Redox potential MCQs With Answer are curated to help B.Pharm students master electrochemistry fundamentals relevant to pharmaceutical analysis, drug stability and metabolic pathways. This introduction and question set emphasizes standard reduction potentials, the Nernst equation, cell notation, potentiometric titrations, concentration and pH effects, and practical pharmaceutical implications like redox-active excipients and electrochemical assays. Questions progress from basic definitions to calculation-based and application-oriented problems so you can build problem-solving skills essential for formulation, analytical methods and understanding drug metabolism. Answers target core pharmacy topics to strengthen exam readiness and applied understanding. Now let’s test your knowledge with 50 MCQs on this topic.
Q1. What is redox potential (reduction potential)?
- The tendency of a species to gain electrons (be reduced) measured in volts
- The energy required to remove an electron from an atom measured in joules
- The rate at which an oxidation reaction occurs
- The concentration of oxidized species in solution
Correct Answer: The tendency of a species to gain electrons (be reduced) measured in volts
Q2. Which electrode is defined as having zero standard potential by convention?
- Silver/silver chloride electrode
- Calomel electrode
- Standard hydrogen electrode
- Platinum inert electrode
Correct Answer: Standard hydrogen electrode
Q3. A more positive standard reduction potential indicates which property?
- Stronger reducing agent
- Stronger oxidizing agent
- Greater tendency to lose electrons
- Lower electronegativity
Correct Answer: Stronger oxidizing agent
Q4. Which equation relates electrode potential to concentration, temperature and reaction quotient?
- Henderson-Hasselbalch equation
- Van’t Hoff equation
- Nernst equation
- Arrhenius equation
Correct Answer: Nernst equation
Q5. What is the relationship between standard Gibbs free energy change (ΔG°) and standard cell potential (E°cell)?
- ΔG° = +nFE°cell
- ΔG° = -nFE°cell
- ΔG° = nRTE°cell
- ΔG° = -RT ln(E°cell)
Correct Answer: ΔG° = -nFE°cell
Q6. In cell notation (anode | anode solution || cathode solution | cathode), which side represents the anode?
- The right side
- The left side
- Both sides simultaneously
- Neither side, anode is external
Correct Answer: The left side
Q7. In a galvanic cell, electrons flow from:
- Cathode to anode through the external circuit
- Anode to cathode through the external circuit
- The salt bridge to the cathode
- The solution to the electrodes
Correct Answer: Anode to cathode through the external circuit
Q8. When the concentration of the oxidized form increases for a reduction half-reaction, the electrode potential for reduction generally:
- Decreases (becomes more negative)
- Increases (becomes more positive)
- Remains unchanged
- Fluctuates randomly
Correct Answer: Increases (becomes more positive)
Q9. Standard cell potential E°cell is calculated as:
- E°anode – E°cathode
- E°cathode + E°anode
- E°cathode – E°anode
- The average of E° values
Correct Answer: E°cathode – E°anode
Q10. A positive cell potential (Ecell > 0) at standard conditions indicates that the reaction is:
- Non-spontaneous
- At equilibrium
- Spontaneous as a galvanic process
- Endothermic
Correct Answer: Spontaneous as a galvanic process
Q11. Which of the following best describes an oxidizing agent?
- A species that donates electrons and gets reduced
- A species that accepts electrons and gets reduced
- A substance that increases pH
- A catalyst for redox reactions
Correct Answer: A species that accepts electrons and gets reduced
Q12. The Nernst equation at 25°C can be commonly expressed as E = E° – (0.05916/n) log Q. What does the 0.05916 represent?
- RT/F multiplied by ln(10) at 298 K
- The Faraday constant in coulombs
- The standard potential of SHE
- The gas constant R at 25°C
Correct Answer: RT/F multiplied by ln(10) at 298 K
Q13. Which biological redox couple is commonly involved in drug metabolism and has a standard potential near -0.32 V at pH 7?
- FAD/FADH2
- NAD+/NADH
- O2/H2O
- Glutathione disulfide/glutathione
Correct Answer: NAD+/NADH
Q14. In acidic solution, permanganate ion (MnO4-) is reduced to which species?
- MnO4^2-
- MnO2
- Mn2+
- Mn3+
Correct Answer: Mn2+
Q15. In basic or neutral medium, permanganate (MnO4-) is commonly reduced to:
- Mn2+
- MnO2 (solid)
- MnO4^2-
- Mn3+
Correct Answer: MnO2 (solid)
Q16. Which of the following is the correct electron transfer number for the reduction MnO4- → Mn2+ in acidic medium?
- 1 electron
- 3 electrons
- 5 electrons
- 2 electrons
Correct Answer: 5 electrons
Q17. Which statement best describes a concentration cell?
- A cell where both electrodes are the same material but have different ion concentrations
- A cell with different electrode materials at equal concentrations
- A cell that uses concentration to heat the system
- A cell that operates without a salt bridge
Correct Answer: A cell where both electrodes are the same material but have different ion concentrations
Q18. The purpose of a salt bridge in an electrochemical cell is to:
- Increase the cell voltage
- Provide a pathway for electron flow
- Maintain electrical neutrality by ion migration
- Act as the electrode
Correct Answer: Maintain electrical neutrality by ion migration
Q19. Which parameter does NOT affect a standard reduction potential (E°) for a given half-cell?
- Temperature (if standard conditions are changed)
- Chemical identity of the species
- Physical state of the species (solid, aqueous, gas at standard state)
- Concentration of species under standard state definition (always 1 M)
Correct Answer: Concentration of species under standard state definition (always 1 M)
Q20. The standard reduction potential of Ag+/Ag is approximately:
- -0.76 V
- +0.34 V
- +0.80 V
- 0.00 V
Correct Answer: +0.80 V
Q21. Which electrode is commonly used as a convenient laboratory reference electrode with a well-defined potential?
- Standard hydrogen electrode
- Silver/silver chloride electrode (Ag/AgCl)
- Zinc electrode
- Iron electrode
Correct Answer: Silver/silver chloride electrode (Ag/AgCl)
Q22. For the half-reaction Cu2+ + 2e- → Cu, increasing [Cu2+] will cause the electrode potential (for reduction) to:
- Become more negative
- Become more positive
- Remain unchanged
- Reverse sign
Correct Answer: Become more positive
Q23. The relation between standard cell potential and equilibrium constant K is:
- E° = (RT/nF) ln K
- E° = -RT ln K
- E° = nF ln K
- E° = -nF ln K
Correct Answer: E° = (RT/nF) ln K
Q24. Which of the following best describes a disproportionation reaction?
- A reaction where an element changes oxidation state by ±1 only
- A reaction where two different elements exchange electrons
- A reaction in which the same element is simultaneously oxidized and reduced
- A reaction that involves only acids and bases
Correct Answer: A reaction in which the same element is simultaneously oxidized and reduced
Q25. Which of the following techniques directly measures redox potential of a solution?
- pH titration
- Potentiometry
- Gravimetry
- Chromatography
Correct Answer: Potentiometry
Q26. The Faraday constant (F) represents:
- The charge of one electron in coulombs
- The charge of one mole of electrons in coulombs
- The potential of the standard hydrogen electrode
- The number of electrons exchanged in a reaction
Correct Answer: The charge of one mole of electrons in coulombs
Q27. Which factor will shift a redox equilibrium that involves H+ ions?
- Change in ionic strength only
- Change in pH
- Change in inert gas atmosphere only
- Change in light intensity
Correct Answer: Change in pH
Q28. Which statement is true about stronger reducing agents in terms of reduction potentials?
- They have more positive standard reduction potentials
- They have more negative standard reduction potentials
- Their potentials are always zero
- They do not appear in electrochemical series
Correct Answer: They have more negative standard reduction potentials
Q29. In a redox titration using iodometry, the titrant commonly used is:
- Sodium thiosulfate
- Potassium permanganate
- Sodium hydroxide
- Ethanol
Correct Answer: Sodium thiosulfate
Q30. At equilibrium for a closed electrochemical cell, the cell potential (Ecell) is:
- Equal to E°cell
- Infinite
- Zero
- Negative of E°cell
Correct Answer: Zero
Q31. Which parameter must be known to convert ΔG° to E° at standard conditions?
- The equilibrium constant only
- The number of moles of solution
- The number of electrons transferred (n) and Faraday constant (F)
- The cell current
Correct Answer: The number of electrons transferred (n) and Faraday constant (F)
Q32. Which of the following redox indicators changes color at a characteristic potential and is used in titrations?
- Methyl orange
- Phenolphthalein
- Ferroin
- Bromothymol blue
Correct Answer: Ferroin
Q33. At 25°C, the Nernst equation is often expressed using 0.05916. For a one-electron process, how much does a tenfold change in Q change the potential?
- 0.05916 V
- 0.005916 V
- 0.5916 V
- 1.0 V
Correct Answer: 0.05916 V
Q34. Which concept explains why some drugs undergo oxidation faster under certain pH conditions?
- Buffer capacity
- pH dependence of redox potential when H+ participates in the half-reaction
- Solubility alone
- Surface tension changes
Correct Answer: pH dependence of redox potential when H+ participates in the half-reaction
Q35. What happens to the sign of a half-reaction potential when you reverse the half-reaction (i.e., from reduction to oxidation)?
- The numerical value doubles
- The sign remains the same
- The sign is inverted (multiplied by -1)
- The potential becomes zero
Correct Answer: The sign is inverted (multiplied by -1)
Q36. Which of the following is an example of an oxidizing agent in common pharmacy labs?
- Sodium thiosulfate
- Potassium iodide
- Potassium permanganate
- Sodium sulfite
Correct Answer: Potassium permanganate
Q37. The oxidation number of sulfur in H2SO4 is:
- +2
- +4
- +6
- -2
Correct Answer: +6
Q38. Which relationship describes the effect of temperature on electrode potential in the Nernst equation?
- Potential is independent of temperature
- Potential increases linearly with absolute temperature in the RT/nF term
- Potential decreases with temperature squared
- Potential oscillates with temperature
Correct Answer: Potential increases linearly with absolute temperature in the RT/nF term
Q39. In electrochemical series, which metal would generally be more easily oxidized?
- Metal with very positive reduction potential
- Metal with very negative reduction potential
- Metal with zero potential
- Non-metal
Correct Answer: Metal with very negative reduction potential
Q40. Which expression gives the cell potential for a concentration cell with identical electrodes?
- E = 0 always
- E = (RT/nF) ln([lower]/[higher])
- E = (RT/nF) ln([higher]/[lower])
- E = E°cathode – E°anode only
Correct Answer: E = (RT/nF) ln([higher]/[lower])
Q41. Which of the following best explains why electrode potentials are tabulated as reduction potentials?
- Because oxidation potentials are meaningless
- For consistency and ease of comparing oxidizing strengths
- Because reduction reactions are faster
- Because electrons cannot be measured in oxidation
Correct Answer: For consistency and ease of comparing oxidizing strengths
Q42. A drug susceptible to oxidation will have its stability predicted by comparing its oxidation potential to that of:
- Solvent boiling point
- Other species present in formulation (oxidants or reductants)
- Melting point of the API
- pH meter reading only
Correct Answer: Other species present in formulation (oxidants or reductants)
Q43. During a redox titration, the end point indicated by a redox indicator corresponds to:
- A pH change
- A characteristic electrode potential at which the indicator changes color
- A sudden change in temperature
- Complete precipitation of the analyte
Correct Answer: A characteristic electrode potential at which the indicator changes color
Q44. Which pair correctly relates electron flow and conventional current in an external circuit of a galvanic cell?
- Electrons flow from cathode to anode; conventional current flows from cathode to anode
- Electrons flow from anode to cathode; conventional current flows from cathode to anode
- Electrons do not flow; only ions move
- Electrons flow from anode to cathode; conventional current flows from anode to cathode
Correct Answer: Electrons flow from anode to cathode; conventional current flows from cathode to anode
Q45. The standard state conditions typically used for tabulating E° values are:
- 1 M concentration, 1 atm gas pressure, 25°C
- 0.1 M concentration, 1 atm, 0°C
- Pure solids only at any temperature
- Any concentration at room temperature
Correct Answer: 1 M concentration, 1 atm gas pressure, 25°C
Q46. Which of the following best describes why activities are used instead of concentrations in precise electrochemical calculations?
- Activities account for non-ideal behavior and ionic interactions in solution
- Activities are easier to measure than concentrations
- Concentrations cannot be used in Nernst equation
- Activities are always equal to molarity
Correct Answer: Activities account for non-ideal behavior and ionic interactions in solution
Q47. The potential of a half-cell measured against a reference electrode at equilibrium corresponds to which thermodynamic quantity?
- Free energy directly
- Electrical potential (electrochemical potential) related to chemical activities
- Entropy only
- Enthalpy directly
Correct Answer: Electrical potential (electrochemical potential) related to chemical activities
Q48. Which of the following is TRUE about redox-active excipients in formulations?
- They never affect drug stability
- They can promote oxidative degradation of APIs depending on redox potential
- They only affect color, not chemical stability
- They always act as antioxidants
Correct Answer: They can promote oxidative degradation of APIs depending on redox potential
Q49. In a cell where E°cell = +0.34 V and n = 2, increasing temperature will generally:
- Always decrease Ecell regardless of reaction
- Have no effect at all on Ecell
- Change the RT/nF term and may alter E under nonstandard conditions
- Convert the cell into an electrolytic cell immediately
Correct Answer: Change the RT/nF term and may alter E under nonstandard conditions
Q50. If ΔG for a redox reaction is negative, what can be said about the cell potential and spontaneity?
- Cell potential is negative and reaction is non-spontaneous
- Cell potential is positive and reaction is spontaneous
- Cell potential is zero and system is at equilibrium
- Cell potential cannot be determined from ΔG
Correct Answer: Cell potential is positive and reaction is spontaneous
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