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Hess' Law

Calculate the...

Question

Calculate the heat of combustion (kJ) of propane, $C_{3} H_{8}$ using the listed standard enthalpy of reaction data:
$C_{3} H_{8} (g) + 5 O_{2} (g) ⟶ 3 C O_{2} (g) + 4 H_{2} O (g)$
$3 C (s) + 4 H_{2} (g) ⟶ C_{3} H_{8} (g) Δ H / k J = - 103.8$
$C (s) + O_{2} (g) ⟶ C O_{2} (g) Δ H / k J = - 393.5$
$H_{2} (g) + 1 / 2 O_{2} (g) ⟶ H_{2} O (g) Δ H / k J = - 241.8$

-2043.9

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-1532.9

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-1021.9

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-739.1

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Solution

The correct option is A -2043.9
$Δ H_{c o m b u s t i o n} = 4 * Δ H_{f} H_{2} O$ $+ 3 * Δ H_{f} C O_{2} - 5 * Δ H_{f} O_{2} - Δ H_{f} C_{3} H_{8}$
$Δ H_{c o m b u s t i o n} =$ = $4 * - 241.83 - 3 * 393.5 - 0 + 103.8 = - 2043.9 k J / m o l$
Hence, option A is correct

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