Which of the reaction defines $Δ H_{f}^{0} ?$

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$N_{2} (g) + 3 H_{2} (g) \overset{}{\to} 2 N H_{3} (g)$

$C O (g) + \frac{1}{2} O_{2} (g) \overset{}{\to} C O_{2} (g)$

C(diamond) $+ O_{2} (g) \overset{}{\to} C O_{2} (g)$

$\frac{1}{2} H_{2} (g) + \frac{1}{2} F_{2} (g) \overset{}{\to} H F (g)$

answer is B.

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Detailed Solution

TIPS/Formulae:
$Δ H_{f}^{o}$ is the enthalpy change when 1 mole of the substance is formed from its elements in their standard states. In (a) carbon is present in diamond however, standard state of carbon is graphite. Again, in (d) $C O$ (g) is involved so it can’t be the right option. Further in (c) 2 moles of $N H_{3}$ are generated. Hence, the correct option is (b).

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