Page 141 - Physical chemistry understanding our chemical world

108 ENERGY AND THE FIRST LAW OF THERMODYNAMICS

bond requires energy to form, and each bond liberates energy when breaking (see
p. 63 ff). Typically, the amount of energy consumed or liberated is characteristic of
the bond involved, so each C–H bond in methane releases about 220 kJ mol −1 of
energy. And, as we have consistently reported, the best macroscopic indicator of a
microscopic energy change is a change in temperature.
Like internal energy, we can never know the enthalpy of a reagent; only the change
in enthalpy during a reaction or process is knowable. Nevertheless, we can think of
changes in H. Consider the preparation of ammonia:

N 2(g) + 3H 2(g) −−→ 2NH 3(g) (3.26)

We obtain the standard enthalpy change on reaction H O as a sum of the molar
r
enthalpies of each chemical participating in the reaction:
O O
O
H = νH − νH (3.27)
r m m
products reactants
The values of ν for the reaction in Equation (3.26) are ν (NH 3 ) =+2,ν (H 2 ) =−3 and
ν (N 2 ) =−1. We obtain the standard molar enthalpy of forming ammonia after inserting
values into Equation (3.27), as

O
O
O
H O = 2H (NH 3 ) − [H (N 2 ) + 3H (H 2 )]
r m m m
O
SAQ 3.8 Write out an expression for H r for the reaction 2NO + O 2 →
2NO 2 in the style of Equation (3.27).
Unfortunately, we do not know the enthalpies of any reagent. All we can know is a
change in enthalpy for a reaction or process. But what is the magnitude of this energy
change? As a consequence of Hess’s law (see p. 98), the overall change in enthalpy
accompanying a reaction follows from the number and nature of the bonds involved.
We call the overall enthalpy change during a reaction the ‘reaction enthalpy’ H r ,
and define it as ‘the change in energy occurring when 1 mol of reaction occurs’. In
−1
consequence, its units are J mol , although chemists will usually want to express
−1
H in kJ mol .
In practice, we generally prefer to tighten the definition of H r above, and look
at reagents in their standard states. Furthermore, we maintain the temperature T at
298 K, and the pressure p at p . We call these conditions standard temperature and
O
pressure, or s.t.p. for short. We need to specify the conditions because temperature and
pressure can so readily change the physical conditions of the reactants and products.
As a simple example, elemental bromine is a liquid at s.t.p., so we say the standard
state of bromine at s.t.p. is Br 2(l) . If a reaction required gaseous bromine Br 2(g) then
we would need to consider an additional energy – the energy of vaporization to effect
the process Br 2(l) → Br 2(g) . Because we restricted ourselves to s.t.p. conditions, we
O
no longer talk of the reaction enthalpy, but the ‘standard reaction enthalpies’ H ,
r
where we indicate the standard state with the plimsoll sign ‘ ’.
O