The situation is this: a copper rod and a carbon rod, touching each other, are submerged in HCl(aq). We're asked to describe the reaction that occurs, but as far as I can see, there's no reaction? The copper wouldn't react with the acid and I don't see why the carbon rod would add anything to the equation. Is there something I'm missing?

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    $\begingroup$ Consider electrolytic corrosion, i.e., a short-circuited galvanic cell. $\endgroup$ Mar 25 at 15:39

2 Answers 2


The carbon as an inert electrode can serve as the conductor for an oxygen electrode due dissolved aerial oxygen.

But the reaction

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

would be very slow.

The respective redox half reaction on the cathode:

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

and anode:

$$\ce{2 Cu(s) -> 2 Cu^2+(aq) + 4 e-}$$

As in any other galvanic cell. Oxidation of copper on the anode, reduction of aerial dissolved oxygen on the cathode. Note that the mentioned reaction happens even just with the copper in acid. But in both cases it very slow reaction.

But it may happen the task author does not consider this, so in such a case, the expected answer may be there is no ongoing reaction.

Carbon as electrochemically inert material acts similarly a platinum merely just as a conductor, providing/ collecting electrons for adjacent redox system, if any present. In our case, it is possibly(not necessarily) $\ce{O2/H2O}$ system.

In a way, it is similar as if there were zinc instead of copper. Without a connected second electrode, zirc would reduce hydrogen ions to hydrogen on its surfof coace. With the electrode, the hydrogen would be formed on carbon, reduced by electrons coming via a wire from the zing electrode.

For a copper, it would be very slow oxidation and dissolving, supported by oxygen.

Theoretically, it is $\ce{Cu/O2}$ galvanic cell,but practically, it would be very soft power source with very low available current, possibly hard to measure by less sensitive devices.

  • $\begingroup$ BG LAU does not speak of oxygen in his experiment. $\endgroup$
    – Maurice
    Mar 25 at 18:41
  • $\begingroup$ @Maurice He does not speak about water either. $\endgroup$
    – Poutnik
    Mar 25 at 22:31
  • $\begingroup$ Sorry, how does this work exactly? I've never seen something similar talked about in class. $\endgroup$
    – BG LAU
    Mar 26 at 4:21
  • $\begingroup$ So could I treat this as some sort of chemical cell? $\endgroup$
    – BG LAU
    Mar 26 at 9:48
  • $\begingroup$ If so, then is the role of the carbon rod simply to make the oxygen some sort of electrode? $\endgroup$
    – BG LAU
    Mar 26 at 9:50

If no air and no oxygen gas is involved in this system, there is no reaction : copper should not react. If oxygen $\ce{O2}$ is present (for example in solution), the reaction proposed by Poutnik occurs, and it is : $$\ce{2 Cu + O2 + 4 H+ -> 2 Cu^{2+} + 2 H2O}$$ It can be considered as a redox reaction, with two half-equations like : $$\ce{Cu -> Cu^{2+} + 2 e-}$$ $$\ce{O2 + 4 H+ + 4 e- -> 2 H2O}$$ These two half-equations can occur in a galvanic cell : the two electrodes being, one copper dipped in water for the first electrode (anode), and a platinum plate (or a carbon rod) in contact with oxygen and water for the second electrode (cathode).


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