Page 141 - Chemical equilibria Volume 4

Determination of the Values Associated with Reactions – Equilibrium Calculations 117
4.2.6.2. Experimental determination of the standard enthalpies of
combustion
The standard enthalpy of combustion is determined on the basis of the
heat of combustion measured by calorimetry at standard pressure.
0
Let Q represent the measured heat of combustion of n 0 moles of the
compound in question at normal pressure, when the reaction starts at
xs
temperature T and finishes at the final temperature T f. If n moles of the excess
O
2
oxygen were consumed, and the standard enthalpy of the reaction at temperature
T is the measured heat of reaction, expressed in relation to 1 mole, decreased by
the amounts of heat which have been used to raise the combustion products and
excess oxygen from temperature T to temperature T f:

Q 0 f T n O xs f T
0
Δ H = − ∑ ∫ ν C dT − 2 ∫ ν C dT [4.21]
comb
T
n 0 products T products P (products) n 0 T O 2 P (O 2 )
Of course, if, in the experiment, the final pressure is different from the
initial pressure, a pressure correction can potentially be applied – particularly
for gaseous species.

4.2.6.3. Use of the standard enthalpies of combustion to calculate the
standard enthalpy of a reaction

If we consider the balance equation of any reaction r, we can always
consider it as the linear combination of the reactions of combustion of the
different components in the reaction under study, labeled with their
stoichiometric coefficients in the reaction in question. We can therefore
write the following for the standard enthalpy of reaction:
0
Δ rT ∑ ν k Δ h = comb H T 0 () k [4.22]
k

4.2.6.4. Use of the standard enthalpies of combustion to calculate the
standard enthalpies of formation

The above calculation can be applied to synthesis reactions, and therefore
the enthalpy of formation can be deduced from the combustion reactions, by:

T ∑
0
H = ν k Δ comb H T 0 () k [4.23]
k