9.1.1 Oxidation and Reduction (Q&A)

Short Answer Questions

1. What is a redox reaction?
   Answer
   A redox reaction is a chemical reaction involving both oxidation and reduction processes occurring simultaneously.
2. State the two words that form the term ‘Redox’.
   Answer
   The term comes from 'Reduction' and 'Oxidation'.
3. Define oxidation in terms of oxygen transfer.
   Answer
   Oxidation is a process where a reactant gains oxygen.
4. Define reduction in terms of hydrogen transfer.
   Answer
   Reduction is a process where a reactant gains hydrogen.
5. What happens to a reactant that undergoes oxidation in terms of electrons?
   Answer
   The reactant loses electrons (e⁻).
6. What is the change in the oxidation number of an element during reduction?
   Answer
   The oxidation number of the element decreases.
7. Identify the oxidising agent in terms of electron transfer.
   Answer
   An oxidising agent is an electron acceptor.
8. Identify the reducing agent in terms of its own change in a reaction.
   Answer
   A reducing agent undergoes oxidation in a redox reaction.
9. Name one example of a common oxidising agent from the halogens.
   Answer
   Chlorine (Cl₂) or Bromine (Br₂).
10. Name one example of a strong reducing agent from electropositive metals.
    Answer
    Magnesium (Mg) or Zinc (Zn).
11. What is the oxidation number of a free element like Cu or N₂?
    Answer
    The oxidation number is zero (0).
12. What is the oxidation number of a Group 1 metal in a compound?
    Answer
    The oxidation number is always +1.
13. What is the total sum of oxidation numbers in a neutral compound?
    Answer
    The total sum is zero (0).
14. What is the oxidation number of oxygen in a peroxide like H₂O₂?
    Answer
    The oxidation number of oxygen is -1.
15. What is the oxidation number of hydrogen in metal hydrides?
    Answer
    The oxidation number is -1.
16. What characterizes a non-redox reaction?
    Answer
    No elements experience a change in their oxidation numbers.
17. Write the half-equation for the oxidation of Potassium (K).
    Answer
    K(s) → K⁺(aq) + e⁻
18. What is the oxidation number of the chlorine ion in MgCl₂?
    Answer
    The oxidation number is -1.
19. Define the term ‘oxidation number’.
    Answer
    An imaginary charge assigned to an atom in a compound if electrons were transferred completely.
20. Give an example of an acidified oxidising agent.
    Answer
    Acidified potassium manganate(VII) (KMnO₄) or acidified potassium dichromate(VI) (K₂Cr₂O₇).

Explanation Questions

1. Explain why the reaction H₂S(g) + Cl₂(g) → S(s) + 2HCl(g) is a redox reaction in terms of hydrogen.
   Answer
   H₂S undergoes oxidation because it loses hydrogen to form Sulfur, while Cl₂ undergoes reduction because it gains hydrogen to form HCl. Since both processes occur simultaneously, it is a redox reaction.
2. How does the transfer of electrons determine if a substance is an oxidising agent?
   Answer
   An oxidising agent causes another substance to be oxidised by accepting the electrons that the other substance loses. By accepting electrons, the oxidising agent itself undergoes reduction.
3. Using half-equations, explain the redox process when Magnesium reacts with Copper(II) oxide.
   Answer
   Magnesium is oxidised by losing electrons: Mg → Mg²⁺ + 2e⁻. Copper(II) ions are reduced by gaining electrons: Cu²⁺ + 2e⁻ → Cu. The exchange of electrons between Mg and Cu²⁺ makes it a redox reaction.
4. Explain the relationship between electropositive metals and their role as reducing agents.
   Answer
   Electropositive metals (like Mg and Zn) have a high tendency to donate electrons. Because they readily give away electrons to reduce other substances, they serve as effective reducing agents.
5. Why is the reaction between an acid and a base (neutralisation) usually not a redox reaction?
   Answer
   In neutralisation, there is no transfer of electrons and the oxidation numbers of the elements involved remain unchanged throughout the reaction.
6. Describe how to determine the oxidation number of a polyatomic ion like Sulfate (SO₄²⁻).
   Answer
   The sum of the oxidation numbers of all atoms in the ion must equal the charge of the ion. For SO₄²⁻, the sum of the oxidation number of Sulfur and four Oxygen atoms must equal -2.
7. Explain the difference between the oxidation process and the oxidising agent.
   Answer
   Oxidation is the process (losing electrons, gaining oxygen, etc.), whereas an oxidising agent is the specific substance that facilitates the oxidation of another material by undergoing reduction itself.
8. Why does the oxidation number of Manganese in KMnO₄ require calculation, whereas Potassium does not?
   Answer
   Potassium is a Group 1 metal and always has a fixed oxidation number of +1 in sebatian. Manganese is a transition metal that can have multiple oxidation states, so its value must be calculated based on the other elements in the neutral compound.
9. How does the “loss of electrons” lead to an “increase in oxidation number”?
   Answer
   Electrons carry a negative charge. When an atom loses these negative particles, the overall balance of the atom becomes more positive, thus increasing its oxidation number.
10. Explain the importance of oxygen being -2 and hydrogen being +1 as standard values in redox chemistry.
    Answer
    These standard values act as a reference point. By knowing these fixed numbers, we can calculate the unknown oxidation numbers of other elements in complex molecules or ions to identify which species are being oxidised or reduced.