Q. \[ \mathrm{Cu} + \mathrm{HCl} \]

Problem statement. Reactants are \( \mathrm{Cu} \) (solid copper) and aqueous \( \mathrm{HCl} \) (hydrochloric acid) . We ask whether a chemical reaction occurs when metallic copper is placed into hydrochloric acid .

Step 1. Identify the kind of reaction that would be required if a reaction were to occur . For a metal plus an acid, the typical reaction is a single displacement in which the metal is oxidized and hydrogen ions are reduced to hydrogen gas . In words, metallic copper would have to be oxidized to copper(II) ions, and hydrogen ions would have to be reduced to hydrogen gas .

Step 2. Write the relevant half-reactions and their standard reduction potentials . Use standard reduction potentials to judge which species is more easily reduced . The copper reduction half-reaction is written as \( \mathrm{Cu^{2+} + 2 e^{-} = Cu} \) with standard potential \( E^{\circ} = +0.34\ \text{V} \) . The hydrogen ion reduction half-reaction is written as \( 2\,\mathrm{H^{+}} + 2 e^{-} = \mathrm{H_{2}} \) with standard potential \( E^{\circ} = 0.00\ \text{V} \) .

Step 3. Compare the reduction potentials and interpret them for the proposed displacement . A species with a more positive standard reduction potential is more readily reduced than one with a less positive potential . Here the copper(II)/copper couple has \( E^{\circ} = +0.34\ \text{V} \) , which is more positive than the hydrogen couple at \( 0.00\ \text{V} \) . That means \( \mathrm{Cu^{2+}} \) is easier to reduce to \( \mathrm{Cu} \) than \( \mathrm{H^{+}} \) is to reduce to \( \mathrm{H_{2}} \) . For a displacement of hydrogen by metallic copper, copper metal would need to be oxidized to \( \mathrm{Cu^{2+}} \) while hydrogen ions are reduced to hydrogen gas . Because copper metal is not willing to lose electrons under these conditions (its oxidation would be the reverse of a favorable reduction) , the displacement is not spontaneous.

Step 4. State the conclusion . Under normal conditions, metallic copper does not react with hydrochloric acid . Therefore no reaction occurs and no hydrogen gas is produced when solid copper is placed in aqueous HCl .

Step 5. Additional notes . Copper will react with acids that are strong oxidizing agents, such as concentrated nitric acid or hot concentrated sulfuric acid, because those acids can oxidize copper by pathways other than simple proton reduction . In the absence of an oxidizing agent, hydrochloric acid does not oxidize copper, so there is no reaction.