Reactions of acids with metals — AQA GCSE Combined Science
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Reactions of acids with metals explained
When an acid reacts with a metal, the metal displaces hydrogen from the acid.
Read the full explanation
The products are a salt and hydrogen gas. For example, magnesium reacts with hydrochloric acid: Mg + 2HCl → MgCl₂ + H₂. The salt formed depends on the acid used: hydrochloric acid gives chloride salts, sulfuric acid gives sulfate salts, and nitric acid gives nitrate salts. The metal must be more reactive than hydrogen for the reaction to occur; metals such as copper, silver and gold do not react with dilute acids. Hydrogen gas is identified by a squeaky pop with a lit splint. For Higher Tier students, this reaction is a redox process: the metal is oxidised (loses electrons) and hydrogen ions are reduced (gain electrons).
explain in terms of gain or loss of electrons, that these are redox reactions
For Higher Tier, candidates must explain metal-acid reactions as redox processes using electron transfer. While all students study the general reaction of acids with metals to produce salts and hydrogen, explaining this in terms of electrons is exclusively a Higher Tier requirement. A redox reaction occurs when oxidation (loss of electrons) and reduction (gain of electrons) happen simultaneously. For example, when magnesium reacts with hydrochloric acid, magnesium atoms lose two electrons to form Mg²⁺ ions (oxidation). Simultaneously, two H⁺ ions from the acid each gain one electron to form a hydrogen molecule, H₂ (reduction). Half-equations clearly show this transfer.
identify which species are oxidised and which are reduced in given chemical equations.
For Higher Tier assessments, students must identify which species are oxidised and reduced in chemical equations by tracking electron transfer. Look at the charges on the atoms and ions. Elements have no charge. In the reaction Mg + 2HCl → MgCl₂ + H₂, magnesium starts as a neutral atom (Mg) and becomes a positive ion (Mg²⁺) in magnesium chloride. It has lost electrons, so it is oxidised. The hydrogen starts as a positive ion (H⁺) in hydrochloric acid and becomes neutral hydrogen gas (H₂). It has gained electrons, so it is reduced. The chloride ions (Cl⁻) remain unchanged and are spectator ions. Always identify the specific ion or atom involved.
Your focus
- Write word and balanced symbol equations for the reaction of an acid with a metal.
- Name the salt produced from a given acid and metal.
- Explain why some metals do not react with dilute acids, using the reactivity series.
Show all 10 objectives
- Explain the reaction in terms of oxidation and reduction of ions (HT only).
- Explain why the reaction between a metal and an acid is classified as a redox reaction.
- Identify that oxidation and reduction occur simultaneously via electron transfer.
- Construct half-equations to describe the specific electron loss by metal atoms and electron gain by hydrogen ions.
- Identify the species that is oxidised in a given chemical equation for a metal-acid reaction.
- Identify the species that is reduced in a given chemical equation for a metal-acid reaction.
- Justify the identification of oxidised and reduced species using changes in ionic charge and electron transfer.
Reactions of acids with metals exam tips
Marking Points
- State the general word equation: acid + metal → salt + hydrogen.
- Name the salt produced from the specific acid and metal, for example magnesium + hydrochloric acid → magnesium chloride + hydrogen.
- Write a balanced symbol equation, including state symbols where appropriate, for example Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g).
- Explain that the metal must be more reactive than hydrogen for the reaction to occur, and that less reactive metals such as copper do not react with dilute acids.
- Describe the test for hydrogen gas: a lit splint produces a squeaky pop.
- Identify the reaction as a redox process in which the metal is oxidised and hydrogen ions are reduced (HT only).
- Define oxidation strictly as the loss of electrons by a chemical species.
- Define reduction strictly as the gain of electrons by a chemical species.
- Explain that a redox reaction involves both oxidation and reduction occurring simultaneously.
- Identify that in metal-acid reactions, metal atoms are oxidised by losing electrons to form positive metal ions.
- Identify that hydrogen ions (H⁺) from the acid are reduced by gaining electrons to form neutral hydrogen gas molecules (H₂).
- Identify the metal atom as the species that is oxidised in a metal-acid reaction.
- Identify the hydrogen ion (H⁺) as the species that is reduced.
- Justify the oxidation of the metal by stating it has lost electrons to form a positive ion.
- Justify the reduction of hydrogen by stating that hydrogen ions have gained electrons to form neutral hydrogen molecules.
Examiner Tips
- 💡Learn the salt names for the three common acids: hydrochloric → chloride, sulfuric → sulfate, nitric → nitrate.
- 💡When writing symbol equations, balance the metal atoms first, then the hydrogen atoms, and finally check the non-metal atoms.
- 💡When asked to 'explain' a redox reaction, always explicitly mention both the loss and gain of electrons to secure full credit.
- 💡Use ionic half-equations (e.g., Mg -> Mg²⁺ + 2e⁻ and 2H⁺ + 2e⁻ -> H₂) to clearly demonstrate the simultaneous electron transfer for both the metal and the hydrogen ions.
- 💡Look for the element that goes from having no charge to a positive charge; this is the species that is oxidised.
- 💡Cross out spectator ions (like Cl⁻ or SO₄²⁻) in the equation to help you focus on the species that actually lose or gain electrons.
Common Mistakes
- Writing the salt as the metal name only, for example 'magnesium' instead of 'magnesium chloride': the salt name combines the metal and the acid's negative ion.
- Forgetting to balance the equation, especially the hydrogen molecules: ensure H₂ is written and the equation balances for both atoms and charge.
- Assuming all metals react with acids: check reactivity; copper, silver and gold do not react with dilute acids because they are less reactive than hydrogen.
- Describing the reaction using oxygen transfer instead of electrons. Correction: Explicitly state which species loses and gains electrons (OIL RIG) for this specific Higher Tier requirement.
- Stating that 'hydrogen gas' is reduced. Correction: Specify that it is the aqueous hydrogen ions (H⁺) that are reduced to form hydrogen gas.
- Forgetting to state that oxidation and reduction happen at the same time. Correction: Use the word 'simultaneously' when defining a redox reaction.
- Identifying the entire acid (e.g., HCl) as being reduced. Correction: Specify that only the hydrogen ions (H⁺) are reduced.
- Stating that the metal compound (e.g., MgCl₂) is oxidised. Correction: Identify the neutral metal reactant (e.g., Mg) as the species that is oxidised.
- Confusing oxidation and reduction. Correction: Use the OIL RIG acronym to remember that Oxidation Is Loss and Reduction Is Gain of electrons.