Acids and alkalis - AQA SynergyReactions of acids with metals

Acids take part in reactions in which salts are produced. In these reactions, the hydrogen ions in the acids are replaced by metal ions.

Learn more on the reactivity series of metals in this podcast.

Listen to the full series on BBC Sounds.

Reactions of acids with metals

When acids react with metals, the products are a salt and hydrogen. In general:

Acid + metal → salt + hydrogen

For example:

Hydrochloric acid + magnesium → magnesium chloride + hydrogen

2HCl(aq) + Mg(s) → MgCl₂(aq) + H₂(g)

Zinc and iron also react with hydrochloric acid.

Magnesium, zinc and iron also react with sulfuric acid. The products are a salt (in this case a sulfate) and hydrogen. For example:

sulfuric acid + iron → iron(II) sulfate + hydrogen

H₂SO₄(aq) + Fe(s) → FeSO₄(aq) + H₂(g)

Testing for hydrogen gas

Hydrogen gas is highly flammable. You can safely test for small quantities of hydrogen gas (eg collected in a test tube) by holding a burning splint near to the top of the test tube. The positive result is a squeaky pop sound as the hydrogen reacts with oxygen in the air in a small explosion.

Explaining the reactions of acids with metals - Higher

The reactions of acids with metals are redox reactions. For example, the ionic equation for the reaction of magnesium with hydrochloric acid is:

2H⁺(aq) + Mg(s) → Mg²⁺(aq) + H₂(g)

This ionic equation can be split into two half equations:

• Mg(s) → Mg²⁺(aq) + 2e^-(oxidation)
• 2H⁺(aq) + 2e^- → H₂(g) (reduction)

Notice that:

• magnesium atoms lose electrons - they are oxidised
• hydrogen ions gain electrons - they are reduced

Reduction and oxidation happen at the same time, so the reactions are called redox reactions.