4.1.1Chemical Changes
Metal Oxides — AQA GCSE Combined Science Trilogy 8464 - Chemistry Revision Notes & Topical Questions
5 min read 8 parts Lesson 2 of 18
Part 1 of 6Metals and oxygen
Formation of oxides
- Formation of oxides Most metals react with oxygen to form a metal oxide when heated or left in air.
- The more reactive the metal, the more readily and vigorously it reacts.
- Gold does not react with oxygen at all, which is why it stays shiny, while sodium reacts rapidly with oxygen in air.
Example: copper
- Example: copper Shiny, red-brown copper turns black when heated in air as copper(II) oxide forms.
- 2Cu + O2 → 2CuO
Example: magnesium
- Example: magnesium Magnesium burns with a bright white flame to give white magnesium oxide.
- 2Mg + O2 → 2MgO
Example: iron
- Example: iron Iron wool glows and forms a dark solid when heated strongly in oxygen, and slowly rusts in damp air.
- 4Fe + 3O2 → 2Fe2O3
Part 2 of 6Defining oxidation and reduction
| Process | In terms of oxygen | Example |
|---|---|---|
| Oxidation | A substance gains oxygen | Cu becomes CuO |
| Reduction | A substance loses oxygen | ZnO becomes Zn |
| Redox | Oxidation and reduction occur together in one reaction | ZnO + C → Zn + CO |
Part 3 of 6Spotting redox in an equation
1
Write the balanced equation and list the reactants and products.
2
Look for a reactant that ends up combined with more oxygen: that substance has been oxidised.
3
Look for a reactant that ends up with less oxygen: that substance has been reduced.
4
Confirm both happen in the same equation; if so it is a redox reaction.
5
In ZnO + C → Zn + CO, zinc oxide loses oxygen (reduced) and carbon gains it (oxidised).
Part 4 of 6Why the two go together
1Oxygen is transferred
- Oxygen is transferred Oxygen cannot simply vanish; it must move from one substance to another.
- The total number of oxygen atoms is the same before and after, only their position has changed.
2One loses, one gains
- One loses, one gains The substance giving up oxygen is reduced.
3Partner gains oxygen
- Partner gains oxygen The substance receiving oxygen is oxidised.
4Simultaneous
- Simultaneous Both changes occur at the same moment, which is why the reaction is called redox.
Part 5 of 6Worked examples of redox
| Reaction | Oxidised | Reduced | Reducing agent |
|---|---|---|---|
| CuO + H2 → Cu + H2O | Hydrogen (gains oxygen to form water) | Copper(II) oxide (loses oxygen) | Hydrogen |
| Fe2O3 + 3CO → 2Fe + 3CO2 | Carbon monoxide | Iron(III) oxide | Carbon monoxide |
| 2Mg + CO2 → 2MgO + C | Magnesium | Carbon dioxide | Magnesium |
Part 6 of 6Applications and exam context
Redox in use
Extraction of metals
- Extraction of metals Heating a metal oxide with carbon reduces the oxide to the metal, as in the blast furnace.
Corrosion
- Corrosion Rusting is oxidation of iron by oxygen and water, so protecting iron prevents this oxidation.
- Painting, oiling or galvanising keeps oxygen and water away from the surface.
Exam technique
- Exam technique Always name both substances, saying which is oxidised and which is reduced, and justify each with gain or loss of oxygen.
Key terms
| Term | Meaning |
|---|---|
| Oxidation | A reaction in which a substance gains oxygen. |
| Reduction | A reaction in which a substance loses oxygen. |
| Redox | A reaction in which oxidation and reduction happen at the same time. |
| Metal oxide | A compound of a metal combined with oxygen. |
| Reducing agent | A substance that removes oxygen from another substance and is itself oxidised. |
| Oxidising agent | A substance that gives oxygen to another substance and is itself reduced. |
Avoid these mistakes
1
Saying oxidation is loss of oxygen — it is gain; reduction is loss.
2
Treating oxidation and reduction as separate reactions; they always occur together.
3
Calling the oxide the thing that is oxidised; in ZnO + C the oxide is reduced.
4
Writing oxygen as O in equations rather than O2.
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