Chem confirmation

[gonelli]

Just been doing some work on acids and bases.

 

I know that when you react an acid with a metal oxide you get a salt and water being produced. But a question I have asks for the equation when you mix lead sulfide and hydrochloric acid. Will you get lead cloride (the salt) and hydrogen sulfide produced?

 

PbS(s) + 2HCl(aq) -----> PbCl2(s) + H2S(g)

[YT2095]

it looks fine to me

[Fuzzwood]

Why not? Would you expect fission?

[John Cuthber]

Well, it should work but lead sulphide is so insoluble that the reaction might be a bit slow and, with dilute enough acid, it might not work at all or the reaction might even go backwards.

[YT2095]

John, it does work, but as you say, there Are solubility issues and heating is required to speed it up a little.

 

I`ve done the same with Antimony trisulphide, it`s either really slow, or you need to heat it, but not enough to get rid of the HCl, Reflux would be the best way.

[John Cuthber]

I wonder; if you got some nice white PbCl2 (I guess damp would work best)and passed H2S over it would it go brown?

[YT2095]

no, it shouldn`t.

[John Cuthber]

I thought that the fact that the H2S could carry away the HCl would force the reaction to give PbS.

Anyway, I tried it and it did.

 

H2S from heated sulphur, parafin wax and sawdust in a test tube blackened a piece of paper smeared with wet PbCl2.

[YT2095]

lol, I suspect you were getting Much more than just H2S from that reaction don`t you?

 

that`s a really Dirty synth to try and generate H2S, and it`s also not the reaction conditions described.

 

lead sulphide in HCl to make lead chloride is a usual step to make the hydroxide with the addition of NaOH later.

 

so yes, his reaction is perfectly Fine!

[John Cuthber]

It's a bit "old school" but that reaction works quite well. The other dominant product is sulphur vapour but I don't think that would blacken PbCl2.

I don't see why it's not the conditions I described; I passed H2S over moist PbCl2 and it went black.

In principle all reactions are reversible so while it's fair to say that the reaction would usually give lead chloride and a nasty smell it can be driven the other way.

[Fuzzwood]

It's a bit "old school" but that reaction works quite well. The other dominant product is sulphur vapour but I don't think that would blacken PbCl2.

I don't see why it's not the conditions I described; I passed H2S over moist PbCl2 and it went black.

In principle all reactions are reversible so while it's fair to say that the reaction would usually give lead chloride and a nasty smell it can be driven the other way.

 

It actually could:

 

S + 2e- --> S^2-

2Cl^- --> Cl₂ + 2e-

 

And no, not all reactions are reversible: can you put the same CO₂ and H₂ back together into C₈H₁₈ and oxygen?

[John Cuthber]

I'm pretty sure that chlorine is a stronger oxidant than sulphur so that reaction is, if anything, less likely than H2S blackening PbCL2.

Burning octane isn't a reaction, it's lots, probably hundreds of reactions. Each of them IS reversible. Did you imagine that there was some sort of concerted attack by a dozen or so oxygen molecules?

Every step in the free radical chain reactions that occur in burning octane is reversible. Getting the whole sequence to run backwards is energetically possible but entropically disfavoured.