Metals and Non-metals Reacting with Dilute Hydrochloric Acid

The behavior of metals and non-metals when reacting with dilute hydrochloric acid (HCl) can vary significantly. Understanding these reactions is crucial for chemistry students and professionals, particularly those dealing with electrochemistry and acid-base reactions.

The Role of Electrochemical Couples

Metals that belong to an electrochemical couple with a potential higher than H3O /H2 couple will not react with dilute hydrochloric acid. As a result, they do not produce hydrogen gas (H2). For instance, metals such as gold (Au) do not react with dilute HCl. This is due to the fact that the potential of the metal-metallated hydronium couple is higher, thereby disfavoring the reduction of H2O to H2

Metals That Do Not React

Metals having a potential lower than H3O /H2 couple, but only under specific conditions, may not react with dilute HCl. These conditions include the passivation of the metal surface or a high overvoltage during hydrogen evolution. For example, metals like lead (Pb) may not react if they effectively passivate or if the evolution of hydrogen on their surface shows a high overvoltage, which typically results in the formation of lead(II) chloride (PbCl2) or other lead(II) compounds.

Platinum Group Metals

Interestingly, the metals of the platinum group (Pt), including platinum, palladium, and platinum salts, do not react with dilute hydrochloric acid either. This is because these metals have a more stable and less reactive surface with HCl. Copper (Cu) and silver (Ag) can, however, react with dilute HCl to produce hydrogen gas, as their potentials are lower than the hydrogen in the H3O /H2 couple.

Non-metals and Their Reactions

There are many non-metals that do not react with dilute hydrochloric acid. These include ammonia (NH3), tin tetraiodide (SnI4), and any form of hydroxide (OH-), in fact, all basic substances do not react. On the other hand, substances that do react often act as hydride donors. Examples include alkali hydrides such as sodium hydride (NaH), aluminum trihydride (AlH3), and dihydridoborane (B2H6).

Reaction Mechanisms

The reaction of metals with dilute HCl follows a clear mechanism. When a metal has a potential higher than the hydrogen ion in HCl, it does not react. Conversely, if the metal has a lower potential, it may react, leading to the evolution of hydrogen gas. This reaction is often represented as:

[Metal] 2 HCl → [Metal Chloride] H2

For instance, copper (Cu) reacts with HCl to produce copper(II) chloride (CuCl2) and hydrogen gas:

Cu 2 HCl → CuCl2 H2

Silver (Ag) similarly reacts to produce silver chloride (AgCl) and hydrogen:

Ag HCl → AgCl H2

However, if the metal does not react, it is often due to the formation of a passivation layer or a high overvoltage barrier preventing the reaction between the metal and the hydrochloric acid.

Conclusion

The ability of metals and non-metals to react with dilute hydrochloric acid is governed by their electrochemical potentials and the specific conditions under which the reaction takes place. Understanding these reactions is essential for various applications in chemistry, electrochemistry, and material science. Whether a substance reacts or not, the underlying mechanisms provide insights into the behavior of these substances in acidic environments.

Keywords: dilute hydrochloric acid, hydrogen production, electrochemical couples