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Copper does not react with hydrochloric acid, according to many sources. However, I found an experiment posted in a video, which I will try to repeat, that shows a copper pipe dissolving into HCl and plating a nail when the copper pipe and the nail are connected to a 12-volt battery, becoming electrodes. No other chemicals, according to the video, are present. How does the electrical current cause the copper to react with the HCl when otherwise it would not?

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A reaction between copper metal and hydrochloric acid would go something like this: $$\ce{Cu + 2HCl -> CuCl2 + H2}$$ ...which we can write as: $$\ce{Cu + 2H+ -> Cu^2+ + H2}$$ If we break this up into half reactions, we get: $$\ce{Cu -> 2e- + Cu} ~~~E°=-0.34~V$$ $$\ce{2H+ + 2e- -> H2} ~~~E°=0.00 ~V$$ If we add the voltages together, we get the cell potential of the combined reaction is $-0.34+0.00=-0.34~V$. Since the cell potential is negative, the reaction is not spontaneous, so the reaction is not going to occur without any artificial interference ($\Delta G°=-nFE°_{cell}$).

However, if you connect a battery (galvanic cell) to this reaction, you can supply the necessary electrons for this reaction to occur (electrolytic cell). Since you are connecting a 12 V battery, there is more than enough voltage to cause this reaction to occur.

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When I want to plate small parts with copper, I submerge lots of waste copper and the steel part into muriatic(HCl) acid. The oxides on both materials dissolves quickly and overnight there deposits a layer of copper on the steel suitable for forming a passivation layer. No electricity involved.

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The short answer is because the electric circuit provides the energy needed to do those reactions. You should know that you could try the same experiment but instead of using HCl, try table salt dissolved in water. That should work (12 V is a whole lot of voltage), but I can't say I actually looked it up to verify it. If that doesn't work, add some vinegar (acid) to the salt solution. Be sure to use the right connections (the copper will only dissolve near one wire). Waiting 5 or 10 minutes should be long enough to see if its working or not. Dr. Who said "Time makes liars of us all." and he was right. It is just not true that HCl is unreactive towards Cu. I found dozens of YouTube videos and testimonies which contradicted that claim. The real world is complicated. Most general statements of the form "A doesn't react with B" are likely to be true in some circumstances and false in others. "Water doesn't react with sugar" for instance. Well, that seems to be true; I can dissolve sugar in water and then evaporate the water and get back the sugar. Well, wait a minute: it turns out that sucrose reacts (very slowly with water at room temperature, it takes years) and converts to a mixture of glucose and fructose. Not only that, but if I heated the water to 2000° I know that dumping some sugar into it would decompose the sugar. So, we need to qualify statements of fact (either implicitly or explicitly). In the case of your question, we need to make sure that the copper and the acid are pure. We need to prevent contamination with any oxidants (such as O2 from the atmosphere) and we need to specify the conditions of temperature and pressure (and protect the system from other interferences, such as radiation, gravity, etc. etc. (admittedly, Earth's surface gravity isn't strong enough to have much effect on reactions at lab scale.). Most copper is coated with various oxides, so cleaning a penny with HCl (which makes it shiny) doesn't prove that the HCl is reacting with the metal, but demonstrates that the HCl is either reacting with the oxide, with the metal, or with the copper alloy. (Copper wire is quite pure, 99.99%, pennies not so much). So, let's say we condition the HCL solution to eliminate all oxygen and other impurities, abrade the surface of a (pure) copper wire, and place the wire into the solution and leave it there at room temperature, 25°C. Will the copper react? Well, let's say that it won't. We can assume this because supposedly the "standard potential" tables indicate it won't. (Be careful here!, what concentration of acid is used to determine "standard potential"??) 1M HCl is ~4% but you can easily buy 30% concentrations. (The devil is in the details.) So, assuming your "sources" are trustworthy, then they are referring to specific conditions, and not some "universal truth". If you doubt it, you could try (the dangerous experiment of) carefully heating the diluted HCl with a scraped-clean-of-oxide piece of Cu wire in it, and seeing if the copper starts to react. It will but it might not be an experiment you should do. HCl is volatile, and can vaporize. The problem with that is that it is also corrosive and toxic. If you breath it, you could die. If it condenses on various metal surfaces around where you're heating it, they might corrode (and lets not even think about what it will do to electronics!).======== OK. So, we've diluted and cleaned the system and if your sources are right, the copper just sits there. But if you connect the metal to an the right side of a 12V transistor battery, you can indeed drive the reaction. Why? Well the flow of current indicates that energy is being used, just like the flow of water. You can use some of that energy (from the battery) to do work. (Incidentally, you could also use a battery charger and do the same thing, even a dc charger or power source which you probably have dozens of (you know, those little black cubes you have to plug into your AC wiring and then plug into your cell phone to charge it, or into thousands of other things that require DC power)). Anyway, a battery by definition works by ionizing something (usually a metal) and guiding the electrons that flow from one electrode to the other out of the battery (if a circuit is made and closed). The opposite of M → M+ + e- which is what powers most batteries (where M is a metal like lead or lithium) is M+ + e- → M. In other words you can use the electrons to ionize some metal, M, if there's enough energy. We measure energy in a circuit by voltage, the higher the voltage, the 'stronger' the energy (voltage, or potential, is like the height of a tank of water - the higher it is, the more work it can do). A 12V battery has a LOT of potential (relative to what's needed to ionize metals) but generally doesn't have much current. That means it can ionize most (I think, all, actually) metals but not very much of them. Electrochemistry is a bit of a strange area of chemistry, because it deals with two different concepts of "potential": the electrical term, same meaning as the word as voltage, more or less and the chemical term, meaning the ability (or potential) to react. Most metals prefer to ionize to the M+ ion, the cation. (By "prefer" I don't mean the atoms are choosing what to do, they do what the Laws of Physics and Chemistry tell them to do, no exceptions) But metals losing electrons is called "oxidation", metal ions gaining electrons (and so going back to the neutral (uncharged) atom, is called reduction. In order for a + ion (a cation) in solution to gain an electron, it needs to take it from something. The easiest place to take it is from a wire which has a negative charge and is "full" of extra electrons (from the battery). It happens at the wire's surface, and this allows the metal to "plate out" on the wire because uncharged (neutral) metal atoms are generally not very soluble in water. The ions are soluble, they are changed to the neutral metal at the electrode surface and plate out (precipitate out) onto that surface. Oh, one last thing. 12 Volts is more than enough to break apart water. In gentle conditions, (not working with explosive mixtures, using small batteries, etc.) the reaction which will occur is the one which uses the least energy. This means that even if you have enough energy to, say, ionize gold or platinum or silver, that they won't dissolve in a dilute acid if the reaction 2H2O → 2H2 + O2 uses less energy. (You can see this by watching bubbles form at the wires. If bubbles form at one wire, you know that something is going on at the other. The water reaction, above, makes hydrogen at one wire (electrode) and oxygen at the other. One explodes, the other we breathe. (Collecting the H2 is mostly safe unless sparks, flames, or heat (or platinum) is present.)

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  • $\begingroup$ Wallet of text. Consider making paragraphs for readability. $\endgroup$ – Yoda Jan 16 '18 at 22:26

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