Metal oxides — AQA GCSE Chemistry
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Metal oxides explained
Metal oxides covers 2 specification points, including Metals react with oxygen to produce metal… and Students should be able to explain reduction….
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Open a point below for its explanation and revision guidance.
Metals react with oxygen to produce metal oxides. The reactions are oxidation reactions because the metals gain oxygen.
Many metals react with oxygen to form metal oxides. For example, magnesium burns in oxygen to form white magnesium oxide: 2Mg + O₂ → 2MgO. Iron reacts more slowly to form iron oxide, and copper reacts on heating to form black copper(II) oxide: 2Cu + O₂ → 2CuO. These are oxidation reactions because the metal atoms gain oxygen. In terms of electrons, the metal atoms also lose electrons to form positive ions, so oxidation can be described as gain of oxygen or loss of electrons. The product is a metal oxide, which is usually a solid at room temperature. The reaction may be vigorous for reactive metals and slower for less reactive metals, but the same pattern applies.
Students should be able to explain reduction and oxidation in terms of loss or gain of oxygen.
This statement asks you to use oxygen transfer as the defining idea for reduction and oxidation in reactions involving metal oxides. Oxidation is the gain of oxygen by a substance; reduction is the loss of oxygen. In a reaction between a metal and a metal oxide, the metal that gains oxygen is oxidised while the metal oxide that loses oxygen is reduced. For example, in 2Mg + CO₂ → 2MgO + C, magnesium gains oxygen and is oxidised, while carbon dioxide loses oxygen and is reduced. The two processes happen together, so such a reaction is a redox reaction. You should be able to inspect a symbol equation, identify which reactant gains oxygen and which loses oxygen, and name the substance oxidised and the substance reduced. This oxygen-based definition is the one required at GCSE; electron transfer is not needed here.
Your focus
- Describe the reaction of metals with oxygen to produce metal oxides.
- Explain why these reactions are oxidation reactions in terms of the metal gaining oxygen.
- Write balanced equations for the formation of metal oxides from metals and oxygen.
Show all 6 objectives
- State that oxidation is the gain of oxygen and reduction is the loss of oxygen.
- Identify from a symbol equation which substance is oxidised and which is reduced.
- Describe a reaction involving a metal oxide as a redox reaction and justify the description using oxygen transfer.
Metal oxides exam tips
Marking Points
- States that metals react with oxygen to produce metal oxides.
- Identifies the reaction as oxidation because the metal gains oxygen.
- Writes a balanced symbol equation for a metal reacting with oxygen, for example 2Mg + O₂ → 2MgO.
- Names the metal oxide formed from a given metal and oxygen reaction.
- Links oxidation to the metal gaining oxygen, and where relevant to the metal losing electrons to form positive ions.
- Oxidation is defined as the gain of oxygen by a substance.
- Reduction is defined as the loss of oxygen from a substance.
- In a given equation, the substance that gains oxygen is identified as oxidised.
- In the same equation, the substance that loses oxygen is identified as reduced.
- A reaction in which one substance is oxidised and another is reduced is described as a redox reaction.
- The explanation is applied correctly to a named example such as 2Mg + CO₂ → 2MgO + C.
Examiner Tips
- 💡Use the phrase 'gain of oxygen' when explaining why a metal reacting with oxygen is oxidation.
- 💡Balance equations for metal oxide formation by checking that the number of each type of atom is the same on both sides.
- 💡Include state symbols where they are known, for example 2Mg(s) + O₂(g) → 2MgO(s), to show the metal oxide is a solid.
- 💡Underline the oxygen atoms in the reactants and products before deciding which substance gains or loses oxygen.
- 💡Write both conclusions explicitly, for example 'magnesium is oxidised and carbon dioxide is reduced', rather than giving only one.
- 💡If asked to explain, include the words gain and loss of oxygen in your answer so the definition is visible to the examiner.
Common Mistakes
- Saying the metal is reduced because it gains oxygen; correct this by stating that gaining oxygen is oxidation.
- Writing an unbalanced equation such as Mg + O₂ → MgO; correct this by balancing to 2Mg + O₂ → 2MgO.
- Confusing oxidation with combustion only; correct this by explaining that oxidation is gain of oxygen, which can occur when a metal reacts with oxygen even without a flame.
- Reversing the definitions by saying oxidation is loss of oxygen; correct this by linking oxidation to gain of oxygen and reduction to loss of oxygen.
- Naming only one of the two changes in a redox reaction; correct this by stating both the substance oxidised and the substance reduced.
- Assuming the metal always gains oxygen without checking the equation; correct this by tracing oxygen atoms from reactants to products before deciding.