Chemical Reactions and Equations · Lesson 7 of 9
Oxidation And Reduction
“Oxygen changes hands and oxidation and reduction happen as an inseparable pair.”
• Define oxidation in terms of gain of oxygen or loss of hydrogen. • Define reduction in terms of loss of oxygen or gain of hydrogen. • Explain copper oxidation and copper oxide reduction. • Explain why oxidation and reduction occur together in redox reactions. • Identify oxidised and reduced substances from chemical equations.
Some reactions are best understood by following oxygen or hydrogen from one substance to another. This gives two linked ideas: oxidation and reduction. The copper-copper oxide system provides a clear visual example because brown copper becomes black copper oxide and can then be changed back to brown copper.
Heating Copper Powder
When brown copper powder is heated in air, its surface becomes coated with black copper(II) oxide. Oxygen has been added to copper.
Oxidation is gain of oxygen or loss of hydrogen by a substance.
Reduction Of Copper Oxide
If hydrogen gas is passed over heated black copper oxide, the black material turns brown because copper is formed again. Copper oxide loses oxygen, so it is reduced. Hydrogen gains that oxygen and forms water, so hydrogen is oxidised.
Reduction is loss of oxygen or gain of hydrogen by a substance.
Redox Reactions
A Redox Reaction is a reaction in which oxidation and reduction occur together.
In CuO + H₂ → Cu + H₂O, oxygen does not disappear. It moves from CuO to H₂. That is why the two processes are linked: CuO loses oxygen and is reduced, while H₂ gains oxygen and is oxidised.
More Redox Examples
Carbon gains oxygen and is oxidised to carbon monoxide. Zinc oxide loses oxygen and is reduced to zinc.
In this reaction, hydrochloric acid is oxidised to chlorine while manganese dioxide is reduced to manganese chloride.
A Step-By-Step Method For Identifying Oxidation And Reduction
First, compare each substance before and after the reaction. Look for oxygen gained or lost. If oxygen tracking is not sufficient, check hydrogen gained or lost. Label the substance gaining oxygen or losing hydrogen as oxidised. Label the substance losing oxygen or gaining hydrogen as reduced. Finally, confirm that both processes occur together in the overall redox reaction.
Problem
Identify oxidation and reduction in CuO + H₂ → Cu + H₂O.
- 1.CuO contains oxygen before the reaction, but product Cu does not. CuO loses oxygen, so CuO is reduced.
- 2.H₂ contains no oxygen before the reaction, while H₂O contains oxygen after the reaction. H₂ gains oxygen, so H₂ is oxidised.
- 3.Both changes occur in the same reaction, so it is a redox reaction.
Problem
Identify the process in 4Na + O₂ → 2Na₂O.
- 1.Sodium begins without oxygen.
- 2.Sodium oxide contains oxygen combined with sodium.
- 3.Sodium has gained oxygen.
- 4.Therefore sodium is oxidised.
Quiz
What happens to copper powder when heated in air?
Which statement defines oxidation?
Which statement defines reduction?
In CuO + H₂ → Cu + H₂O, which substance is reduced?
Why is CuO + H₂ → Cu + H₂O a redox reaction?
Practice Problems
- Explain the colour changes when copper is heated in air and then copper oxide is heated in hydrogen.
- In CuO + H₂ → Cu + H₂O, identify the oxidised and reduced substances and explain each decision.
- In ZnO + C → Zn + CO, identify the oxidised substance and the reduced substance.
- Use gain or loss of oxygen to explain oxidation of sodium in 4Na + O₂ → 2Na₂O.
- State both definitions of oxidation and both definitions of reduction used in the lesson.
- Explain why oxidation and reduction are described as occurring together in a redox reaction.
Key Takeaways
• Oxidation means gain of oxygen or loss of hydrogen. • Reduction means loss of oxygen or gain of hydrogen. • Heating copper in air oxidises copper to black copper oxide. • Hydrogen reduces copper oxide to copper while hydrogen itself is oxidised to water. • Redox reactions contain oxidation and reduction at the same time.