[Oblihatni]

I am having trouble with the specific reactions involved in an experiment I did in synthesizing PbI2 from PbS.

I will (very briefly otherwise this post may become tedious) describe the experiment as well as some observations, followed by my own conclusions and then my questions. Unfortunately, I forgot my Lab Notebook (which I kept for the first time in my life and highly recommend for any experiment!), so my observations will be mostly from memory and some other notes I jotted down on a paper.

Experiment:
1) PbS (solid) plus HCl (concentrated). Heat and gently boil.
    Black PbS dissolves. A smelly red gas evaporates.

2) Remove from heat and add HNO3 (concentrated). Boil again.
    A precipitate forms which dissolves after a while. Yellowish gas evaporates which become clear (colourless) after about 10-15 minutes.

3) Add H2O & cool.

4) Add NH3 (concentrated).
    A white precipitate forms.

5) Add HNO3 (dilute).
    Precipitate dissolves.

6) Add an excess of H2O.

7) Add KI dissolved in water.
    Golden yellow flakes form.

Heat. Cool down to room temperature and then on ice-bath.
    Precipitate dissolves and recrystallization occurs.

9) Dry.

My conclusions:
I currently have the following reactions:

1) PbS (s) + 2HCl (l) → PbCl₂ (aq) + H₂S (g)

2) PbCl₂ (aq) + 2HNO₃ (l) → Pb(NO₃)₂ (s) + 2HCl (g)

3) Pb(NO₃)₂ (s) + H₂O (l) → [Pb(OH₂)₂]²⁺ (aq) + NO₃^- (aq)

4) [Pb(OH₂)]²⁺ + 2NH₃ (l) → [Pb(NH₃)₂]²⁺ + 2H2O (l)

5) [Pb(NH3)2]2+ + KI2 (aq) → PbI2 (s) + NH3 (aq)

I now that some things are wrong. So I present the following questions:

Firstly, I suspect that Pb²⁺ + NH₃ forms an octahedral coordination [Pb(NH₃)₆]²⁺ compound and not a covalent one as I currently indicated. If this is true, why? And at what stage does the Pb-complex form? My wild guess would be when water is added.

Secondly, I know that a reaction exists where [Pb(NO₃)₂]²⁺ (aq) + KI₂ (aq) → PbI₂ (s) + NO₃^- (aq)
Why then was the water step and the ammonia step added for my experiment? Water may only be there as a solvent, but from discussions with other people and observations I am sure that a reaction happened. For ammonia a reaction definitely did happen.

In fact, following the second question, any remarks on the function of any of the steps would be helpful.

I know that PbCl₂ is insoluble in water, but no precipitate was obtained. From the numerous books I consulted I concluded that this is due to the acid hydrolysis of a halide. I still cannot seem to figure out the reaction that fits into my current hypothesis for a reaction scheme.

Finally, a question we have, is why is KI not to be added in excess? What I currently believe is that an excess of iodide ion would cause an coordination compound ([PbI₆]^4-) to form, but this does not intuitively feel right.

That is most of my questions. I used to be good at inorganic chemistry, but seem to have lost the touch However, I am determined to get it back again. So please, do not just give me answers, I would really appreciate explanations and discussions. Currently I have information to at least pass, but I want to learn and understand. I will still continue this discussion even after I have handed in my report.

Thank you.