CHAPTER 11 REACTIONS AND OTHER CHEMICAL PROCESSES
11.5 REACTIONCALORIMETRY 333
11.5 Reaction Calorimetry
Reaction calorimetry is used to evaluate the molar integral reaction enthalpyÅHm(rxn) of a
reaction or other chemical process at constant temperature and pressure. The measurement
actually made, however, is a temperature change.
Sections11.5.1and11.5.2will describe two common types of calorimeters designed for
reactions taking place at either constant pressure or constant volume. The constant-pressure
type is usually called areaction calorimeter, and the constant-volume type is known as a
bomb calorimeterorcombustion calorimeter.
In either type of calorimeter, the chemical process takes place in a reaction vessel sur-
rounded by an outer jacket. The jacket may be of either the adiabatic type or the isothermal-
jacket type described in Sec.7.3.2in connection with heat capacity measurements. A
temperature-measuring device is immersed either in the vessel or in a phase in thermal
contact with it. The measured temperature change is caused by the chemical process, in-
stead of by electrical work as in the determination of heat capacity. One important way in
which these calorimeters differ from ones used for heat capacity measurements is that work
is kept deliberately small, in order to minimize changes of internal energy and enthalpy
during the experimental process.
11.5.1 The constant-pressure reaction calorimeter
The contents of a constant-pressure calorimeter are usually open to the atmosphere, so this
type of calorimeter is unsuitable for processes involving gases. It is, however, a convenient
apparatus in which to study a liquid-phase chemical reaction, the dissolution of a solid or
liquid solute in a liquid solvent, or the dilution of a solution with solvent.
The process is initiated in the calorimeter by allowing the reactants to come into contact.
The temperature in the reaction vessel is measured over a period of time starting before the
process initiation and ending after the advancement has reached a final value with no further
change.
The heating or cooling curve (temperature as a function of time) is observed over a
period of time that includes the period during which the advancementchanges. For an
exothermic reaction occurring in an adiabatic calorimeter, the heating curve may resemble
that shown in Fig.7.3on page 169 , and the heating curve in an isothermal-jacket calorimeter
may resemble that shown in Fig.7.4on page 171. Two points are designated on the heating
or cooling curve: one at temperatureT 1 , before the reaction is initiated, and the other atT 2 ,
afterhas reached its final value. These points are indicated by open circles in Figs.7.3
and7.4.
Figure11.11on the next page depicts three paths at constant pressure. The enthalpy
change of the experimental process, in which reactants at temperatureT 1 change to products
at temperatureT 2 , is denotedÅH(expt).
The value ofÅH(expt) at constant pressure would be zero if the process were perfectly
adiabatic and the only work were expansion work, but this is rarely the case. There may be
unavoidable work from stirring and from electrical temperature measurement. We can eval-
uateÅH(expt) by one of the methods described in Sec.7.3.2. For an adiabatic calorimeter,
the appropriate expression isÅH(expt)Dr.t 2 t 1 /(Eq.7.3.19on page 170 withwel
set equal to zero), whereis the energy equivalent of the calorimeter,ris the slope of the