75 free MCQs on Redox Reactions, each with its own worked answer and explanation. Master electron transfer in chemistry: assign oxidation states, balance half-reactions, identify oxidising and reducing agents, and connect redox to everyday reactions.
Take the timed Redox Reactions chapterwise test →75 practice questions on Redox Reactions, sorted Easy → Hard. Try each one first, then open its answer page for the worked explanation. Want the full theory first? Read the Redox Reactions notes.
Electron transfer in a redox reaction: zinc loses electrons (oxidised, acts as reducing agent) and copper(II) ions gain those same electrons (reduced, acts as oxidising agent).
Q1.
The oxidation number of oxygen in oxygen difluoride (OF2) is:
Q2.
In a redox reaction, oxidation involves:
Q3.
The oxidation number of hydrogen in sodium hydride (NaH) is:
Q4.
During a reaction, the reducing agent is itself:
Q5.
The oxidation number of oxygen in hydrogen peroxide (H2O2) is:
Q6.
Oxidation is defined as:
Q7.
Reduction is defined as:
Q8.
What is the oxidation state of oxygen in most compounds?
Q9.
What is the oxidation state of hydrogen in water (H2O)?
Q10.
In the reaction 2Mg + O2 → 2MgO, magnesium is:
Q11.
An oxidising agent:
Q12.
What is the oxidation state of a free element (e.g., O2, Fe, Cl2)?
Q14.
In the reaction Zn + CuSO4 → ZnSO4 + Cu, zinc is:
Q16.
Which of the following is a common oxidising agent?
Q18.
Rusting of iron is an example of:
Q19.
In a redox reaction, the substance that gets oxidised is called the:
Q20.
What is the oxidation state of fluorine in all its compounds?
Q21.
In the electrolytic decomposition of water: 2H2O → 2H2 + O2, oxygen is:
Q23.
A disproportionation reaction is one in which:
Q24.
In the reaction 2H2O2 → 2H2O + O2, H2O2 undergoes:
Q25.
Which of the following processes is an example of reduction?
Q26.
The oxidation number of manganese in the manganate ion MnO42− is:
Q27.
The average oxidation number of sulphur in sodium thiosulphate Na2S2O3 is:
Q28.
In acidic medium, for MnO4− → Mn2+, the number of electrons gained per MnO4− is:
Q29.
Among the halogens, the strongest oxidising agent is:
Q30.
The n-factor of oxalic acid acting as a reductant (C2O42− → 2CO2) is:
Q31.
What is the oxidation state of chromium in K2Cr2O7 (potassium dichromate)?
Q32.
Balance the redox half-reaction in acidic medium: MnO4- → Mn2+. How many H+ ions are needed?
Q33.
In the reaction: Cr2O72- + Fe2+ → Cr3+ + Fe3+ (acidic), how many Fe2+ ions are oxidised per one Cr2O72- ion?
Q35.
When Cu reacts with dilute H2SO4, no reaction occurs. But with concentrated H2SO4, the reaction gives:
Q36.
In the electrochemical series, which of the following is the strongest reducing agent?
Q37.
Balance the redox reaction in basic medium: MnO4- + I- → MnO2 + I2. The balanced equation is:
Q38.
In the reaction: Fe + dilute H2SO4 → FeSO4 + H2, the element that acts as the reducing agent is:
Q39.
What is the oxidation state of iron in Fe3O4?
Q40.
The reaction Cl2 + 2NaOH → NaCl + NaOCl + H2O is an example of:
Q41.
Which of the following can act as both an oxidising agent and a reducing agent?
Q43.
In the ion-electron method of balancing, when balancing the following half-reaction in acidic medium: Cr2O72- → Cr3+, how many electrons are transferred?
Q44.
In the reaction: 2F2 + 2H2O → 4HF + O2, fluorine is:
Q45.
What is the change in oxidation state of sulphur when H2S is converted to SO2?
Q46.
The standard electrode potential for the H+/H2 couple is:
Q47.
In the reaction: Zn + 2HCl → ZnCl2 + H2, identify the oxidising agent:
Q48.
What is the oxidation state of carbon in glucose (C6H12O6)?
Q49.
Bleaching powder decolourises coloured substances because:
Q50.
The method of balancing redox reactions that involves calculating the change in oxidation numbers and multiplying coefficients to make total increase = total decrease is called:
Q51.
In the balanced acidic-medium reaction MnO4− + Fe2+ + H+ → Mn2+ + Fe3+ + H2O, the mole ratio of MnO4− to Fe2+ is:
Q52.
The oxidation number of chromium in chromium pentoxide CrO5 (containing two peroxide linkages) is:
Q53.
The equivalent weight of KMnO4 (molar mass ≈ 158 g/mol) acting as an oxidant in acidic medium is:
Q54.
Chlorine exhibits an oxidation state of +5 in which of the following ions?
Q55.
For a spontaneous redox reaction, the standard cell potential E°cell and ΔG° must be, respectively:
Q56.
Balance the following redox reaction in acidic medium: MnO4- + C2O42- → Mn2+ + CO2. What are the stoichiometric coefficients (MnO4- : C2O42-)?
Q57.
The disproportionation of H3PO3 (phosphorous acid) on heating gives H3PO4 and:
Q58.
What is the oxidation state of sulphur in peroxomonosulphuric acid (Caro acid, H2SO5)?
Q59.
In the reaction: I2 + 2S2O32- → 2I- + S4O62-, the oxidation state of sulphur in S4O62- is:
Q60.
The comproportionation reaction (reverse of disproportionation) has the general form: A(higher OS) + A(lower OS) → A(intermediate OS). Which of the following is an example?
Q61.
In the permanganate titration of ferrous ions in acidic medium, the end point is determined by:
Q62.
What is the n-factor (equivalents per mole) of KMnO4 in neutral/faintly alkaline medium?
Q63.
Assign the oxidation state of each element in [Fe(CN)6]4- and determine the oxidation state of iron:
Q64.
Balance the redox reaction in basic medium: Cr3+ + H2O2 → CrO42- + H2O. What is the oxidising agent?
Q65.
The equivalent weight of K2Cr2O7 in acidic medium (where Cr goes from +6 to +3) is:
Q66.
In the reaction of F2 with water: F2 + H2O → HF + HOF, fluorine:
Q67.
In calculating n-factor for Na2S2O3 in its reaction with I2 (where products include Na2S4O6), the n-factor of Na2S2O3 is:
Q68.
What happens to the oxidising power of KMnO4 as the pH of the medium decreases (more acidic)?
Q69.
The oxidation state of nitrogen in NH2OH (hydroxylamine) is:
Q70.
During iodometric back-titration, Na2S2O3 is used to titrate liberated I2. The reaction is: 2S2O32- + I2 → S4O62- + 2I-. What is the role of Na2S2O3 here?
Q71.
The reaction: 5H2O2 + 2KMnO4 + 3H2SO4 → 2MnSO4 + K2SO4 + 5O2 + 8H2O shows that H2O2 is acting as:
Q72.
In the reaction: 3Cl2 + 6NaOH (hot and conc.) → 5NaCl + NaClO3 + 3H2O, which of the following statements is correct?
Q73.
A sample of iron ore weighing 1.00 g is dissolved in acid and all iron converted to Fe2+. The solution requires 25.0 mL of 0.0200 M KMnO4 for titration. The percentage of Fe in the ore is approximately:
Q74.
In photo-oxidation of silver halides during photography, the reaction Ag+ + e- → Ag involves:
Q75.
Using the standard electrode potentials E°(Fe3+/Fe2+) = +0.77 V and E°(I2/I-) = +0.54 V, predict whether Fe3+ will oxidise I- ions:
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