CHAP. 5: THERMOCHEMISTRY [CONTENTS] 135
Solution
From the result of the preceding example we know that the enthalpy of combustion of methane
(while gaseous water originates) atT= 298.15 K is−802.324 kJ/mol. We use equation (5.16)
for conversion to temperature 1500 K:
∆cH◦(T= 1500K) = ∆cH◦(T= 298. 15 K) +
∫ 1500
298. 15
∆CpdT ,
where
∆Cp = Cpm(CO 2 ) + 2Cpm(H 2 O)−Cpm(CH 4 )− 2 Cpm(O 2 )
= 51.0 + 2× 39. 8 − 66. 9 − 2 × 33 .7 =− 3. 7 J/mol.
By substituting into the preceding equation and integrating we obtain
∆cH◦(T= 1500K) =−802 324− 3. 7 ×(1500− 298 .15) =−806 771J mol−^1.
It follows from the reaction enthalpy definition that the enthalpy of combustion does not depend
on whether the burning is driven by oxygen or air.
