Why do acids react with some metals to release hydrogen gas?

The reaction between acids and certain metals to release hydrogen gas is an oxidation-reduction (redox) reaction, not a simple acid–base neutralization.12

When a strong acid dissolves in water, it releases hydrogen ions in the form of protons ($H^{+}$). The process of releasing hydrogen gas occurs because the metal acts as a reducing agent, transferring electrons to these hydrogen ions. This reduction can be represented by the half-reaction:132

$$H^{+}+e^{-}\to \frac{1}{2}{H}_2$$

Whether a metal will react depends on its reduction potential. Metals with a negative reduction potential—meaning they are more easily oxidized than hydrogen—can displace hydrogen from an acidic solution. For example, in the reaction between zinc and a strong acid, zinc atoms give up electrons to the hydrogen ions, forming zinc ions ($Z{n}^{2+}$) and gaseous hydrogen ($H_2$):12

$$Zn+2H^{+}\to Z{n}^{2+}+H_2$$

Metals with positive reduction potentials, such as silver, are generally inert toward aqueous hydrogen ions and do not release hydrogen gas in this manner.1