16
$\begingroup$

From just the IB chemistry bonding chapter information, you would normally deduce that $\ce{PCl3}$ and $\ce{PCl5}$ are both covalent molecules.

However, as $\ce{PCl3}$ has a dipole moment, it has dipole-dipole intermolecular attractions, implying that $\ce{PCl3}$ has a higher melting point than $\ce{PCl5}$, which only has London dispersion forces.

So why is it that $\ce{PCl5}$ has a higher melting point than $\ce{PCl3}$ (I saw online that $\ce{PCl5}$ is an ionic solid)?

Improve this question
$\endgroup$
4
29
$\begingroup$

Because $\ce{PCl5}$ does something which is not immediately obvious from its molecular formula: it autoionizes and becomes an ionic solid $\ce{PCl4+PCl6-}$. As such, it has much stronger interactions than $\ce{PCl3}$ with its mere dipole-dipole attractions, hence the higher melting point.

If not for that fact, you deduction should have worked just fine.

Improve this answer
$\endgroup$
2
  • $\begingroup$ Is there any mathematical way or just method to deduce that $\ce {PCl5}$ is an ionic solid? Or is that from observation. $\endgroup$
    – Stone
    Dec 12 '17 at 15:23
  • 3
    $\begingroup$ No, I don't think this can be deduced via logic alone. As for more mathematical way, in chemistry this usually implies that you are going to end up in a nocturnal abyss of quantum chemistry, which is not what you want, believe me. $\endgroup$
    – Ivan Neretin
    Dec 12 '17 at 15:33

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.