Question 1

What is the number of electrons gained by Cl atom when n mole of ClO _4^- is transformed into n mole of ClO _2^- ion?

Accepted Answer

Let the oxidation state of Cl in ClO _4^- be x. x + 4(-2) = -1 x = +7 Let the oxidation state of Cl in ClO _2^- be y. y + 2(-2) = -1 y = +3 The change in oxidation state of Cl is from +7 to +3. This means each Cl atom gains 4 electrons during the transformation. For 1 mole of ClO _4^-, 4 moles of electrons are gained. Therefore, for n moles of ClO _4^-, the number of moles of electrons gained is 4n. Answer: 4n

Question 2

What is the oxidation number of Cl in HOClO _2?

Accepted Answer

Let the oxidation number of Cl in HOClO _2 (which is HClO _3) be x. The oxidation number of hydrogen ( H ) is +1 and that of oxygen ( O ) is -2. The sum of the oxidation numbers of all atoms in a neutral molecule is zero. +1 + x + 3(-2) = 0 x - 5 = 0 x = +5 The oxidation number of Cl in HOClO _2 is +5. Answer: +5

Question 3

What is the oxidation state of Xe in XeOF _4?

Accepted Answer

Let the oxidation state of Xe in XeOF _4 be x. The oxidation state of oxygen (O) is -2 and that of fluorine (F) is -1. Since the molecule is neutral, the sum of the oxidation states of all atoms must be zero. x + (-2) + 4(-1) = 0 x - 2 - 4 = 0 x - 6 = 0 x = +6 The oxidation state of Xe in XeOF _4 is +6. Answer: +6

Question 4

What is the difference in oxidation number of nitrogen when nitric acid is converted into nitrous oxide ?

Accepted Answer

The chemical formula of nitric acid is HNO_3. Let the oxidation state of nitrogen in HNO_3 be x. +1 + x + 3(-2) = 0 x = +5 The chemical formula of nitrous oxide is N_2O. Let the oxidation state of nitrogen in N_2O be y. 2y + (-2) = 0 y = +1 The difference in the oxidation number of nitrogen is 5 - 1 = 4. Answer: +4

Question 5

Identify change in oxidation state of oxidising agent in following redox reaction. 3 H _3 AsO _ 3(aq) + BrO _ 3(aq) ^- Br _ (aq) ^- + 3 H _3 AsO _ 4(aq)

Accepted Answer

The given reaction is 3 H _3 AsO _ 3(aq) + BrO _ 3(aq) ^- Br _ (aq) ^- + 3 H _3 AsO _ 4(aq) Let the oxidation state of As in H _3 AsO _3 be x. 3(+1) + x + 3(-2) = 0 x = +3 Let the oxidation state of Br in BrO _3^- be y. y + 3(-2) = -1 y = +5 Let the oxidation state of As in H _3 AsO _4 be z. 3(+1) + z + 4(-2) = 0 z = +5 The oxidation state of Br in Br ^- is -1. The oxidation state of As increases from +3 to +5, so H _3 AsO _3 undergoes oxidation and acts as a reducing agent. The oxidation state

Question 6

What is the oxidation number of carbon in oxalic acid ?

Accepted Answer

The chemical formula of oxalic acid is H_2C_2O_4. Let the oxidation number of carbon be x. The oxidation number of hydrogen is +1 and that of oxygen is -2. The sum of oxidation numbers of all atoms in a neutral molecule is zero. 2(+1) + 2(x) + 4(-2) = 0 2 + 2x - 8 = 0 2x - 6 = 0 2x = 6 x = +3 The oxidation number of carbon in oxalic acid is +3. Answer: +3

Question 7

What happens in the oxidation number of Mn during working of dry cell.

Accepted Answer

In a dry cell, the reaction at the cathode involves the reduction of manganese dioxide (MnO_2). The cathode reaction is: MnO_2 + NH_4^+ + e^- MnO(OH) + NH_3 The oxidation state of Mn in MnO_2 is +4. The oxidation state of Mn in MnO(OH) is +3. The oxidation number of Mn changes from +4 to +3. Thus, the oxidation number of Mn decreases by 1. Answer: decreases by 1

Question 8

Identify colour change when NaCl solution is titrated against AgNO _3 solution using fluorescein indicator.

Accepted Answer

The titration of NaCl against AgNO _3 using fluorescein indicator is an example of Fajan's method (precipitation titration). Fluorescein acts as an adsorption indicator. In the aqueous solution, it exists as the fluoresceinate anion, which imparts a pale yellow (or yellowish-green) colour to the solution. During the titration, AgCl precipitates. Before the equivalence point, the AgCl particles adsorb excess Cl ^- ions, acquiring a negative charge that repels the fluoresceinate anions. Just after

Question 9

Identify oxidation number of Sulphur respectively in SO _2 and SO _4^ 2- .

Accepted Answer

Let the oxidation number of Sulphur in SO _2 be x. x + 2(-2) = 0 x = +4 Let the oxidation number of Sulphur in SO _4^ 2- be y. y + 4(-2) = -2 y - 8 = -2 y = +6 The oxidation numbers of Sulphur in SO _2 and SO _4^ 2- are +4 and +6 respectively. Answer: +4, +6

Question 10

Which of the following reactions is NOT an example of redox reaction?

Accepted Answer

A redox reaction involves a change in the oxidation states of the elements involved. In the reaction Cl _2 + 2 H _2 O + SO _2 4 H ^+ + SO _4^ 2- + 2 Cl ^-, the oxidation state of Cl changes from 0 to -1 and S changes from +4 to +6. This is a redox reaction. In the reaction Cu ^ 2+ + Zn Cu + Zn ^ 2+ , the oxidation state of Cu changes from +2 to 0 and Zn changes from 0 to +2. This is a redox reaction. In the reaction 2 H _2 + O _2 2 H _2 O , the oxidation state of H changes from 0 to +1 and O cha

Question 11

Which of the following phenomena represents oxidation?

Accepted Answer

Oxidation is defined as the addition of oxygen or any other electronegative element, the removal of hydrogen or any other electropositive element, or the loss of electrons. Removal of oxygen represents reduction. Addition of an electronegative element represents oxidation. Addition of an electropositive element represents reduction. Gain of electrons represents reduction. Answer: Addition of electronegative element

Question 12

What is oxidation state of chlorine in chlorous acid?

Accepted Answer

The chemical formula of chlorous acid is HClO_2. Let the oxidation state of chlorine be x. The oxidation state of hydrogen is +1 and that of oxygen is -2. Since the molecule is neutral, the sum of the oxidation states of all atoms is zero. +1 + x + 2(-2) = 0 1 + x - 4 = 0 x - 3 = 0 x = +3 The oxidation state of chlorine in chlorous acid is +3. Answer: +3

Question 13

What is the oxidation number of oxygen in KO _2 ?

Accepted Answer

Let the oxidation number of oxygen in KO _2 be x. Potassium is an alkali metal, so its oxidation state is always +1 in its compounds. The sum of oxidation states of all atoms in a neutral molecule is zero. +1 + 2x = 0 2x = -1 x = -1/2 The oxidation number of oxygen in KO _2 is -1/2. Answer: -1/2

Question 14

For the following redox reaction, find the correct statement. Sn ^ 2+ + 2 Fe ^ 3+ Sn ^ 4+ + 2 Fe ^ 2+

Accepted Answer

The given reaction is Sn ^ 2+ + 2 Fe ^ 3+ Sn ^ 4+ + 2 Fe ^ 2+ The oxidation state of Sn increases from +2 to +4, which indicates that Sn ^ 2+ undergoes oxidation. The oxidation state of Fe decreases from +3 to +2, which indicates that Fe ^ 3+ undergoes reduction. Therefore, Sn ^ 2+ is undergoing oxidation. Answer: Sn ^ 2+ is undergoing oxidation.

Question 15

What is oxidation state of oxygen in OF _2 and in KO _2 respectively?

Accepted Answer

In OF _2, fluorine is more electronegative than oxygen and always has an oxidation state of -1. Let the oxidation state of oxygen in OF _2 be x. x + 2(-1) = 0 x = +2 In KO _2, potassium is an alkali metal and always has an oxidation state of +1. Let the oxidation state of oxygen in KO _2 be y. +1 + 2y = 0 y = -1/2 The oxidation states of oxygen in OF _2 and KO _2 are +2 and -1/2 respectively. Answer: +2 and -1/2

Question 16

Which of the following is NOT a redox reaction?

Accepted Answer

In the reaction Zn + H _2 SO _4 ZnSO _4 + H _2, the oxidation state of Zn changes from 0 to +2 and H changes from +1 to 0. It is a redox reaction. In the reaction Al(OH) _3 + 3 HCl AlCl _3 + 3 H _2 O , the oxidation states of all elements remain the same (Al is +3, O is -2, H is +1, Cl is -1). It is an acid-base neutralization reaction, not a redox reaction. In the reaction Mg + 1/2 O _2 Mg ^ 2+ + O ^ 2- , the oxidation state of Mg changes from 0 to +2 and O changes from 0 to -2. It is a redox r

Question 17

Find the oxidation number of Cl in ClO _4^ -1

Accepted Answer

Let the oxidation number of Cl be x. The oxidation number of oxygen is generally -2. The sum of oxidation numbers of all atoms in a polyatomic ion is equal to its net charge. x + 4(-2) = -1 x - 8 = -1 x = +7 Answer: +7

Redox Reactions — Previous Year Questions & Solutions