The ideal answer must be copper, however why doesn"t copper react via hydrochloric acid while the other metals do?


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You might desire to look up some terms, such as

Here, reaction implies that

hydrogen gas is formedthe steel is dissolved

In order to form hydrogen, prolots have to be diminished to hydrogen atoms which then incorporate to $ceH2$.

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$$ce2 H+ + 2 e- -> H2$$

The steel serves as an electron donor and also is oxidized, e.g.

$$ceZn -> Zn^2+ + 2 e- $$

The more noble a steel is, the even more reluctant it is to lose electrons. This is the instance for copper, which is therefore not oxidized under these conditions.


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In principle, non-oxidizing acids cannot directly oxidize copper given that the redox potentials $E$ for $mathrmpH = 0$ present that $ceH+$ cannot oxidize $ceCu$ to $ceCu^2+$ or to $ceCu+$:

$$eginalignat2ce2H+ + 2e- ;& H2quad &&E^circ = +0.000 mathrmV\ceCu^2+ + 2e- ;& Cuquad &&E^circ = +0.340 mathrmV\ceCu+ + e- ;& Cuquad &&E^circ = +0.521 mathrmVendalignat$$

Keep in mind that copper(II) is normally even more secure than copper(I) in aqueous remedies.

However, the situation is slightly complicated because of the low solubility of $ceCuCl$ in dilute hydrochloric acid (in focused hydrochloric acid, copper forms chloricarry out complexes such as $ce-$ and also $ce^2-$):

$$K_mathrmsp = left cdot left = 1.72 imes 10^-7$$

The corresponding efficient redox potential may be estimated as follows:

$$eginalignedE&=E_ceCu+^circ+fracRTFcdotlnleft\ &=E_ceCu+^circ+fracRTFcdotlnfracK_mathrmspleft \ E_ceCuCl^circ&=E_ceCu+^circ+fracRTFcdotln K_mathrmsp\&= E_ceCu+^circ+0.0592 mathrmVcdotlog K_mathrmsp\&=0.521 mathrmV+0.0592 mathrmVcdotlogleft(1.72 imes 10^-7 ight)\&=0.121 mathrmVendaligned $$

Thus, oxidation of $ceCu$ to $ceCu+$ is favoured in dilute hydrochloric acid. Nonetheless, $ceH+$ is still not solid sufficient to oxidize $ceCu$.

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However, $ceCu$ deserve to be oxidized by $ceO2$:

$$ceO2 + 4H+ + 4e- 2H2Oquad E^circ = +1.229 mathrmV$$

As such, copper is progressively oxidized in dilute hydrochloric acid in call via air.