Chemistry question

benny.123 SG

Why can phosphorus form both PCl3 and PCl5 at the same time? Please help me, I have a assignment to submit tmr!

yongcheng_10 SG

Hey benny. I recently saw a question that is similar to yours. The answer can be found here.

UltimaOnline SG

If Cl2 is present in extremely limited supply, PCl3 is the major product. If Cl2 is present in large excess, PCl5 is the major product. If Cl2 is present in amounts anywhere in between, a mixture of PCl3 and PCl5 results.

Regardless, you must always write both equations separately, one equation for formation of PCl3, and one equation for formation of PCl5. (This is similar to the necessity of writing two equations for incomplete combustion, one to generate CO, the other to generate CO2. A common costly student error, is to write a single equation for both CO and CO2, which is based on the erroneous and ridiculous assumption that CO and CO2 are generated in equal amounts).

For A level H2 Chemistry purposes, if you're not told whether Cl2 is limited or in large excess, the exam-smart student will write both equations (separately, of course).

UltimaOnline SG

(Of course, it's always possible, probably even, that both "Benny" and "YongCheng" newly registered here on SgForums just to promote their colab.sg website.)

It seems that your question is poorly phrased. What you mean, "Why does phosphorus form two different chlorides, PCl3 and PCl5", rather than asking in the context of an A level exam, in which the worried student is unsure which equation to write. This was the context I assumed, and replied accordingly in my previous post.

Even then, the question should be phrased more appropriately or pointedly, in a way that can be asked for an A levels examination context, eg. "Why is it that nitrogen cannot form NCl5, but phosphorus can form PCl5?"

Unlike nitrogen, which cannot accommodate an expanded octet using on its 2s and 2p orbitals, phosphorus has vacant, energetically accessible 3d orbitals that can be used to accommodate an expanded octet, and hence increase its coordination number from 3 to 5.

Furthermore, because phosphorus has one additional electron shell, it has a significantly larger atomic radius compared to N, and so it is able to increase its coordination number from 3 to 5 (ie. having more atoms around and bonded to the large central P atom), without experiencing severe steric van der Waals strain or repulsion, as would be the case for N.

Both of these reasons, allow phosphorus to form two different chlorides, PCl3 and PCl5, depending on whether Cl2 is limited or in excess. In contrast, N can only form one chloride NCl3, and NCl5 does not exist due to the two reasons described above (lack of energetically accessible d orbitals, and the resulting steric van der Waals strain or repulsion, when attempting to increase its coordination number beyond 4).

benny.123 SG

Thanks mr ultima for the detailed explanation. The exact question that i got was:

Explain why phosphorus can form PCl5 even though it has only 3 unpaired electrons.

I think you answered my question nicely