Yes, hydrochloric acid (HCl) can break down certain metals through chemical reactions, particularly those that are more reactive than hydrogen. The acid dissolves the metal by oxidizing it and releasing hydrogen gas.
Which metals react with HCl?
Metals above hydrogen in the reactivity series, such as magnesium, zinc, iron, and aluminum, react with hydrochloric acid. These metals lose electrons more easily than hydrogen, allowing them to displace hydrogen ions from the acid. The result is a salt (a metal chloride) and hydrogen gas.
For example, zinc reacts with HCl to form zinc chloride and hydrogen gas:
- Zn + 2HCl → ZnCl₂ + H₂
The reaction is often visible as bubbling due to the release of hydrogen gas.
How iron and steel are affected
Iron reacts with HCl to form iron(II) chloride and hydrogen gas:
- Fe + 2HCl → FeCl₂ + H₂
This reaction is used in industry to clean steel surfaces, a process called acid pickling. It removes rust and mill scale before coating or painting. However, prolonged exposure weakens the metal, as the acid continues to eat into the base material.
Why aluminum resists at first
Aluminum is high on the reactivity series, yet it resists immediate attack by HCl. This is because it forms a thin, protective layer of aluminum oxide (Al₂O₃) when exposed to air. This layer must be dissolved by the acid before the underlying metal can react.
Once the oxide layer is removed, aluminum reacts rapidly:
- 2Al + 6HCl → 2AlCl₃ + 3H₂
The delay in reaction is due to this passivation effect.
Metals that resist HCl
Metals below hydrogen in the reactivity series do not react with hydrochloric acid under normal conditions. These include copper, silver, gold, and platinum.
Copper, for example, does not react with pure HCl because hydrogen ions are not strong enough oxidizers to remove electrons from copper atoms. However, if oxygen or another oxidizing agent is present, slow corrosion can occur.
How the reaction works
The process is an electrochemical reaction. The metal acts as a reducing agent, donating electrons to hydrogen ions (H⁺) from the acid. This reduces H⁺ to hydrogen gas (H₂), while the metal is oxidized to positive ions that dissolve in the solution.
Chloride ions in HCl also help by forming soluble complexes with metal ions, aiding in the dissolution process. The overall reaction depends on the metal's tendency to lose electrons, which is predictable from its position in the reactivity series.