Thermodynamics and Chemistry

CHAPTER 11 REACTIONS AND OTHER CHEMICAL PROCESSES

11.3 MOLARREACTIONENTHALPY 318

11.2.3 Standard molar reaction quantities

If a chemical process takes place at constant temperature while each reactant and product
remains in its standard state of unit activity, the molar reaction quantityÅrXis called the
standard molar reaction quantityand is denoted byÅrX. For instance,ÅvapHis a
standard molar enthalpy of vaporization (already discussed in Sec.8.3.3), andÅrGis the
standard molar Gibbs energy of a reaction.
From Eq.11.2.15, the relation between a standard molar reaction quantity and the stan-
dard molar quantities of the reactants and products at the same temperature is

ÅrX defD

X

i

iXi (11.2.18)

Two comments are in order.

1.Whereas a molar reaction quantity is usually a function ofT,p, and, astandard

molar reaction quantity is a function only ofT. This is evident because standard-state

conditions imply that each reactant and product is in a separate phase of constant

defined composition and constant pressurep.

2.Since the value of a standard molar reaction quantity is independent of, the standard

molar integral and differential quantities are identical (page 317 ):

ÅXm(rxn)DÅrX (11.2.19)

These general concepts will now be applied to some specific chemical processes.

11.3 Molar Reaction Enthalpy

Recall thatÅHm(rxn) is a molar integral reaction enthalpy equal toÅH(rxn)=Å, and that
ÅrHis a molar differential reaction enthalpy defined by

P

iiHiand equal to.@H=@/T;p.

11.3.1 Molar reaction enthalpy and heat

During a process in a closed system at constant pressure with expansion work only, the
enthalpy change equals the energy transferred across the boundary in the form of heat:
dH D∂q(Eq.5.3.7). Thus for the molar reaction enthalpyÅrH D.@H=@/T;p, which
refers to a process not just at constant pressure but also at constant temperature, we can
write

ÅrHD

∂q

d

(11.3.1)

(constantTandp,∂w^0 D 0 )

Note that when there is nonexpansion work (w^0 ), such as electrical work, the enthalpy
change is not equal to the heat. For example, if we compare a reaction taking place in a
galvanic cell with the same reaction in a reaction vessel, the heats at constantT andpfor
a given change ofare different, and may even have opposite signs. The value ofÅrHis
the same in both systems, but the ratio of heat to advancement,∂q=d, is different.
Anexothermicreaction is one for whichÅrHis negative, and anendothermicreaction
is one for whichÅrHis positive. Thus in a reaction at constant temperature and pressure