Page 148 - Physical chemistry understanding our chemical world

ENTHALPY 115

We first draw out the reaction in full:
The butyl ethanoate
produced by Equation
H
(3.30) is an ester, and
OH O smells of pear drops.
+
CH 3
O
(3.30)

O O
+
CH 3 H H
O
Second, we look for those parts that change and those that remain unchanged. In this
example, the bonds that cleave are the O–H bond on the acid and the C–O bond on the
alcohol. Such cleavage will require energy. The bonds that form are an O–H bond (to
yield water) and a C–O bond in the product ester. All bonds release energy as they form.
In this example, the bonds outside the box do not change and hence do not change their
energy content, and can be ignored.
The value of H r relates to bond changes. In this example, equal numbers of O–H
and C–O bonds break as form, so we expect an equal amount of energy to be released as
is consumed, leading to an enthalpy change of zero. In fact, the value of H r is tiny at
−1
−12 kJ mol .

Worked Example 3.12 is somewhat artificial, because most reac- O
tions proceed with differing numbers of bonds breaking and form- In some texts, H BE is
written simply as ‘BE’.
ing. A more rigorous approach quantifies the energy per bond – the
O
‘bond enthalpy’ H BE (also called the ‘bond dissociation energy’).
H O is the energy needed to cleave 1 mol of bonds. For this reason, values of H O
BE BE
are always positive, because energy is consumed.
The chemical environment of a given atom in a molecule will influence the mag-
nitude of the bond enthalpy, so tabulated data such as that in Table 3.3 represent
average values.
We can calculate a value of H r O with an adapted form of Equation (3.29):

products reactants

O
O
H r O =− ν H BE − ν H BE (3.31)
i i
where the subscripted i means those bonds that cleave or form within each reactant
or product species during the reaction. We need the minus sign because of the way
we defined the bond dissociation enthalpy. All the values in Table 3.3 are positive
because H O relates to bond dissociation.
BE
The stoichiometric numbers ν here can be quite large unless the molecules are small.
The combustion of butane, for example, proceeds with the loss of 10 C–H bonds. A