Chapter 8 - Redox Reactions

Assertion (A): In the reaction between potassium permanganate and
potassium iodide, permanganate ions act as oxidising agent.
Reason (R) : Oxidation state of manganese changes from +2 to +7 during
the reaction.
(i) Both A and R are true and R is the correct explanation of A.
(ii) Both A and R are true but R is not the correct explanation of A.
(iii) A is true but R is false.
(iv) Both A and R are false.

In the following questions a statement of assertion (A) followed by a statement
of reason (R) is given. Choose the correct option out of the choices given
below each question.

 Assertion (A) : Among halogens fluorine is the best oxidant.
Reason (R) : Fluorine is the most electronegative atom.
(i) Both A and R are true and R is the correct explanation of A.
(ii) Both A and R are true but R is not the correct explanation of A.
(iii) A is true but R is false.
(iv) Both A and R are false.

Match the items in Column I with relevant items in Column II.
Column I                                                                           Column II
(i) Ions having positive charge                                                  (a) +7
(ii) The sum of oxidation number                                                (b) –1
of all atoms in a neutral molecule                                                 (c) +1
(iii) Oxidation number of hydrogen ion                                         (H+) (d) 0
(iv) Oxidation number of fluorine in NaF                                      (e) Cation
(v) Ions having negative charge                                                (f) Anion

Define a redox couple.

Which reaction occurs at cathode in a galvanic cell?

What is the electrode potential of a standard hydrogen electrode?

If reduction potential of an electrode is 1.28V. What will be its oxidation
potential?

The E⁰ of Cu²⁺ / Cu is + 0.34V. What does it signify?

An electrochemical cell is constituted by combining Al electrode (E⁰ = - 1.66v)

and Cu electrode (E⁰ = + 0.34v). Which of these electrodes will work as

cathode and why?

What is the role of a salt bridge in an electro chemical cell?

Can we store copper sulphate in an iron vessel?

Set up an electrochemical cell for the redox reaction

Ni²⁺ (aq) + Fe(S) → Ni(S) + Fe²⁺ (aq)

Define half – cell.

All decomposition reactions are not redox reactions. Give reason.

Which gas is produced when less reactive metals like Mg and Fe react with
steam?

Why ClO₄

-does not show disproportionation reaction where as ClO-, ClO₂- ,ClO₃- shows?

Define disproportionation reaction.

Why F- ions Cannot be converted to F₂ by chemical means?

Which is the strongest oxidizing agent?

The displacement reactions of Cl, Br, I using fluorine are not generally
carried out in aqueous solution. Give reason.

Name the different types of reductions.

Name the different types of reductions.    

Find the oxidation state of sulphur in the following compounds : H₂S, H₂SO₄, S2O₄^2- S₂O₈²- and HSO_3-.

A metal ion M³⁺ loses 3 electrons. What will be its oxidation number?

Indicate the oxidizing and reducing agents in the following reaction :

2Cu²⁺ + 4I- → 2CuI + I₂.is the oxidation number of Mn in KMnO₄?

Name one compound in which oxidation number of Cl is + 4.

What happens to the oxidation number of an element in oxidation?

What is the oxidation number of Mn in KMnO₄?

What is meant by reducing? Name the best reducing agent.

Define an oxidizing agent. Name the best reducing agent

What is meant by reduction?

Define oxidation in terms of electron transfer.

Explain why

3Fe₃O₄ (S) +8Al(S) → 9Fe (S) +4Al₂O₃

Is an oxidation reaction. ?

Why the following reaction is an example of oxidation reaction?

CH₄ (g) +2O₂ (g) →CO₂ (g) + 2H₂O

In the reaction

MnO₂ + 4HCl →  Mn Cl₂ + Cl₂ +2H₂ O

Which species is oxidized?

What is the most essential conditions that must be satisfied in a redox
reaction?

In the reactions given below, identify the species undergoing oxidation and

reduction.

H₂S (g) + Cl₂ (g)  →2HCl (g) + S (S)

Define reduction reaction?

Depict the galvanic cell in which the reaction Zn(s) + 2Ag⁺(aq) → Zn²⁺(aq) + 2Ag(s) takes place, further show:

(i) which of the electrode is negatively charged,

(ii) the carriers of the current in the cell, and

(iii) individual reaction at each electrode.

Given the standard electrode potentials,

K⁺/K = -2.93V, Ag⁺/Ag = 0.80V,      

Hg²⁺/Hg = 0.79V

Mg²⁺/Mg = -2.37V. Cr³⁺/Cr = -0.74V

Arrange these metals in their increasing order of reducing power.

Arrange the following metals in the order in which they displace each other from the solution of their salts.

Al, Cu, Fe, Mg and Zn.

Predict the products of electrolysis in each of the following:

(i) An aqueous solution of AgNO₃ with silver electrodes

(ii) An aqueous solution AgNO₃ with platinum electrodes

(iii) A dilute solution of H₂SO₄ with platinum electrodes

(iv) An aqueous solution of CuCl₂ with platinum electrodes.

Using the standard electrode potentials given in the Table 8.1, predict if the reaction between the following is feasible:

(a) Fe³⁺(aq) and I^-(aq)

(b) Ag⁺(aq) and Cu(s)

(c) Fe³⁺ (aq) and Cu(s)

(d) Ag(s) and Fe³⁺(aq)

(e) Br₂(aq) and Fe²⁺(aq)

In Ostwald's process for the manufacture of nitric acid, the first step involves the oxidation of ammonia gas by oxygen gas to give nitric oxide gas and steam. What is the maximum weight of nitric oxide that can be obtained starting only with 10.00 g. of ammonia and 20.00 g of oxygen?

Refer to the periodic table given in your book and now answer the following questions:

(a) Select the possible non metals that can show disproportionation reaction.

(b) Select three metals that can show disproportionation reaction.

Chlorine is used to purify drinking water. Excess of chlorine is harmful. The excess of chlorine is removed by treating with sulphur dioxide. Present a balanced equation for this redox change taking place in water.

Consider the elements:

Cs, Ne, I and F

(a) Identify the element that exhibits only negative oxidation state.

(b) Identify the element that exhibits only postive oxidation state.

(c) Identify the element that exhibits both positive and negative oxidation states.

(d) Identify the element which exhibits neither the negative nor does the positive oxidation state.

The Mn³⁺ ion is unstable in solution and undergoes disproportionation to give Mn²⁺, MnO₂, and H⁺ ion. Write a balanced ionic equation for the reaction.

Consider the reactions:

(a) H₃PO₂(aq) + 4 AgNO₃(aq) + 2 H₂O(l) → H₃PO₄(aq) + 4Ag(s) + 4HNO₃(aq)

(b) H₃PO₂(aq) + 2CuSO₄(aq) + 2 H₂O(l) → H₃PO₄(aq) + 2Cu(s) + H₂SO₄(aq)

(c) C₆H₅CHO(l) + 2[Ag (NH₃)₂]⁺(aq) + 3OH^-(aq) → C₆H₅COO^-(aq) + 2Ag(s) + 4NH₃(aq) + 2 H₂O(l)

(d) C₆H₅CHO(l) + 2Cu²⁺⁽aq) + 5OH^-(aq) → No change observed.

What inference do you draw about the behaviour of Ag⁺and Cu²⁺from these reactions?

Justify giving reactions that among halogens, fluorine is the best oxidant and among hydrohalic compounds, hydroiodic acid is the best reductant.

How do you count for the following observations?

(a) Though alkaline potassium permanganate and acidic potassium permanganate both are used as oxidants, yet in the manufacture of benzoic acid from toluene we use alcoholic potassium permanganate as an oxidant. Why? Write a balanced redox equation for the reaction.

(b) When concentrated sulphuric acid is added to an inorganic mixture containing chloride, we get colourless pungent smelling gas HCl, but if the mixture contains bromide then we get red vapour of bromine. Why?

Whenever a reaction between an oxidising agent and a reducing agent is carried out, a compound of lower oxidation state is formed if the reducing agent is in excess and a compound of higher oxidation state is formed if the oxidising agent is in excess. Justify this statement giving three illustrations.

The compound AgF₂ is an unstable compound. However, if formed, the compound acts as a very strong oxidizing agent. Why?

Consider the reactions:

(a) 6 CO₂(g) + 6H₂O(l) → C₆H₁₂O₆(aq) + 6O₂(g)

(b) O₃(g) + H₂O₂(l) → H₂O(l) + 2O₂(g)

Why it is more appropriate to write these reactions as:

(a) 6CO₂(g) + 12H₂O(l) → C₆H₁₂O₆(aq) + 6H₂O(l) + 6O₂(g)

(b) O₃(g) + H₂O₂(l) → H₂O(l) + O₂(g) + O₂(g)

Also suggest a technique to investigate the path of the above (a) and (b) redox

reactions.

While sulphur dioxide and hydrogen peroxide can act as oxidising as well as reducing agents in their reactions, ozone and nitric acid act only as oxidants. Why?

Suggest a list of the substances where carbon can exhibit oxidation states from -4 to +4 and nitrogen from -3 to +5.

Write the formulae for the following compounds:

(a) Mercury(II) chloride                        (b) Nickel(II) sulphate

(c) Tin(IV) oxide                                 (d) Thallium(I) sulphate

(e) Iron(III) sulphate                           (f) Chromium(III) oxide

What are the oxidation numbers of the underlined elements in each of the following and how do you rationalise your results?

(a) KI₃ (b) H₂S₄O₆ (c) Fe₃O₄ (d) CH₃CH₂OH (e) CH₃COOH

Fluorine reacts with ice and results in the change:

H₂O(s) + F₂(g) → HF(g) + HOF(g)

Justify that this reaction is a redox reaction.

Justify that the following reactions are redox reactions:

(a) CuO(s) + H₂(g) → Cu(s) + H₂O(g)

(b) Fe₂O₃(s) + 3CO(g) → 2Fe(s) + 3CO₂(g)

(c) 4BCl₃(g) + 3LiAlH₄(s) → 2B₂H₆(g) + 3LiCl(s) + 3 AlCl₃ (s)

(d) 2K(s) + F₂(g) → 2K+F- (s)

(e) 4 NH₃(g) + 5 O₂(g) → 4NO(g) + 6H₂O(g)

What are the oxidation numbers of the underlined elements in each of the following and how do you rationalise your results?

(a) KI₃ (b) H₂S₄O₆ (c) Fe₃O₄ (d) CH₃CH₂OH (e) CH₃COOH

Assign oxidation numbers to the underlined elements in each of the following species:

(a) NaH₂PO₄ (b) NaHSO₄ (c) H₄P₂O₇ (d) K₂MnO₄

(e) CaO₂ (f) NaBH₄ (g) H₂S₂O₇ (h) KAl(SO₄)₂.12 H₂O