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Hess' Law

Calculate Δ...

Question

Calculate $Δ H / k J$ for the following reaction using the listed standard enthalpy of reaction data.
$2 N_{2} (g) + 5 O_{2} (g) ⟶ 2 N_{2} O_{5} (s)$

$N_{2} (g) + 3 O_{2} (g) + H_{2} (g) ⟶ 2 H N O_{3} (a q); Δ H / k J = - 414.0$
$N_{2} O_{5} (s) + H_{2} O (l) ⟶ 2 H N O_{3} (a q); Δ H / k J = - 86.0$
$2 H_{2} (g) + O_{2} (g) ⟶ 2 H_{2} O (l); Δ H / k J = - 571.6$

-84.4

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-243.6

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-71.2

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-121.8

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Solution

The correct option is A -84.4
$2 N_{2} (g) + 5 O_{2} (g) ⟶ 2 N_{2} O_{5} (s)$ ----- a

Given that,
$N_{2} (g) + 3 O_{2} (g) + H_{2} (g) ⟶ 2 H N O_{3} (a q); Δ H / k J = - 414.0$ -------- 1
$N_{2} O_{5} (s) + H_{2} O (l) ⟶ 2 H N O_{3} (a q); Δ H / k J = - 86.0$ -------- 2

$2 H_{2} (g) + O_{2} (g) ⟶ 2 H_{2} O (l); Δ H / k J = - 571.6$ ------------- 3
Applying Hess's law,
Eqn (a) = 2 x Eqn (1) - 2 x Eqn (2) - Eqn (3)

$Δ H_{r} = [2 (- 414) + 2 (86) + 571.6]$
$Δ H_{r} = - 84.4 k J$

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Δ H / k J

2 N_{2} (g) + 5 O_{2} (g) ⟶ 2 N_{2} O_{5} (s)

N_{2} (g) + 3 O_{2} (g) + H_{2} (g) ⟶ 2 H N O_{3} (a q) Δ H / k J = - 414.0

N_{2} O_{5} (s) + H_{2} O (I) ⟶ 2 H N O_{3} (a q) Δ H / k J = - 86.0

2 H_{2} (g) + O_{2} (g) ⟶ 2 H_{2} O (I) Δ H / k J = - 571.6

Δ H / k J

4 C O (g) + 8 H_{2} (g) ⟶ 3 C H_{4} (g) + C O_{2} (g) + 2 H_{2} O (l)

C (g r a p h i t e) + 1 / 2 O_{2} (g) ⟶ C O (g); Δ H / k J = - 110.5 k J

C O (g) + 1 / 2 O_{2} (g) ⟶ C O_{2} (g); Δ H / k J = - 282.9 k J

H_{2} (g) + 1 / 2 O_{2} (g) ⟶ H_{2} O (l); Δ H / k J = - 285.8 k J

C (g r a p h i t e) + 2 H_{2} (g) ⟶ C H_{4} (g); Δ H / k J = - 74.8 k J

Δ H / k J

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

Δ H / k J

4 C O (g) + 8 H_{2} (g) ⟶ 3 C H_{4} (g) + C O_{2} (g) + 2 H_{2} O (l)

C (g r a p h i t e) + 1 / 2 O_{2} (g) ⟶ C O (g); Δ H / k J = - 110.5 k J

C O (g) + 1 / 2 O_{2} (g) ⟶ C O_{2} (g); Δ H / k J = - 282.9 k J

H_{2} (g) + 1 / 2 O_{2} (g) ⟶ H_{2} O (l); Δ H / k J = - 285.8 k J

C (g r a p h i t e) + 2 H_{2} (g) ⟶ C H_{4} (g); Δ H / k J = - 74.8 k J

C_{3} H_{8}

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

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