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reactofurnace
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sad.gif posted on 28-8-2020 at 10:34
Copper sulphate preparation question


I'm working on a collaboration with another channel and he asked if I could explain the reason of adding H2O2 for the reaction of Cu and H2SO4 to progress. I thought of it and researched it but couldn't find any convincing answers. I know that H2O2 acts as an oxidizer, presumably oxidizing the copper to Cu2+.
I have a slight hunch that H2O2 decreases the Gibbs free energy of the system making the reaction more thermodynamically spontaneous. However, until I've calculated it I'm not sure if that would be correct. Any ideas on how this reaction works:
Cu + H2SO4 + H2O2 ---> CuSO4 + 2 H2O
and why this reaction is slow:
Cu + 2 H2SO4 ---> CuSO4 + 2 H2O + SO2

[Edited on 28-8-2020 by reactofurnace]
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[*] posted on 28-8-2020 at 10:53


An important part of the reason is
SO2 + 2 H2O2 --> H2SO4 + H2O
Which means you use less acid (because some of it is regenerated from the SO2
And you don't have to take care of lots of SO2.
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[*] posted on 28-8-2020 at 11:57


Cu + H2O2 --> CuO + H2O

CuO + H2SO4 --> CuSO4 + H2O

This is what actually happen. Copper can't be dissolve in nonoxidizing acids because nascent hydrogen reduce Cu(II) back to its metalic state. However you can dissolve it in conc. H2SO4 (>90%), because this acid have oxidizing properties.

Cu + 2 H2SO4 ---> CuSO4 + 2 H2O + SO2

But this reaction take place only with concentrated sulfuric acid, not with dilute. Btw. this reaction is simplified, in reality is much more complex, look at this:

http://pubs.sciepub.com/wjce/2/2/1/




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[*] posted on 28-8-2020 at 12:02


Acids attack less noble metals by a replacement reaction where it switches place with hydrogen. For iron the reaction will be something like:
Fe + H2SO4 => FeSO4 + H2
But this only happens if the metal is more reactive than hydrogen (https://en.wikipedia.org/wiki/Reactivity_series). Copper is more noble, so acids do not attack it directly.
It is however readily oxidized into CuO by both H2O2 (and air for that matter). Once oxidized the acid have no problem attacking it through a different reaction:
CuO + H2SO4 => CuSO4 + H2O



Unionseed: What are you talking about???




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[*] posted on 28-8-2020 at 12:28


Quote: Originally posted by Fulmen  


Unionseed: What are you talking about???

Which bit don't you understand.
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[*] posted on 28-8-2020 at 12:42


@Unionseed: Everything. As Bedlasky points out H2SO4 isn't oxidizing except for in extreme cases. Also, you only presented part of the reaction. Where does the SO2 come from? The starting ingredients are H2SO4, Cu and H2O2, any intermediate steps should be included in a well presented argument.



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[*] posted on 28-8-2020 at 13:35


Fulmen's analysis is correct.
Copper is sufficiently inert toward sulfuric acid it won't displace hydrogen.
H2O2 will react with sulfuric acid forming caro's acid aka Piranha solution which is a stronger oxidizer.
That reacts with copper forming copper(II) oxide which then reacts forming copper(II) sulfate.
H2O2 and Sulfuric acid are very very slow to attack copper individually.
Peracetic acid does a similar reaction.

[Edited on 28-8-2020 by macckone]
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[*] posted on 28-8-2020 at 13:47


I don't think it's correct to say that the hydrogen peroxide reacts with copper to form copper(II) oxide, which then reacts with the acid to form copper(II) sulphate. Acidic hydrogen peroxide will react with copper metal to give water and copper(II) ions.

2 H+ + H2O2 + Cu ==> 2 H2O + Cu(2+)

The sulphate is a spectator ion.




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[*] posted on 28-8-2020 at 15:24


DraconicAcid,
You are correct technically once the copper (ii) oxide is attacked by the hydronium ions to yield copper ions and water.
In this reaction the sulfate ions are first converted to monopersulfate, so they aren't exactly spectators.
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[*] posted on 28-8-2020 at 16:10


Quote: Originally posted by macckone  
DraconicAcid,
You are correct technically once the copper (ii) oxide is attacked by the hydronium ions to yield copper ions and water.
In this reaction the sulfate ions are first converted to monopersulfate, so they aren't exactly spectators.


You'd have to have some hard evidence to convince me that copper(II) oxide is formed as an intermediate at all in this reaction.

The conversion of sulphate to a persulphate is believable, but I'm not convinced that it's a required step. Has it been shown to be involved in this reaction?




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[*] posted on 28-8-2020 at 18:50


Quote: Originally posted by Bedlasky  
Cu + H2O2 --> CuO + H2O

CuO + H2SO4 --> CuSO4 + H2O

Cu + 2 H2SO4 ---> CuSO4 + 2 H2O + SO2



http://pubs.sciepub.com/wjce/2/2/1/

I
Bedlasky is correct.the copper has to react to CuO first. the peroxide will help speed this up and if it stops the rxn from giving off so2 that's a good thing.also don't start with copper pipe.use headphone cords or some other thin electric wire as boiling sulfuric acid with copper pipe would take a week to dissolve.thin wire dissolves in a day or less.use a wash bottle with caustic soda to dissolve any fumes
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[*] posted on 28-8-2020 at 21:15


Here is the link to a 1962 paper on it:
https://www.nrcresearchpress.com/doi/pdf/10.1139/v62-227

It has to form a copper oxide before it will react with the sulfuric acid.
But neither sulfuric acid nor hydrogen peroxide by itself will oxidize copper in a timely manner.
Therefore the intermediate monopersulfuric acid has to be present.
Bubbling oxygen into sulfuric acid is known to form monopersulfuric acid which is detected as peroxide in solution.
Also in the provided paper.

As I said previously peracetic acid does the same type of oxidation.
This is true for copper and other metals below hydrogen on the reactivity scale.
In the case of lead, the sulfate is insoluble so you have to go the peracetic route.
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[*] posted on 28-8-2020 at 21:34


Here is one that goes more into persulfates:
Simpkins, D. C. (1979). Hydrogen Peroxide/Sulphuric Acid Etching Systems. Circuit World, 6(1), 54–57. doi:10.1108/eb043607

It is available from you favorite hub of scientific papers.
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[*] posted on 28-8-2020 at 21:38


If you read the end of the paper where they talk about the proposed mechanism, they don't suggest that copper oxide gets formed- just copper(I) and copper(II) ions, the former of which gets oxidized by hydrogen peroxide.

I can accept that neither hydrogen peroxide nor sulphuric acid alone will oxidize copper at a reasonable rate. The fact that they do so together can be adequately explained by the fact that acidic hydrogen peroxide is a stronger and more rapid oxidizing agent than neutral hydrogen peroxide, without postulating the involvement of peracids or the formation of copper(II) oxide.

As far as I know, hydrogen peroxide will corrode copper quite well in the presence of *any* acid. Hydrochloric acid certainly won't form an oxyacid with hydrogen peroxide, and will react with hydrogen peroxide and copper. Anyone have any fluoboric acid to test the reaction of that with copper and hydrogen peroxide? It won't form a peroxyacid, neither will it coordinate the copper ions.




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[*] posted on 28-8-2020 at 22:17


Quote: Originally posted by draculic acid69  
Quote: Originally posted by Bedlasky  
Cu + H2O2 --> CuO + H2O

CuO + H2SO4 --> CuSO4 + H2O

Cu + 2 H2SO4 ---> CuSO4 + 2 H2O + SO2



http://pubs.sciepub.com/wjce/2/2/1/

I
Bedlasky is correct.the copper has to react to CuO first. the peroxide will help speed this up and if it stops the rxn from giving off so2 that's a good thing.also don't start with copper pipe.use headphone cords or some other thin electric wire as boiling sulfuric acid with copper pipe would take a week to dissolve.thin wire dissolves in a day or less.use a wash bottle with caustic soda to dissolve any fumes


I try to say, that if you want to dissolve copper in dilute sulfuric acid you need some oxidizer, because dilute sulfuric acid haven't oxidizing properties. So there isn't SO2 formation at all. Dilute sulfuric acid alone just don't react with copper. This can do only concentrated sulfuric acid which have oxidizing properties (so SO2 is produced as a product of reduction of sulfuric acid). This needs heating for reasonable speed of the reaction. Of course that you can add some peroxide for speeding things up, but it isn't necessary.

Quote: Originally posted by DraconicAcid  
Anyone have any fluoboric acid to test the reaction of that with copper and hydrogen peroxide? It won't form a peroxyacid, neither will it coordinate the copper ions.


I think that perchloric acid is more common nonoxidizing and noncoordinating acid, which doesn't form peroxyacid. I have some NaClO4 and I plan make some HClO4 from it so I can try it.

[Edited on 29-8-2020 by Bedlasky]




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[*] posted on 29-8-2020 at 00:22


Quote: Originally posted by unionised  
An important part of the reason is
SO2 + 2 H2O2 --> H2SO4 + H2O
Which means you use less acid (because some of it is regenerated from the SO2
And you don't have to take care of lots of SO2.


If you add sufficient even 50% H2O2 to concentrated sulphuric acid for it to be stoichiometric the 50% H2O of the 50% H2O2 will dilute the acid to about 75% and therefor little or no SO2 formed to be oxidised, assuming that about 75% acid does not oxidise copper to produce SO2.

If 30% H2O2 is used the acid will be diluted to only about 50%.




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[*] posted on 29-8-2020 at 03:22


OK, so I actually did the experiment.
I got some copper , cleaned it up and put it in aqueous hydrogen peroxide overnight.
It is definitely blackened.

So CuO is a possible intermediate.
I can't rule out the idea that Cu2O is the actual intermediate.
The reaction H2SO4 + H2O2 <--> H2SO5 + H2O lies fairly well to the left in aqueous conditions.
I doubt any peroxysulfate is an intermediate.
Not least because it's not obvious why persulphate would do a better job of oxidising copper than peroxide.

In response to Fulmen's question "Where does the SO2 come from?" I suggest that he re-reads the OP to which my reply was a direct response; and which gives the equation for the production of SO2.
To be fair you do need a relatively high concentration of sulphuric acid to produce much SO2 (If I get round to it, I will find out how high), but since the OP was talking about it, I assumed that he had observed it.

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[*] posted on 29-8-2020 at 07:24


Rather high and quite hot, to get it started. Not for the fearful or careless.

[Edited on 29-8-2020 by S.C. Wack]




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[*] posted on 29-8-2020 at 17:20


Quote: Originally posted by unionised  
OK, so I actually did the experiment.
I got some copper , cleaned it up and put it in aqueous hydrogen peroxide overnight.
It is definitely blackened.

So CuO is a possible intermediate.
I can't rule out the idea that Cu2O is the actual intermediate.
The reaction H2SO4 + H2O2 <--> H2SO5 + H2O lies fairly well to the left in aqueous conditions.
I doubt any peroxysulfate is an intermediate.
Not least because it's not obvious why persulphate would do a better job of oxidising copper than peroxide.

In response to Fulmen's question "Where does the SO2 come from?" I suggest that he re-reads the OP to which my reply was a direct response; and which gives the equation for the production of SO2.
To be fair you do need a relatively high concentration of sulphuric acid to produce much SO2 (If I get round to it, I will find out how high), but since the OP was talking about it, I assumed that he had observed it.




Interesting result. Copper (II) oxide could have formed but unfortunately doesn't rule out formation of persulphuric acid.

(1) H2SO4 + H2O2 <--> H2SO5 + H2O
(2) H2SO5 + Cu ---> CuSO4 + H2O
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[*] posted on 30-8-2020 at 00:04


Quote: Originally posted by unionised  

I doubt any peroxysulfate is an intermediate.
Not least because it's not obvious why persulphate would do a better job of oxidising copper than peroxide.


That's my conclusion as well. We know that copper is oxidized by H2O2 (and air for that matter), and that acids readily attacks copper oxide.


Quote:

In response to Fulmen's question "Where does the SO2 come from?" I suggest that he re-reads the OP

You're right, my bad. I completely missed that part of the post.




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[*] posted on 30-8-2020 at 02:38


Quote: Originally posted by reactofurnace  

Interesting result. Copper (II) oxide could have formed but unfortunately doesn't rule out formation of persulphuric acid.

(1) H2SO4 + H2O2 <--> H2SO5 + H2O
(2) H2SO5 + Cu ---> CuSO4 + H2O


Yes, some very little amount of H2SO5 is formed. So there will be two reactions with copper metal - major with H2O2 and minor with H2SO5.




If you are interested in aqueous inorganic chemistry look at https://colourchem.wordpress.com/main-page/

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