PCl5 is a stable molecule. In contrast, antimony pentachloride, SbCl5, is unstable and spontaneously loses?

Question

Cl2 to from antimony trichloride, SbCl3. Why is SbCl5 not as stable as PCl5

Answer 1

It has to do with the heats of reactions for the different compounds and the activation energy barrier between ACl3 <-> ACl5 where A= Sb or P.