CHAPTER 11 REACTIONS AND OTHER CHEMICAL PROCESSES
11.5 REACTIONCALORIMETRY 335
system in order for the temperature to return to its initial value. IfT 2 is less thanT 1 (the
process is endothermic),ÅHm(rxn) ispositive.
Most reactions cause a change in the composition of one or more phases, in which case
ÅHm(rxn) is not the same as the molar differential reaction enthalpy,ÅrHD.@H=@/T;p,
unless the phase or phases can be treated as ideal mixtures (see Sec.11.2.2). Correc-
tions, usually small, are needed to obtain the standard molar reaction enthalpyÅrHfrom
ÅHm(rxn).
11.5.2 The bomb calorimeter
A bomb calorimeter typically is used to carry out the complete combustion of a solid or
liquid substance in the presence of excess oxygen. The combustion reaction is initiated with
electrical ignition. In addition to the main combustion reaction, there may be unavoidable
side reactions, such as the formation of nitrogen oxides if N 2 is not purged from the gas
phase. Sometimes auxiliary reactions are deliberately carried out to complete or moderate
the main reaction.
From the measured heating curve and known properties of the calorimeter, reactants,
and products, it is possible to evaluate the standard molar enthalpy of combustion,ÅcH,
of the substance of interest at a particular temperature called the reference temperature,Tref.
(Trefis often chosen to be298:15K, which is25:00C.) With careful work, using temper-
ature measurements with a resolution of 1 10 ^4 K or better and detailed corrections, the
precision ofÅcHcan be of the order of0:01percent.
Bomb calorimetry is the principal means by which standard molar enthalpies of com-
bustion of individual elements and of compounds of these elements are evaluated. From
these values, using Hess’s law, we can calculate the standard molar enthalpies offormation
of the compounds as described in Sec.11.3.2. From the formation values of only a few com-
pounds, the standard molar reaction enthalpies of innumerable reactions can be calculated
with Hess’s law (Eq.11.3.3on page 320 ).
Because of their importance, the experimental procedure and the analysis of the data
it provides will now be described in some detail. A comprehensive problem (Prob. 11. 7 )
based on this material is included at the end of the chapter.
There are five main steps in the procedure of evaluating a standard molar enthalpy of
combustion:
1.The combustion reaction, and any side reactions and auxiliary reactions, are carried
out in the calorimeter, and the course of the resulting temperature change is observed.
2.The experimental data are used to determine the value ofÅU.IBP; T 2 /, the internal
energy change of the isothermal bomb process at the final temperature of the reaction.
Theisothermal bomb processis the idealized process that would have occurred if
the reaction or reactions had taken place in the calorimeter at constant temperature.
3.The internal energy change of the isothermal bomb process is corrected to yield
ÅU.IBP; Tref/, the value at the reference temperature of interest.
4.The standard molar internal energy of combustion,ÅcU.Tref/, is calculated. This
calculation is calledreduction to standard states.
5.The standard molar enthalpy of combustion,ÅcH.Tref/, is calculated.These five steps are described below.