For a particular isomer of C8H18, the combustion reaction produces 5104.1 kJ of heat per mole of C8H18 (g) consumed, under standard conditions. C8H18 (g) + 252O2 (g) ⟶8CO2 (g) + 9H2O (g) ΔH∘rxn=-5104.1 kJ/mol What is the standard enthalpy of formation of this isomer of C8H18 (g) ?

Hello,

Considering the chemical reaction, the enthalpy of reaction is given by:

ΔH°rxn=ΔfHCO2+ΔfHH2O-ΔfHC8H18

(ΔfHO2=0)

Taking into account that the reaction produces energy, ΔH°rxn is negative. No, solving for ΔfHC8H18:

ΔfHC8H18=-ΔH°rxn+8*ΔfHCO2+9*ΔfHH2O

ΔfHC8H18 = - (-5104.1 kJ/mol) + 9 * (-292.74kJ/mol) + 8 * (-393.5 kJ/mol)

ΔfHC8H18=-678.56 kJ/mol

Best regards.