Which one of the following represents standard enthlpy of formation

Easy

A

$H_{2} (g) + \frac{1}{2} O_{2} (g) \overset{}{\to} H_{2} O (g), Δ H = - 245 k J$
B

$S (β) + O_{2} (g) \overset{}{\to} S O_{2} (g), Δ H = + 300 k J$
C

$N O (g) + \frac{1}{2} O_{2} (g) \overset{}{\to} N O_{2} (g), Δ H = - 90 k J$
D

$\frac{1}{2} N_{2} (g) + \frac{3}{2} H_{2} (g) \overset{}{\to} N H_{3} (g), Δ H = - 44 k J$

Solution

(1) $H_{2} O (g) ...$ Not a standard state.
(2) $S_{8} (α) ...$ is standard but not S( $β$ )
more over $Δ H$ must be -ve
(3) ‘NO’ is compound.
$Δ^{o} H_{f}$ must involve only elements on the reactant side

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