Thermodynamics and Thermochemistry 2 Question 17
17. Estimate the average $\mathrm{S}-\mathrm{F}$ bond energy in $\mathrm{SF}{6}$. The values of standard enthalpy of formation of $\mathrm{SF}{6}(g), \mathrm{S}(g)$ and $\mathrm{F}(g)$ are : $-1100,275$ and $80 \mathrm{~kJ} \mathrm{~mol}^{-1}$ respectively. (1999, 3M)
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Solution:
- $\mathrm{SF}_{6}(g) \longrightarrow \mathrm{S}(g)+6 \mathrm{~F}(g)$
$\Delta H=\Sigma \Delta H_{f}^{\circ}$ (products) $-\Sigma \Delta H_{f}^{\circ}$ (reactants)
$$ =275+6 \times 80+1100=1855 \mathrm{~kJ} $$
$\Rightarrow$ Average $\mathrm{S}-\mathrm{F}$ bond energy $=\frac{1855}{6}=309.16 \mathrm{~kJ} / \mathrm{mol}$
