Chapter 8

Which of the following is not an example of redox reaction?

• (i)CuO + H2 → Cu + H2O
• (ii)Fe2O3 + 3CO → 2Fe + 3CO2
• (iii)2K + F2 → 2KF
• (iv)BaCl2 + H2SO4 → BaSO4 + 2HCl

The more positive the value of E , the greater is the tendency of the species to get reduced. Using the standard electrode potential of redox couples given below find out which of the following is the strongest oxidising agent. V – E values: Fe3+/Fe2+ = + 0.77; I2(s)/I = + 0.54; Cu2+/Cu = + 0.34; Ag+/Ag = + 0.80V

• (i)Fe3+
• (ii)I2(s)
• (iii)Cu2+
• (iv)Ag +

E values of some redox couples are given below. On the basis of these values choose the correct option. V – E values : Br2/Br = + 1.90; Ag+ /Ag(s) = + 0.80 – Cu2+/Cu(s) = + 0.34; I2(s)/I = + 0.54 –

• (i)Cu will reduce Br
• (ii)Cu will reduce Ag –
• (iii)Cu will reduce I
• (iv)Cu will reduce Br2

Using the standard electrode potential, find out the pair between which redox reaction is not feasible. V 3+ 2+ – E values : Fe /Fe = + 0.77; I2/I = + 0.54; 2+ + Cu /Cu = + 0.34; Ag /Ag = + 0.80 V 3+ –

• (i)Fe and I +
• (ii)Ag and Cu 3+
• (iii)Fe and Cu 3+
• (iv)Ag and Fe

Thiosulphate reacts differently with iodine and bromine in the reactions given below: 2– 2– – 2S2O3 + I2 → S4O6 + 2I 2– 2– – + S2O3 + 2Br2 + 5H2O → 2SO4 + 2Br + 10 H Which of the following statements justifies the above dual behaviour of thiosulphate?

• (i)Bromine is a stronger oxidant than iodine.
• (ii)Bromine is a weaker oxidant than iodine.
• (iii)Thiosulphate undergoes oxidation by bromine and reduction by iodine in these reactions.
• (iv)Bromine undergoes oxidation and iodine undergoes reduction in these reactions.

The oxidation number of an element in a compound is evaluated on the basis of certain rules. Which of the following rules is not correct in this respect?

• (i)The oxidation number of hydrogen is always +1.
• (ii)The algebraic sum of all the oxidation numbers in a compound is zero.
• (iii)An element in the free or the uncombined state bears oxidation number zero.
• (iv)In all its compounds, the oxidation number of fluorine is – 1.

In which of the following compounds, an element exhibits two different oxidation states.

• (i)NH2OH
• (ii)NH4NO3
• (iii)N2H4
• (iv)N3H

Which of the following arrangements represent increasing oxidation number of the central atom? 2– –

• (i)CrO–2 , ClO3– , CrO4 , MnO4 – 2– – –
• (ii)ClO3 , CrO4 , MnO4 , CrO2 105 Redox Reactions – – – 2–
• (iii)CrO2 , ClO3 , MnO4 , CrO4 2– – – –
• (iv)CrO4 , MnO4 , CrO2 , ClO3

The largest oxidation number exhibited by an element depends on its outer electronic configuration. With which of the following outer electronic configurations the element will exhibit largest oxidation number? 1 2

• (i)3d 4s 3 2
• (ii)3d 4s 5 1
• (iii)3d 4s 5 2
• (iv)3d 4s

Identify disproportionation reaction

• (i)CH4 + 2O2 → CO2 + 2H2O
• (ii)CH4 + 4Cl2 → CCl4 + 4HCl – –
• (iii)2F2 + 2OH → 2F + OF2 + H2O – – –
• (iv)2NO2 + 2OH → NO2 + NO3 + H2O

Which of the following elements does not show disproportionation tendency?

• (i)Cl
• (ii)Br
• (iii)F
• (iv)I

Which of the following statement(s) is/are not true about the following decomposition reaction. 2KClO3 → 2KCl + 3O2

• (i)Potassium is undergoing oxidation
• (ii)Chlorine is undergoing oxidation
• (iii)Oxygen is reduced
• (iv)None of the species are undergoing oxidation or reduction

Identify the correct statement (s) in relation to the following reaction: Zn + 2HCl → ZnCl2 + H2

• (i)Zinc is acting as an oxidant
• (ii)Chlorine is acting as a reductant
• (iii)Hydrogen ion is acting as an oxidant
• (iv)Zinc is acting as a reductant

The exhibition of various oxidation states by an element is also related to the outer orbital electronic configuration of its atom. Atom(s) having which of the following outermost electronic configurations will exhibit more than one oxidation state in its compounds.

• (i)3s 1 2
• (ii)3d 4s 2 2
• (iii)3d 4s 2 3
• (iv)3s 3p

Identify the correct statements with reference to the given reaction – – P4 + 3OH + 3H2O PH3 + 3H2PO2

• (i)Phosphorus is undergoing reduction only.
• (ii)Phosphorus is undergoing oxidation only.
• (iii)Phosphorus is undergoing oxidation as well as reduction.
• (iv)Hydrogen is undergoing neither oxidation nor reduction.

Which of the following electrodes will act as anodes, when connected to Standard Hydrogen Electrode? 3+

• (i)Al/Al E = –1.66 2+
• (ii)Fe/Fe E = – 0.44 2+
• (iii)Cu/Cu E = + 0.34 –
• (iv)F2 (g)/2F (aq) E = + 2.87

The reaction Cl2 (g) + 2OH (aq) ClO (aq) + Cl (aq) + H2O (l ) – – – represents the process of bleaching. Identify and name the species that bleaches the substances due to its oxidising action. –

MnO 24 – undergoes disproportionation reaction in acidic medium but MnO4 does not. Give reason.

PbO and PbO2 react with HCl according to following chemical equations : 2PbO + 4HCl 2PbCl2 + 2H2O PbO2 + 4HCl PbCl2 + Cl2 + 2H2O Why do these compounds differ in their reactivity?

Nitric acid is an oxidising agent and reacts with PbO but it does not react with PbO2. Explain why? 107 Redox Reactions

Write balanced chemical equation for the following reactions: –

• (i)Permanganate ion (MnO4 ) reacts with sulphur dioxide gas in acidic medium to produce Mn2+ and hydrogensulphate ion. (Balance by ion electron method) –
• (ii)Reaction of liquid hydrazine (N2H4) with chlorate ion (ClO3 ) in basic medium produces nitric oxide gas and chloride ion in gaseous state. (Balance by oxidation number method)
• (iii)Dichlorine heptaoxide (Cl2O7) in gaseous state combines with an aqueous solution of hydrogen peroxide in acidic medium to give chlorite ion – (ClO2 ) and oxygen gas. (Balance by ion electron method)

Calculate the oxidation number of phosphorus in the following species. 2– 3–

• (a)HPO 3 and
• (b)PO 4

Calculate the oxidation number of each sulphur atom in the following compounds:

• (a)Na2S2O3
• (b)Na2S4O6
• (c)Na2SO3
• (d)Na2SO4

Balance the following equations by the oxidation number method. Fe2+ + H+ + Cr2 O7 Cr + Fe + H2O 2– 3+ 3+

• (i)I2 + N O3 NO2 + I O 3 – –
• (ii)I2 + S2 O3 I + S4 O6 2– – 2–
• (iii)MnO2 + C2 O 4 Mn + CO2 2– 2+
• (iv)

Identify the redox reactions out of the following reactions and identify the oxidising and reducing agents in them.

• (i)3HCl(aq) + HNO3 (aq) Cl2 (g) + NOCl (g) + 2H2O (l )
• (ii)HgCl2 (aq) + 2KI (aq) HgI2 (s) + 2KCl (aq)
• (iii)Δ → 2Fe (s) + 3CO (g) Fe2O3 (s) + 3CO (g) ⎯⎯⎯ 2
• (iv)PCl3 (l) + 3H2O (l) 3HCl (aq) + H3 PO3 (aq) (v) 4NH3 + 3O2 (g) 2N2 (g) + 6H2O (g)

Balance the following ionic equations Cr2 O7 + H + I Cr + I2 + H2O 2– + – 3+

• (i)Cr2 O7 + Fe + H Cr + Fe + H2O 2– 2+ + 3+ 3+
• (ii)Mn O 4 + S O 3 + H Mn + S O 4 + H2O – 2– + 2+ 2–
• (iii)Mn O 4 + H + Br Mn + Br2 + H2O – + – 2+
• (iv)

Match Column I with Column II for the oxidation states of the central atoms. Column I Column II 2– (i) Cr2O 7

• (a)+3 – (ii) MnO 4
• (b)+4 – (iii) VO3
• (c)+5 3– (iv) FeF6
• (d)+6 (e) +7

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

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.

• (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.

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. 109 Redox Reactions

• (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. 109 Redox Reactions

Assertion (A): The decomposition of hydrogen peroxide to form water and oxygen is an example of disproportionation reaction.

Reason (R): The oxygen of peroxide is in –1 oxidation state and it is converted to zero oxidation state in O2 and –2 oxidation state in H2O. (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.

• (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.

Assertion (A): Redox couple is the combination of oxidised and reduced form of a substance involved in an oxidation or reduction half cell.

Reason (R): In the representation E Fe3+ /Fe2+ and ECu2+ /Cu , Fe3+/ Fe2+ and Cu2+ / Cu are redox couples. (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.

• (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.

Explain redox reactions on the basis of electron transfer. Give suitable examples.

On the basis of standard electrode potential values, suggest which of the following reactions would take place? (Consult the book for E value). Cu + Zn Cu + Zn 2+ 2+

• (i)Mg 2+ 2+
• (ii)Mg + Fe + Fe Br2 + 2Cl Cl2 + 2Br – –
• (iii)Fe + Cd Cd + Fe 2+ 2+
• (iv)

Why does fluorine not show disporportionation reaction?

Write redox couples involved in the reactions (i) to (iv) given in question 34.

Find out the oxidation number of chlorine in the following compounds and arrange them in increasing order of oxidation number of chlorine. NaClO4, NaClO3, NaClO, KClO2, Cl2O7, ClO3, Cl2O, NaCl, Cl2, ClO2. Which oxidation state is not present in any of the above compounds?

Which method can be used to find out strength of reductant/oxidant in a solution? Explain with an example.