H .
f
To conclude: are diamonds forever? No. They convert slowly into graphite, which
is the stablest form of carbon. Graphite has the lowest energy for any of the allotropes
of carbon, and will not convert to diamond without the addition of energy.

Why do we burn fuel when cold?

Enthalpies of combustion

A common picture in any book describing our Stone Age forebears shows short, hairy
people crouched, warming themselves round a flickering fire. In fact, fire was one of
the first chemical reactions discovered by our prehistoric ancestors. Primeval fire was
needed for warmth. Cooking and warding off dangerous animals with fire was a later
‘discovery’.
But why do they burn wood, say, when cold? The principal reactions occurring
when natural materials burn involve chemical oxidation, with carbohydrates
combining with elemental oxygen to yield water and carbon dioxide. Nitrogen