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Revision notes for AQA GCSE Chemistry Reactions of acids with metals. Open the guide for explanations and worked examples. Written against the AQA GCSE Chemistry (8462) specification, so the content matches what's examinable rather than general Chemistry background.

Reactions of acids with metals

What you'll learn:

  • How to predict the products when a metal is added to an acid.
  • How to name the salts produced by hydrochloric and sulfuric acids.
  • How to write balanced symbol equations with state symbols for these reactions.
  • (Higher Tier) How to prove these are redox reactions using half-equations.

The General Equation

When you drop a piece of metal into an acid, a chemical reaction usually takes place. The metal atoms displace the hydrogen from the acid.

You will see fizzing (effervescence) as hydrogen gas is produced, and the metal will slowly disappear as it dissolves to form a chemical called a salt.

Key Idea

The General Equation

Whenever a reactive metal reacts with an acid, the products are always a salt and hydrogen gas. Metal + Acid →\to→ Salt + Hydrogen

Tip

MASH

An easy way to remember this general equation is the acronym MASH: Metal + Acid →\to→ Salt + Hydrogen.

Which metals will react?

Not all metals react with dilute acids. For a metal to react, it must be more reactive than hydrogen. For your GCSE, you only need to know about the reactions of three specific metals with acids:

  • Magnesium (Mg) – Reacts vigorously, producing lots of bubbles very quickly.
  • Zinc (Zn) – Reacts steadily, producing a moderate stream of bubbles.
  • Iron (Fe) – Reacts slowly, producing only a few tiny bubbles.

Test tubes showing reaction rates of magnesium, zinc, and iron in acid

Naming the salts

The name of the salt produced depends on the metal and the acid you start with. The first part of the salt's name comes from the metal. The second part comes from the acid:

  • Hydrochloric acid produces salts ending in chloride.
  • Sulfuric acid produces salts ending in sulfate.

If you react magnesium with hydrochloric acid, you get magnesium chloride. If you react zinc with sulfuric acid, you get zinc sulfate.

Common Mistake

Forgetting the hydrogen

When writing word equations, students often just write the name of the salt and forget that hydrogen gas is also released. Remember the H in MASH!

Writing balanced symbol equations

Once you know the word equation, you can convert it into a balanced symbol equation. You need to remember that hydrogen gas travels in pairs as a diatomic molecule (H2H_2H2​), and you must include state symbols:

  • (s) for the solid metal.
  • (aq) for the acid and the dissolved salt.
  • (g) for the hydrogen gas.
Example

Writing a balanced symbol equation

Write the balanced symbol equation, including state symbols, for the reaction between magnesium and hydrochloric acid.

  1. Write the unbalanced equation with correct chemical formulas. Magnesium is MgMgMg, hydrochloric acid is HClHClHCl, magnesium chloride is MgCl2MgCl_2MgCl2​, and hydrogen gas is H2H_2H2​.
Mg+HCl→MgCl2+H2 \text{Mg} + \text{HCl} \to \text{MgCl}_2 + \text{H}_2 Mg+HCl→MgCl2​+H2​
  1. Count the atoms to balance the equation. There is one chlorine atom on the left, but two on the right. There is one hydrogen atom on the left, but two on the right. Place a large 222 in front of the HClHClHCl to balance both the hydrogen and the chlorine atoms.
Mg+2HCl→MgCl2+H2 \text{Mg} + 2\text{HCl} \to \text{MgCl}_2 + \text{H}_2 Mg+2HCl→MgCl2​+H2​
  1. Add the state symbols. Magnesium is a solid, hydrochloric acid is in aqueous solution, the salt remains dissolved in the aqueous solution, and hydrogen is a gas.
Mg(s)+2HCl(aq)→MgCl2(aq)+H2(g) \text{Mg(s)} + 2\text{HCl(aq)} \to \text{MgCl}_2\text{(aq)} + \text{H}_2\text{(g)} Mg(s)+2HCl(aq)→MgCl2​(aq)+H2​(g)

Higher Tier Only: Redox Reactions

If you are studying Higher Tier, you need to look closer at what happens to the electrons during these reactions.

When a metal reacts with an acid, the metal atoms lose electrons to become positive ions, while the hydrogen ions from the acid gain those electrons to become hydrogen gas. Because oxidation and reduction are happening at the same time, this is called a redox reaction.

Definition

Oxidation and Reduction

  • Oxidation is the loss of electrons.
  • Reduction is the gain of electrons.
Tip

OIL RIG

Use the famous mnemonic OIL RIG to remember electron transfer: Oxidation Is Loss, Reduction Is Gain.

To see the electron transfer clearly, we ignore the "spectator ions" (like chloride or sulfate ions) because they don't change during the reaction. Instead, we write an ionic equation. Let's look at magnesium and hydrochloric acid again.

The full equation is:

Mg(s)+2HCl(aq)→MgCl2(aq)+H2(g) \text{Mg(s)} + 2\text{HCl(aq)} \to \text{MgCl}_2\text{(aq)} + \text{H}_2\text{(g)} Mg(s)+2HCl(aq)→MgCl2​(aq)+H2​(g)

The net ionic equation (removing the chloride spectator ions) is:

Mg(s)+2H+(aq)→Mg2+(aq)+H2(g) \text{Mg(s)} + 2\text{H}^+\text{(aq)} \to \text{Mg}^{2+}\text{(aq)} + \text{H}_2\text{(g)} Mg(s)+2H+(aq)→Mg2+(aq)+H2​(g)

We can split this ionic equation into two half-equations to see exactly what is being oxidised and what is being reduced.

Example

Identifying oxidation and reduction species

Determine which species is oxidised and which is reduced in the reaction between zinc and sulfuric acid.

  1. Write the net ionic equation. Zinc reacts with the hydrogen ions in the acid. The sulfate ions are spectators.
Zn(s)+2H+(aq)→Zn2+(aq)+H2(g) \text{Zn(s)} + 2\text{H}^+\text{(aq)} \to \text{Zn}^{2+}\text{(aq)} + \text{H}_2\text{(g)} Zn(s)+2H+(aq)→Zn2+(aq)+H2​(g)
  1. Write the half-equation for the metal. The zinc atom starts with no charge and becomes a Zn2+\text{Zn}^{2+}Zn2+ ion, meaning it must have lost 2 electrons. Because it loses electrons, zinc is oxidised.
Zn→Zn2++2e− \text{Zn} \to \text{Zn}^{2+} + 2\text{e}^- Zn→Zn2++2e−
  1. Write the half-equation for the hydrogen. Two H+\text{H}^+H+ ions gain 1 electron each (2 electrons total) to form a neutral H2\text{H}_2H2​ molecule. Because they gain electrons, the hydrogen ions are reduced.
2H++2e−→H2 2\text{H}^+ + 2\text{e}^- \to \text{H}_2 2H++2e−→H2​
Common Mistake

Naming the reduced species

Be very precise when naming the species. It is the hydrogen ions (H+\text{H}^+H+) that are reduced, not "hydrogen gas" or "the acid".

Exam technique

In the exam

  1. Always read the question carefully to see which acid is being used. Hydrochloric acid means you must write a formula ending in Cl2Cl_2Cl2​ (for group 2 or transition metals) or ClClCl, while sulfuric acid means it ends in SO4SO_4SO4​.
  2. Never write hydrogen gas as just HHH. It is always the diatomic molecule H2H_2H2​.
  3. When asked to add state symbols to a MASH equation, remember that the salt is almost always aqueous (aq)\text{(aq)}(aq) because it dissolves in the water that was mixed with the acid.
  4. (Higher Tier) If an exam question asks "Which species is oxidised?", state the reactant (e.g. "Magnesium" or MgMgMg), not the product ("Magnesium ions" or Mg2+Mg^{2+}Mg2+). The reactant is the thing that undergoes the change.
Self review

Check yourself

  • Write the word equation for the reaction between iron and sulfuric acid.
  • Why is it important to put a (g) state symbol next to the hydrogen in the chemical equation?
  • (Higher Tier) Look at this half equation: Fe→Fe2++2e−\text{Fe} \to \text{Fe}^{2+} + 2\text{e}^-Fe→Fe2++2e−. Does this represent oxidation or reduction?
  • (Higher Tier) What is the net ionic equation for the reaction between magnesium and an acid?
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