9.1 Oxidation and Reduction

Short Answer Questions

1. What is a redox reaction?
   Answer
   A redox reaction is a chemical reaction that involves both oxidation and reduction processes occurring simultaneously with the transfer of electrons.
2. What are the four ways oxidation can be defined?
   Answer
   Oxidation can be defined as the addition of oxygen, removal of hydrogen, loss of electrons, or an increase in oxidation number.
3. Give an example of oxidation involving the addition of oxygen.
   Answer
   Magnesium reacts with oxygen to form magnesium oxide: Mg + O₂ → MgO.
4. What happens to an element’s oxidation number during oxidation?
   Answer
   The oxidation number of the element increases.
5. What are the four ways reduction can be defined?
   Answer
   Reduction can be defined as the removal of oxygen, addition of hydrogen, gain of electrons, or a decrease in oxidation number.
6. Give an example of reduction involving the removal of oxygen.
   Answer
   Copper(II) oxide (CuO) is reduced to copper (Cu).
7. What is an oxidising agent?
   Answer
   An oxidising agent is a substance that causes oxidation by accepting electrons from another substance.
8. Give two examples of oxidising agents.
   Answer
   Oxygen, chlorine, and potassium manganate(VII) are examples of oxidising agents.
9. What is a reducing agent?
   Answer
   A reducing agent is a substance that causes reduction by donating electrons to another substance.
10. What is a half-equation?
    Answer
    A half-equation shows either the oxidation or the reduction process separately in a redox reaction.
11. Write the oxidation half-equation for magnesium.
    Answer
    Mg → Mg²⁺ + 2e⁻
12. What is the oxidation number of an element in its free state?
    Answer
    The oxidation number of an element in its free state is zero.
13. What is the oxidation number of oxygen in most compounds?
    Answer
    The oxidation number of oxygen is -2, except in peroxides where it is -1.
14. How do oxidation numbers change during oxidation and reduction?
    Answer
    In oxidation, the oxidation number increases, while in reduction, the oxidation number decreases.
15. What is the electrochemical series?
    Answer
    The electrochemical series is a list of elements arranged in order of their tendency to lose electrons (electropositivity).
16. What is the relationship between a metal’s position in the electrochemical series and its ability to be oxidised?
    Answer
    Metals at the top of the series are more electropositive and easily oxidised, making them strong reducing agents.
17. What happens when zinc is placed in a solution of copper(II) sulfate?
    Answer
    Zinc displaces copper from copper(II) sulfate solution because zinc is more electropositive: Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s).
18. Give an example of a redox reaction involving halogens.
    Answer
    Chlorine reacts with hydrogen sulfide to form sulfur and hydrogen chloride: H₂S + Cl₂ → S + 2HCl.
19. Why are neutralisation reactions not considered redox reactions?
    Answer
    Neutralisation reactions do not involve a change in oxidation number or electron transfer.

Explanation Questions

1. Why do redox reactions always involve both oxidation and reduction?
   Answer
   Electrons lost in oxidation must be gained by another substance, meaning oxidation and reduction occur simultaneously.
2. How does an oxidising agent cause oxidation?
   Answer
   An oxidising agent causes oxidation by accepting electrons from another substance, allowing it to be oxidised.
3. Explain how a reducing agent works in a redox reaction.
   Answer
   A reducing agent donates electrons to another substance, reducing it while itself undergoing oxidation.
4. Why do metals at the top of the electrochemical series act as strong reducing agents?
   Answer
   These metals have a high tendency to lose electrons and form positive ions, making them easily oxidised.
5. How do oxidation numbers help identify redox reactions?
   Answer
   Changes in oxidation numbers indicate whether oxidation (increase) or reduction (decrease) has occurred in a reaction.
6. Explain the importance of half-equations in representing redox reactions.
   Answer
   Half-equations show the separate oxidation and reduction processes, clarifying electron transfer in a redox reaction.
7. Why does magnesium react with copper(II) sulfate in a redox reaction?
   Answer
   Magnesium is more electropositive than copper, so it donates electrons to Cu²⁺ ions, reducing them to copper metal.
8. What role does oxygen play in combustion reactions?
   Answer
   Oxygen acts as an oxidising agent, accepting electrons from the fuel and allowing combustion to occur.
9. Why do electrochemical cells produce an electrical current?
   Answer
   Electrons flow from the oxidised metal (anode) to the reduced metal (cathode), generating an electric current.
10. How can you determine if a reaction is a redox reaction?
    Answer
    Check if there is a change in oxidation numbers or electron transfer between reactants.