Reactions of acids with metals — AQA GCSE Chemistry
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Reactions of acids with metals explained
Reactions of acids with metals covers 2 specification points, including Acids react with some metals to produce salts… and Explain in terms of gain or loss of electrons,….
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Open a point below for its explanation and revision guidance.
Acids react with some metals to produce salts and hydrogen.
When an acid reacts with a reactive metal, the metal atoms lose electrons and hydrogen ions in the acid gain electrons, producing hydrogen gas. The other product is a salt formed from the metal ion and the acid's negative ion. For example, magnesium reacts with hydrochloric acid to give magnesium chloride and hydrogen: Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g). The salt name depends on the acid: hydrochloric acid gives chlorides, sulfuric acid gives sulfates. Not all metals react; those below hydrogen in the reactivity series, such as copper, do not react with dilute acids. The reaction is observed as effervescence as hydrogen bubbles form, and the metal gradually dissolves. Testing the gas with a lit splint gives a squeaky pop, confirming hydrogen.
(HT only) Students should be able to: explain in terms of gain or loss of electrons, that these are redox reactions identify which species are oxidised and which are reduced in given chemical equations. Knowledge of reactions limited to those of magnesium, zinc and iron with hydrochloric and sulfuric acids.
In the reaction between a metal and an acid, the metal atoms lose electrons and are oxidised, while hydrogen ions from the acid gain electrons and are reduced. Because both oxidation and reduction occur together, the reaction is a redox reaction. For example, with magnesium and hydrochloric acid: Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g). The magnesium loses two electrons to form Mg²⁺, so magnesium is oxidised. The hydrogen ions gain electrons to form H₂, so hydrogen is reduced. The same reasoning applies to zinc and iron with hydrochloric or sulfuric acid. To identify the species, compare the metal atom with its ion and the hydrogen ion with hydrogen gas, tracking electron loss and gain.
Your focus
- Write word and balanced symbol equations for reactions of acids with metals.
- Name the salt produced from a given metal and acid.
- Describe the test and observation for hydrogen gas.
Show all 6 objectives
- Explain that reactions of metals with acids are redox reactions in terms of electron gain and loss.
- Identify which species is oxidised and which is reduced in a given equation.
- Apply electron-transfer reasoning to magnesium, zinc and iron with hydrochloric and sulfuric acids.
Reactions of acids with metals exam tips
Marking Points
- Name the salt from the metal and the acid used, for example magnesium sulfate from magnesium and sulfuric acid.
- Write a balanced symbol equation with the correct state symbols where required.
- State that hydrogen gas is produced and describe the test for hydrogen.
- Explain that the metal must be more reactive than hydrogen for the reaction to occur.
- Describe observations such as effervescence and the metal dissolving.
- State that oxidation is loss of electrons and reduction is gain of electrons.
- Identify the metal as the species oxidised because it loses electrons to form positive ions.
- Identify hydrogen ions as the species reduced because they gain electrons to form hydrogen gas.
- Explain that a reaction is redox when both oxidation and reduction occur.
- Apply the explanation to magnesium, zinc or iron with hydrochloric or sulfuric acid.
Examiner Tips
- 💡Write the word equation first to fix the salt name, then convert to a balanced symbol equation.
- 💡Include state symbols when the question asks for a full equation.
- 💡Link the squeaky pop test directly to hydrogen rather than to oxygen or carbon dioxide.
- 💡Write the half equations for the metal and for hydrogen to make electron transfer explicit.
- 💡Use the phrase 'loss of electrons is oxidation, gain of electrons is reduction' in your explanation.
- 💡Restrict examples to magnesium, zinc and iron with hydrochloric or sulfuric acid as required by the specification.
Common Mistakes
- Naming the salt from the acid only, for example calling the product 'hydrochloric acid salt'; the correction is to combine the metal name with the acid-derived ending, such as magnesium chloride.
- Writing an unbalanced equation such as Mg + HCl → MgCl₂ + H₂; the correction is to balance to Mg + 2HCl → MgCl₂ + H₂.
- Claiming all metals react with acids; the correction is that metals below hydrogen in the reactivity series, such as copper, do not react with dilute acids.
- Saying hydrogen is oxidised because it is a product; the correction is that hydrogen ions gain electrons and are reduced.
- Confusing the metal ion with the metal atom; the correction is to compare the metal atom, which loses electrons, with the metal ion formed.
- Describing the reaction as only oxidation or only reduction; the correction is that both occur together, so it is a redox reaction.