Stability Constants - UNSOLVED PRACTICE SET
Chapter: Coordination Compounds | Topic: Stability Constants
STABILITY CONSTANTS - UNSOLVED PRACTICE SET
Topic: Stability Constants
Multiple Choice Questions
Q1. The stability constant (formation constant) of a complex is defined as:
- The rate of formation of the complex
- The equilibrium constant for the formation of the complex from its constituents
- The activation energy for complex formation
- The enthalpy change of complex formation
Q2. A higher value of the stability constant (K) indicates:
- A less stable complex
- A more stable complex
- A faster reaction rate
- A higher temperature requirement
Q3. The stepwise stability constants (K₁, K₂, K₃...) for the formation of a complex MLₙ generally:
- Increase with each step
- Decrease with each step
- Remain constant for all steps
- Are unrelated to each other
Q4. The overall stability constant (βₙ) is related to stepwise stability constants by:
- βₙ = K₁ + K₂ + K₃ + ... + Kₙ
- βₙ = K₁ × K₂ × K₃ × ... × Kₙ
- βₙ = K₁ / K₂ / K₃ / ... / Kₙ
- βₙ = K₁ – K₂ + K₃ – ... ± Kₙ
Q5. Chelate complexes generally have:
- Lower stability constants than similar non-chelate complexes
- Higher stability constants due to the chelate effect
- The same stability as non-chelate complexes
- No stability at all
Q6. The instability constant is:
- The same as the stability constant
- The reciprocal of the stability constant
- Equal to the square of the stability constant
- Equal to the square root of the stability constant
Short Answer Questions
Q7. Define stability constant and instability constant. How are they related?
Q8. What is the chelate effect? Explain why [Ni(en)₃]²⁺ is more stable than [Ni(NH₃)₆]²⁺.
Q9. Write the expression for the overall stability constant (β₄) for the formation of [Cu(NH₃)₄]²⁺ from Cu²⁺ and NH₃.
Q10. The stability constant of [Ag(CN)₂]⁻ is very high (K ≈ 10²¹). What does this tell you about the stability of this complex?
Q11. Why do polydentate ligands generally form more stable complexes than monodentate ligands of similar donor strength?
Q12. The stepwise stability constants for [Cd(NH₃)₄]²⁺ are: K₁ = 300, K₂ = 100, K₃ = 50, K₄ = 20. Calculate the overall stability constant β₄.
Long Answer Questions
Q13. Explain the concept of stability constants in coordination chemistry. Discuss:
(a) Stepwise and overall stability constants
(b) The chelate effect and its thermodynamic origin
(c) Factors affecting the stability of coordination compounds
Q14. (a) For the formation of [Cu(NH₃)₄]²⁺, the stepwise stability constants are:
log K₁ = 4.0, log K₂ = 3.2, log K₃ = 2.7, log K₄ = 2.0
Calculate the overall stability constant β₄ and log β₄.
plain
(b) Explain why the stepwise stability constants decrease as more ligands are added.
Q15. (a) What is meant by the chelate effect? Explain with an example why chelate complexes are more stable.
(b) Compare the stability of [Ni(NH₃)₆]²⁺ and [Ni(en)₃]²⁺. Given that log β for [Ni(NH₃)₆]²⁺ is 8.6 and for [Ni(en)₃]²⁺ is 18.3, calculate the ratio of their stability constants. What does this ratio indicate?
Numerical / Application-Based Problems
Q16. The formation of [Ag(NH₃)₂]⁺ can be represented as:
Ag⁺ + 2NH₃ ⇌ [Ag(NH₃)₂]⁺
The overall stability constant β₂ = 1.6 × 10⁷.
(a) Write the expression for β₂.
(b) If 0.10 M AgNO₃ is mixed with 1.0 M NH₃, calculate the equilibrium concentration of free Ag⁺ ions. (Assume that almost all Ag⁺ is converted to the complex.)
(c) Explain why this complex is used in Tollens' reagent for silver mirror test.
(d) If NaCl is added to this solution, will AgCl precipitate? (Ksp of AgCl = 1.8 × 10⁻¹⁰.) Justify your answer with calculation.
Q17. Consider the following complexes and their log β values:
| Complex | log β |
|---|---|
| [Fe(CN)₆]⁴⁻ | 35 |
| [Fe(CN)₆]³⁻ | 43 |
| [FeF₆]³⁻ | 16 |
| [Fe(SCN)]²⁺ | 2.1 |
(a) Arrange these complexes in order of increasing stability.
(b) Which complex is the most stable? Which is the least stable?
(c) The very high stability of [Fe(CN)₆]⁴⁻ explains why it is non-toxic despite containing CN⁻ ions. Explain this paradox.
(d) Why is [Fe(SCN)]²⁺ much less stable than the cyano complexes?
(e) In qualitative analysis, Fe³⁺ is detected using SCN⁻ to form a blood-red complex. Why is this test sensitive even though [Fe(SCN)]²⁺ has a low stability constant?
Q18. In water treatment plants, EDTA is used to remove hardness caused by Ca²⁺ and Mg²⁺ ions. The stability constants are:
log β for [Ca(EDTA)]²⁻ = 10.7
log β for [Mg(EDTA)]²⁻ = 8.7
(a) Write the formation reaction for [Ca(EDTA)]²⁻ and its stability constant expression.
(b) Calculate the ratio of stability constants of the calcium complex to the magnesium complex.
(c) Explain why EDTA is particularly effective for removing hardness. (Consider the chelate effect.)
(d) If water contains 0.001 M Ca²⁺ and excess EDTA is added, calculate the approximate concentration of free Ca²⁺ ions at equilibrium.
(e) Why is it important to control pH when using EDTA for water softening?