Question

A principal constituent of petrol (gasoline) is iso-octane, C8H18. From the following thermodynamic data at

298 K what is the
standard molar enthalpy of combustion of iso-octane in excess oxygen
at 298 K?
C«H;8(1) + 12702() +8C02(g) +91,0(1)
Substance AfHn/kJ mol"}
C8H8(1)
-258.07
02(8)
0
CO2(8)
-393.51
H2O(1)
-285.83

Answer 1

Answer: The enthalpy of combustion of iso-octane in excess oxygen at 298 K is -5462.2kJ/mol

Step-by-step explanation:

The balanced reaction for combustion of isooctane is:

`C_8H_(18)(l)+(25)/(2)O_2(g)\rightarrow 8CO_2(g)+9H_2O(l)`

The equation for the enthalpy change of the above reaction is:

`\Delta H^o_(rxn)=[(8* \Delta H^o_f_((CO_2(g))))+(9* \Delta H^o_f_((H_2O(l))))]-[(1* \Delta H^o_f_{(C_8H_(18)(g))})+((25)/(2)* \Delta H^o_f_((O_2(g))))]`

We are given:

`\Delta H^o_f_((H_2O(l)))=-285.83kJ/mol\\\Delta H^o_f_((O_2(g)))=0kJ/mol\\\Delta H^o_f_((CO_2(g)))=-393.51kJ/mol\\\Delta H^o_{C_8H_(18)(l)}=-258.07kJ/mol`

Putting values in above equation, we get:

`\Delta H^o_(rxn)=[(8* (-393.51))+(9* (-285.8))]-[(1* (-258.07))+((25)/(2)* (0))]\\\Delta H^o_(rxn)=-5462.2kJ/mol`

The enthalpy of combustion of iso-octane in excess oxygen at 298 K is -5462.2kJ/mol