Group 15 elements question (acid versus base)

[Dhanush Shivaramaiah]

Why is it that all group 15 element's trihalides except Nitrogen on hydrolysis gives an acid while Nitrogen trichloride give ammonia which is a base on hydrolysis?

 

[snorkack]

Why is it that all group 15 element's trihalides except Nitrogen on hydrolysis gives an acid while Nitrogen trichloride give ammonia which is a base on hydrolysis?

The more important factor is that of oxidation state. All other "halides" on hydrolysis give hydrogen halides. The compound of nitrogen and chlorine gives hypochlorous acid.
By which excuse is the compound of chlorine and nitrogen called nitrogen chloride and not chlorine nitride?

 

[TeethWhitener]

The more important factor is that of oxidation state.

But all the pnictides have an oxidation state of +3 in the trichlorides.

By which excuse is the compound of chlorine and nitrogen called nitrogen chloride and not chlorine nitride?

Because nitrogen has a lower electronegativity than chlorine.

In simple terms, you can think of the reaction as a straight substitution of OH^- for Cl^-:
$$ ACl_3+3H_2O\to A(OH)_3+3HCl$$
This works as advertised for arsenic and phosphorus, although phosphorus mainly converts to the tautomeric (OH)₂HP=O, phosphorous acid. The problem with the nitrogen case is that N(OH)₃ is completely unstable. You're never going to see it in solution. Instead, you might expect to see a complex mixture of NH₂OH or various nitrogen oxides, as well as a mixture of chloramines. Nitrogen in its +3 oxidation state is also a pretty good oxidizer, so what ultimately ends up happening is that you "oxidize" water to HOCl. (The oxidation state of O doesn't change in this case, but Cl goes from -1 to +1.) Ammonia is simply the stable end of a long road.

 

[snorkack]

But all the pnictides have an oxidation state of +3 in the trichlorides.

By which evidence?

Because nitrogen has a lower electronegativity than chlorine.

By which evidence?

In simple terms, you can think of the reaction as a straight substitution of OH^- for Cl^-:
$$ ACl_3+3H_2O\to A(OH)_3+3HCl$$
This works as advertised for arsenic and phosphorus, although phosphorus mainly converts to the tautomeric (OH)₂HP=O, phosphorous acid. The problem with the nitrogen case is that N(OH)₃ is completely unstable. You're never going to see it in solution. Instead, you might expect to see a complex mixture of NH₂OH or various nitrogen oxides, as well as a mixture of chloramines. Nitrogen in its +3 oxidation state is also a pretty good oxidizer,

And NH₂OH is not oxidation state +3, it is -1. While N(OH)₃ is completely unstable, a common long-lived species in dilute aqueous solutions is HNO₂. So if NCl₃ were nitrogen trichloride, you might expect the substitution:
$$ NCl_3+2H_2O\to HNO_2+3HCl$$
But that´s not what happens.

so what ultimately ends up happening is that you "oxidize" water to HOCl. (The oxidation state of O doesn't change in this case, but Cl goes from -1 to +1.)

And therefore it would be chlorine that is oxidized, not water.
But what is the evidence that the oxidation state of Cl is -1 in the compound with nitrogen in the first place?

 

[Borek]

"Oxidation state" is a weak argument no matter which way you will try to use it - there is no way to measure it, in general it is more of an accounting device that any real property of an atom.

 

[TeethWhitener]

By which evidence?

A decent piece of evidence is the fact that the dipole moment in NCl₃ points in the opposite direction of NH₃. This means the electron distribution is shifted toward the nitrogen in NH₃ but away from the nitrogen in NCl₃.

"Oxidation state" is a weak argument no matter which way you will try to use it - there is no way to measure it, in general it is more of an accounting device that any real property of an atom.

This would be fine, but the concept of oxidation state 1) works well in a lot of cases, and 2) the electric charge in the vicinity of a nucleus tracks oxidation states rather closely and absolutely can be measured (via XPS, for example). Oxidation state has its problems, but it's a pretty decent shorthand for horribly complicated electrostatics.

 

[snorkack]

How is the hydrolysis mechanism of monochloramine?
Hydroxylamine is a well described species.
Why does the reaction
NH₂OH+HCl<->NH₂Cl+H₂O
not happen, in either direction, so that the only reaction is
NH₃+HClO<->NH₂Cl+H₂O?

 

[TeethWhitener]

How is the hydrolysis mechanism of monochloramine?
Hydroxylamine is a well described species.
Why does the reaction
NH₂OH+HCl<->NH₂Cl+H₂O
not happen, in either direction, so that the only reaction is
NH₃+HClO<->NH₂Cl+H₂O?

According to this JACS article:
http://pubs.acs.org/doi/abs/10.1021/ja00869a004
At least in alkaline solution, you do get hydroxylamine (and HNO₂ to boot).
As for the mechanism, I suspect that, like most things in chemistry, it's more complicated than simple theories make it sound.