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Hess' Law

Calculate the...

Question

Calculate the value of $Δ H / k J$ for the following reaction using the listed thermochemical equations:
$2 C (s) + H_{2} (g) ⟶ C_{2} H_{2} (g)$
$2 C_{2} H_{2} (g) + 5 O_{2} (g) ⟶ 4 C O_{2} (g) + 2 H_{2} O (I) Δ H / k J = - 2600 k J$
$C (s) + O_{2} (g) ⟶ C O_{2} (g) Δ H / k J = - 390 k J$
$2 H_{2} (g) + O_{2} (g) ⟶ 2 H_{2} O (I) Δ H / k J = - 572 k J$

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Solution

$Δ H_{r} = [- \frac{1}{2} {(Δ H_{f})}_{C_{2} H_{2}} + 2 {(Δ H_{f})}_{C O_{2}} + \frac{1}{2} {(Δ H_{f})}_{H_{2} O}]$
$Δ H_{r} = - \frac{1}{2} (- 1300) + 2 (- 390) - \frac{1}{2} \times 572$
$Δ H_{r} = 234$

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