Question

The average $S-F$ bond energy in $S{F}_6$ if the $\Delta H_f^o$ values are $-1100,+275$ and $+80$ kJ/mol respectively for $S{F}_6$(g), $S$(g) and $F$(g) is:

• A. $390.1$ kJ/mol
• B. $103.9$ kJ/mol
• C. $903.1$ kJ/mol
• D. $309.1$ kJ/mol

Answer 1

Answer: (D)

$309.1$ kJ/mol

Given that:

$S_{\left(s\right)}+3F_{2\left(g\right)}\to S{F}_{6\left(g\right)};\Delta H=-1100kJ\left(1\right)$

$S{}_{\left(s\right)}\to S_{\left(g\right)};\Delta H=275kJ\left(2\right)$

$\frac{1}{2}{F}_{2\left(g\right)}\to F_{\left(g\right)};\Delta H=80kJ\left(3\right)$

To have $S{F}_{6\left(g\right)}\to S_{\left(g\right)}+6F_{\left(g\right)}$ we can proceed as

$\text{equation}\left(2\right)+6\times \text{equation}\left(3\right)-\text{equation}\left(1\right)$

$S{F}_{6\left(g\right)}\to S_{\left(g\right)}+6F_{\left(g\right)};\Delta H=1855kJ$

Thus average bond energy for $S-F$ bond$=\frac{1855}{6}=309.16kJ$.