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Hess' Law

C s + O 2 g →...

Question

$C (s) + O_{2} (g) \to C O_{2} (g), Δ_{r} H = - 94.3 k c a l / m o l$

$C O (g) + \frac{1}{2} O_{2} (g) \to C O_{2} (g), Δ_{r} H = - 67.4 k c a l / m o l$

$O_{2} (g) \to 2 O (g), Δ_{r} H = 117.4 K c a l / m o l$

$C O (g) \to C (g) + O (g), Δ_{r} H = 230.6 K c a l / m o l$

Calculate: $Δ_{r} H f o r C (s) \to C (g) i n K c a l / m o l$

A

$165 k c a l / m o l$

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B

$145 k c a l / m o l$

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C

$185 k c a l / m o l$

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D

$205 k c a l / m o l$

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Solution

The correct option is B
$145 k c a l / m o l$

$C (s) + O_{2} (g) \to C O_{2} (g), Δ_{r} H_{1} = - 94.3 k c a l / m o l \dots e q . (1)$

$C O (g) + \frac{1}{2} O_{2} (g) \to C O_{2} (g), Δ_{r} H_{2} = - 67.4 k c a l / m o l . \dots e q . (2)$

$O_{2} (g) \to 2 O (g), Δ_{r} H_{3} = 117.4 K c a l / m o l . \dots e q . (3)$

$C O (g) \to C (g) + O (g), Δ_{r} H_{4} = 230.6 K c a l / m o l \dots e q . (4)$

adding equation (1) and equation (4), we get

$C (s) + O_{2} (g) + C O (g) \to C O_{2} (g) + C (g) + O (g) \dots . e q . (5)$

reversing the equation (2), we get

$C O_{2} (g) \to C O (g) + \frac{1}{2} O_{2} (g) \dots e q . (6)$

Now, adding equation (5) and equation (6), we get

$C (s) + \frac{1}{2} O_{2} (g) \to C (g) + O (g) \dots e q . (7)$

Now, on reversing the equation (3) and multiplying it by $\frac{1}{2}$ , we get

$O (g) \to \frac{1}{2} O_{2} (g) \dots e q . (8)$

Now, adding equation (7) and equation (8), we get

$C (s) \to C (g)$

so, using the Hess law,
$Δ_{r} H$ for $C (s) \to C (g)$ is
$Δ_{r} H = Δ_{r} H_{1} - Δ_{r} H_{2} - \frac{1}{2} \times Δ_{r} H_{3} + Δ_{r} H_{4}$

$Δ_{r} H = (- 94.3 + 67.4 - \frac{1}{2} \times 117.4 + 230.6)$ $Δ_{r} H = 145 k c a l / m o l$ .

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Similar questions

$C (s) + O_{2} (g) \to C O_{2} (g), Δ_{r} H = - 94.3 k c a l / m o l$

$C O (g) + \frac{1}{2} O_{2} (g) \to C O_{2} (g), Δ_{r} H = - 67.4 k c a l / m o l$

$O_{2} (g) \to 2 O (g), Δ_{r} H = 117.4 K c a l / m o l$

$C O (g) \to C (g) + O (g), Δ_{r} H = 230.6 K c a l / m o l$

Calculate: $Δ_{r} H f o r C (s) \to C (g) i n K c a l / m o l$

C (s) + O_{2} (g) \to C O_{2}, (g); △ H = - 94.3 k c a l / m o l

C O (g) + \frac{1}{2} O_{2} (g) \to C O_{2} (g); △ H = - 67.4 k c a l / m o l

O_{2} (g) \to 2 O (g); △ H = 117.4 k c a l / m o l

C O (g) \to C (g) + O (g); △ H = 230.6 k c a l / m o l

△ H

C (s) \to C (g)

k c a l / m o l

C (s) + O_{2} (g) \to C O_{2}, (g); △ H = - 94.3 k c a l / m o l

C O (g) + \frac{1}{2} O_{2} (g) \to C O_{2} (g); △ H = - 67.4 k c a l / m o l

O_{2} (g) \to 2 O (g); △ H = 117.4 k c a l / m o l

C O (g) \to C (g) + O (g); △ H = 230.6 k c a l / m o l

△ H

C (s) \to C (g)

k c a l / m o l

C (s) + O_{2} (g) \to C O_{2} (g); Δ H = - 94.3

C O (g) + 1 / 2 O_{2} (g) \to C O_{2} (g); Δ H = - 67.4

O_{2} (g) \to 2 O (g); Δ H = 117.4

C O (g) \to C (g) + O (g); Δ H = 230.6

Δ H

C (s)

\to

C (g)

Q. Calculate the enthalpy of formation of methane from the following data:

C (s) + O_{2} (g) \to C O_{2} (g)

Δ_{r} H^{\circ} = - 393.5 k J

2 H_{2} (g) + O_{2} (g) \to 2 H_{2} O (l)

Δ_{r} H^{\circ} = - 571.8 k J

C H_{4} (g) + 2 O_{2} (g) \to C O_{2} (g) + 2 H_{2} O (l)

Δ_{r} H^{\circ} = - 890.3 k J

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