Metals are elements that readily donate electrons, while acids release hydrogen ions ($\text{H}^+$) when dissolved in water. The interaction between them is governed by the metal’s ability to transfer its electrons onto the available hydrogen ions. This electron exchange is the basis of a broad class of chemical reactions, and most metals will participate in this interaction.
The Core Chemical Mechanism
The typical reaction between a metal and a common acid, such as hydrochloric or sulfuric acid, is a single replacement reaction. During this process, the metal atoms lose electrons (oxidation) to become positively charged metal ions that dissolve into the solution.
The freed electrons are transferred to the hydrogen ions ($\text{H}^+$) supplied by the acid (reduction). By gaining these electrons, two hydrogen ions combine to form neutral hydrogen gas ($\text{H}_2$). The resulting metal ion pairs with the acid’s remaining anion to produce a salt, which typically remains soluble in the solution.
The products of this interaction are a metal salt and hydrogen gas. For instance, when zinc metal reacts with hydrochloric acid, it produces zinc chloride salt and bubbles of hydrogen gas. The bubbling, or effervescence, serves as a visible indicator of the reaction taking place.
What Determines Reactivity
The ability of a metal to displace hydrogen from an acid is determined by its inherent chemical properties, organized in the Electromotive Series (reactivity series). This series ranks metals based on their tendency to lose electrons. A metal must be more reactive than hydrogen to successfully transfer electrons to the $\text{H}^+$ ions and displace the hydrogen from the acid.
Metals high on the series, such as alkali metals like potassium or sodium, react quickly and exothermically, often resulting in a violent explosion or fire. Moderately reactive metals, including zinc, iron, and tin, react at a controlled pace, producing a manageable stream of hydrogen bubbles.
Metals situated below hydrogen in the reactivity series, known as noble metals (copper, silver, gold, and platinum), do not react with common dilute acids like hydrochloric acid. These metals have a lower tendency to lose electrons than hydrogen has to accept them, making the required electron transfer chemically unfavorable. This resistance is why metals like gold and silver are prized for their resistance to tarnishing and corrosion.
Exceptions and Special Cases
Not all metal-acid interactions follow the predictable rules of the reactivity series. Some metals, including aluminum, chromium, and lead, are theoretically reactive but appear inert due to passivation. This occurs because the metal instantly reacts with oxygen or the acid to form an extremely thin, tough layer of metal oxide on its surface.
This protective oxide layer shields the underlying metal from further contact with the acid. The metal is “passivated” by its own corrosion product. This stability makes metals like aluminum suitable for many structural applications despite their high position on the reactivity series.
Strong Oxidizing Acids
Strong oxidizing acids, such as concentrated nitric acid ($\text{HNO}_3$), behave differently from common acids. Nitric acid acts as a powerful electron acceptor; when it reacts with a metal, it is reduced to various nitrogen oxides ($\text{NO}_2$ or $\text{NO}$) instead of producing hydrogen gas.
Aqua Regia
To dissolve truly inert noble metals like gold and platinum, a specialized mixture called aqua regia is necessary. This mixture combines concentrated nitric acid with hydrochloric acid in a three-to-one volume ratio. The nitric acid oxidizes the metal, while chloride ions from the hydrochloric acid form a soluble complex ion, preventing the metal from reforming.
Practical Implications and Safety
The metal-acid reaction mechanism is the underlying cause of acid corrosion, a significant concern in industrial and environmental settings. Acidic rainwater or chemicals can dissolve metal infrastructure like pipes, containers, and bridges, compromising their structural integrity. Mitigating this requires controlling acidity and choosing corrosion-resistant alloys.
Conversely, this reaction is used beneficially in various industrial processes. It is employed in metal processing for “pickling,” which cleans the surface to remove rust or scale before manufacturing. The etching of metals for decorative or technical purposes also relies on the controlled dissolution of the surface metal by an acid solution.
Safety precautions are necessary when handling metals and acids. The production of highly flammable hydrogen gas requires adequate ventilation to prevent explosive concentrations. Furthermore, most metal-acid reactions are exothermic, releasing heat that can cause the solution to boil or splatter, necessitating appropriate protective equipment and careful control of reaction conditions.