Thermodynamic treatment of stability constants MCQs With Answer

Thermodynamic treatment of stability constants MCQs With Answer is a focused review for B. Pharm students exploring how thermodynamics explains metal–ligand complexation. This introduction links stability constants, formation (β) and stepwise constants, Gibbs free energy, enthalpy, entropy, van’t Hoff relationships, and conditional stability under physiological pH and ionic strength. Emphasis is on practical determination methods (potentiometry, spectrophotometry, ITC), interpretation of log K values, chelate effect, and relevance to drug formulation and metal–drug interactions. Clear conceptual understanding of ΔG°, ΔH°, ΔS°, activity effects, and common errors will boost exam readiness and lab competence. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is the thermodynamic relation between standard Gibbs free energy change and the equilibrium constant?

• ΔG° = RT ln K
• ΔG° = -RT ln K
• ΔG° = K / RT
• ΔG° = -K RT

Correct Answer: ΔG° = -RT ln K

Q2. Which quantity increases when a complex formation is entropy-driven?

• ΔH° becomes large and negative
• ΔS° is positive
• Gibbs free energy increases
• Equilibrium constant decreases

Correct Answer: ΔS° is positive

Q3. In the van’t Hoff equation ln K = -ΔH°/RT + ΔS°/R, what does a negative ΔH° imply about temperature dependence of K?

• K increases with temperature
• K decreases with temperature
• K is independent of temperature
• ΔS° must be negative

Correct Answer: K decreases with temperature

Q4. What is an overall formation constant (β) for a metal-ligand system?

• The ratio of free ligand to complex concentration
• The equilibrium constant for formation of a complex from metal and n ligands
• An instrumental calibration constant in spectrophotometry
• Activity coefficient for the metal ion

Correct Answer: The equilibrium constant for formation of a complex from metal and n ligands

Q5. How does ionic strength affect measured stability constants?

• Ionic strength has no effect on apparent constants
• Higher ionic strength reduces activity coefficients and can change apparent constants
• Ionic strength only affects instrumental noise
• Ionic strength always increases true thermodynamic K

Correct Answer: Higher ionic strength reduces activity coefficients and can change apparent constants

Q6. What is a conditional (or apparent) stability constant?

• A constant measured under specific pH and ionic strength conditions
• A theoretical constant at infinite dilution only
• The intrinsic constant corrected for activity coefficients
• A constant valid only for solid complexes

Correct Answer: A constant measured under specific pH and ionic strength conditions

Q7. Which experimental method directly measures heat change to determine ΔH° and K simultaneously?

• Spectrophotometry
• Isothermal titration calorimetry (ITC)
• Potentiometry
• Mass spectrometry

Correct Answer: Isothermal titration calorimetry (ITC)

Q8. What does the chelate effect describe?

• Formation of weaker complexes with polydentate ligands
• Enhanced stability of complexes formed by multidentate ligands compared to equivalent monodentate ligands
• Ligand decomposition during complexation
• Preference for ionic bonding over covalent bonding

Correct Answer: Enhanced stability of complexes formed by multidentate ligands compared to equivalent monodentate ligands

Q9. Which relation links pKd and stability constant K?

• pKd = -log K
• pKd = log K
• pKd = K
• pKd = 1/K

Correct Answer: pKd = -log K

Q10. How are stepwise formation constants (K1, K2, …) defined?

• Equilibrium constants for sequential addition of ligands to the metal
• Constants for dissociation into elemental metal
• Constants for ligand protonation only
• Rates of ligand exchange reactions

Correct Answer: Equilibrium constants for sequential addition of ligands to the metal

Q11. Which effect would you expect when temperature increases for an endothermic complexation (ΔH° > 0)?

• Equilibrium constant K decreases
• Equilibrium constant K increases
• No change in K
• ΔS° becomes negative

Correct Answer: Equilibrium constant K increases

Q12. Why are activity coefficients important in thermodynamic treatment of stability constants?

• They correct concentration to effective chemical activity for accurate thermodynamic K
• They measure reaction speed
• They are only needed for solids
• They are irrelevant in solution chemistry

Correct Answer: They correct concentration to effective chemical activity for accurate thermodynamic K

Q13. Which ligand property most strongly increases complex stability with a metal ion?

• Lower denticity
• Higher denticity (multidentate)
• Higher molecular weight only
• Greater volatility

Correct Answer: Higher denticity (multidentate)

Q14. Job’s method (method of continuous variations) is used to determine what?

• Stoichiometry of a metal–ligand complex
• Absolute Gibbs free energy
• pKa values of ligands
• Ionic strength of solution

Correct Answer: Stoichiometry of a metal–ligand complex

Q15. How is the conditional constant affected by pH when ligand has ionizable groups?

• Conditional constant is independent of pH
• Conditional constant varies with pH due to ligand protonation equilibria
• pH only affects metal concentration
• Conditional constant becomes equal to intrinsic K at any pH

Correct Answer: Conditional constant varies with pH due to ligand protonation equilibria

Q16. Which definition best describes Kf (formation constant) used in pharmaceutical complexes?

• Equilibrium constant for dissociation of complex
• Equilibrium constant for formation of complex from free metal and ligand
• Rate constant for complex formation
• Partition coefficient of complex

Correct Answer: Equilibrium constant for formation of complex from free metal and ligand

Q17. What is the practical importance of stability constants in drug formulation?

• Predicting color changes only
• Assessing metal binding in excipients, stability, and bioavailability
• Measuring pH meter calibration
• Determining melting point of drugs

Correct Answer: Assessing metal binding in excipients, stability, and bioavailability

Q18. Which statement about stepwise constants (K1, K2) and overall constant (β2) is correct?

• β2 = K1 + K2
• β2 = K1 × K2
• β2 = K1 / K2
• β2 = K2 – K1

Correct Answer: β2 = K1 × K2

Q19. Which instrument-based method is most sensitive for monitoring metal–ligand complex formation using changes in absorbance?

• Potentiometry
• Spectrophotometry (UV–Vis)
• Conductometry
• Gravimetry

Correct Answer: Spectrophotometry (UV–Vis)

Q20. According to Pearson’s HSAB concept, which ligand type prefers soft metal ions?

• Hard ligands (e.g., F−, OH−)
• Soft ligands (e.g., S donors, phosphines)
• Neutral ligands only
• Inert gases

Correct Answer: Soft ligands (e.g., S donors, phosphines)

Q21. When reporting thermodynamic K, what standard state is typically assumed?

• 1 M for solutes and pure solids/liquids for condensed phases
• 1 atm for all solutes
• 0 K standard state
• Activity of 0

Correct Answer: 1 M for solutes and pure solids/liquids for condensed phases

Q22. Which phenomenon can cause apparent stability constants to differ from intrinsic values in biological media?

• Ligand volatility
• Competition from other ligands and varying ionic strength and pH
• Pure theoretical calculations only
• Lack of any competing species

Correct Answer: Competition from other ligands and varying ionic strength and pH

Q23. In potentiometric determination of stability constants, which species is typically monitored?

• Optical absorbance
• Electrode potential related to free metal or proton concentration
• Mass of precipitate
• Viscosity changes

Correct Answer: Electrode potential related to free metal or proton concentration

Q24. What does a large positive log K indicate about a complex?

• The complex is very unstable
• The complex is very stable
• The reaction is nonspontaneous
• The complex is gaseous

Correct Answer: The complex is very stable

Q25. Which parameter directly indicates whether complex formation is spontaneous under standard conditions?

• Positive ΔG°
• Negative ΔG°
• Zero ΔH°
• Large molecular weight

Correct Answer: Negative ΔG°

Q26. How does formation of inner-sphere complexes typically differ from outer-sphere complexes?

• Inner-sphere involves direct ligand–metal bonding; outer-sphere involves electrostatic interaction with solvent-separated ions
• Inner-sphere is always weaker
• Outer-sphere requires covalent bonding
• There is no difference

Correct Answer: Inner-sphere involves direct ligand–metal bonding; outer-sphere involves electrostatic interaction with solvent-separated ions

Q27. Which term describes the equilibrium between protonated ligand forms and metal-bound forms affecting observed K?

• Redox potential
• Protonation equilibria or ligand speciation
• Boiling point elevation
• Dielectric constant

Correct Answer: Protonation equilibria or ligand speciation

Q28. Benesi–Hildebrand plots are used to analyze which type of data for stability constants?

• Direct calorimetric heats
• UV–Vis absorbance changes for 1:1 complexes
• pH titrations only
• Mass spectrometric fragmentation

Correct Answer: UV–Vis absorbance changes for 1:1 complexes

Q29. In chelation therapy, why is knowledge of stability constants important?

• To design ligands that form sufficiently strong complexes with toxic metals for excretion
• To make metal complexes more volatile
• To improve color of the therapy solution
• Stability constants are irrelevant in therapy

Correct Answer: To design ligands that form sufficiently strong complexes with toxic metals for excretion

Q30. Which of the following best describes an apparent formation constant measured without activity corrections?

• Intrinsic thermodynamic constant
• Conditional or apparent constant influenced by ionic strength
• Always larger than true constant
• Independent of concentration units

Correct Answer: Conditional or apparent constant influenced by ionic strength

Q31. Which factor commonly stabilizes metal–ligand complexes through entropy gain?

• Release of coordinated solvent molecules on complexation
• Decrease in translational freedom only
• Formation of a rigid structure with fewer degrees of freedom
• Absorption of heat from surroundings

Correct Answer: Release of coordinated solvent molecules on complexation

Q32. How does the charge of a metal ion affect stability constants with anionic ligands?

• Higher positive charge typically decreases stability
• Higher positive charge typically increases electrostatic attraction and stability
• Charge has no influence
• Anionic ligands repel highly charged metals

Correct Answer: Higher positive charge typically increases electrostatic attraction and stability

Q33. What is the main advantage of using spectrophotometric titration to find stability constants?

• It cannot detect minor species
• It provides sensitive monitoring of complex formation via absorbance changes
• It only works for colorless complexes
• It measures heat directly

Correct Answer: It provides sensitive monitoring of complex formation via absorbance changes

Q34. Which quantity is directly obtained from a van’t Hoff plot of ln K versus 1/T?

• ΔS° from slope
• ΔH° from slope and ΔS° from intercept
• ΔG° from intercept only
• Activity coefficients

Correct Answer: ΔH° from slope and ΔS° from intercept

Q35. In metal–ligand equilibria, what is meant by “competition”?

• Ligands compete with solvent for instrumental time
• Different ligands or ions compete to bind the same metal, affecting apparent stability
• Molecules compete to crystallize
• Competition refers to kinetic barriers only

Correct Answer: Different ligands or ions compete to bind the same metal, affecting apparent stability

Q36. Which method helps determine speciation and conditional constants in complex biological matrices?

• Simple gravimetric analysis only
• Combined techniques with modeling (speciation software using potentiometric and spectroscopic data)
• Pure theoretical estimation without experiments
• Flame photometry alone

Correct Answer: Combined techniques with modeling (speciation software using potentiometric and spectroscopic data)

Q37. If ΔG° = -5.7 kJ·mol−1 at 298 K, what is the approximate value of K (use R = 8.314 J·mol−1·K−1)?

• K ≈ 1
• K ≈ 10
• K ≈ 100
• K ≈ 0.1

Correct Answer: K ≈ 10

Q38. Which observation suggests a complex formation controlled mainly by enthalpy?

• K increases significantly with temperature
• K decreases with temperature and ΔH° is negative
• Entropy change is strongly positive
• No temperature dependence

Correct Answer: K decreases with temperature and ΔH° is negative

Q39. Why is EDTA commonly used as a standard ligand in stability constant studies?

• It is monodentate and weakly binding
• It is a well-characterized hexadentate chelator with predictable behavior
• It forms unstable complexes under all conditions
• It is only useful in gas phase studies

Correct Answer: It is a well-characterized hexadentate chelator with predictable behavior

Q40. Which is true for a complex with very large negative ΔG°?

• Complex formation is nonspontaneous
• Complex formation is highly spontaneous and favors product
• K will be close to zero
• ΔH° must be positive

Correct Answer: Complex formation is highly spontaneous and favors product

Q41. What does the term “formation quotient” refer to?

• Rate of complex formation
• Another name for the equilibrium formation constant β
• Quantity used only in gas equilibria
• Solubility product of a salt

Correct Answer: Another name for the equilibrium formation constant β

Q42. Which analytical technique can provide information on complex stoichiometry and mass directly?

• Mass spectrometry
• pH meter
• Paper chromatography
• Refractometry

Correct Answer: Mass spectrometry

Q43. How can protonation constants of a ligand affect its metal binding constant?

• Protonation reduces available binding sites and lowers conditional metal binding
• Protonation always increases metal binding
• Protonation has no effect
• Protonation converts ligand to gas

Correct Answer: Protonation reduces available binding sites and lowers conditional metal binding

Q44. What does a negative slope in a plot of log K versus ionic strength commonly indicate?

• Stability constant increases with ionic strength
• Stability constant decreases with ionic strength due to activity effects
• Ionic strength has no measurable effect
• Plot is unrelated to activity coefficients

Correct Answer: Stability constant decreases with ionic strength due to activity effects

Q45. Which is a limitation when determining stability constants by UV–Vis spectroscopy?

• Requires complexes to have distinct absorbance changes
• It measures heat, not absorbance
• It cannot be used with colored species
• It ignores pH effects entirely

Correct Answer: Requires complexes to have distinct absorbance changes

Q46. When are conditional stability constants most relevant in pharmacy?

• When modeling drug–metal interactions under physiological pH and ionic strength
• Only in vacuum conditions
• When measuring vapor pressure
• When ignoring biological media effects

Correct Answer: When modeling drug–metal interactions under physiological pH and ionic strength

Q47. Which factor can make a complex formation kinetically slow despite being thermodynamically favorable?

• Low activation energy
• High activation energy or ligand substitution barriers
• Very large negative ΔG°
• High temperature always speeds it up

Correct Answer: High activation energy or ligand substitution barriers

Q48. In data fitting to obtain stability constants, why is model selection important?

• Any model gives same constants regardless of data
• Correct model of stoichiometry and speciation ensures reliable K values
• Models are unnecessary with modern instruments
• Model selection only affects color of graphs

Correct Answer: Correct model of stoichiometry and speciation ensures reliable K values

Q49. What is the significance of measuring stepwise dissociation constants for a polydentate ligand?

• They provide insight into sequential ligand binding strength and cooperativity
• They are meaningless for polydentate ligands
• They only apply to monodentate ligands
• They equal the overall β raised to step number

Correct Answer: They provide insight into sequential ligand binding strength and cooperativity

Q50. Which approach best corrects apparent stability constants to thermodynamic (intrinsic) constants?

• Ignore ionic strength and use concentrations directly
• Apply activity coefficient corrections and account for protonation and competing equilibria
• Assume all species behave ideally without correction
• Use only visual inspection of spectra

Correct Answer: Apply activity coefficient corrections and account for protonation and competing equilibria

G S Sachin

I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.

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