The answer to whether acids react with metals to produce hydrogen gas is generally yes. This chemical interaction is fundamental in chemistry. Acids donate hydrogen ions (\(\text{H}^+\)) when dissolved in water, and metals readily lose electrons to form positive ions. When these two classes interact, the reaction displaces hydrogen from the acid compound, releasing it as a gas.
The Chemistry of Acid-Metal Reactions
The reaction between an acid and a metal is classified as a single displacement reaction. The metal takes the place of the hydrogen in the acid compound, forming a new metal salt and liberating hydrogen gas (\(\text{H}_2\)). This transformation is driven by a reduction-oxidation (Redox) reaction, involving the transfer of electrons between the metal and the hydrogen ions.
Metal atoms undergo oxidation, losing electrons to become positively charged metal ions that dissolve into the solution. These liberated electrons are immediately gained by the hydrogen ions (\(\text{H}^+\)) from the acid, which is the reduction half of the reaction. The hydrogen ions gain electrons, become neutral hydrogen atoms, and then pair up to form diatomic hydrogen gas (\(\text{H}_2\)), observed as bubbling. For example, zinc reacting with hydrochloric acid produces zinc chloride and hydrogen gas, confirming that the metal is oxidized as the hydrogen is reduced.
Ranking Metals by Reactivity
Whether a metal can displace hydrogen from an acid depends on its position in the Electrochemical Series, also called the Reactivity Series. This series ranks metals based on their tendency to lose electrons and determines their chemical reactivity. Hydrogen is included as a benchmark to predict the outcome of acid-metal reactions.
Any metal positioned above hydrogen in the series is more reactive and will readily react with an acid to produce hydrogen gas and a salt. Highly reactive metals like potassium, sodium, and calcium react vigorously. Metals like magnesium, zinc, and iron also react effectively, though at a slower pace. Conversely, metals ranked below hydrogen, such as copper, silver, and gold, are less reactive and will not react with common dilute acids to produce hydrogen gas.
The ability of a metal to displace hydrogen is a direct function of its electron-losing capability; the higher up the series, the more easily the metal atom is oxidized. This explains why gold, which is near the bottom, resists reaction with most acids, while zinc, which is above hydrogen, reacts easily. The predictability offered by the Reactivity Series is a fundamental concept used to understand these displacement reactions.
Why Certain Acids Fail to Produce Hydrogen Gas
Certain acids fail to produce hydrogen gas even when reacting with highly reactive metals. The most common exceptions involve oxidizing acids, specifically nitric acid (\(\text{HNO}_3\)) and concentrated sulfuric acid (\(\text{H}_2\text{SO}_4\)). These acids are powerful oxidizing agents, meaning they readily accept electrons.
When a metal reacts with nitric acid, the acid acts as a stronger electron acceptor than the hydrogen ions. Any hydrogen gas that might form is immediately oxidized by the nitric acid to produce water (\(\text{H}_2\text{O}\)). The nitric acid is reduced, and its reduction products are typically various oxides of nitrogen, such as nitrogen monoxide (\(\text{NO}\)) or nitrogen dioxide (\(\text{NO}_2\)), rather than hydrogen gas.
A second exception occurs through a phenomenon called passivation, where a metal’s surface becomes chemically non-reactive. Metals like aluminum, chromium, and iron form a thin, protective oxide layer when exposed to concentrated oxidizing acids. This oxide layer acts as a physical barrier, preventing the underlying bulk metal from contacting the acid and halting the reaction. This is why concentrated nitric acid is sometimes used in the industrial cleaning process to create this passivated surface on stainless steel.