Sciencemadness Discussion Board

Acid strength

Random - 11-7-2013 at 15:29

How do we exactly know which acid is stronger and if it will push out weaker acid out of it's salt?

For example oxalic acid is called a weak acid, sulfuric acid is called strong acid yet oxalic acid can push out sulphate and produce sulfuric acid.

ElectroWin - 11-7-2013 at 15:41

doesn't that depend on solubility of the respective salts? like calcium sulfate (ksp = 4.93×10^-5) vs oxalate (ksp = 2.32×10^-9)


[Edited on 2013-7-11 by ElectroWin]

Random - 11-7-2013 at 15:46

Quote: Originally posted by ElectroWin  
doesn't that depend on solubility of the respective salts? like calcium sulfate vs oxalate


I thought so, but then it came to my mind that H2CO3 should react with CaSO4 to produce sulfuric acid which clearly doesn't work. Why is is that?

Metacelsus - 11-7-2013 at 16:18

1) CaSO4 isn't very soluble in water to begin with.
2) H2CO3 exists in equilibrium with H2O and CO2.

The reverse reaction is what usually takes place: CaCO3(s) + H2SO4(l) -> CaSO4(s) + H2O(l) + CO2(g)
Try mixing some limestone chips with sulfuric acid.

ElectroWin - 11-7-2013 at 16:24

ok, let's also compare properties of these acids:

sulfuric pKa1 < 0, pKa2 1.92;
oxalic pKa1= 1.2 pKa2= 4.2;
carbonic pKa1 = 6.37 and pKa2 = 10.33

so, carbonic is *much* weaker than oxalic. not a complete explanation i suppose, but a strong clue

annaandherdad - 11-7-2013 at 18:24

Another example is H2S + ZnSO4 -> ZnS + H2SO4. It's a way of making zinc sulfide.

Metacelsus - 11-7-2013 at 19:36

Really? Sulfides tend to react with acids (or sometimes even water!) liberating hydrogen sulfide gas, not the other way around.

https://en.wikipedia.org/wiki/Iron(II)_sulfide
https://en.wikipedia.org/wiki/Aluminum_sulphide
http://www.juniata.edu/services/ScienceInMotion/chem/labs/ga...
(the final link cites ZnS reacting with HCl to produce H2S and ZnCl2; I assume it would be similar with H2SO4)

Do you have any references to the contrary?

Magpie - 11-7-2013 at 19:44

Compare two sulfate salts, each mixed with oxalic acid in two separate dilute aqueous solutions.

case 1: The salt is Na2SO4, which is completely dissolved.
case 2: The salt is CaSO4, which is virtually undissolved.

H+ ions will exist in both solutions due to the partial dissolution of the oxalic acid.

1. In the Na2SO4 solution the SO4-- ions will exist since Na2SO4 is quite soluble. SO4-- anions were not "pushed out." Does sulfuric acid exist here? Sure, but what anion really owns those H+ ions? It's arbitrary whether you say they belong to oxalate, sulfate, or bisulfate anions. The real answer is they belong to none of them.

2. In the CaSO4 case there are virtually no SO4-- ions due to the insolubility of CaSO4. So no sulfuric acid exists.



Random - 11-7-2013 at 20:52

Quote: Originally posted by Magpie  
Compare two sulfate salts, each mixed with oxalic acid in two separate dilute aqueous solutions.

case 1: The salt is Na2SO4, which is completely dissolved.
case 2: The salt is CaSO4, which is virtually undissolved.

H+ ions will exist in both solutions due to the partial dissolution of the oxalic acid.

1. In the Na2SO4 solution the SO4-- ions will exist since Na2SO4 is quite soluble. SO4-- anions were not "pushed out." Does sulfuric acid exist here? Sure, but what anion really owns those H+ ions? It's arbitrary whether you say they belong to oxalate, sulfate, or bisulfate anions. The real answer is they belong to none of them.

2. In the CaSO4 case there are virtually no SO4-- ions due to the insolubility of CaSO4. So no sulfuric acid exists.





But if that is the case, then the method of obtaining sulfuric acid from oxalic and CaSO4 wouldn't work, yet it works. As seen in attachment provided.

But also doesn't work with H2S and ZnSO4.

Given that H2S and oxalic acid are both weaker than sulfuric, and both ZnS and calcium oxalate are insoluble, which explains driving the equilibrium to their side, yet one forms sulfuric and other doesn't.

Is it the difference in their strengths? Given the fact that H2CO3 and H2S are extremly weak acids in relation to oxalic?

Attachment: chemicals_sulfuric_acid_preparation_from_oxalic_acid_thompson1849.pdf (209kB)
This file has been downloaded 295 times

Magpie - 11-7-2013 at 21:32

The solubility of CaSO4 is 2.1g/L (20°C) whereas that for CaC2O4 is only 0.007 g/L (20°C), per Wiki. So this is what is driving the formation of CaC2O4 as an insoluble residue when CaSO4 is added to oxalic acid (H2C2O4). This then leaves sulfuric acid in solution.

So I picked an incorrect example to illustrate my point. In this case you might indeed say that sulfate is "pushed out" of the relatively less insoluble salt, CaSO4.

I should have looked into the solubility of CaC2O4 before posting. I knew it was insoluble as it was a big problem in a paper mill I once worked in. The insoluble CaC2O4 was clogging up the pulp washing equipment.

I think relative salt solubility is more relevant than relative acid strength for the reason behind formation of sulfuric acid in this case.

Random - 11-7-2013 at 21:47

Quote: Originally posted by Magpie  

I think relative salt solubility is more relevant than relative acid strength for the reason behind formation of sulfuric acid in this case.


So I guess the reaction with H2S + ZnSO4 -> H2SO4 + ZnS doesn't work because H2S is a gas and escapes out of solution, also it has low solubility.
And the reaction with oxalic works because oxalic acid can't escape the solution.

If we see the reaction from another side, calcium oxalate is so insoluble that it could barely get dissolved and even if it does, oxalate ions again quickly precipiatate with calcium ion because it cannot escape out of solution and thus equilibrium works to the sulfuric acid side. One could say, sufluric acid overall reacts way slower with barely soluble oxalate than oxalate reacts with calcium ion which is a very fast reaction.

ZnS on the other side, as soon as a small amount of it reacts with sulfuric acid, H2S being relatively insoluble, escapes the solution and only thing that remains in the solution are Zn+ and (SO4)2- ions which have no way to escape out. Thus eqilibrium gets in favor of ZnSO4. Same with H2CO3. One could say here that H2S escapes the solution faster than it can actually react with ZnSO4 again due to it's not so high solubility and a faster reaction speed of H2SO4 with ZnS.

Is this correct?

[Edited on 12-7-2013 by Random]

annaandherdad - 12-7-2013 at 07:39

Quote: Originally posted by Cheddite Cheese  
Really? Sulfides tend to react with acids (or sometimes even water!) liberating hydrogen sulfide gas, not the other way around.

https://en.wikipedia.org/wiki/Iron(II)_sulfide
https://en.wikipedia.org/wiki/Aluminum_sulphide
http://www.juniata.edu/services/ScienceInMotion/chem/labs/ga...
(the final link cites ZnS reacting with HCl to produce H2S and ZnCl2; I assume it would be similar with H2SO4)

Do you have any references to the contrary?


I learned about this from the video by Gyzmodium, https://www.youtube.com/watch?v=EKpLFz2isrI
who posts on SM as Squall181. I was surprised, too, since as you say sulfides react with HCl to make H2S.

Then I did a systematic search with the SM search engine on zinc sulfide, and found an independent reference to the fact that the reaction H2S + ZnSO4 -> ZnS + H2SO4 does take place. However I don't remember where I found that or who said it, just that it's somewhere on SM.

That's the extent of my information on this topic, but I'm currently planning to try it myself, as soon as I can figure out how to deal safely with the H2S.

ElectroWin - 12-7-2013 at 09:58

i've seen reports on these reactions going both ways. H2S will precipitate sulfides, and acidifying sulfides will liberate H2S. so i guess equilibrium depends on specific conditions.

re: oxalic acid, there is probably a maximum concentration of H2SO4 obtained?

Magpie - 12-7-2013 at 13:10

As an interesting side note:

For qualitative analysis schemes:

Group II cations (Pb2+, Bi3+, Cu2+, Cd2+, Hg2+, As3+, Sb3+, Sn4+) will all precipitate their sulfides in H2S in an aqueous solution 0.3M in H+. Zn2+ will not.

Group III cations Co2+, Ni2+, Mn2+, Fe3+, and Zn2+ will precipitate their sulfides in a solution of NH3 and H2S, however.

In the Gp II test the S-- concentration is not high enough to ppt ZnS. But in the Gp III test it is.