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System, Surroundings, Intensive, Extensive Properties

H2 g + 1/2 O2...

Question

$H_{2} (g) + \frac{1}{2} O_{2} (g) = H_{2} O (l); Δ H_{298 K} = - 68.32 K c a l .$
Heat of vapourisation of water at 1 atm and 25 $^{o}$ C is 10.52 Kcal. The standard heat of formation (in Kcal) of 1 mole of water vapour at 25 $^{o}$ C is:

10.52

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-78.84

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57.8

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-57.8

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Solution

The correct option is D -57.8
The thermochemical reactions are as given below.
$H_{2} (g) + \frac{1}{2} O_{2} (g) = H_{2} O (l); Δ H_{298 K} = - 68.32 K c a l .$
$H_{2} O (l) = H_{2} O (v); Δ H_{298 K} = 10.52 K c a l .$
Both reactions are added to obtain the reaction for the formation of water vapour which is $H_{2} (g) + \frac{1}{2} O_{2} (g) = H_{2} O (v)$ .
Hence, the heat of formation of water vapour is $- 68.32 + 10.52 = 57.8 K c a l$
Therefore,option D is correct.

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