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 (OF<sub>2</sub>) 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 (H<sub>2</sub>O<sub>2</sub>) 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 (H<sub>2</sub>O)?
Q10.
In the reaction 2Mg + O<sub>2</sub> → 2MgO, magnesium is:
Q11.
An oxidising agent:
Q12.
What is the oxidation state of a free element (e.g., O<sub>2</sub>, Fe, Cl<sub>2</sub>)?
Q14.
In the reaction Zn + CuSO<sub>4</sub> → ZnSO<sub>4</sub> + Cu, zinc is:
Q15.
The oxidation state of sulphur in H<sub>2</sub>SO<sub>4</sub> is:
Q16.
Which of the following is a common oxidising agent?
Q17.
What is the oxidation state of nitrogen in NH<sub>3</sub>?
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: 2H<sub>2</sub>O → 2H<sub>2</sub> + O<sub>2</sub>, oxygen is:
Q22.
The oxidation state of manganese in KMnO<sub>4</sub> is:
Q23.
A disproportionation reaction is one in which:
Q24.
In the reaction 2H<sub>2</sub>O<sub>2</sub> → 2H<sub>2</sub>O + O<sub>2</sub>, H<sub>2</sub>O<sub>2</sub> undergoes:
Q25.
Which of the following processes is an example of reduction?
Q26.
The oxidation number of manganese in the manganate ion MnO<sub>4</sub><sup>2−</sup> is:
Q27.
The average oxidation number of sulphur in sodium thiosulphate Na<sub>2</sub>S<sub>2</sub>O<sub>3</sub> is:
Q28.
In acidic medium, for MnO<sub>4</sub><sup>−</sup> → Mn<sup>2+</sup>, the number of electrons gained per MnO<sub>4</sub><sup>−</sup> is:
Q29.
Among the halogens, the strongest oxidising agent is:
Q30.
The n-factor of oxalic acid acting as a reductant (C<sub>2</sub>O<sub>4</sub><sup>2−</sup> → 2CO<sub>2</sub>) is:
Q31.
What is the oxidation state of chromium in K<sub>2</sub>Cr<sub>2</sub>O<sub>7</sub> (potassium dichromate)?
Q32.
Balance the redox half-reaction in acidic medium: MnO<sub>4</sub><sup>-</sup> → Mn<sup>2+</sup>. How many H+ ions are needed?
Q33.
In the reaction: Cr<sub>2</sub>O<sub>7</sub><sup>2-</sup> + Fe<sup>2+</sup> → Cr<sup>3+</sup> + Fe<sup>3+</sup> (acidic), how many Fe<sup>2+</sup> ions are oxidised per one Cr<sub>2</sub>O<sub>7</sub><sup>2-</sup> ion?
Q34.
What is the oxidation state of phosphorus in H<sub>3</sub>PO<sub>4</sub>?
Q35.
When Cu reacts with dilute H<sub>2</sub>SO<sub>4</sub>, no reaction occurs. But with concentrated H<sub>2</sub>SO<sub>4</sub>, 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: MnO<sub>4</sub><sup>-</sup> + I- → MnO<sub>2</sub> + I<sub>2</sub>. The balanced equation is:
Q38.
In the reaction: Fe + dilute H<sub>2</sub>SO<sub>4</sub> → FeSO<sub>4</sub> + H<sub>2</sub>, the element that acts as the reducing agent is:
Q39.
What is the oxidation state of iron in Fe<sub>3</sub>O<sub>4</sub>?
Q40.
The reaction Cl<sub>2</sub> + 2NaOH → NaCl + NaOCl + H<sub>2</sub>O is an example of:
Q41.
Which of the following can act as both an oxidising agent and a reducing agent?
Q42.
The oxidation state of nitrogen in NH<sub>4</sub><sup>+</sup> ion is:
Q43.
In the ion-electron method of balancing, when balancing the following half-reaction in acidic medium: Cr<sub>2</sub>O<sub>7</sub><sup>2-</sup> → Cr<sup>3+</sup>, how many electrons are transferred?
Q44.
In the reaction: 2F<sub>2</sub> + 2H<sub>2</sub>O → 4HF + O<sub>2</sub>, fluorine is:
Q45.
What is the change in oxidation state of sulphur when H<sub>2</sub>S is converted to SO<sub>2</sub>?
Q46.
The standard electrode potential for the H+/H<sub>2</sub> couple is:
Q47.
In the reaction: Zn + 2HCl → ZnCl<sub>2</sub> + H<sub>2</sub>, identify the oxidising agent:
Q48.
What is the oxidation state of carbon in glucose (C<sub>6</sub>H<sub>12</sub>O<sub>6</sub>)?
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 MnO<sub>4</sub><sup>−</sup> + Fe<sup>2+</sup> + H<sup>+</sup> → Mn<sup>2+</sup> + Fe<sup>3+</sup> + H<sub>2</sub>O, the mole ratio of MnO<sub>4</sub><sup>−</sup> to Fe<sup>2+</sup> is:
Q52.
The oxidation number of chromium in chromium pentoxide CrO<sub>5</sub> (containing two peroxide linkages) is:
Q53.
The equivalent weight of KMnO<sub>4</sub> (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°<sub>cell</sub> and ΔG° must be, respectively:
Q56.
Balance the following redox reaction in acidic medium: MnO<sub>4</sub><sup>-</sup> + C<sub>2</sub>O<sub>4</sub><sup>2-</sup> → Mn<sup>2+</sup> + CO<sub>2</sub>. What are the stoichiometric coefficients (MnO<sub>4</sub><sup>-</sup> : C<sub>2</sub>O<sub>4</sub><sup>2-</sup>)?
Q57.
The disproportionation of H<sub>3</sub>PO<sub>3</sub> (phosphorous acid) on heating gives H<sub>3</sub>PO<sub>4</sub> and:
Q58.
What is the oxidation state of sulphur in peroxomonosulphuric acid (Caro acid, H<sub>2</sub>SO<sub>5</sub>)?
Q59.
In the reaction: I<sub>2</sub> + 2S<sub>2</sub>O<sub>3</sub><sup>2-</sup> → 2I- + S<sub>4</sub>O<sub>6</sub><sup>2-</sup>, the oxidation state of sulphur in S<sub>4</sub>O<sub>6</sub><sup>2-</sup> 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 KMnO<sub>4</sub> in neutral/faintly alkaline medium?
Q63.
Assign the oxidation state of each element in [Fe(CN)<sub>6</sub>]4- and determine the oxidation state of iron:
Q64.
Balance the redox reaction in basic medium: Cr<sup>3+</sup> + H<sub>2</sub>O<sub>2</sub> → CrO<sub>4</sub><sup>2-</sup> + H<sub>2</sub>O. What is the oxidising agent?
Q65.
The equivalent weight of K<sub>2</sub>Cr<sub>2</sub>O<sub>7</sub> in acidic medium (where Cr goes from +6 to +3) is:
Q66.
In the reaction of F<sub>2</sub> with water: F<sub>2</sub> + H<sub>2</sub>O → HF + HOF, fluorine:
Q67.
In calculating n-factor for Na<sub>2</sub>S<sub>2</sub>O<sub>3</sub> in its reaction with I<sub>2</sub> (where products include Na<sub>2</sub>S<sub>4</sub>O<sub>6</sub>), the n-factor of Na<sub>2</sub>S<sub>2</sub>O<sub>3</sub> is:
Q68.
What happens to the oxidising power of KMnO<sub>4</sub> as the pH of the medium decreases (more acidic)?
Q69.
The oxidation state of nitrogen in NH<sub>2</sub>OH (hydroxylamine) is:
Q70.
During iodometric back-titration, Na<sub>2</sub>S<sub>2</sub>O<sub>3</sub> is used to titrate liberated I<sub>2</sub>. The reaction is: 2S<sub>2</sub>O<sub>3</sub><sup>2-</sup> + I<sub>2</sub> → S<sub>4</sub>O<sub>6</sub><sup>2-</sup> + 2I-. What is the role of Na<sub>2</sub>S<sub>2</sub>O<sub>3</sub> here?
Q71.
The reaction: 5H<sub>2</sub>O<sub>2</sub> + 2KMnO<sub>4</sub> + 3H<sub>2</sub>SO<sub>4</sub> → 2MnSO<sub>4</sub> + K<sub>2</sub>SO<sub>4</sub> + 5O<sub>2</sub> + 8H<sub>2</sub>O shows that H<sub>2</sub>O<sub>2</sub> is acting as:
Q72.
In the reaction: 3Cl<sub>2</sub> + 6NaOH (hot and conc.) → 5NaCl + NaClO<sub>3</sub> + 3H<sub>2</sub>O, 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 Fe<sup>2+</sup>. The solution requires 25.0 mL of 0.0200 M KMnO<sub>4</sub> 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°(Fe<sup>3+</sup>/Fe<sup>2+</sup>) = +0.77 V and E°(I<sub>2</sub>/I-) = +0.54 V, predict whether Fe<sup>3+</sup> will oxidise I- ions:
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