Why did hydrochloric acid cause copper to gain weight after soaking in it overnight?

Question

I recently did an experiment involving 2M hydrochloric acid and two different metals, iron and copper. This experiment was to investigate how the metals reacted to the acid by letting them sit in the acid overnight. I used relatively small pieces of metal (slightly larger than your thumbnail) an I placed them in 5 mL of 2M hydrochloric acid. They were left in test tubes overnight, and when I went to measure the weight of the metals, the iron showed an average weight change of -0.10 grams (which was expected) (I did two pieces of each metal), but the copper showed an average weight change of +0.04 grams, which I did not expect. The starting weights of each of the metals were Iron 1: 0.48g, Iron 2: 0.47g, Copper 1: 0.11g, Copper 2: 0.15g. The end weights of each of these metals were Iron 1: 0.38g, Iron 2: 0.38g, Copper 1: 0.17g, Copper 2: 0.17g. I could not find information on how the copper gained weight after sitting in the hydrochloric acid, and it is confusing me. I would like to know how the copper gained weight after sitting in the hydrochloric acid overnight.

Here is my method and materials:

Equipment/materials: 4x test tube 2x piece of iron 2x piece of copper 3x pairs of gloves 15mL of 2M hydrochloric acid 1x scale 1x test tube holder 1x tweezers 1x beaker

method: 1. gather and prepare equipment and materials 2. pour 5mL of hydrochloric acid in the test tube 3. measure and record the mass of the iron and copper pieces using the scale 4. insert the pieces of metal into the test tubes using tweezers 5. leave the metals in the hydrochloric acid overnight 6. after leaving the metals overnight, retrieve the metals from the test tubes using tweezers 7. record the mass of each piece of metal using the scale 8. pour the left-over hydrochloric acid from the test tubes into a safe beaker

And here is my table of results:

|||||||| Trial 1 (Iron)|Trial 2 (Iron)|Averages (Iron)|Trial 1 (Copper)|Trial 2 (Copper)|Averages (Copper)

Mass Before Experiment |0.48g |0.47g |0.475g |0.11g |0.15g |0.13g

Mass After Experiment |0.38g |0.38g |0.38g |0.17g |0.17g |0.17g

Change in Mass |-0.10g |-0.09g |-0.095g |+0.06g |+0.02g |+0.04g

Answer 1

The fact that the copper sample changed color implies that it did indeed react, which goes against what the experiment was presumably supposed to show.

This is because air was allowed to get into the acid, and while copper will not react with nonoxidizing acids alone, the combination of acid and air qualifies as oxidizing to copper*.

Multiple reactions are involved in this copper plus acid plus air interaction. The dissolved air first acts in combination with acid to give soluble copper(II) ions:

$\ce{2 Cu + 4 H^+(aq) + O2 -> 2 Cu^{2+} + 2 H2O}$

With hydrochloric acid and additional copper metal, reprecipitation of the copper onto the metal as copper(I) chloride is favored:

$\ce{Cu^{2+} + Cu + 2 Cl^- -> 2 CuCl(s)}$

The copper(I) chloride is white, but again it can react with oxygen from the air to give the basic salt mentioned by Maurice ($\ce{CuO•CuCl2}$ or $\ce{Cu2OCl2}$), which contains copper(II) again and thus goes back to being bluish-green. Most likely the material on the copper sample is some combination of copper(I) and basic copper(II) chlorides.

To perform the intended experiment accurately you must exclude air. This means bubbling the acid with an inert gas such as nitrogen to displace the dissolved air, then doing the actual test in a box with an inert atmosphere. The air-free technique is also good for the iron part of the experiment, for it prevents the oxidation of dissolved iron(II) which could make iron(III)-bearing precipitates.

*For example, when steel is remade from recycled scrap and contains increased amounts of copper from this scrap, the copper can be dissolved into hydrochloric acid-based (and air-exposed) pickle liquors and potentially replate onto other steel strips, discoloring them.