Page 145 - Physical chemistry understanding our chemical world

112 ENERGY AND THE FIRST LAW OF THERMODYNAMICS

compounds yield nitrogen oxide, and sulphur compounds yield sulphur dioxide, which
itself oxidizes to form SO 3 .
Let us simplify and look at the combustion of the simplest hydrocarbon, methane.
CH 4 reacts with oxygen according to

CH 4(g) + 2O 2(g) −−→ CO 2(g) + 2H 2 O (g) (3.28)

The reaction is very exothermic, which explains why much of the developed world
employs methane as a heating fuel. We can measure the enthalpy change accompany-
ing the reaction inside a calorimeter, or we can calculate a value with thermochemi-
cal data.
This enthalpy has a special name: we call it the enthalpy of
Most authors abbre- combustion, and define it as the change in enthalpy accompanying
viate ‘combustion’ to the burning of methane, and symbolize it as H (combustion) or just
just ‘c’, and symbolize H c . In fact, we rarely perform calculations with H c but with the
the enthalpy change standard enthalpy of combustion H , where the plimsoll symbol
O
c
O
as H c . Others write ‘ ’ implies s.t.p. conditions.
O
H O . O
(comb) Table 3.2 contains values of H for a few selected organic
c
compounds. The table shows how all value of H c O are nega-
tive, reminding us that energy is given out during a combustion reaction. We say
combustion is exothermic, meaning energy is emitted. All exothermic reactions are
O
characterized by a negative value of H .
c
O
But we do not have to measure each value of H : we can
c
We can use equations calculate them if we know the enthalpies of formation of each
like Equation (3.27) for chemical, product and reactant, we can adapt the expression in
any form of enthalpy, eq. (3.27), saying:
not just combustion.
O O
O
H = ν H − ν H (3.29)
c f f
products reactants
We could not perform
cycles of this type where each H term on the right-hand side of the equation is a
unless enthalpy was molar enthalpy of formation, which can be obtained from tables.
a state function.
Worked Example 3.11 The wood mentioned in our title question
is a complicated mixture of organic chemicals; so, for simplicity, we
update the scene. Rather than prehistoric men sitting around a fire, we
The word ‘calorific’ consider the calorific value of methane in a modern central-heating
◦
means heat contain- system. Calculate the value of H c for methane at 25 Cusing molar
O
ing, and comes from enthalpies of formation H .
f
the Latin calor,mean- The necessary values of H O are:
ing ‘heat’. f
Species (all as gases) CH 4 O 2 CO 2 H 2 O
O
H /kJ mol −1 −74.81 0 −393.51 −285.83
f