Sciencemadness Discussion Board - Molar calculations - lame question

Molar calculations - lame question

G.Lowell - 3-4-2015 at 10:35

I am making CuCl2 - Cu (s) + 2 HCl (aq) -> CuCl2 + H2

My calculation is this:

Molar weight of copper is ~63 g/mol thus I have 10/63= ~ 0.16 mol of copper.

Making CuCl2 from it should take 2*0.16 ~ 0.32 mol of acid.

32% solution HCl ~ 10 mol which means 1mol ~ 100ml so I need 0.32*100ml ~ 32 ml of HCl.

But after a week only 3 grams of copper are dissolved

Sorry for so simple question, but what could be wrong with my calculation ?

aga - 3-4-2015 at 10:42

I'd be surprised if that even worked at all.

Perhaps 3g of something else has been consumed by the acid ?

Edit :

As per the wiki :
http://en.wikipedia.org/wiki/Copper%28II%29_chloride

"Copper(II) chloride is prepared commercially by the action of chlorination of copper:

Cu + Cl2 + 2 H2O → CuCl2(H2O)2

Copper metal itself cannot be oxidised by hydrochloric acid, but copper-containing bases such as the hydroxide, oxide, or copper(II) carbonate can be reacted with hydrochloric acid."

[Edited on 3-4-2015 by aga]

G.Lowell - 3-4-2015 at 10:46

The piece of copper was 10 gram. Now is 7. Solution is brown.
+air was bubbling through the solution.

[Edited on 3-4-2015 by G.Lowell]

[Edited on 3-4-2015 by G.Lowell]

aga - 3-4-2015 at 11:01

It still can't defy the laws of physics.

Most likely is that the copper isn't pure or 'clean' and the HCl has dissolved 3g of crud that was in your Cu to start with.

Your reasoning on the Molar calculations is perfectly sound, just that Cu does not react with HCl is all.

As far as i know, the only common acid that will react with Cu is HNO3.

Edit:

Here's a nice prep for CuCl2 :
http://amateurchemistry.weebly.com/synthesis-of-copper-ii-ch...

[Edited on 3-4-2015 by aga]

gdflp - 3-4-2015 at 11:34

Yes aga, hydrochloric acid can dissolve copper for the following reason. Air from the bubbler oxidizes the surface of the copper by the following process 2Cu + O₂ --> 2CuO, the hydrochloric acid then attacks the copper oxide by CuO + 2HCl --> CuCl₂ + H₂O. This leaves a fresh surface of copper which can then be oxidized and the process repeats. G.Lowell your math looks correct, the only thing that I would recommend is diluting the acid to ~10% with water to increase the solubility of the copper chloride and thus increase the rate of the reaction. Because the oxidation of copper is a slow process, this reaction does take a long time.

blogfast25 - 3-4-2015 at 12:12

Firstly, the brown colour described by G.Lowell is almost certainly due to a mixed oxidation state Cu(I)/Cu(II) complex that's been described on this forum several times (I'll see if I can dig it up). I've seen it myself and it is well described in the peer reviewed literature. Copper(II) hexaqua ions are blue, Cu(I) is generally colourless.

Copper will dissolve slowly in HCl, but only IF there's an oxidising agent like hydrogen peroxide or air oxygen present. Copper is not an electropositive element and cannot be oxidised by H3O+ cations (the only oxidising species present in dilute solutions of HCl and other acids with 'non-oxidising' conjugated bases).

CuO doesn't form in this process, not in acidic conditions, instead the likely oxidation reaction in acid conditions is:

Cu(s) + 1/2 O2(g) + 2 H+(aq) ===> Cu2+(aq) + H2O(l)

Copper metal reacts with Cu2+ to form Cu(I), the source of the Cu(I)/Cu(II) complex most likely. Removing the excess copper and allowing the Cu(I) to be oxidised to Cu(II) should remove the brown complex.

There's little reason to believe diluter HCl would work faster here.

In the link in aga's post, copper carbonate is represented by CuCO3. That's incorrect: it's copper basic carbonate, Cu2CO3(OH)2.

A few experiments by woelen on the copper(I,II) mixed oxidation complex:

http://woelen.homescience.net/science/chem/riddles/copperI+c...

http://woelen.homescience.net/science/chem/riddles/copperI+c...

http://woelen.homescience.net/science/chem/riddles/copperI+c...

[Edited on 3-4-2015 by blogfast25]

aga - 3-4-2015 at 12:32

So No, HCl doesn't dissolve Cu.

Glad that's cleared up !

Sorry to have missed the significant Cu + O2 reaction going on.

blogfast25 - 3-4-2015 at 12:45

HCl and other acids with non-oxidising conjugated bases will not dissolve any non-electropositive elements.

Simply put, the general reaction:

M(s) + z H+ === > Mz+ + z/2 H2

…cannot proceed if the Reduction Potential of the metal M is positive (> 0 V) (somewhat paradoxically perhaps these metals are NOT electropositive).

But ‘external’ oxidisers can dissolve these metals in acids anyway (generally more slowly). See e.g. the dissolution of lead in acetic acid and hydrogen peroxide.

[Edited on 3-4-2015 by blogfast25]

Loptr - 3-4-2015 at 13:01

Quote: Originally posted by aga

So No, HCl doesn't dissolve Cu.

Glad that's cleared up !

Sorry to have missed the significant Cu + O2 reaction going on.

A lot of times the system being observed isn't exactly as we understand it to be. So if we observe behavior that doesn't line up with a given law or accepted mechanism, there is more than likely more to our particular reaction than we are initially aware of. I hate when I see claims regarding how something is breaking a law of physics or what have you, as most of the time, it turns out there is indeed something proceeding, just under different circumstances and not according to our understanding. We tend to be the break in the chain majority of the time.

Are your reagents pure? Have you excluded moisture? Are your additions allowing for temperatures higher or localized concentrations than anticipated? Are there stabilizing agents added to your chemicals. (phosphoric acid for h2o2, etc., etc.)

[Edited on 3-4-2015 by Loptr]

[Edited on 3-4-2015 by Loptr]

brubei - 3-4-2015 at 14:19

just look at the electropotentiel of your redox equations https://en.wikipedia.org/wiki/Standard_electrode_potential_(data_page)

Cu2+ + 2e- => Cu +0.33V
2 H+ + 2 e- => H2(g) +0.00V

The gamma rule says you need oxydant with greater potential than the Cué+/Cu couple. Of course the couple H+/H2 is not one of them.

Non minéral or halogenic couple with higher potentiel requires oxygenated water:

H2O2(aq) + 2 H+ + 2 e− => 2 H2O +1.78V

Atmospheric O2 dissolved in your water explain the slow dissolution of copper despite the thermodynamics rules.

[Edited on 3-4-2015 by brubei]

G.Lowell - 5-4-2015 at 00:19

When extra water was added into solution reaction started again, copper has only 4 grams now. To speed reaction up I added H2O2. Solution changed color to green which is exactly what I need.

Thanks to all!

Deathunter88 - 5-4-2015 at 03:40

Quote: Originally posted by aga

It still can't defy the laws of physics.

Most likely is that the copper isn't pure or 'clean' and the HCl has dissolved 3g of crud that was in your Cu to start with.

Your reasoning on the Molar calculations is perfectly sound, just that Cu does not react with HCl is all.

As far as i know, the only common acid that will react with Cu is HNO3.

Edit:

Here's a nice prep for CuCl2 :
http://amateurchemistry.weebly.com/synthesis-of-copper-ii-ch...

[Edited on 3-4-2015 by aga]

In the URL you linked the author used Copper (II) Carbonate with the formula CuCO3, but most copper carbonate that is available is basic copper carbonate. Will using basic copper carbonate give the same result?

blogfast25 - 5-4-2015 at 06:51

Quote: Originally posted by Deathunter88

It's not 'most', it's ALL: CuCO3 simply does not exist.

Copper basic carbonate will also dissolve in HCl:

Cu2CO3(OH)2(s) + 4 HCl(aq) === > 2 CuCl2(aq) + 3 H2O(l) + CO2(g)

[Edited on 5-4-2015 by blogfast25]

gdflp - 5-4-2015 at 08:19

Quote: Originally posted by blogfast25

CuCO3 simply does not exist

At STP. At high temperatures and pressures in a CO₂ atmosphere, CuCO₃ has been synthesized, it just only exists under these extreme conditions. Here is a link to the abstract of one article http://onlinelibrary.wiley.com/doi/10.1002/zaac.19744100207/...

aga - 5-4-2015 at 11:26

What does 20kb mean in terms of pressure ?

DraconicAcid - 5-4-2015 at 12:42

Copper will slowly react with hydrochloric acid in the presence of air, as blogfast says. This reaction is autocatalytic- the more copper ions there are in solution, the faster it goes.

blogfast25 - 5-4-2015 at 12:46

Quote: Originally posted by aga

What does 20kb mean in terms of pressure ?

I assume it means 20 kilobar, which is about 20 atm. But there's no SI unit symbol 'kb'. Bad Science by the author.

[Edited on 5-4-2015 by blogfast25]