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When aluminum (foil) is placed in an aqueous solution of copper(II) chloride, the aluminum atoms lose electrons to the copper(II) ions and replace them to form aluminum chloride, and to free up metallic copper.

The metallic copper has a tendancy to float on the surface of the solution, being buoyed by gas bubbles. The bubbles appear to evolve from the interface of the aluminum foil with the solution. What is the source and nature of the gas bubbles?

I have two hypotheses:

  1. The bubbles are coming from dissolved $\ce{CO2}$ that is coming out of solution due to an increase in temperature close to the site of reactions.

    enter image description here

  2. The bubbles are $\ce{O2}$ bubbles coming from the layer of aluminum oxide on the surface of the aluminum foil.

If this is not the case, what happens to the portion of the portion of the foil that was in oxide form prior to the reaction. How would the aluminum oxide react with copper(II) chloride?

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    $\begingroup$ See chemistry.stackexchange.com/questions/68745/redox-mgcopperii. Copper salts give weakly acidic solutions from which strongly electropositive metals can displace hydrogen. $\endgroup$ Mar 21, 2017 at 2:11
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    $\begingroup$ I think the gas is hydrogen(H2). $\endgroup$
    – user50688
    Aug 12, 2017 at 15:44
  • $\begingroup$ Do not think but think it out and give a chemical reason. $\endgroup$
    – jimchmst
    Apr 15, 2023 at 23:20

4 Answers 4

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The currently accepted answer does not consider the possibility discussed here, where magnesium is the metal and again there are bubbles. Transition metal salts of strong acids are weakly acidic in water, and that weak acid enables a strongly electropositive metal which reacts only slowly or not at all with pure water to begin displacing hydrogen more readily. In the case of copper(II) chloride tetrahydrate a 50 mg/L solution has a pH between 3 and 4 according to supplier material data. Thus the aluminum in this case is displacing hydrogen (from the acid produced by hydrolysis) as well as copper.

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  • $\begingroup$ They do not appear to be flammable. There is no insoluble Aluminum oxide or hydroxide at the end of the process. Could you show me what all of the predicted products would be based on your hypothesis? $\endgroup$ Sep 17, 2019 at 13:18
  • $\begingroup$ Aluminum --> hydroxo complexes , $\ce{Al(OH)_2^+}$ etc. Magnesium --> cation stays in solution below pH 8 or 9. $\endgroup$ Sep 17, 2019 at 13:23
  • $\begingroup$ Some people are never satisfied. I am one of those people when I see a drive-by downvote with no suggested alternative or where the answer needs to be improved. $\endgroup$ Apr 16, 2023 at 1:18
  • $\begingroup$ You stated an observation with no backup. Read my answer below. These reactions will happen between steel and stainless steel with even slight conductivity. The steel corrodes. The hydrogen is literally absorbed into the nickel containing SS. Agreed acid hastens the process but the catalytic overvoltage lowering hastens it more. $\endgroup$
    – jimchmst
    Apr 16, 2023 at 6:22
  • $\begingroup$ Then I am returning the favor. Where isthe backup for your claim? In another answer you comment that aluminum does not form an amalgam, which is false. Good day! $\endgroup$ Apr 16, 2023 at 7:47
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Metallic aluminium reacts with water somewhat like sodium $\ce{Na}$, according to $$\ce{2Al + 6 H2O -> 2 Al(OH)3 + 3 H2}$$ So it produces a lot of gaseous hydrogen $\ce{H2}$.

Of course, usually, aluminum metallic pieces do not react with water. This is due to a thin, waterproof and continuous layer of aluminium oxide $\ce{Al2O3}$ which is automatically produced on aluminium metal by the atmospheric oxygen. It prevents the contact metal-water. This oxide layer can be removed by dipping the aluminium piece into a $\ce{HgCl2}$ solution. It makes an $\ce{Al}$ amalgam reacting quickly with $\ce{H2O}$ and produces long white filaments of $\ce{Al(OH)3}$ growing at a quick glance, as long as there is some metallic mercury remaining on the aluminium surface.

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  • $\begingroup$ You almost got it but did not consider why the mercury remains in contact. If Na or K amalgams are representative amalgamation lowers activity and Al supposedly does not form an amalgam. $\endgroup$
    – jimchmst
    Apr 15, 2023 at 23:39
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The gas bubbles are very likely just air that was dissolved in the water previously and now gets released. I would expect most of the bubbles to actually be nitrogen, followed by oxygen, argon and then some carbon dioxide — that is assuming you used deionised water which should not have significant amounts of carbonate and hydrogencarbonate dissolved.

The copper particles merely supply suitable nucleation sites for the gas.

The theory on aluminium oxide being oxidised to pure oxygen is not correct. Oxygen is a very oxidative element meaning that you require strong oxidants to oxidise oxide anions back to oxygen. With ‘strong’, we’re talking the oxidative strength of potassium permanganate. Aluminium’s oxidative powers are nowhere near that strong.

Copper(II) chloride and aluminium oxide do not react with each other; only metallic aluminium beneath the passivisation layer is able to react with copper(II) ions.

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    $\begingroup$ I was thinking that given the temperature it could also be water vapor. $\endgroup$ Dec 20, 2016 at 21:19
  • $\begingroup$ @JosephHirsch Water vapour does not form bubbles in water at room temperature. You didn’t mention that you were boiling your solution. $\endgroup$
    – Jan
    Dec 20, 2016 at 21:27
  • $\begingroup$ Not boiling. The reaction is exothermic. Not violently but I suspect that it gets warm enough at the reaction sites. $\endgroup$ Dec 20, 2016 at 21:34
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    $\begingroup$ @JosephHirsch No. $\endgroup$
    – Jan
    Dec 20, 2016 at 21:40
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    $\begingroup$ I object. This is most probably hydrogen. $\endgroup$ Jan 22, 2019 at 9:24
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An active metal such as iron, zinc, Aluminum, magnesium reacts slowly with water to form H2 and the metal ion or oxide-hydroxide. The rates and products depend on the hydrogen ion concentration. There is an over voltage to this reaction that is reduced if a more noble metal is in contact with the active metal. This is how sacrificial anodes work; the protected metal is more noble. Commonly Zn, Al or Mg are used to protect the more noble Fe and the production of H2 is ignored. Typical noble metals are iron, stainless steels, nickel, copper, the palladium and platinum groups; for some reason gold and silver do not work as well. Any situation where different metals are in contact, with an electrolyte present, will result in corrosion and usually the reduction of hydrogen ion to give H2 gas on the more noble metal.

In the reaction of Cu++ ions with Al metal some copper immersion-plates on the aluminum. This combination catalyzes the reaction of the aluminum metal with the aqueous solution, generating H2. There are two competing reactions.

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