GCSE · Chemistry · AQA · Spec 8462

Reactions of acids with metals

Drop magnesium into dilute acid and it fizzes, then slowly disappears. Where does the metal go? And where do all those bubbles come from?

Acids and metals · Six reactions

Pick a metal. Pick an acid. What do you get?

Walk every route. Before you tap the acid, say the salt's name out loud, then check it. Keep an eye on the second product each time.

Metal → Acid

Three metals, two acids. Each metal can meet each acid, so count the ends of the tree: that's every reaction in this topic.

3 × 2 = 6 possible reactions, and the tree ends 6 times.

On A metal meets a dilute acid. 3 branches to choose from.

The tree makes you choose the metal first, but that's just how it's drawn. Magnesium in sulfuric acid is the same reaction whichever you think of first.

Watch out: Iron's salts carry a (II): iron(II) chloride and iron(II) sulfate. Leave it out and the name is incomplete.

Where did the fizz come from?

What really happens to the metal?

A piece of zinc goes into dilute sulfuric acid. It fizzes, and bit by bit the zinc gets smaller until you can't see it. Four learners explain what they saw.

Which explanation is closest to what you think right now?

The zinc is just dissolving, like sugar in tea. It's still zinc, only spread out in the water.The bubbles are gas escaping from inside the zinc.The zinc is destroyed. Once it's gone, it no longer exists.The zinc atoms turn into zinc ions that stay in the solution as zinc sulfate, and the hydrogen comes from the acid.

How sure are you?

I'm sureI think soI'm guessing

The zinc is just dissolving, like sugar in tea. It's still zinc, only spread out in the water.

This is a common misconception

Sugar dissolving is a physical change: it's still sugar, and no gas is made. Here a gas IS made, and that's the giveaway that a new substance has formed. The zinc atoms have changed into zinc ions, which are now part of a new substance, the salt zinc sulfate, dissolved in the solution.

The bubbles are gas escaping from inside the zinc.

This is a common misconception

Look at the formulas. Zinc is Zn: there's no hydrogen in it at all. Sulfuric acid is H₂SO₄, and hydrochloric acid is HCl: both acids contain hydrogen. So the hydrogen gas has to come from the acid. The same is true for magnesium and iron.

The zinc is destroyed. Once it's gone, it no longer exists.

This is a common misconception

Atoms aren't destroyed in a chemical reaction. Every zinc atom is still there, just changed: it has become a zinc ion. You can't see the ions because they are spread through the solution as dissolved zinc sulfate.

The zinc atoms turn into zinc ions that stay in the solution as zinc sulfate, and the hydrogen comes from the acid.

This matches the evidence

Exactly right. The zinc hasn't vanished; its atoms have become ions in the salt solution. And the fizz is hydrogen from the acid. Hold on to 'metal atoms become metal ions': it's the key to explaining these reactions in terms of electrons.

The metal isn't lost and the gas isn't hiding inside it. Metal atoms become metal ions in the salt solution, and the hydrogen comes from the acid.

Higher

Following the electrons

Reason it through

Why is magnesium + hydrochloric acid a redox reaction?

Link 1 of 4

First link · your turn

A magnesium atom becomes a magnesium ion, Mg²⁺. What does the atom have to lose to end up with a 2+ charge?

Locked — reveal the link above first

Higher

Redox · Who loses, who gains?

Oxidised, reduced, or unchanged?

Pick a species, then choose its column.

Still to sort

Oxidised (0)

Loses electrons

Reduced (0)

Gains electrons

Unchanged (0)

The same ion before and after

Where the line is: Ending up in the salt doesn't make an ion reduced. Check whether it actually gained or lost electrons.

7 of 7 still to sort.

Each item is a species from one of three equations. Place it, then read why.

Exam line: Oxidised: the metal atom. Reduced: the hydrogen ion. Chloride and sulfate ions are unchanged, so they are called spectator ions.

Watch out: Don't name H₂ as the species reduced. H₂ is what the hydrogen ions become; the species that gained electrons is H⁺.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Three metals, two acids, six reactions. One pattern runs through all of them, and once you spot it you can name every salt yourself.

What you need to know

Acids react with some metals to produce a salt and hydrogen: metal + acid → salt + hydrogen.
The six reactions to know are magnesium, zinc and iron, each with hydrochloric acid and with sulfuric acid.
Hydrochloric acid makes chlorides and sulfuric acid makes sulfates; iron makes iron(II) salts.
The hydrogen gas comes from the acid, and the metal atoms become metal ions in the salt solution.
(Higher) These reactions are redox: metal atoms lose electrons (oxidation) and hydrogen ions gain electrons (reduction).

The big picture

Acids react with some metals to make a salt and hydrogen. You need six of these reactions: magnesium, zinc or iron with hydrochloric or sulfuric acid. Every one makes hydrogen, and it comes from the acid. The salt's name is built from the reactants: the metal gives the first word, the acid the second (chloride or sulfate), and iron's salts are iron(II) salts. The metal atoms become metal ions in the salt solution. (Higher) These are redox reactions: metal atoms lose electrons (oxidised), hydrogen ions gain them (reduced), and chloride or sulfate ions are unchanged.

Key points

1Hydrogen is made in all six reactions. Only the salt changes.

2Build the salt name from the reactants: metal first, then chloride or sulfate from the acid.

3Chloride, not chlorine; sulfate, not sulfur. And iron needs its (II).

4(Higher) Oxidation is loss of electrons, reduction is gain of electrons. In these reactions the metal is oxidised and the hydrogen ion is reduced.

5(Higher) Chloride and sulfate ions are unchanged in these reactions, so they are neither oxidised nor reduced.

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Worked example

Problem

A student writes: iron + sulfuric acid → iron sulfur + water. Find the mistakes and write the correct word equation.

⚠ Watch out

Saying the bubbles come out of the metal, or writing water as the second product. The gas is hydrogen, and it comes from the acid.

🧠

Memory hook

Metal first, acid second, hydrogen always. For the electrons: OIL RIG, Oxidation Is Loss, Reduction Is Gain.

✓

Check yourself

Cover the page. Name the salt and the gas made when magnesium meets dilute sulfuric acid. Higher: in Zn + 2HCl → ZnCl₂ + H₂, what is oxidised, what is reduced, and why?

Flashcards

(10)

What two kinds of product form when magnesium, zinc or iron reacts with a dilute acid?

A salt and hydrogen.

Which kind of salt does hydrochloric acid make?

Chlorides.

Which kind of salt does sulfuric acid make?

Sulfates.

Zinc + hydrochloric acid → ?

Zinc chloride + hydrogen.

Name the two salts iron makes with hydrochloric acid and with sulfuric acid.

Iron(II) chloride and iron(II) sulfate.

Where does the hydrogen gas come from when a metal reacts with an acid?

From the acid (its hydrogen ions), not from the metal.

What happens to the metal atoms when the metal reacts with an acid?

They become metal ions, which stay in the solution as part of the salt.

(Higher) Define oxidation and reduction in terms of electrons.

Oxidation is loss of electrons; reduction is gain of electrons.

(Higher) Why is a metal + acid reaction a redox reaction?

Metal atoms lose electrons (oxidised) and hydrogen ions gain them (reduced), both at the same time.

(Higher) What happens to chloride or sulfate ions when a metal reacts with an acid?

Nothing: they are unchanged spectator ions, neither oxidised nor reduced.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Pick a metal. Pick an acid. What do you get?

What really happens to the metal?

Oxidised, reduced, or unchanged?

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