Heats of neutralization $(△ H)$ of $N H_{4} O H$ and $H F$ are $- 51.5$ and $68.6 k J$ respectively. Calculate their heats of dissociation.

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Solution

$H C l + N a O H (a q) \to N a C l (a q) + H_{2} O$ $△ H = - 57.3 K J$
$H C l (a q) + N H_{4} O H (a q) \to N H_{4} C l (a q) + H_{2} O$ $△ H = - 51.5 K J$
Weak base
$∴$ Heat of dissociation of $N H_{4} O H$
$△ H = - 51.5 - (- 57.3) = 5.8 K J$
Similarly, we have
$H F (a q) + N a O H (a q) \to N a F (a q) + H_{2} O$ $△ H = + 68.6 K J$
$∴$ Heat of dissociation of $H F$ ,
$△ H = + 68.6 - (- 57.3) = 125.9 K J$

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