PHYSICAL CHEMISTRY IN BRIEF

CHAP. 5: THERMOCHEMISTRY [CONTENTS] 137

TAto the temperature of the reactionTreaction, the second member is the enthalpy needed for the
heating of substance B, the third member is the enthalpy supplied to the system by the reaction
at temperatureTreaction, and the other members are the enthalpies needed for the heating of
the products from temperatureTreactionto temperatureTprodat the exit from the reaction.

Note:Equation (5.17) does not represent the most general enthalpy balance of a system

in which a chemical reaction occurs. We might consider cases in which the products exit

the reactor at different temperatures, when the reaction does not proceed quantitatively,

when the reactants are not in a stoichiometric ratio, or when an inert substance is present

in the reacting mixture. Generalization of equation (5.17) to these cases is quite easy.

If the reaction proceeds at constant volume of the system, we balance the internal energy
instead of enthalpy. In the balance equation (5.17) we substitute ∆rHwith the reaction internal
energy ∆rU, and the isobaric heat capacitiesCpwith the isochoric capacitiesCV.

5.4.1 Adiabatic temperature of reaction.

If we prevent a system in which a chemical reaction occurs from exchanging heat with its
surroundings, we speak about an adiabatic course of the reaction. In such a case we write
[compare with (5.17)]

0 = ∆rH(T 1 ) +

∫Tprod

T 1

r Cpm(R) dT+

∫Tprod

T 1

s Cpm(S) dT ···

−

∫TA

T 1

a Cpm(A) dT−

∫TB

T 1

b Cpm(B) dT− ···, [p], (5.18)

with the unknown temperatureTprodtermed theadiabatic temperature of reaction. The
term adiabatic temperature of reaction is usually applied to exothermic reactions. Equation
(5.18) may then be interpreted as follows: the amount of heat contained in the reactants and
the reaction heat of the reaction are used to heat the products to the adiabatic temperature of
reaction.