Suppose the enthalpy of formation of CO2(.g.) and H2O(.l.) are -394kJ/mol and -286 kJ/mol respectively. The standard enthalpy of formation of octane (.C8H18.) is -250kJ/mol . Then calculate the standard enthalpy of combustion of C8H18 :

Question

Suppose the enthalpy of formation of CO2(.g.) and H2O(.l.) are -394kJ/mol and -286 kJ/mol respectively. The standard enthalpy of formation of octane (.C8H18.) is -250kJ/mol . Then calculate the standard enthalpy of combustion of C8H18 : (A) -5200kJ/mol (B) -5726kJ/mol (C) -5476kJ/mol (D) -5310kJ/mol

Tardigrade Admin · Accepted Answer

The Balanced reaction from the combustion of

C_{8} H_{18}

:

C_{8} H_{18} + \frac{25}{2} O_{2} \to 8 C O_{2} + 9 H_{2} O

^{$(Δ)_{r} H^{0} = 8 \times ((ΔH)^{0})_{(CO)_{2}} + 9 \times ((ΔH)^{0})_{H_{2} O} - (((ΔH)^{0})_{C_{8} H_{18}} + \frac{25}{2} ((ΔH)^{0})_{(O_{2})}) ∴ ((ΔH)^{0})_{O_{2}} = 0$} (standard enthalpy of formation of

O_{2}

)
(standard enthalpy of reaction = standard enthalpy of combustion)

(Δ)_{r} H^{o} = 8 \times (- 394) + 9 \times (- 286) - (- 250)

= - 5476 k J / m o l

Q. Suppose the enthalpy of formation of $C O_{2} (g)$ and $H_{2} O (l)$ are $- 394 k J / m o l$ and -286 kJ/mol respectively. The standard enthalpy of formation of octane $(C_{8} H_{18})$ is $- 250 k J / m o l$ . Then calculate the standard enthalpy of combustion of $C_{8} H_{18}$ :

$- 5200 k J / m o l$

$- 5726 k J / m o l$

$- 5476 k J / m o l$

$- 5310 k J / m o l$

Q. Suppose the enthalpy of formation of CO2​(g) and H2​O(l) are −394kJ/mol and -286 kJ/mol respectively. The standard enthalpy of formation of octane (C8​H18​) is −250kJ/mol . Then calculate the standard enthalpy of combustion of C8​H18​ :

−5200kJ/mol

−5726kJ/mol

−5476kJ/mol

−5310kJ/mol

Q. Suppose the enthalpy of formation of $C O_{2} (g)$ and $H_{2} O (l)$ are $- 394 k J / m o l$ and -286 kJ/mol respectively. The standard enthalpy of formation of octane $(C_{8} H_{18})$ is $- 250 k J / m o l$ . Then calculate the standard enthalpy of combustion of $C_{8} H_{18}$ :

$- 5200 k J / m o l$

$- 5726 k J / m o l$

$- 5476 k J / m o l$

$- 5310 k J / m o l$