Relationship between free energy, enthalpy and entropy MCQs With Answer

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This concise introduction explains the relationship between free energy, enthalpy and entropy for B. Pharm students preparing for exams and competitive tests. Understanding Gibbs free energy (ΔG = ΔH − TΔS), enthalpy changes (ΔH), and entropy changes (ΔS) is vital in pharmaceutical chemistry to predict reaction spontaneity, drug stability, and formulation behavior. This topic links thermodynamics, equilibrium constants, temperature dependence, and entropy of the universe—key concepts in drug design and quality control. Clear conceptual mastery and practice with targeted MCQs improve problem-solving and exam performance. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. Which equation correctly relates Gibbs free energy to enthalpy and entropy?

  • ΔG = ΔH + TΔS
  • ΔG = ΔH − TΔS
  • ΔG = TΔS − ΔH
  • ΔG = ΔH/ΔS

Correct Answer: ΔG = ΔH − TΔS

Q2. For a process at constant temperature and pressure, a negative ΔG indicates:

  • The process is non-spontaneous
  • The process is at equilibrium
  • The process is spontaneous
  • The process is exothermic only

Correct Answer: The process is spontaneous

Q3. Which quantity is a measure of disorder or energy dispersal in a system?

  • Enthalpy (ΔH)
  • Gibbs free energy (ΔG)
  • Entropy (ΔS)
  • Work (w)

Correct Answer: Entropy (ΔS)

Q4. At constant pressure, the heat exchanged with surroundings equals:

  • ΔG
  • ΔS
  • ΔH
  • −ΔH

Correct Answer: ΔH

Q5. For an exothermic reaction with ΔH < 0 and ΔS < 0, spontaneity depends on:

  • Low temperature
  • High temperature
  • Temperature has no effect
  • Pressure only

Correct Answer: Low temperature

Q6. The standard free energy change ΔG° is related to the equilibrium constant K by:

  • ΔG° = RT ln K
  • ΔG° = −RT ln K
  • ΔG° = R ln K
  • ΔG° = −R/K

Correct Answer: ΔG° = −RT ln K

Q7. If ΔG = 0 for a reaction at a given temperature, then:

  • The reaction proceeds spontaneously forward
  • The reaction proceeds spontaneously backward
  • The reaction is at equilibrium
  • The reaction is irreversible

Correct Answer: The reaction is at equilibrium

Q8. The entropy change of the surroundings at constant pressure is given by:

  • ΔSsurr = ΔHsys/T
  • ΔSsurr = −ΔHsys/T
  • ΔSsurr = ΔGsys/T
  • ΔSsurr = −ΔGsys/T

Correct Answer: ΔSsurr = −ΔHsys/T

Q9. Which of the following is a state function?

  • Work (w)
  • Heat (q)
  • Enthalpy (H)
  • Path of reaction

Correct Answer: Enthalpy (H)

Q10. For melting (solid → liquid) at the melting point, which statement is true?

  • ΔS = 0 and ΔG < 0
  • ΔG = 0 and ΔH = TΔS
  • ΔH = 0 and ΔS > 0
  • ΔG > 0 and ΔH < 0

Correct Answer: ΔG = 0 and ΔH = TΔS

Q11. Which term represents the maximum non-expansion work obtainable from a system at constant T and P?

  • ΔH
  • ΔG
  • ΔS
  • ΔA (Helmholtz free energy)

Correct Answer: ΔG

Q12. When ΔH > 0 and ΔS > 0, the reaction is spontaneous at:

  • All temperatures
  • No temperatures
  • High temperatures
  • Low temperatures

Correct Answer: High temperatures

Q13. Which of the following correctly describes Hess’s law in relation to enthalpy?

  • ΔH depends on the path
  • Total ΔH for a reaction is independent of the route
  • ΔH is directly proportional to entropy
  • ΔH equals ΔG at all times

Correct Answer: Total ΔH for a reaction is independent of the route

Q14. The unit of entropy in SI is:

  • J
  • J K−1
  • J K−1 mol−1
  • J mol−1

Correct Answer: J K−1 mol−1

Q15. For a chemical reaction, ΔG° = +5.7 kJ mol−1 at 298 K. The equilibrium constant K is:

  • Greater than 1
  • Equal to 1
  • Less than 1
  • Infinite

Correct Answer: Less than 1

Q16. The sign of ΔS for the vaporization of a liquid is typically:

  • Negative
  • Zero
  • Positive
  • Undefined

Correct Answer: Positive

Q17. Which free energy is most useful at constant volume and temperature?

  • Gibbs free energy (G)
  • Enthalpy (H)
  • Helmholtz free energy (A or F)
  • Entropy (S)

Correct Answer: Helmholtz free energy (A or F)

Q18. If ΔG° for a reaction is highly negative, the equilibrium lies:

  • Towards reactants
  • Close to 50:50
  • Towards products
  • At unstable state

Correct Answer: Towards products

Q19. Which relationship describes temperature dependence of ΔG for a reaction if ΔH and ΔS are constant?

  • ΔG increases linearly with T
  • ΔG is independent of T
  • ΔG decreases linearly with T as ΔG = ΔH − TΔS
  • ΔG varies quadratically with T

Correct Answer: ΔG decreases linearly with T as ΔG = ΔH − TΔS

Q20. The entropy change of the universe for a spontaneous process is:

  • Negative
  • Zero
  • Positive
  • Undefined

Correct Answer: Positive

Q21. Which is true for a reversible process at equilibrium?

  • ΔSuniv > 0
  • ΔSuniv = 0
  • ΔSuniv < 0
  • ΔSsys is maximum

Correct Answer: ΔSuniv = 0

Q22. For a reaction where ΔH = −40 kJ mol−1 and ΔS = −100 J K−1 mol−1, ΔG at 298 K is approximately:

  • −10.2 kJ mol−1
  • +9.8 kJ mol−1
  • −70 kJ mol−1
  • +40 kJ mol−1

Correct Answer: −10.2 kJ mol−1

Q23. Which process increases entropy the most:

  • Freezing water
  • Condensing steam
  • Dissolving a salt into water
  • Compressing a gas

Correct Answer: Dissolving a salt into water

Q24. The molar entropy of an ideal gas increases with:

  • Decrease in temperature
  • Decrease in volume
  • Increase in temperature and volume
  • No change with T or V

Correct Answer: Increase in temperature and volume

Q25. In pharmaceutical formulations, predicting stability often requires knowledge of:

  • Only enthalpy changes
  • Only entropy changes
  • Gibbs free energy changes
  • Only equilibrium constant K

Correct Answer: Gibbs free energy changes

Q26. Which statement about ΔH°f (standard enthalpy of formation) is correct?

  • It is zero for all elements in their standard states
  • It varies arbitrarily for elements in standard states
  • It is always positive for compounds
  • It equals ΔG°f for compounds

Correct Answer: It is zero for all elements in their standard states

Q27. For a reaction where ΔS > 0 and ΔH < 0, the reaction is:

  • Always non-spontaneous
  • Spontaneous at all temperatures
  • Spontaneous only at low temperatures
  • Spontaneous only at high pressures

Correct Answer: Spontaneous at all temperatures

Q28. The relation ΔG = ΔG° + RT ln Q uses Q, which is:

  • The reaction quotient at current conditions
  • The equilibrium constant
  • The partition function
  • The heat capacity

Correct Answer: The reaction quotient at current conditions

Q29. Which thermodynamic function is minimized at constant T and V for spontaneous change?

  • Gibbs free energy (G)
  • Helmholtz free energy (A)
  • Enthalpy (H)
  • Internal energy (U)

Correct Answer: Helmholtz free energy (A)

Q30. Entropy change for an ideal gas during isothermal expansion is given by:

  • ΔS = nR ln(V2/V1)
  • ΔS = nC_v ln(T2/T1)
  • ΔS = ΔH/T
  • ΔS = 0

Correct Answer: ΔS = nR ln(V2/V1)

Q31. Which factor does NOT directly appear in the ΔG = ΔH − TΔS expression?

  • Temperature
  • Entropy
  • Pressure
  • Enthalpy

Correct Answer: Pressure

Q32. For a spontaneous biochemical reaction in a cell at constant T and P, the sign of ΔG must be:

  • Positive
  • Zero
  • Negative
  • Variable without significance

Correct Answer: Negative

Q33. When calculating ΔG° from tabulated ΔG°f values, the correct procedure is:

  • Add product ΔG°f and subtract reactant ΔG°f
  • Add reactant ΔG°f and subtract product ΔG°f
  • Only sum reactant ΔG°f
  • Only use ΔH values

Correct Answer: Add product ΔG°f and subtract reactant ΔG°f

Q34. Which effect increases the spontaneity of an endothermic reaction with positive ΔS?

  • Lowering temperature
  • Increasing temperature
  • Removing products
  • Decreasing entropy

Correct Answer: Increasing temperature

Q35. Standard state for gases in thermodynamic tables is usually taken at:

  • 1 atm pressure
  • 1 Pa pressure
  • 1 M concentration for gases
  • 0 K temperature

Correct Answer: 1 atm pressure

Q36. The spontaneity criterion at constant T and P is that ΔG must be:

  • Maximized
  • Minimized
  • Equal to ΔH
  • Equal to ΔS

Correct Answer: Minimized

Q37. Which of the following best explains why freezing can be spontaneous at low temperatures despite ΔS < 0?

  • ΔH becomes sufficiently negative so ΔG < 0
  • Entropy of universe decreases
  • Pressure increases dramatically
  • Temperature dependence is irrelevant

Correct Answer: ΔH becomes sufficiently negative so ΔG < 0

Q38. In the relation ΔG° = −RT ln K, as K increases, ΔG° becomes:

  • More positive
  • More negative
  • Zero
  • Unchanged

Correct Answer: More negative

Q39. Which statement about entropy and molecular complexity is generally true?

  • Simpler molecules have higher entropy than complex ones
  • Larger and more complex molecules tend to have higher molar entropy
  • Entropy is independent of molecular structure
  • All gases have lower entropy than liquids

Correct Answer: Larger and more complex molecules tend to have higher molar entropy

Q40. For a reversible isothermal process, the change in entropy is equal to:

  • q_rev/T
  • q_irrev/T
  • ΔH/T only for solids
  • 0 always

Correct Answer: q_rev/T

Q41. Which of the following is TRUE about ΔG° and reaction spontaneity under standard conditions?

  • ΔG° < 0 guarantees reaction is spontaneous under any conditions
  • ΔG° > 0 means reaction cannot proceed under any conditions
  • ΔG° indicates spontaneity under standard conditions only
  • ΔG° equals reaction rate

Correct Answer: ΔG° indicates spontaneity under standard conditions only

Q42. The contribution of entropy to spontaneity increases with:

  • Lower temperature
  • Higher temperature
  • Lower ΔS
  • Constant ΔH

Correct Answer: Higher temperature

Q43. Which of the following is an extensive property?

  • Temperature
  • Pressure
  • Enthalpy (H)
  • Density

Correct Answer: Enthalpy (H)

Q44. A catalyst affects which of the following?

  • ΔG of reaction
  • Equilibrium constant K
  • Activation energy and reaction rate
  • ΔH and ΔS values

Correct Answer: Activation energy and reaction rate

Q45. For dissolution where ionic solute dissociates into many particles, entropy change is typically:

  • Negative due to ordering
  • Positive due to increased dispersal
  • Zero because energy is conserved
  • Undefined

Correct Answer: Positive due to increased dispersal

Q46. Which expression gives ΔS for a reversible heating of a substance from T1 to T2?

  • ΔS = ∫C_p dT/T from T1 to T2
  • ΔS = C_p (T2 − T1)
  • ΔS = ΔH/T1
  • ΔS = −ΔG/T

Correct Answer: ΔS = ∫C_p dT/T from T1 to T2

Q47. In pharmaceutical reactions coupled to ATP hydrolysis, the coupling works because ATP hydrolysis has:

  • A large positive ΔG
  • A large negative ΔG
  • No ΔG change
  • Only enthalpic change

Correct Answer: A large negative ΔG

Q48. If ΔG for a reaction is −20 kJ mol−1 at 298 K, the reaction quotient Q compared to K is:

  • Q > K
  • Q = K
  • Q < K
  • Q = 0

Correct Answer: Q < K

Q49. Which statement is correct about entropy at absolute zero according to the third law?

  • Entropy of a perfect crystal is zero
  • Entropy is infinite
  • Entropy depends on pressure
  • Entropy is maximal

Correct Answer: Entropy of a perfect crystal is zero

Q50. For a pharmaceutical degradation reaction, measuring ΔH and ΔS allows prediction of:

  • Only reaction kinetics
  • Temperature range over which degradation is spontaneous
  • Chemical structure changes
  • Molecular weight of products

Correct Answer: Temperature range over which degradation is spontaneous

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