Chemical And Ionic Equilibrium Result Question 9
9. Which of the following are Lewis acids?
(2018 Main)
(a) $\mathrm{PH} _3$ and $\mathrm{BCl} _3$
(b) $\mathrm{AlCl} _3$ and $\mathrm{SiCl} _4$
(c) $\mathrm{PH} _3$ and $\mathrm{SiCl} _4$
(d) $\mathrm{BCl} _3$ and $\mathrm{AlCl} _3$
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Answer:
Correct Answer: 9. ( d )
Solution:
- Key Idea: Lewis acids are defined as,
“Electron deficient compounds which have the ability to accept atleast one lone pair.”,
The compound given are
$\mathbf{P H} _3$ -Octet complete although $\mathrm{P}$ has vacant $3 d$-orbital but does not have the tendency to accept lone pair in it. Hence, it cannot be considered as Lewis acid.
$\mathbf{B C l} _3$ -Incomplete octet with following orbital picture.
Hence, vacant $p$-orbital of B can accept one lone pair thus it can be considered as Lewis acid.
$\mathrm{AlCl}_3$-Similar condition is visible in $\mathrm{AlCl}_3$ as well i.e.
$\mathrm{Al}($ Valence orbital only $)=$
Hence this compound and also be considered as Lewis acid.
$\mathbf{S i C l}_4$ - Although this compound does not have incomplete octet but it shows the tendency to accept lone pair of electrons in its vacant $d$-orbital. This tendency of $\mathrm{SiCl}_4$ is visible in following reaction.
Thus, option (b) and (d) both appear as correct but most suitable answer is (d) as the condition of a proper Lewis acid is more well defined in $\mathrm{BCl}_3$ and $\mathrm{AlCl}_3$.
BETA
