Reactions of Acids with Metals

This lesson covers the reactions between acids and metals for Edexcel GCSE Chemistry (1CH0). You need to know the general equation, be able to write word and balanced symbol equations, describe observations, test for hydrogen gas, and explain why some metals do not react with acids.

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General Equation

When a metal reacts with an acid, the products are a salt and hydrogen gas:

acid + metal → salt + hydrogen

The metal atoms lose electrons to form positive metal ions (this is oxidation), and the hydrogen ions from the acid gain electrons to form hydrogen gas (this is reduction).

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Naming the Salt

The name of the salt produced depends on:

1. The metal used (gives the first part of the salt name)
2. The acid used (gives the second part)

Acid	Salt Type	Example Reaction	Salt Produced
Hydrochloric acid (HCl)	Chloride	Mg + HCl	Magnesium chloride
Sulfuric acid (H₂SO₄)	Sulfate	Zn + H₂SO₄	Zinc sulfate
Nitric acid (HNO₃)	Nitrate	Fe + HNO₃	Iron nitrate

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Examples with Word and Symbol Equations

Magnesium + Hydrochloric Acid

Word equation: magnesium + hydrochloric acid → magnesium chloride + hydrogen

Balanced symbol equation: Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)

Observations: Vigorous fizzing/effervescence; magnesium ribbon dissolves; solution warms up; colourless solution produced.

Zinc + Sulfuric Acid

Word equation: zinc + sulfuric acid → zinc sulfate + hydrogen

Balanced symbol equation: Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)

Observations: Steady fizzing; zinc pieces dissolve slowly; solution warms up; colourless solution produced.

Iron + Hydrochloric Acid

Word equation: iron + hydrochloric acid → iron(II) chloride + hydrogen

Balanced symbol equation: Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g)

Observations: Slow fizzing; iron dissolves slowly; solution turns pale green (due to Fe²⁺ ions); some warmth produced.

Magnesium + Sulfuric Acid

Word equation: magnesium + sulfuric acid → magnesium sulfate + hydrogen

Balanced symbol equation: Mg(s) + H₂SO₄(aq) → MgSO₄(aq) + H₂(g)

Observations: Very vigorous fizzing; magnesium dissolves rapidly; solution becomes very warm; colourless solution produced.

Exam Tip: When writing observations, use precise language. Say "fizzing" or "effervescence" (not "bubbling"), state whether the metal dissolves, and describe any colour change of the solution. Avoid saying the solution "turns clear" — say "colourless" (clear means see-through, not the same as colourless).

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Testing for Hydrogen Gas

When a metal reacts with an acid, hydrogen gas is produced. To confirm that the gas is hydrogen:

1. Collect the gas in a test tube held upside down over the reaction (hydrogen is less dense than air and rises).
2. Bring a lighted splint to the mouth of the test tube.
3. If hydrogen is present, it burns with a characteristic squeaky pop sound.

The chemical equation for the combustion of hydrogen is:

2H₂(g) + O₂(g) → 2H₂O(l)

Exam Tip: The test for hydrogen is one of the required gas tests. You must state both parts: (1) apply a lighted/burning splint, and (2) the result is a squeaky pop. Both parts are needed for the mark.

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Comparing the Reactivity of Metals with Acids

The speed and vigour of the reaction between a metal and an acid depends on the position of the metal in the reactivity series.

Metal	Reactivity	Reaction with Dilute HCl	Observation
Magnesium	Very reactive	Very vigorous	Rapid fizzing; dissolves quickly; gets hot
Aluminium	Reactive	Slow to start (oxide layer), then moderate	Little reaction initially, then steady fizzing
Zinc	Moderately reactive	Moderate	Steady fizzing; dissolves over several minutes
Iron	Low reactivity	Slow	Slow fizzing; dissolves very slowly; pale green solution
Tin	Very low reactivity	Very slow	Very slow fizzing; barely reacts
Copper	Below hydrogen	No reaction	No fizzing; metal unchanged
Silver	Well below hydrogen	No reaction	No fizzing; metal unchanged
Gold	Very unreactive	No reaction	No fizzing; metal unchanged

Why Don't Some Metals React?

Metals below hydrogen in the reactivity series (copper, silver, gold, platinum) do not react with dilute acids. This is because they are less reactive than hydrogen — they cannot displace hydrogen from the acid.

Only metals that are more reactive than hydrogen can displace it from an acid and therefore react.

Linking to the Reactivity Series

The observations from acid-metal reactions can be used to deduce the order of reactivity of metals. If magnesium fizzes vigorously and zinc fizzes only moderately, we can conclude that magnesium is more reactive than zinc.

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Practical Considerations

Metals That Are Too Dangerous to React with Acids

• Potassium, sodium and lithium react too violently with acids to be used safely in the laboratory. They can ignite or explode.
• These reactions should only be carried out as teacher demonstrations with appropriate safety measures.

Aluminium and the Oxide Layer

• Aluminium has a thin, tough layer of aluminium oxide (Al₂O₃) on its surface.
• This protective layer must be removed or broken through before the metal can react with the acid.
• Once the oxide layer is breached, aluminium reacts quite vigorously because it is actually quite reactive.

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Oxidation and Reduction in Acid-Metal Reactions (Higher)

In these reactions:

• The metal is oxidised — it loses electrons to form positive ions.
  • Example: Mg → Mg²⁺ + 2e⁻ (oxidation — loss of electrons)
• The hydrogen ions are reduced — they gain electrons to form hydrogen gas.
  • Example: 2H⁺ + 2e⁻ → H₂ (reduction — gain of electrons)

These are half equations that show what happens to each species separately.

Remember: OIL RIG — Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons).

Exam Tip: If an exam question asks you to write half equations for the reaction of a metal with an acid, write one for the metal (showing loss of electrons) and one for the hydrogen ions (showing gain of electrons). Include state symbols and make sure the electrons balance.

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Summary