Question

For a particular isomer of C8H18, the following reaction produces 5113.3 kJ of heat per mole of C8H18(g) consumed, under standard conditions.

C8H12(g)+25/2O₂(g)------->8CO₂(g)+9H₂O(g) Delta Hrxn=-5113.3 KJ/mol

What is the standard enthalpy of formation of this isomer of C8H18(g)?

Answer 1

The balanced equation is-

C8H18(g) + 25/2(O2)(g) -> 8CO2(g) + 9H2O(g)
STANDARD ENTHALPY VALUES (H):
02(g) = 0 kj/mol
CO2 (g) = -393.5 kj/mol
H20(g) = -241.8 kj/mol
C8H18(g) = ?
Delta Hrxn=-5113.3 KJ/mol

H_f^o of C8H18 (g) is your unknown, so call that x.
use: Heat of reaction = (Heat of formation of products – Heat of formation of reactants) at Standard condition.
so, -5113.3 kJ = [8(CO2) + 9(H2O)] - [x +12.5(O2)]
plug in your standard enthalpy values.
-5113.3 kJ = [8(-393.5) + 9(-241.8)] - [x + 12.5(0)]
-5113.3   kJ = [-3148 + (-2176.2)] - [x + 0]
-5113.3   kJ = --5324.2- x

or x = - 5324.2 –(-5113.3) = - 210.9 kJ/mol
The standard enthalpy of formation of this isomer of C8H18(g) = - 210.9 kJ/mol