Question

Cars burn gasoline as fuel according to the equation: C8H18(l)+O2(g)=CO2(g)+H2O(g). (∆Hrxn=-5074KJ). Write a balanced equation for the reaction. Calculate the Standard enthalpy of formation for gasoline.

Answer 1

Ans. Balanced reaction:       2 C8H18 + 25 O2(g) = 16 CO2(g) + 18 H2O(g)

Using Hess’s Law, the standard enthalpy of reaction, dHrxn is given by summation of standard enthalpy of formation of products subtracted by summation of standard enthalpy of formation of reactants.

So, dHrxn⁰ = [16 x dH_f⁰ (CO2, g) + 18 x dH_f⁰ (H2O, g)] –

[2 x dH_f⁰ (C8H18. l) + 25 x dH_f⁰ (O2, g)]

Or, - 5074 kJ = [16 x (-396.5 kJ) + 18 x (- 241.8 kJ)] –

[ 2 x dH_f⁰ (C8H18. l) + 25 x 0 kJ]

Or, - 5074 kJ = 10696.4 kJ - [ 2 x dH_f⁰ (C8H18. l)

Or, 2 x dH_f⁰ (C8H18. l) = 10696.4 kJ + 5074 kJ = 15770.4 kJ

Or, dH_f⁰ (C8H18. l) = 15770.4 kJ / 2 = 7885.2 kJ

Therefore, standard enthalpy of formation of gasoline, dH_f⁰ (C8H18. l) = 7885.2 kJ