Why does PCl5 in the solid state exist as cation and anion, i.e. in the form of PCl4^+ and PCl6^-?

1 Answer
Ernest Z.
Dec 11, 2017

Increased lattice energy and better packing efficiency.

Explanation:

We must ask ourselves three questions:

  • What does the #"PCl"_5# lose by doing this?
  • What does the #"PCl"_5# gain by doing this?
  • Does the gain exceed the loss and therefore lower the energy of the system?

What does the #"PCl"_5# lose?

It takes energy to form ions from #"PCl"_5#.

#"PCl"_5 +"PCl"_5 + "energy" → "PCl"_4^"+" + "PCl"_6^"-"#

What does the #"PCl"_5# gain?

(a) Lattice energy

The attractive forces in an ionic crystal are much greater than the dipole-dipole or dispersion in crystals of covalent substances.

(b) Greater symmetry

The tetrahedral # "PCl"_4^"+" ("T"_text(d))# and octahedral #"PCl"_6^"-" ("O"_text(h))# ions are more symmetrical than the trigonal bipyramidal #"PCl"_5 ("D"_text(3d))# molecules.

In particular, all the #"P-Cl"# bonds in each ion have become identical.

This enables more efficient packing in the crystal lattice and further increases the interionic attractions.

Do the gains outweigh the losses?

Yes! Lattice energy is usually the largest factor in determining the stability of an ionic solid.

The extra energy gained by the lattice energy more than compensates for the energy needed to transfer a chloride ion from one #"PCl"_5# molecule to another.

Thus, #"PCl"_5# exists as an ionic solid.

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