Variable Oxidation States - UNSOLVED PRACTICE SET
Chapter: d and f Block Elements | Topic: Variable Oxidation States
VARIABLE OXIDATION STATES - UNSOLVED PRACTICE SET
Topic: Variable Oxidation States
Multiple Choice Questions
Q1. Transition elements show variable oxidation states because:
- They have a variable number of valence electrons
- The energy difference between (n-1)d and ns orbitals is small
- They can lose electrons from both s and d orbitals
- Both (b) and (c)
Q2. The maximum oxidation state shown by Mn is:
- +2
- +4
- +6
- +7
Q3. Which of the following elements shows only one non-zero oxidation state?
- Sc
- Ti
- V
- Cr
Q4. The most common oxidation state of first row transition elements is:
- +1
- +2
- +3
- Variable
Q5. In the 3d series, the stability of the +2 oxidation state:
- Decreases from left to right
- Increases from left to right
- Remains constant
- Shows no regular trend
Q6. Which transition metal shows the highest number of oxidation states?
- Cr
- Mn
- Fe
- Co
Short Answer Questions
Q7. Why do transition metals exhibit variable oxidation states? Explain with reference to the energy levels of (n-1)d and ns electrons.
Q8. Scandium shows only +3 oxidation state. Explain why it does not show variable oxidation states like other transition metals.
Q9. Why is the +2 oxidation state more stable than the +3 oxidation state towards the end of the first transition series?
Q10. Manganese shows oxidation states from +2 to +7. Write the electronic configuration of Mn in its +2 and +7 oxidation states.
Q11. Why does chromium prefer to form Cr³⁺ rather than Cr²⁺, even though both are possible?
Q12. Copper shows +1 and +2 oxidation states. Which is more stable in aqueous solution? Give reason.
Long Answer Questions
Q13. (a) Explain why transition elements exhibit variable oxidation states.
(b) Compare the oxidation states of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn.
(c) Why does the number of oxidation states increase from Sc to Mn and then decrease from Mn to Zn?
(d) Which element in the 3d series shows the maximum number of oxidation states? Explain with examples.
Q14. (a) What is the most common oxidation state of the first-row transition elements? Why is this oxidation state particularly stable?
(b) Explain why:
(i) Mn²⁺ is more stable than Mn³⁺
(ii) Fe³⁺ is more stable than Fe²⁺
(iii) Cu²⁺ is more stable than Cu⁺ in aqueous solution
(c) Write the electronic configurations of Ti²⁺, Ti³⁺, and Ti⁴⁺. Which of these is most stable and why?
Q15. (a) Discuss the stability of various oxidation states of iron. Which is most stable and why?
(b) A student finds two bottles in the lab: one containing FeSO₄ (green) and another containing Fe₂(SO₄)₃ (yellow).
(i) What are the oxidation states of iron in these two compounds?
(ii) Which compound is more easily oxidized?
(iii) What precaution should be taken while storing the FeSO₄ solution?
Numerical / Application-Based Problems
Q16. The standard electrode potentials for some transition metal ions are given below:
| Half-Reaction | E° (V) |
|---|---|
| Mn²⁺ + 2e⁻ → Mn | -1.18 |
| Fe²⁺ + 2e⁻ → Fe | -0.44 |
| Co²⁺ + 2e⁻ → Co | -0.28 |
| Ni²⁺ + 2e⁻ → Ni | -0.25 |
| Cu²⁺ + 2e⁻ → Cu | +0.34 |
| Zn²⁺ + 2e⁻ → Zn | -0.76 |
(a) Arrange these metals in order of increasing ease of oxidation.
(b) Which metal is the strongest reducing agent? Which is the weakest?
(c) A solution contains Fe²⁺ and Cu²⁺ ions. If a strip of zinc metal is dipped into this solution, which ion will be reduced first? Explain using E° values.
(d) Calculate the standard cell potential for the reaction: Cu²⁺ + Zn → Cu + Zn²⁺
(e) Why does the standard reduction potential become less negative from Mn to Cu, despite the increasing nuclear charge?
Q17. The following compounds of manganese are found in the laboratory:
MnO (manganese(II) oxide)
MnO₂ (manganese(IV) oxide)
K₂MnO₄ (potassium manganate)
KMnO₄ (potassium permanganate)
(a) Determine the oxidation state of manganese in each compound.
(b) Write the electronic configuration of Mn in each oxidation state.
(c) Which compound is the strongest oxidizing agent? Explain using oxidation state.
(d) MnO₂ is used in dry cells (Leclanché cells). Write the reaction at the cathode involving MnO₂.
(e) A student mixes KMnO₄ with FeSO₄ in acidic medium. Write the balanced redox reaction and identify the oxidizing and reducing agents.
Q18. In a school chemistry lab, students perform an experiment to study the variable oxidation states of iron.
(a) They add potassium ferricyanide K₃[Fe(CN)₆] to a solution of FeSO₄. A dark blue precipitate (Turnbull's blue) forms. Write the reaction and identify the oxidation states of iron in the reactants and products.
(b) They then add potassium permanganate (KMnO₄) dropwise to FeSO₄ solution in acidic medium. The purple colour disappears and the solution turns yellow. Explain the colour change with the balanced redox equation.
(c) The students prepare Mohr's salt (FeSO₄·(NH₄)₂SO₄·6H₂O) for titration. Why is Mohr's salt preferred over FeSO₄ for volumetric analysis?
(d) A student notices that FeSO₄ solution turns brown on standing in air. What chemical change has occurred? Write the reaction.
(e) The teacher asks: "If you need to store Fe²⁺ solution for a long time, what would you add and why?" Give your answer with chemical reasoning.