PHYSICAL CHEMISTRY IN BRIEF

CHAP. 5: THERMOCHEMISTRY [CONTENTS] 136

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@R



Tprod

q≡∆H

A(TA)

B(TB)

Reactor

Treaction

Obr.5.1:The enthalpy balance of a system in which the chemical reaction A + B = products takes
place. Substances A and B enter the reactor at temperaturesTA andTB, the products leave the
reactor at temperatureTprod. The temperature in the reactor isTreaction. ∆His the enthalpy (heat)
exchanged between the reactor and its surroundings.

5.4 Enthalpy balances.

Let the reaction (5.1) take place in a reactor. Substance A enters the reactor at temperature
TA, substance B at temperatureTB... and the products exit the reactor at temperatureTprod;
the reaction itself proceeds at temperatureTreaction, see Figure5.1.
The change in enthalpy ∆Hneeded for this process to take place is

∆H =

∫Treaction

TA

a Cpm(A) dT+

∫Treaction

TB

b Cpm(B) dT+ ···

+∆rH(Treaction)

+

∫Tprod

Treaction

r Cpm(R) dT+

∫Tprod

Treaction

s Cpm(S) dT+ ···

= ∆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], (5.17)

whereT 1 is the temperature (most often 298.15 K) at which we know the reaction enthalpy of
the given reaction. The first equation can be verbally interpreted as follows: the first member on
the right side is the enthalpy needed for the heating of the initial substance A from temperature