Redox Reactions — 50 MCQs

1. Oxidation is defined as:

1. Gain of electrons
2. Loss of electrons ✅
3. Increase in mass
4. Formation of precipitate

2. Which species is reduced in the reaction: Cu2+ + Zn → Cu + Zn2+ ?

1. Zn
2. Cu2+ ✅
3. Zn2+
4. Cu

3. The oxidizing agent is the species that:

1. Donates electrons
2. Is always a metal
3. Accepts electrons ✅
4. Is always a gas

4. Assign oxidation number of S in H2S:

1. -1
2. -2 ✅
3. +2
4. 0

5. In the half-reaction method in acidic medium, which species are used to balance O and H?

1. H2 and O2
2. OH- and H2O
3. H2O and H+ ✅
4. Na+ and Cl-

6. Balanced redox reaction: MnO4- + Fe2+ → Mn2+ + Fe3+ in acidic medium, which is the oxidizing agent?

1. Fe2+
2. MnO4- ✅
3. Mn2+
4. Fe3+

7. Which of the following elements has oxidation state +2 in its common ion?

1. Cl
2. Ca ✅
3. O
4. F

8. In electrochemical cell notation, which side is the anode?

1. Right side
2. Left side ✅
3. Depends on concentration only
4. Neither

9. Standard cell potential E°cell is defined as:

1. E°anode - E°cathode
2. E°cathode - E°anode ✅
3. Sum of electrode potentials
4. Always zero

10. Which metal will be oxidized (react) with HCl: Cu, Zn, Ag?

1. Cu and Ag
2. Zn ✅
3. Ag only
4. All of them

11. In the reaction 2Cl- → Cl2 + 2e-, chlorine is being:

1. Reduced
2. Oxidized ✅
3. Neutralized
4. Hydrolyzed

12. The species acting as reducing agent in Zn + 2H+ → Zn2+ + H2 is:

1. H+
2. Zn ✅
3. Zn2+
4. H2

13. Which statement about electrolytic cells is TRUE?

1. They produce electrical energy
2. They use electrical energy to drive non-spontaneous reactions ✅
3. They always have positive E°cell
4. They do not involve electron transfer

14. Faraday’s constant F equals approximately:

1. 96.5 C·mol⁻¹ ✅
2. 8.314 J·mol⁻¹·K⁻¹
3. 1.602×10^-19 C
4. 0.05916 V

15. In Daniell cell (Zn|Zn2+||Cu2+|Cu), which electrode is cathode?

1. Zn
2. Cu ✅
3. Salt bridge
4. Both are cathodes

16. The Nernst equation can be used to calculate:

1. Standard potentials only
2. Cell potential under non-standard conditions ✅
3. Equilibrium constant from ΔH only
4. Mass of product in electrolysis without current

17. Which half-reaction will occur at the cathode during electrolysis of molten NaCl?

1. Na → Na+ + e-
2. Na+ + e- → Na ✅
3. Cl2 → Cl- + e-
4. Cl- + e- → Cl2

18. Standard reduction potential for hydrogen electrode (SHE) is defined as:

1. +1.00 V
2. 0.00 V ✅
3. -0.76 V
4. 0.59 V

19. Which metal is more easily oxidized: Mg or Cu?

1. Cu
2. Mg ✅
3. Both equal
4. Depends on concentration only

20. In balancing redox in basic medium, after adding H2O and H+ you must:

1. Leave H+ as is
2. Add OH- to both sides to neutralize H+ ✅
3. Convert OH- to H2O only on one side
4. Multiply entire equation by 2

21. The reaction 2Ag+ + Cu → 2Ag + Cu2+ is spontaneous because:

1. Ag has more positive reduction potential than Cu ✅
2. Cu has more positive reduction potential than Ag
3. Both have same potential
4. No electron transfer occurs

22. Which of these processes increases oxidation number of an element?

1. Gain of electrons
2. Loss of electrons ✅
3. Formation of complex by donation of lone pair
4. Precipitation

23. The number of electrons transferred in reaction: Cr2O7^2- + 14H+ + 6e- → 2Cr3+ + 7H2O is:

1. 2
2. 6 ✅
3. 3
4. 0

24. Which electrochemical cell converts chemical energy into electrical energy?

1. Electrolytic cell
2. Galvanic (voltaic) cell ✅
3. Both
4. Neither

25. In electroplating, metal is deposited at the:

1. Anode
2. Cathode ✅
3. Salt bridge
4. Both electrodes equally

26. The sign of ΔG° and E°cell are related by:

1. ΔG° = +nFE°cell
2. ΔG° = -nFE°cell ✅
3. ΔG° = nFE°cell / T
4. No relation

27. Which halogen is the strongest oxidizing agent in aqueous solution?

1. Iodine
2. Chlorine
3. Fluorine ✅
4. Bromine

28. Electrolysis of aqueous NaCl produces at the anode:

1. H2 gas
2. Cl2 gas (in concentrated brine) ✅
3. Na metal
4. O2 gas only

29. If E°cell is negative, the reaction as written is:

1. Spontaneous as written
2. Non-spontaneous as written ✅
3. At equilibrium
4. Impossible

30. Which of the following increases during galvanic cell discharge?

1. Mass of anode
2. Mass of cathode (metal deposited) ✅
3. Potential energy of system
4. Concentration of oxidized species at anode decreases

31. In the Nernst equation at 25°C, the term (0.05916/n) appears; what does n represent?

1. Number of moles of reactant
2. Number of electrons transferred in the cell reaction ✅
3. Number of species in reaction
4. Valency of metal only

32. Which metal is least likely to be oxidized (most noble) among: K, Fe, Ag, Al?

1. K
2. Fe
3. Ag ✅
4. Al

33. During electrolysis, the amount of substance produced is proportional to:

1. Voltage only
2. Total charge passed (current × time) ✅
3. Temperature only
4. Concentration only

34. Which ion will be preferentially discharged at the cathode in aqueous solution containing Cu2+ and H+ under typical conditions?

1. H+
2. Cu2+ (if its reduction potential is higher) ✅
3. Both equally
4. Neither

35. Which of the following statements is TRUE about corrosion of iron?

1. It is purely chemical with no electron transfer
2. It is an electrochemical oxidation process accelerated by moisture and oxygen ✅
3. It only occurs at high temperatures
4. It can be reversed easily without energy input

36. The activity series helps predict:

1. Solubility of salts
2. Which metals will displace others from solution ✅
3. Acid strength order
4. Boiling points

37. In the reaction Ag+ + e- → Ag, E° = +0.80 V. This means Ag+ is:

1. Strong reducing agent
2. Good oxidizing agent (tends to be reduced) ✅
3. Neutral species
4. Unreactive

38. Which statement about standard electrode potentials is TRUE?

1. They depend on concentration
2. They are measured under standard conditions and tabulated as reduction potentials ✅
3. They are always negative for metals
4. They depend on cell size

39. Which of the following is reduced in the reaction: Cl2 + 2e- → 2Cl- ?

1. e-
2. Cl2 ✅
3. Cl-
4. Nothing

40. In galvanic cell notation, a single vertical bar '|' indicates:

1. Salt bridge
2. Phase boundary (e.g., electrode | electrolyte) ✅
3. Electrode potential sign
4. Concentration change

41. Which product is formed at the anode during electrolysis of water using inert electrodes?

1. Hydrogen gas
2. Oxygen gas ✅
3. Metallic sodium
4. Chlorine gas

42. If a redox reaction involves transfer of 2 electrons per mole of reaction and E°cell = 1.10 V, ΔG° (kJ·mol⁻¹) is approximately:

1. -212 kJ·mol⁻¹
2. -212 kJ·mol⁻¹ ✅ (ΔG° = -nFE°, n=2, F≈96485 C·mol⁻¹)
3. +212 kJ·mol⁻¹
4. 0 kJ·mol⁻¹

43. Corrosion prevention by cathodic protection works by:

1. Making metal the anode
2. Making metal the cathode so it does not oxidize ✅
3. Increasing oxygen concentration
4. Heating the metal

44. Which species will be oxidized preferentially at the anode in aqueous solution containing Cl- and Br-?

1. Br- (more easily oxidized)
2. Cl- (if its oxidation potential is lower under conditions) ✅
3. Both equally
4. Neither

45. Which of these statements about standard hydrogen electrode (SHE) is FALSE?

1. It is assigned 0.00 V by convention
2. It is used as a reference that depends on temperature and pressure and cannot be standardized ✅
3. It involves H2 gas and 1 M H+ solution
4. It serves as a reference for tabulating standard potentials

46. Which process is NOT a redox reaction?

1. Zn + Cu2+ → Zn2+ + Cu
2. Fe2+ → Fe3+ + e-
3. Precipitation of BaSO4 from Ba2+ + SO4^2- ✅ (no change in oxidation states)
4. Combustion of methane

47. In a cell where E°cell = 0.34 V and Q = 1, the cell is:

1. At equilibrium
2. Spontaneous under standard conditions ✅
3. Non-spontaneous
4. Explosive

48. The electrochemical equivalent (mass deposited per coulomb) depends on:

1. Voltage only
2. Charge on ion and molar mass (via Faraday's laws) ✅
3. Temperature only
4. Concentration only

49. Which of the following increases the cell potential for a redox reaction?

1. Increasing temperature always
2. Changing concentrations to favor products (per Nernst equation) ✅
3. Adding inert gas at constant pressure
4. Adding catalyst