How Do Acids React with Reactive Metals to Produce Hydrogen Gas?

For both SL and HL, a reactive metal transfers electrons to hydrogen ions in a dilute acid. The metal forms positive ions, producing a salt, while the hydrogen ions form hydrogen gas.

The Redox Mechanism

This is a redox reaction involving electron transfer. The metal is oxidized because it loses electrons, while hydrogen ions are reduced because they gain electrons.

For magnesium reacting with hydrochloric acid:

$$\text{Mg(s)} + 2\text{HCl(aq)} \rightarrow \text{MgCl}_2\text{(aq)} + \text{H}_2\text{(g)}$$

The process can be separated into half-equations:

• Oxidation: $\text{Mg(s)} \rightarrow \text{Mg}^{2+}\text{(aq)} + 2e^-$
• Reduction: $2\text{H}^{+}\text{(aq)} + 2e^- \rightarrow \text{H}_2\text{(g)}$

Magnesium changes oxidation state from 0 to +2, while hydrogen changes from +1 to 0. Magnesium is therefore the reducing agent, and hydrogen ions are the oxidizing agent. Chloride ions are spectator ions, but they combine with magnesium ions when the salt is represented in the molecular equation.

The net ionic equation for a divalent metal is:

$$\text{M(s)} + 2\text{H}^{+}\text{(aq)} \rightarrow \text{M}^{2+}\text{(aq)} + \text{H}_2\text{(g)}$$

This makes clear that the acid's anion does not participate in the electron transfer. A more reactive metal generally reacts faster because it loses electrons more readily. However, reaction rate also depends on acid concentration, metal surface area, and temperature; these factors change collision frequency or energy, not the reaction's basic redox identity.

Only sufficiently reactive metals, generally those above hydrogen in the reactivity series, displace hydrogen from typical dilute acids.

Observable evidence includes effervescence, the metal gradually dissolving, and often a temperature increase. Hydrogen gives a characteristic squeaky pop with a lighted splint.

A common misconception is that oxidation must involve oxygen. Here, oxidation is correctly defined as loss of electrons, even though no oxygen is transferred.

Exam Technique

For a “deduce” question, identify the salt, balance the equation, and include state symbols. For an “explain” question, state the electron transfer and oxidation-state changes rather than only describing the bubbles.