Question

What happens when $PC{l_5}$ is heated?

Answer 1

Hint: Before answering this question, we must recall the type of halides phosphorus forms. We should mainly concentrate on their thermal stability and determine if Phosphorus pentachloride is stable or not.

Complete step by step solution:
Phosphorus generally forms 3 series of halides with oxidation state +5, +3 and +2. But with chlorine, it only forms pentahalides and trihalides. They are usually prepared by direct reaction of the elements, or by trans halogenation. They are mostly utilized as ligands in coordination complexes.
Phosphorus pentachloride is a colourless compound which is extremely water sensitive. Its Lewis acid nature is responsible for many of its chemical properties like autoionization, chlorinations, and hydrolysis. Remember that $PC{l_5}$ is a dangerous substance as it reacts violently with water. It is also corrosive when in contact with skin and can be fatal when inhaled.
We should know that phosphorus forms two kinds of halides, namely trihalides and pentahalides. In $PC{l_5}$, phosphorus has $s{p^3}$ hybridization and has a trigonal bipyramidal structure. It has three equatorial P-Cl bonds and two axial P-Cl bonds which are different in length and hence stability. The axial bonds (219pm) are larger than equatorial bonds (204pm). This means that the equatorial bonds are stronger than axial ones. Trihalides are far more stable thermally than pentahalides. Hence, $PC{l_5}$ decomposes to form its trihalide when strongly heated.
$PC{l_5} \to PC{l_3} + C{l_2}$

Note: We should note that on heating lightly, phosphorus pentachloride generally sublimates, that is the solid substance changes to its gaseous state. It is only on heating strongly that it decomposes.