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08 Redox Reactions Chapter 08: Redox Reactions 8.0 Introduction 8.3 Oxidation number 8.1 Concept of oxidation and reduction 8.4 Change in oxidation number 8.2 Redox reactions reactions) 8.5 Balancing redox reactions in terms of loss and gain of electrons (Oxidation reduction 8.0 Introduction Chemical reaction: A reaction in which two substances chemically react with each other to give products is called chemical reaction. Chemical reactions Precipitation reactions i. ii. iii. iv. v. vi. vii. Oxidation-reduction or Redox reactions Acid-base Neutralization reactions Importance of redox reactions: The redox reactions are a very important group of reactions which occur with almost every element. These reactions are involved in large number of processes in nature (biological and industrial). They take place in burning of fuels such as gasoline, oil, natural gas, organic substances of carbon and hydrogen and so on. The redox reactions are involved in household bleaching. The metallic elements are extracted from their ores by oxidation-reduction reactions. The functioning of batteries is based on redox reactions. The corrosion of metals involve redox reactions. 8.1 Concept of oxidation and reduction i. ii. iii. Oxidation: According to the classical concept, “Oxidation is defined as the addition of oxygen or any other electronegative element or removal of hydrogen or any other electropositive element.” According to the modern concept, “Oxidation is a process which involves loss of electrons. It is also called de-electronation.” Oxidation increases the oxidation number of the element in the given substance and it involves, a. Addition of oxygen. C + O2 CO2 b. Addition of an electronegative element. Zn + S ZnS c. Removal of hydrogen. H2S + Br2 2HBr + S d. Removal of an electropositive element/radical. 2KI + Cl2 2KCl + I2 1 Chemistry Vol ‐ 1.2 (Med. and Engg.) i. ii. iii. i. Reduction: According to the classical concept, “Reduction is defined as the addition of hydrogen or any other electropositive element or removal of oxygen or any other electronegative element” According to the modern concept, “Reduction is a process which involves gain of electrons. It is also called electronation.” Reduction decreases the oxidation number of the element in the given substance and it involves, a. Addition of hydrogen. Cl2 + H2 2HCl b. Addition of an electropositive element. 2HgCl2 + SnCl2 Hg2Cl2 + SnCl4 c. Removal of oxygen. ZnO + C Zn + CO d. Removal of an electronegative element/ radical. 2FeCl3 + H2 2FeCl2 + 2HCl Oxidizing agent: “The substance which gains one or more electrons (i.e.itself gets reduced) is called oxidizing agent or oxidant.” Oxidizing agent increases the oxidation number of an element in a given substance. Eg. O2, Cl2, Br2, H2O2, HNO3, CO2, etc. ii. i. Reducing agent: “The substance which loses one or more electrons (i.e.,itself gets oxidized) is called reducing agent or reductant.” Reducing agent lowers the oxidation number of an element in a given substance. Eg. H2, Fe, Cu, C, LiAIH4, Sodium amalgam etc. ii. 8.2 Redox reactions (Oxidation reduction reactions) i. Redox reactions: “The reactions in which both oxidation and reduction reactions occur simultaneously (together) are called redox reactions.” In these reactions, one substance acts as a reducing agent and itself gets oxidized while another substance acts as an oxidizing agent and itself gets reduced. Eg. Reaction between zinc and copper (II) salt occuring in a battery. In this reaction, zinc loses electrons and gets oxidized, whereas Cu2+ ions gain electrons and get reduced. ii. loss of 2e : Oxidation 2 Zn(s) + Cu (aq) 2 + Cu(s) Zn (aq) gain of 2e : Reduction iii. iv. v. vi. 2 In the direct redox reaction, the transferance of electrons is limited to very small distances and therefore, no useful electrical work could be obtained. In these reactions, chemical energy appears as heat. If the transferance of electrons from zinc to copper ions is allowed to occur through some metallic wires, useful electrical work could be performed. Such redox reactions are called Indirect redox reactions and electrical energy is produced during such spontaneous reactions instead of heat energy. In this case, zinc acts as a reducing agent or reductant, while Cu2+ ions act as an oxidizing agent or oxidant. Other examples of redox reactions are, Zn + 2HCl ZnCl2 + H2 5Fe2+ + 8H+ + MnO 4 5Fe3+ + Mn2+ + 4H2O Examples for substances which can act both as oxidising as well as reducing agents are SO2, H2O2, HNO2 etc. Chapter 08: Redox Reactions i. ii. Half reactions: “Every redox reaction according to the electronic concept consists of two reactions known as half-reactions.” These are: Oxidation half reaction (loss of electrons). Reduction half reaction (gain of electrons). Eg. The reaction involving zinc and dilute HCl. The two half reactions are: (oxidation half reaction) Zn(s) Zn 2aq + 2e (reduction half reaction) H2(g) 2H aq + 2e Zn(s) + 2H aq (overall reaction) Zn 2aq + H2(g) Types of redox reactions: Redox reactions Combination reactions Decomposition reactions Disproportionation reactions Displacement reactions Metal displacement Non-metal displacement i. Combination reactions: a. “The chemical reactions in which two or more substances (elements or compounds) combine to form a single substance are called combination reactions.” b. A combination reaction may be expressed as, A + B C c. For combination reaction to be a redox reaction one or both ‘A’ and ‘B’ must be in the elementary form. 0 0 +4 2 0 +2 3 Eg. 0 CO2(g) ; 3Mg(s) + N2(g) C(s) + O2(g) Mg3N2(s) ii. Decomposition reactions: a. “The chemical reactions in which a compound breaks up into two or more simple substances are called decomposition reactions.” b. The decomposition reactions are the opposite of combination reactions. c. A decomposition reaction is the breakdown of a compound into two or more compounds atleast one of which must be in the elemental state. +1 +5 2 +1 2 0 0 +1 1 0 Eg. Δ Δ 2H2O(l) 2H2(g) + O2(g) ; 2KClO3(s) 2KCl(s) + 3O2(g) Note: All decomposition reactions are not redox reactions. Eg. Decomposition of calcium carbonate is not a redox reaction. +2 +4 2 +2 2 Δ CaCO3(s) CaO(s) + iii. +4 2 CO2(g) Displacement reactions: a. “The reactions in which one ion (or atom) in a compound is replaced by an ion (or atom) of other element are called displacement reactions.” b. Displacement reactions may be expressed as: X + YZ XZ + Y 3 Chemistry Vol ‐ 1.2 (Med. and Engg.) c. 1. These reactions are of two types: Metal displacement: In these reactions, a metal in a compound can be displaced by another metal in the uncombined state. 0 0 +2 +6 2 Eg. +2 +6 2 CuSO4(aq) + Zn(s) Cu(s) + ZnSO4(aq) +5 2 2. i. ii. iii. iv. iv. 0 0 +2 2 V2O5(s) + 5Ca(s) 2V(s) + 5CaO(s) Non-metal displacement: The non-metal displacement redox reactions include hydrogen displacement and a rarely occurring reaction involving oxygen displacement. All alkali metals and some alkaline earth metals (Ca, Sr and Ba) which are very good reductants, will displace hydrogen from cold water. 0 +1 2 +1 2 +1 0 Eg. 2Na(s) + 2H2O(l) 2NaOH(aq) + H2(g) Less active metals such as magnesium and iron react with steam to produce hydrogen gas. 0 +1 2 +2 2 +1 0 Eg. Mg(s) + 2H2O(l) Mg(OH)2(aq) + H2(g) Many metals, including those which do not react with cold water, are capable of displacing hydrogen from acids. 0 +1 1 +2 1 0 Eg. Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g) Metals like cadmium and tin do not react with steam, but react with acids to displace hydrogen gas. 0 +1 1 +2 1 0 Eg. Cd(s) + 2HCl(aq) CdCl2(aq) + H2(g) Disproportionation reactions: a. In a disproportionation reaction, an element in one oxidation state is simultaneously oxidized and reduced. b. Atleast one reacting substance in a disproportionation reaction always contains an element that has both higher and lower oxidation states existing for that element. Eg. The decomposition of hydrogen peroxide is an example of the reaction, where oxygen element experiences disproportionation. Reduced +1 1 +1 2 2H2O(l) 2H2O2(aq) 0 O2(g) + Oxidized In this reaction, oxidation number of ‘O’ decreases from 1 to 2 (in H2O) and increases from 1 to 0 (in O2). Note: 1. All redox reactions are exothermic. 2. Phosphorus, sulphur and chlorine undergo disproportionation in the alkaline medium as shown below: Reduced 0 P4(s) + 3OH (aq) 3 + 3H2O(l) +1 PH3(g) + 3H 2 PO 2(aq) Oxidized In this reaction, oxidation number of ‘P’ increases from 0 to + 1 (in H 2 PO 2 ) and decreases from 0 to 3 (in PH3). 4 Chapter 08: Redox Reactions 8.3 Oxidation number i. ii. iii. iv. v. i. ii. iii. iv. v. vi. vii. Oxidation number: “The charge which an atom appears to have, when all other atoms are removed from it as ions is known as oxidation number.” OR “The oxidation number or oxidation state of an atom in a molecule or ion is defined as the number of charges it would carry if electrons were completely transferred.” Metals invariably have positive oxidation states, while non-metals may have positive or negative oxidation states. Transition metals usually display several oxidation states. The highest positive oxidation state for s-block elements is equal to its group number but for p-block elements it is equal to group number minus 10 (except for noble gases). However, the highest negative oxidation state for p-block elements is equal to eight minus (8) the number of electrons in the valence shell. Eg. In the third period, the highest positive oxidation state or number increases from + 1 to + 7 as shown below: Group Element 1 Na 2 Mg 13 Al 14 Si 15 P 16 S 17 Cl Compounds Oxidation state of the underlined element in the compound NaCl MgSO4 AlF3 SiCl4 IF5, P4O10 SF6, SO3 HClO4, Cl2O7 +1 +2 +3 +4 +5 +6 +7 Rules to assign oxidation number or oxidation states to an atom: If there is a covalent bond between, a. two same atoms then, oxidation number of both the atoms will be zero. Eg. In a chlorine molecule, the oxidation number of both the chlorine atoms is zero. b. two different atoms then, electrons are counted more towards electronegative atom and, 1. more electronegative atom will be negative. 2. less electronegative atom will be positive. Eg. In a HCl molecule, the oxidation number of chlorine (more electronegative atom) is 1 and that of hydrogen (lesser electronegative atom) is +1. If there is a co - ordinate bond between two atoms then, a. Oxidation number of acceptor atom will be 2. b. Oxidation number of donor atom will be + 2. The oxidation number of an element in the free or elementary state or in any of its allotropic forms is always zero. Eg. Oxidation numbers of helium in He, hydrogen in H2, Oxygen in O2 or O3 are zero. The oxidation number of an element in a single (monoatomic) ion is same as the charge on the ion. Eg. Oxidation number of K+ is + 1, and Ca2+ is + 2. In binary compounds of metals and non-metals the oxidation number of metals is always positive while that of non-metals is negative. Eg. In NaCl, the oxidation number of sodium is + 1 and that of chlorine is 1. In compounds formed by the combination of non-metallic atoms, the atom with higher electronegativity is given negative oxidation number. Eg. In HCl, the oxidation number of chlorine is 1 because of it’s high electronegativity. In all compounds of hydrogen, the oxidation number of hydrogen is + 1 except in hydrides of active metals such as LiH, NaH, KH, MgH2, etc., where hydrogen has the oxidation number of 1. 5 Chemistry Vol ‐ 1.2 (Med. and Engg.) viii. The oxidation number of oxygen is 2 in most of the compounds. However, there are two exceptions. a. The first exception is peroxides and superoxides in which oxygen atoms are directly linked to each other. Eg. In peroxides i.e. H2O2, each oxygen atom is assigned an oxidation number of 1 and in superoxides i.e. KO2, RbO2 etc. each oxygen atom is assigned an oxidation number of 1/2. b. The second exception is found in compounds in which oxygen is bonded to fluorine. Eg. In OF2 (oxygen difluoride) the oxidation no. of O is +2 and in O2F2 (dioxygen fluoride), the oxidation no. of each O is +1. ix. The most electronegative element, fluorine always has an oxidation number 1. For other halogens, the oxidation number is generally 1, but there are exceptions, when these are bonded to a more electronegative halogen atom or oxygen. Eg. In HI, the oxidation number of ‘I’ is 1 but in IF5, it is + 5 and in IF7, it is + 7. x. For neutral molecule, the sum of the oxidation numbers of all the atoms is equal to zero. Eg. In NH3, Nitrogen is in 3 oxidation state, whereas hydrogen is in + 1 oxidation state and there are three hydrogen atoms present. Therefore the net charge is zero i.e. 3 and + 3 equals to zero. 8.4 Change in oxidation number i. ii. iii. iv. The oxidation number is more oftenly termed as oxidation state. In an oxidation process, the oxidation number of the element increases in the given substance whereas in a reduction process, the oxidation number of the element decreases in the given substance. Oxidising agent increases the oxidation number of an element in a given substance whereas Reducing agent lowers the oxidation number of an element in a given substance. Redox reactions involve the change in oxidation number of interacting species. Eg. The reaction between zinc and hydrochloric acid. The oxidation number of all the atoms are written above their respective symbols, Oxidized +1 1 0 Zn + 2HCl 2+ 1 ZnCl2 0 + H2 Reduced In this reaction, the oxidation number (O. N.) of zinc increases from 0 to + 2 and that of hydrogen decreases from + 1 to 0, while that of chlorine remains unchanged. Thus, zinc is oxidized while hydrogen is reduced. 8.5 Balancing redox reactions in terms of loss and gain of electrons Methods for balancing redox reactions Oxidation number method Ion-electron method (Half-reaction method) Oxidation number method: The oxidation number method for balancing the redox reactions follows the basic principle that the total increase in oxidation number must be equal to total decrease in oxidation number. Steps involved in balancing of redox reactions by the oxidation number method are given as follows: Write the unbalanced net equation for the redox reaction. Balance the equation for all atoms except ‘H’ and ‘O’. i. 6 Chapter 08: Redox Reactions ii. iii. iv. v. vi. vii. Assign the oxidation numbers to all atoms in the reactants and the products, using the rules to assign oxidation number. Identify the atoms undergoing change in oxidation numbers. Show an increase in oxidation number per atom of the oxidized species and hence, the net increase in oxidation number. Similarly, show a decrease in oxidation number per atom of the reduced species and the net decrease in oxidation number. Determine the factor that will make the total increase and decrease in oxidation numbers equal. Insert these coefficients into the equation. Balance oxygen atoms by adding H2O to the side containing less O atoms, one H2O for one O atom. Balance H atoms by adding H+ ions, to the side with less H atoms. If the reaction occurs in basic medium, then add OH ions, equal to the number of H+ on both sides of the equation. The H+and OH ions appearing on the same side of the reaction are combined to give H2O molecules. Check the balanced equation to make sure that the reaction is balanced with respect to both the number of atoms of each element and the charges. Note: For a reaction in alkaline medium all the steps are applicable. However, if the reaction occurs in acidic medium, point (vi) is omitted. a. Reaction occurring in acidic medium: 2 3+ H2O2(aq) + Cr2O 7(aq) O2(g) + Cr(aq) 1. Balance Cr atom and assign oxidation number to each atom. 2 3+ H2O2(aq) + Cr2 O7(aq) O2(g) + 2Cr(aq) 2. +1 1 +6 2 0 +3 Identify the atoms undergoing change in oxidation number. The oxidation number of Cr reduces from + 6 to + 3. i.e., undergoes reduction by gain of three electrons and is an oxidizing agent. The oxidation number of ‘O’ increases from 1 to 0 by loss of two electrons. i.e., it acts as a reducing agent and itself gets oxidized. 2 3+ H2O2(aq) + Cr2 O7(aq) O2(g) + 2Cr(aq) 1 3. 4. +6 0 +3 Gain of e Loss of e Find the total increase in oxidation number for the oxidized atom and total decrease in oxidation number for the reduced atom. i. Increase in oxidation number : 2O ( 1) 2O (0). Increase per atom = + 1 Net increase = + 2. ii. Decrease in oxidation number : 2Cr (+ 6) 2Cr (+ 3) Decrease per atom = 3 Net decrease = 6. Choose the factors that will make net increase and decrease equal. The net increase must be multiplied by 3. Hence, the coefficient 3 is needed for the O species on both sides. 2 3+ 3O2(g) + 2Cr(aq) 3H2O2(aq) + Cr2 O7(aq) 5. Balance the equation for O atoms by adding H2O to the side with less O atoms. There are 13 (O) atoms on the left side and 6 (O) atoms on the right side. Hence, add 7 H2O molecule to the right side. 2 3+ 3H2O2(aq) + Cr2 O7(aq) 3O2(g) + 2Cr(aq) + 7H2O(l) 6. Balance the equation for H atoms by adding H+ ions on the side with less H atoms. There are 6 (H) atoms on left side whereas 14 (H) atoms on right side. Hence add 8H+ to the left side. 2 + 3+ + 8H (aq) 3O2(g) + 2Cr(aq) + 7H2O(l) 3H2O2(aq) + Cr2 O7(aq) Thus, the equation is balanced for both charge as well as atoms. 7 Chemistry Vol ‐ 1.2 (Med. and Engg.) b. Reaction occurring in basic medium: 2 Zn(s) + NO 3(aq) NH3(g) + Zn(OH)6(aq) 1. Assign oxidation number to all atoms. Zn(s) + NO3(aq) NH3(g) + Zn(OH)62(aq) 0 2. +5 2 3 +1 +4 –2 +1 Identify the atoms undergoing changes in oxidation numbers. The oxidation number of ‘N’ reduces from + 5 to 3 and that of ‘Zn’ increases from 0 to + 4. Hence, N is an oxidizing agent and is itself reduced by the gain of electrons. ‘Zn’ acts as a reducing agent and is itself oxidized by loss of electrons. 2 Zn(s) + NO3(aq) NH3(g) + Zn(OH)6(aq) 0 +5 3 +4 Gain of e Loss of e 3. 4. Find the net increase in oxidation number for the oxidized atom and net decrease in oxidation number for the reduced atom. i. Increase in oxidation number : Zn (0) Zn (4) Increase per atom = Net increase = +4 ii. Decrease in oxidation number : N (+5) N (3) Decrease per atom = Net decrease = 8. Choose the factors that will make the net increase equal to the net decrease. The net increase is multiplied by 2. Hence, a coefficient 2 is needed for both species of Zn. 2 2Zn(s) + NO3(aq) NH3(g) + 2Zn(OH)6(aq) 5. Balance the ‘O’ atoms by adding H2O molecule to the side with less ‘O’ atoms. The left side has 3 oxygen atoms and right side has 12 oxygen atoms. Therefore, balance the equation for ‘O’ atoms by adding 9H2O to the left side. 2 2Zn(s) + NO3(aq) + 9H2O(l) NH3(g) + 2Zn(OH)6(aq) 6. ‘H’ atoms are balanced by adding H+ ions to the side with less ‘H’ atoms. There are 18 H atoms on the left side and 15 on the right side. Add 3H+ ions to the right side to balance ‘H’ atoms. 2 + + 9H2O(l) NH3(g) + 2Zn(OH)6(aq) + 3H (aq) 2Zn(s) + NO3(aq) 7. Since the reaction occurs in basic medium, add OH ions equal to the number of H+ ions, on both sides of the equation. Hence add 3OH ions on both sides. 2 + 2Zn(s) + NO3(aq) + 9H2O(l) + 3OH (aq) NH3(g) + 2Zn(OH)6(aq) + 3H (aq) + 3OH(aq) 8. 3H+ and 3OH on the right side will be neutralized to form 3H2O. 2 + 9H2O(l) + 3OH (aq) NH3(g) + 2Zn(OH)6(aq) + 3H2O(l) 2Zn(s) + NO3(aq) 9. Cancel 3H2O molecules on both sides of equation. The final equation is 2 NH3(g) + 2Zn(OH)6(aq) 2Zn(s) + NO 3(aq) + 6H2O(l) + 3OH (aq) The equation is balanced for both atoms and charges. 8 Chapter 08: Redox Reactions Ion-electron method (Half reaction method): The Ion-electron method for balancing the redox reactions follows the principle of balancing the equations using half reactions. Steps involved in balancing of redox reaction by the ion-electron method are given as follows: i. Write the unbalanced equation for the redox reaction and assign the oxidation number to all the atoms in the reactants and the products. ii. Divide the equation into two half equations. One half equation describes oxidation, which involves increase in oxidation number of an oxidized species and the other half equation describes reduction in which the oxidation number of the reduced species decreases. iii. Balance the atoms except ‘O’ and ‘H’ in each half equation. Balance oxygen atoms by adding H2O to the side with less ‘O’ atoms. iv. Balance ‘H’ atoms by adding ‘H+’ ions to the side with less ‘H’ atoms. v. Balance the charges by adding appropriate number of electrons to the right side of oxidation half equation and to the left of reduction half equation. vi. Multiply half equations by suitable factors to equalize the number of electrons in the two half equations. Then add two half equations and cancel the number of electrons on both sides of the equation. vii. If the reaction occurs in basic medium, then add OH ions, equal to the number of H+ ions, on both sides of the equation. H+ and OH ions appearing on the same side of the equation are combined to give H2O molecules. viii. Check that the equation is balanced for both, the atoms and the charges. Note: For the reaction in an alkaline medium, all the steps are applicable. However, if the reaction occurs in acidic medium, point (vii) is omitted. a. Reaction occurring in acidic medium: 3+ 2+ 2 SO2(g) + Fe(aq) Fe(aq) + SO 4(aq) (acidic) 1. Write the unbalanced equation and assign oxidation number to all atoms. 2 SO2(g) + Fe3(aq) Fe(aq) + SO 24(aq) +4 2 +3 +2 +6 2 Gain of e Loss of e 2. Divide the equation into two half equations, an oxidation half equation and a reduction half equation. The oxidation number of ‘S’ increases from + 4 to + 6 and that of Fe decreases from + 3 to + 2. Hence, S is oxidized and Fe is reduced. Oxidation half equation: SO2(g) SO 24(aq) 2 Fe(aq) Reduction half equation: Fe3(aq) 3. Balance the half equations for ‘O’ atoms by adding H2O to the side with less ‘O’ atoms. Hence, add 2H2O to the left side of oxidation half equation whereas no addition of H2O to reduction half equation. Oxidation: SO2(g) + 2H2O(l) SO 24(aq) 2 Fe(aq) Reduction: Fe3(aq) 4. Balance ‘H’ atoms by adding ‘H+’ ions to the side with less ‘H’ atoms. Hence, add ‘4H+’ to the right side of oxidation half equation and no addition to reduction half equation. + 4H (aq) Oxidation: SO2(g) + 2H2O(l) SO 24(aq) 2 Reduction: Fe3(aq) Fe(aq) 5. Add 2e to the right side of oxidation half equation and le to the left side of reduction half equation to balance the charges. + 4H (aq) +2e Oxidation: SO2(g) + 2H2O(l) SO 24(aq) 2 Reduction: Fe3(aq) + e Fe(aq) 9 Chemistry Vol ‐ 1.2 (Med. and Engg.) 6. Multiply reduction half equation by ‘2’ to equalize the number of electrons in the two half equations and then add the two half equations. SO2(g) + 2H2O(l) SO 24(aq) + 4H (aq) + 2e 2 2Fe3(aq) + 2e 2Fe(aq) 2 SO2(g) + 2Fe3(aq) + 2H2O(l) SO 24(aq) + 2Fe(aq) + 4H (aq) b. 1. Hence, the equation is balanced in terms of number of atoms and charges. Reaction occurring in basic medium: 2+ I2(aq) S 4O 6(aq (basic) S 2 O 23(aq) ) + I (aq) Balance ‘S’ and ‘I’ atoms and assign oxidation number to all atoms. 22+ I2(aq) S4 O6(aq.) + 2 I-(aq) 2S2 O3(aq) +2 2 0 +2.5 2 1 Loss of e Gain of e 2. Divide the equation into two half equations, an oxidation half equation and a reduction half equation. The oxidation number of ‘S’ increases from + 2 to + 2.5 and that of ‘I’ decreases from 0 to 1. Therefore ‘S’ is oxidized and ‘I’ is reduced. 22i. Oxidation half equation: 2S2 O3(aq) S4 O 6(aq) ii. 3. 4. Reduction half equation: I2(aq) 2I-(aq) Number of oxygen atoms is already balanced for the oxidation step. In the reduction step, there is no oxygen atom involved. Balancing for the charge: Add 2 electrons on the R.H.S. of the oxidation half equation and 2 electrons on the L.H.S. of the reduction half equation. 22Oxidation: 2S2 O3(aq) S4 O6(aq) + 2e Reduction: I2(aq) + 2e 2I-(aq) Adding two half equations we get, 22 2S2 O3(aq) + I2(aq) S4 O6(aq) + 2I (aq) Thus, the equation is balanced for all atoms and charges. Note: Stock Notation: i. The oxidation number/state of a metal in a compound is sometimes presented according to the notation given by German chemist, Alfred Stock. It is popularly known as Stock notation. ii. According to this, the oxidation number is expressed by putting a Roman numeral representing the oxidation number in parenthesis after the symbol of the metal in the molecular formula. iii. Thus, Aurous chloride and Auric chloride are written as Au(I)Cl and Au(III)Cl3. Similarly, Stannous chloride and Stannic chloride are written as Sn(II)Cl2 and Sn(IV)Cl4. This change in oxidation number implies change in oxidation state, which helps to identify whether the species is present in oxidized form or reduced form. No. Compounds Oxidation state of metal ion Representation or stock notation i. HAuCl4 +3 HAu(III)Cl4 ii. FeO +2 Fe(II)O iii. Fe2O3 +3 Fe2(III)O3 iv. CuI +1 Cu(I)I v. CuO +2 Cu(II)O vi. MnO +2 Mn(II)O vii MnO2 +4 Mn(IV)O2 10 Chapter 08: Redox Reactions Difference between valency and oxidation number: No. Valency Oxidation number (O.N.) i. It is the combining capacity of the element. No O.N. is the charge (real or imaginary) which an atom has or appears to have when all the atoms (+) or () sign is attached to it. are removed as ions. It may have (+) or () sign. ii. Valency of an element is usually fixed. O.N. of an element may have different values. It depends on the nature of compound in which it is present. iii. Valency is always a whole number. O.N. of the element may be a whole number or fractional. iv. Valency of the element cannot be zero except O.N. of the element may be zero. noble gases. Note: Electrode potential: i. The electrical potential difference set up between the metal and its solution is known as electrode potential. ii. The electrode potential is a measure of tendency of an electrode in a half cell to gain or lose electrons. iii. If the concentration of the ions is 1 mol L1 solution, the electrode potential is called standard electrode potential (E) iv. Higher the standard electrode potential (E), stronger is the oxidizing agent. v. All standard potentials are taken as reduction potentials. i. Applications of redox reactions: Metallurgy: In metallurgy, metals are extracted from their ores and they are then purified. The extraction and purification of metals use redox reactions in different steps. Eg. a. Sulphide minerals are converted to oxides by roasting. heat 2ZnS(s) + 3O2(g) ¾¾ 2ZnO(s) + 2SO2(g) +2 2 0 +2 2 +4 2 The oxidation number of ‘S’ increases from 2 to + 4 and that of ‘O’ decreases from 0 to 2. b. ZnO is reduced by coke to prepare Zn. heat ZnO(s) + C(s) ¾¾ Zn(s) + CO(g) ii. 0 +2 0 +2 The oxidation number of ‘Zn’ decreases from + 2 to 0 and that of ‘C’ increases from 0 to + 2. Batteries: The electricity produced in batteries or galvanic cells is due to redox reactions occurring in them. Eg. Daniel cell involves the transfer of electrons from Zn to Cu through a wire connecting the two electrodes which are dipped in the solutions of their own ions. Zn is oxidized by transferring two electrons to Cu2+ ions which are reduced. Thus, 2+ 2+ Zn(s) + Cu (aq) Zn (aq) + Cu(s) 0 iii. iv. +2 0 +2 The oxidation number of Zn increases from 0 to + 2 and that of Cu decreases from + 2 to 0. Bleaching: Decolourization or lightening of coloured material uses redox reaction and is called bleaching. Eg. a. NaOCl is used as an oxidizing agent in bleaching of clothes to remove stains. b. Chlorine is used as an oxidizing agent to bleach wood pulp into white paper. c. H2O2 is used as an oxidizing agent to bleach dark hair by a redox reaction. Combustion: Burning of a substance with oxygen in air is called combustion. Eg. CH4 burns by oxidation with oxygen. CH4(g) + 2O2(g) CO2(g) + 2H2O(l) 4 +1 0 +4 2 +1 2 11 Chemistry Vol ‐ 1.2 (Med. and Engg.) v. The oxidation number of ‘C’ increases from 4 to + 4 and that of ‘O’ decreases from 0 to 2. Hence, it is a redox reaction. CH4 is a reducing agent and O2 is an oxidizing agent. Corrosion: Corrosion is the destruction of metals by oxidation. Eg. Rusting of iron is its oxidation by oxygen of air in presence of moisture. Moisture 4Fe(s) + 3O2(g) 2Fe2O3.H2O(s) H2O Corroded Fe vi. The method employed in preventing corrosion of iron is by zinc coating. This method is called Galvanization. Respiration: a. The process of breathing and using oxygen for various biological redox reactions is called respiration. b. The biological redox reactions provide energy required for living organisms. The overall effect of respiration is similar to combustion. Eg. Glucose reacts with oxygen to give CO2 and H2O releasing energy. C6H12O6 + 6O2 6CO2 + 6H2O + energy 0 0 +4 –2 –2 The oxidation number of ‘C’ increases from 0 to + 4 and that of ‘O’ decreases from 0 to 2. Hence, it is a redox reaction. Quick Review Oxidation: L Loss of Reduction: G Gain of Oxidizing agent: (Oxidant) O Oxidizing agent Reducing agent: (Reductant) R E Electrons is E O Eg. Na(s) Na (aq) + e Eg. Cl2(g) + 2e 2Cl(aq) Eg. F2(g) + 2e 2F(aq) Oxidation R Electrons is Reduction E A (F2 is an oxidizing agent) Electron E Acceptor D Eg. Reducing agent 12 2 Fe(aq) Fe3(aq) + e (Fe2+ is a reducing agent) Electron Donor Chapter 08: Redox Reactions Oxidation numbers of some of the elements in their compounds: Oxidation Element Number Element Oxidation Number Oxidation Element Number H +1, 1 Co +3, +2 Fe +3, +2 Li +1 Ni +2 Sn +4, +2 Be +2 Cu +2, +1 Sb +5, +3, 3 B +3 Zn +2 Te +6, +4, 2 C +4, +2, 4 Ga +3 I Ge +4, 4 Xe +6, +4, +2 N +5, +4, +3 +2, +1, 3, 2, 1 +7, +5, +1, 1 O +1, +2, 1/2, 1, 2 As +5, +3, 3 Cs +1 F 1 Se +6, +4, 2 Ba +2 Na +1 Br +5, +3, +1, 1 La +3 Mg +2 Kr +4, +2 Hf +4 Al +3 Rb +1 Ta +5 Si +4, 4 Sr +2 W +6, +4 P +5, +3, 3 Y +3 Re +7, +6, +4 S +6, +4, +2, 2 Zr +4 Os +8, +4 Nb +5, +4 Ir +4, +3 Cl +7, +6, +5, +4, +3, +1, 1 K +1 Mo +6, +4, +3 Pt +4, +2 Ca +2 Tc +7, +6, +4 Au +3, +1 Sc +2 ,+3 Ru +8, +6, +4, +3 Hg +2, +1 Ti +4, +3, +2 Pd +4, +2 Tl +3, +1 V +5, +4, +3, +2 Ag +1 Pb +4, +2 Cr +6, +5, +4, +3 Cd +2 Bi +5, +3 Po +2 In +3 At 1 +7, +6, +4, Mn +3, +2 13 Chemistry Vol ‐ 1.2 (Med. and Engg.) Multiple Choice Questions 8. Change of hydrogen into proton is _______. (A) Oxidation of hydrogen (B) Acid-base reaction (C) Reduction of hydrogen (D) Displacement reaction 9. In the reaction, H2S+NO2 H2O + NO + S, H2S is _______. (A) Oxidized (B) Reduced (C) Precipitated (D) None of these 10. In a reaction between zinc and iodine, in which zinc iodide is formed, what is being oxidized? [NCERT 1975] (A) Zinc ions (B) Iodide ions (C) Zinc atom (D) Iodine atom 11. When a piece of copper wire is dipped in AgNO3 solution, the colour of the solution turns blue due to _______. (A) Formation of soluble complex (B) Oxidation of copper (C) Oxidation of silver (D) Reduction of copper 12. Which one of the following does NOT get oxidized by bromine water? (B) Cu+ to Cu2+ (A) Fe2+ to Fe3+ (C) Mn2+ to MnO 4 (D) Sn2+ to Sn4+ 13. In which of the following reactions, the underlined substance has been oxidized? (A) Br2 + H2S 2HBr + S (B) 2HgCl2 + SnCl2 Hg2Cl2+ SnCl4 (C) Cl2 + 2KI 2KCl + I2 (D) 2Cu2+ + 4I Cu2I2 + I2 14. Reduction is a process which involves _______. (A) gain of electrons (B) loss of electronegative element (C) decrease in the oxidation number of one of the atoms (D) all of these 15. Reduction is defined as _______. (A) Increase in positive valency (B) Gain of electrons (C) Loss of protons (D) Decrease in negative valency 16. H2O2 reduces MnO 4 ion to _______. [KCET (Med.) 2000] (A) Mn+ (B) Mn2+ (C) Mn3+ (D) Mn 8.0 Introduction 1. The metallic elements are extracted from their ores by _______. (A) precipitation reactions (B) acid-base neutralization (C) redox reactions (D) complexometric titrations 8.1 Concept of oxidation and reduction 2. 3. 4. 5. 6. 7. 14 According to classical concept, oxidation involves which of the following? (A) Addition of oxygen. (B) Addition of electronegative radical. (C) Removal of either hydrogen or some electropositive radical. (D) All of these. According to modern concept, oxidation is which of the following? (A) Electronation (B) De-electronation (C) Addition of oxygen (D) Addition of electronegative element Oxidation involves _______. [NCERT 1971, 81; CPMT 1980, 82, 83; MP PMT 1983] (A) Loss of electrons (B) Gain of electrons (C) Increase in the valency of negative part (D) Decrease in the valency of positive part Oxidation is a process which involves _______. (A) gain of electrons (B) loss of an electronegative radical (C) gain of an electropositive radical (D) increase in the oxidation number of one of the atoms When Sn2+ changes to Sn4+ in a reaction, _______. [CPMT 1981] (A) it loses two electrons (B) it gains two electrons (C) it loses two protons (D) it gains two protons Consider the reaction, Zn+Cu2+Zn2+ + Cu With reference to the above, which one of the following is the CORRECT statement? (A) Cu is oxidized to Cu2+. (B) Zn is oxidized to Zn2+. (C) Zn2+ is oxidized to Zn. (D) Cu2+ is oxidized to Cu. Chapter 08: Redox Reactions 17. When a sulphur atom becomes a sulphide ion, _______. [AMU 1999] (A) There is no change in the composition of atom (B) It gains two electrons (C) The mass number changes (D) None of these 25. An oxidizing agent is a substance which _______. (A) gains electrons (B) loses an electronegative radical (C) undergoes decrease in the oxidation number of one of its atoms (D) undergoes any one of the above changes 18. The reaction of KMnO4 and _______. (A) Oxidation of Mn in production of Cl2 (B) Reduction of Mn in production of H2 (C) Oxidation of Mn in production of H2 (D) Reduction of Mn in production of Cl2 26. In the course of a chemical reaction, an oxidant _______. [MP PMT 1986] (A) Loses electrons (B) Gains electrons (C) Loses as well as gains electron (D) Gains an electronegative radical. 27. Which of the following is an oxidizing substance? [CPMT 1997] (B) CO (A) C2H2O2 (D) CO2 (C) H2S 28. In a conjugate pair of reductant and oxidant, the oxidant has _______. (A) Higher oxidation number (B) Lower oxidation number (C) Same oxidation number (D) Either of these. 29. In which of the following reactions, hydrogen acts as an oxidizing agent? (A) With iodine to give hydrogen iodide (B) With lithium to give lithium hydride (C) With nitrogen to give ammonia (D) With sulphur to give hydrogen sulphide 30. In the reaction, 2FeCl3 + H2S 2FeCl2 + 2HCl + S (A) FeCl3 acts as an oxidizing agent (B) Both H2S and FeCl3 are oxidized (C) FeCl3 is oxidized while H2S is reduced (D) H2S acts as an oxidizing agent 31. Of the four oxyacids of chlorine the strongest oxidizing agent in dilute aqueous solution is _______. (B) HClO3 (A) HClO4 (C) HClO2 (D) HClO1 Which of the following CANNOT act as an oxidizing agent? [CPMT 1996] (B) KMnO4 (A) O2 (D) K2Cr2O7 (C) I2 HCl results in KMnO4 and KMnO4 and KMnO4 and KMnO4 and 19. The conversion of PbS to Pb is _______. (A) dissociation (B) reduction (C) oxidation (D) electrolysis 20. In which of the following reactions, the underlined substance has been reduced? (A) CO + CuO CO2 + Cu (B) CuO + 2HCl CuCl2 + H2O (C) 4H2O (g) + 3Fe 4H2 (g) + Fe3O4 (D) C + 4HNO3 CO2 + 2H2O + 4NO2 21. In the reaction, 4Zn+ NO3 +7H2O 4Zn2+ + NH 4 +10OH, the substance reduced is _______. (A) Zn (B) H2O (C) NO3 (D) NH 4 22. 23. 24. Which of the following is an example of reduction? (A) CuO Cu2O (B) [Fe(CN)6]4– [Fe(CN)6]3– (C) KI I2 (D) H2S S In the chemical reaction, Ag2O + H2O + 2e 2Ag + 2OH (A) water is oxidized. (B) electrons are reduced. (C) silver is oxidized. (D) silver is reduced. In which one of the following reactions, nitrogen is NOT reduced? (A) NO2 NO 2 (B) NO3 NO (C) NO3 NH 4 (D) NH 4 N2 32. 33. Reducing agent is a substance which can _______. [CPMT 1971, 74, 76, 78, 80; NCERT 1976] (A) accept electrons (B) accept protons (C) donate electrons (D) donate protons 15 Chemistry Vol ‐ 1.2 (Med. and Engg.) 34. Sodium amalgam is useful as which of the following? (A) Oxidizing agent (B) Reducing agent (C) Catalyst (D) Bleaching agent 35. Oxidation number of chlorine increases from 1 to 0. Hence HCl is _______ agent. (A) redox (B) oxidizing (C) reducing (D) complex 36. In a conjugate pair of reductant and oxidant, the reductant has _______. (A) Lower oxidation number (B) Higher oxidation number (C) Same oxidation number (D) Either of these 37. 38. 39. Nitric oxide acts as a reducing agent in which of the following reaction? (A) 4NH3 + 5O2 4NO + 6H2O (B) 2NO + 3I2 + 4H2O 2 NO3 + 6I + 8H+ (C) 2NO + H2SO3 N2O + H2SO4 (D) 2NO + H2S N2O + S + H2O Strongest reducing agent is _______. [CPMT 1977; BHU 1984, 96; MP PET 1990; AMU 1999] (B) Cl (A) F (C) Br (D) I Which substance is serving as a reducing agent in the following reaction? 14H+ + Cr2 O72 + 3Ni2Cr3+ +7H2O+ 3Ni2+ [CBSE PMT 1994; AFMC 2000; DPMT 2001] (B) Ni (A) H2O (D) Cr2 O72 (C) H+ 40. In the reaction between ozone and hydrogen peroxide, H2O2 acts as _______. [RPET 2000] (A) Oxidizing agent (B) Reducing agent (C) Bleaching agent (D) Both oxidizing and bleaching agent 41. In the reaction, Ag2O + H2O2 2Ag + H2O + O2, H2O2 acts as _______. [BHU 2004] (A) Reducing agent (B) Oxidizing agent (C) Bleaching agent (D) None of the above 16 42. In C + H2O CO + H2, H2O acts as : [AFMC 1988] (A) Oxidizing agent (B) Reducing agent (C) Both oxidizing agent and reducing agent. (D) Neither oxidizing agent nor reducing agent. 8.2 Redox reactions 43. A chemical reaction in which oxidation and reduction processes take place simultaneously is known as ________ reaction. (A) redox (B) precipitation (C) complexometric (D) titration 44. Which of the following behaves as both oxidising and reducing agent? [AFMC 1995] (B) SO2 (A) H2SO4 (C) H2S (D) HNO3 45. The compound that can work both as oxidising and reducing agent is [CPMT 1986; MP PET 2000] (A) KMnO4 (B) H2O2 (D) K2Cr2O7 (C) BaO2 46. A redox reaction is _______. (A) exothermic (B) endothermic (C) can neither be exothermic nor endothermic i.e. H = 0 (D) can be either exothermic or endothermic 47. Which of the following statements is CORRECT? (A) Oxidation of a substance is followed by reduction of another. (B) Reduction of a substance is followed by oxidation of another. (C) Oxidation and reduction are complementary reactions. (D) It is necessary that both oxidation and reduction should take place in the same reaction. 48. Which of the following is NOT an example of redox reaction? [NCERT Exemplar] (A) CuO + H2 Cu + H2O (B) Fe2O3 + 3CO 2Fe + 3CO2 (C) 2K + F2 2KF (D) BaCl2 + H2SO4 BaSO4 + 2HCl Chapter 08: Redox Reactions 49. 50. In the reaction, P + NaOH PH3 + NaH2PO2 [MP PET 2004] (A) P is oxidised only (B) P is reduced only (C) P is oxidised as well as reduced (D) Na is reduced In the following reaction, 4P + 3KOH + 3H2O 3KH2PO2 + PH3, Which of the following is TRUE? [Pb. PMT 2002] (A) P is oxidised as well as reduced (B) P is reduced only (C) P is oxidised only (D) None of these 51. 2CuI Cu + CuI2, the reaction is _______. [RPMT 1997] (A) Redox (B) Neutralisation (C) Oxidation (D) Reduction 52. 2 2 Co(s) + Cu (aq) Co(aq) + Cu(s). The reaction is an example for which of the following? (A) Oxidation reaction (B) Reduction reaction (C) Redox reaction (D) None of these 53. 54. 55. Which is the best description of the behaviour of bromine in the reaction given below? H2O + Br2 HOBr + HBr [CBSE PMT 2004] (A) It is oxidised only (B) It is reduced only (C) It acts as a proton acceptor only (D) It gets both oxidised and reduced Which of the following represents a redox reaction? (A) NaOH + HCl NaCl + H2O (B) BaCl2 + H2SO4 BaSO4 + 2HCl (C) CuSO4 + 2H2O Cu(OH)2 + H2SO4 (D) Zn + 2HCl ZnCl2 + H2 Which of the following is a redox reaction? [CBSE PMT 1997] (A) H2SO4 with NaOH. (B) In atmosphere, O3 from O2 by lightning. (C) Nitrogen oxides from nitrogen and oxygen by lightning. (D) Evaporation of H2O. 56. Of the following reactions, only one is a redox reaction. Identify it (A) Ca(OH)2 + 2HCl CaCl2 + 2H2O (B) BaCl2 + MgSO4 BaSO4 + MgCl2 (C) 2S2 O72 + 2H2O 4SO 24 + 4H+ (D) Cu2S + 2FeO 2Cu + 2Fe + SO2 57. Which of the following is a redox-reaction? [AIEEE 2002] (A) 2Na[Ag(CN)2] + Zn Na2[Zn(CN)4] + Ag (B) BaO2 + H2SO4 BaSO4 + H2O2 (C) N2O5 + H2O 2HNO3 (D) AgNO3 + K Ag2+ + KNO3 58. Which of the following is a redox reaction? [MP PMT 2003] (A) P2O5 + 2H2O H4P2O7 (B) 2AgNO3 + BaCl2 2AgCl + Ba(NO3)2 (C) BaCl2 + H2SO4 BaSO4 + 2HCl (D) Cu + 2AgNO3 2Ag + Cu(NO3)2 59. What is ‘A’ in the following reaction 2 2 2Fe3(aq) Sn (aq) 2Fe(aq) A? (A) Sn 3 (aq) (B) [MP PET 2003] 4 Sn (aq) (C) 2 Sn (aq) (D) Sn 60. Which of the following reactions involves oxidation-reduction? [NCERT 1972; AFMC 2000; Pb. CET 2004; CPMT 2004] (A) NaBr + HCl NaCl + HBr (B) HBr + AgNO3 AgBr + HNO3 (C) H2 + Br2 2HBr (D) 2NaOH + H2SO4 Na2SO4 + 2H2O 61. Which of the following is NOT a redox reaction? (A) 2Na + Cl2 2NaCl (B) C + O2 CO2 (C) AgNO3 + NaCl AgCl + NaNO3 (D) Zn + H2SO4 ZnSO4 + H2 62. Which of the following is a redox reaction? (A) NaCl + KNO3 NaNO3 + KCl (B) CaC2O4+ 2NH4Cl CaCl2 + H2C2O4 (C) Mg(OH)2+2NH4ClMgCl2+2NH4OH (D) Zn + 2AgCN 2Ag + Zn(CN)2 17 Chemistry Vol ‐ 1.2 (Med. and Engg.) 63. 64. 65. The violent reaction between sodium and water is an example of _______. (A) reduction (B) oxidation (C) redox reaction (D) neutralisation reaction The reaction 2H2O2(aq) 2H2O(l) + O2(g) is an example for which of the following? (A) oxidation (B) reduction (C) disproportionation (D) neither oxidation nor reduction In the reaction, 3Cl2 + 6NaOH NaClO3 + 5NaCl + 3H2O the element which loses as well as gains electrons is _______. (A) Na (B) O (C) Cl (D) None of these 66. Which of the following elements does NOT show disproportionation tendency? [NCERT Exemplar] (A) Cl (B) Br (C) F (D) I 67. Identify disproportionation reaction. [NCERT Exemplar] (A) CH4 + 2O2 CO2 + 2H2O (B) CH4 + 4Cl2 CCl4 + 4HCl (C) 2F2 + 2OH 2F– + OF2 + H2O (D) 2NO2 + 2OH NO 2 NO3 H 2 O 68. Which of the following change represents a disproportionation reaction ? (A) Cl2 + 2OH– ClO– + Cl– + H2O (B) Cu2O + 2H+ Cu + Cu2+ + H2O 3 (C) (D) 71. 72. P4(s)+ 3OH (aq) +3H2O(l) P H3 g + 3H 2 P O 2(aq) (D) All of the above Which of the following halogens always show only one oxidation state ? (A) Cl (B) F (C) Br (D) I 74. Fluorine does not show positive oxidation state due to which of the following reasons? (A) Absence of s-orbitals (B) Absence of p-orbitals (C) Absence of d-orbitals (D) Highest electronegativity 75. The characteristic oxidation in free metals is _______. (A) Minus one (B) (C) One (D) 76. The oxidation number of hydrogen in LiH is _______. (A) + 1 (B) – 1 (C) 2 (D) 0 77. Hydrogen can have oxidation number/s of _______. (A) 1 only (B) + 1 only (C) 0 only (D) 1, 0, + 1 78. O.N. of hydrogen in KH, MgH2 and NaH respectively would be _______. (A) –1, – 1 and –1 (B) +1, + 1, and + 1 (C) +2, +1 and –2 (D) –2, –3 and –1 79. In which of the following oxidation number of chlorine is + 5? (A) Cl (B) ClO (C) ClO 2 (D) ClO 3 80. In which of the following compounds, an element exhibits two different oxidation states? [NCERT Exemplar] (A) NH2OH (B) NH4NO3 (D) N3H (C) N2H4 8.3 Oxidation number 69. 70. 18 Oxidation number of metals is always _______. (A) positive (B) negative (C) zero (D) neutral 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? [NCERT Exemplar] (A) The oxidation number of hydrogen is always +1. (B) The algebraic sum of all the oxidation numbers in a compound is zero. The oxidation number of oxygen in peroxide is _______. (A) 2 (B) 1 (C) +1 (D) +2 The element with atomic number 9 can exhibit oxidation state of _______. (A) + 1 (B) + 3 (C) 1 (D) + 5 73. 1 (C) An element in the free or the uncombined state bears oxidation number zero. In all its compounds, the oxidation number of fluorine is – 1. number of atoms [NCERT 1975] Two Zero Chapter 08: Redox Reactions 81. In which of the following compounds, iron has the lowest oxidation number? [MNR 1984] (A) Fe(CO)5 (B) Fe2O3 (C) K4[Fe(CN)6] (D) FeSO4.(NH4)2SO4.6H2O 82. A metal ion M3+ loses 3 electrons, its oxidation number will become _______. [CPMT 2002] (A) + 3 (B) + 6 (C) 0 (D) 3 83. The oxidation number of P in HP2 O7 ion is _______. (A) + 5 (B) + 6 (C) + 7 (D) + 3 84. The oxidation number of Fe in [Fe(CN)6]3 ion is _______. (A) + 2 (B) + 3 (C) 2 (D) 3 85. 86. 87. 88. The oxidation number of Mn is + 7 in _______. (A) manganese dioxide (B) manganese chloride (C) manganese sulphate (D) potassium permanganate In the conversion of K2Cr2O7 to K2CrO4, the oxidation number of chromium _______. (A) remains same (B) increases (C) decreases (D) none of these The oxidation number of C in sucrose (C12H22O11) is _______. (A) + 4 (B) + 3 (C) + 2 (D) zero The change in oxidation number of Pb and Cl in the reaction, PbO2 +ClClO+Pb OH 3 respectively are _______. (A) + 2, + 2 (C) + 2, 1 (B) (D) 2, + 2 2, + 1 89. The oxidation state of S in S2 O82 is _______. (A) + 2 (B) + 4 (C) + 6 (D) + 7 90. Oxidation number of iodine varies from _______. [CPMT 1982] (A) – 1 to + 1 (B) – 1 to + 7 (C) + 3 to + 5 (D) – 1 to + 5 91. Match List I with List II and select the correct answer using the codes given below the lists: List I List II (Compound) (Oxidation state of N) (A) NO2 i. + 5 (B) HNO ii. – 3 (C) NH3 iii. + 4 (D) N2O5 iv. + 1 Codes : (A) A B C D ii. iii. iv. i. (B) A B C D iii. i. ii. iv. (C) A B C D iii. iv. ii. i. (D) A B C D ii. iii. i. iv. 92. When SO2 is passed through acidic solution of potassium dichromate, then chromium sulphate is formed. Change in valency of chromium is _______. [CPMT 1979] (A) + 4 to + 2 (B) + 5 to + 3 (C) + 6 to + 3 (D) + 7 to + 2 93. The oxidation number of sulphur in S8, S2F2, H2S respectively are _______. [IIT 1999] (A) 0, + 1 and 2 (B) + 2, + 1 and 2 (C) 0, + 1 and + 2 (D) 2, + 1 and 2 94. Among the following, identify the species with an atom in + 6 oxidation state. [IIT Screening 2000] (A) MnO 4 (B) Cr(CN)36 (C) NiF62 (D) CrO2Cl2 95. The oxidation number of Cr in CrO5 is _______. (A) +3 (B) +5 (C) +6 (D) 0 96. Calculate the oxidation number of the underlined element Ba2XeO6. (A) + 4 (B) + 8 (C) + 5 (D) + 6 97. Oxidation number of C in CH3OH, CH2O, HCOOH and C2H2 is respectively _______. (B) +2, 0, +2, 2 (A) 2, 0, +2, 1 (C) 2, 0, +2, 0 (D) 2, 4, +2, 2 98. The oxidation state of oxygen in H2O is _______. (A) 1 (B) +2 (C) 2 (D) +1 19 Chemistry Vol ‐ 1.2 (Med. and Engg.) 99. Which of the following arrangements represent increasing oxidation number of the central atom? [NCERT Exemplar] 2 (A) CrO 2 , ClO3 , CrO 4 , MnO 4 (B) ClO3 , CrO 24 , MnO 4 , CrO 2 (C) CrO 2 , ClO3 , MnO 4 , CrO 42 (D) CrO 24 , MnO 4 , CrO 2 , ClO3 100. The compound in which oxidation state of metal is zero is _______. (A) Fe2(CO)9 (B) Ni(CO)4 (C) Fe3(CO)9 (D) All of the above 101. The oxidation state of phosphorus is + 3 in which of the following? (A) Orthophosphorous acid (B) Orthophosphoric acid (C) Pyrophosphoric acid (D) Metaphosphoric acid 102. In the compounds KMnO4 and K2Cr2O7, the highest oxidation state is of which of the following element? (A) Potassium (B) Manganese (C) Chromium (D) Oxygen 103. Oxidation number of nitrogen in NH3 is : [CPMT 1979; Pb CET 2004] (A) – 3 (B) + 3 (C) 0 (D) + 5 104. The oxidation number of chlorine in HOCl is _______. (A) – 1 (B) 0 (C) + 1 (D) + 2 105. Sulphur has lowest oxidation number in which of the following? [EAMCET 1993] (A) H2SO3 (B) SO2 (C) H2SO4 (D) H2S 106. Oxidation number of oxygen in ozone (O3) is _______. [MP PET 2000; MP PMT 2001] (A) + 3 (B) – 3 (C) – 2 (D) 0 109. Oxidation number of Pt in [Pt(C2H4)Cl3]– is _______. [MLNR 1993] (A) +1 (B) +2 (C) +3 (D) +4 110. The sum of the oxidation numbers of all the carbons in C6H5CHO is _______. [EAMCET 1986] (A) + 2 (B) 0 (C) + 4 (D) – 4 111. Calculate the oxidation number of the underlined element K4P2O7 (A) +3 (B) 5 (C) +5 (D) +6 112. In which compound, oxidation state of nitrogen is +1? [MP PMT 1989] (A) NO (B) N2O (D) N2H4 (C) NH2OH 113. Carbon is in the lowest oxidation state in _______. [NCERT 1979; MH CET 1999] (A) CH4 (B) CCl4 (D) CO2 (C) CF4 114. The oxidation states of sulphur in the anions SO32 ,S2 O 42 and S2 O62 follows the order: [CBSE PMT 2003] 2 2 2 (A) S2 O 6 S2 O 4 SO3 (B) S2 O 24 SO32 S2 O62 (C) SO32 S2 O 42 S2 O62 (D) S2 O 24 S2 O 62 SO32 115. H2S acts only as a reducing agent while SO2 can act both as a reducing and oxidising agent because _______. [AMU 1999] (A) S in H2S has – 2 oxidation state and S in SO2 has oxidation state + 4 (B) Hydrogen in H2S is more +ve than oxygen (C) Oxygen is more – ve in SO2 (D) None of the above 107. Oxidation number of cobalt in K[Co(CO)4] is [KCET 1996] _______. (A) + 1 (B) + 3 (C) – 1 (D) – 3 116. In the conversion Br2 BrO3 , the oxidation state of bromine changes from [EAMCET 1990; AMU 1999; _______. RPMT 2002] (A) – 1 to 0 (B) 0 to – 1 (C) 0 to + 5 (D) 0 to – 5 108. The charge on cobalt in [Co(CN)6]3– is [CPMT 1985, 93] _______. (A) – 6 (B) – 3 (C) + 3 (D) + 6 117. Oxidation state of chlorine in perchloric acid is _______. [EAMCET 1989] (A) – 1 (B) 0 (C) – 7 (D) + 7 20 Chapter 08: Redox Reactions 118. Sn++ loses two electrons in a reaction. What will be the oxidation number of tin after the reaction? (A) + 2 (B) Zero (C) + 4 (D) – 2 119. Oxidation number of carbon in H2C2O4 is _______. [CPMT 1982] (A) + 4 (B) + 3 (C) + 2 (D) – 2 120. The oxidation state of Cr in [Cr(NH3)4Cl2]+ [AIEEE 2005] is _______. (A) +3 (B) +2 (C) +1 (D) 0 121. The oxidation number of Fe and S in iron pyrites are _______. [RPMT 1997] (A) 4, – 2 (B) 2, – 1 (C) 3, – 1.5 (D) 3, – 1 + 122. The oxidation number of N in N2H5 is [Pb. PMT 2001] _______. (A) – 3 (B) – 2 (C) – 1 (D) + 2 6 123. The oxidation state of I in H 4 IO is _______. [CBSE PMT 1994] (A) + 7 (B) + 5 (C) + 1 (D) – 1 124. The brown ring complex compound is formulated as [Fe(H2O)5NO]SO4. The oxidation state of iron is _______. [EAMCET 1987; IIT 1987; MP PMT 1994; AIIMS 1997; DCE 2000] (A) 1 (B) 2 (C) 3 (D) 0 125. Oxidation state of oxygen in F2O is _______. [BHU 1982; UPSEAT 2001; MH CET 2002] (A) + 1 (B) + 2 (C) –1 (D) –2 126. Oxidation number of P in KH2PO2 is which of the following? [CPMT 1987; MH CET 1999] (A) + 1 (B) + 3 (C) + 5 (D) – 4 127. The oxidation number of phosphorus in Ba(H2PO2)2 is _______. [Kurukshetra CEE 1998; DCE 2004] (A) – 1 (B) + 1 (C) + 2 (D) + 3 128. Oxidation number of Xenon in XeOF2 is _______. (A) 0 (B) 2 (C) 4 (D) 3 129. In XeO3 and XeF6 the oxidation state of Xe is _______. [MP PET 2003] (A) + 4 (B) + 6 (C) + 1 (D) + 3 130. Oxidation numbers of the two nitrogen atoms present in ammonium nitrate are respectively ? (A) +3 and +3 (B) 0 and 0 (C) –3 and +5 (D) –1 and –1 131. Oxidation numbers of two Cl atoms in bleaching powder, CaOCl2 are _______. (A) – 1, – 1 (B) + 1, – 1 (C) + 1, + 1 (D) 0, – 1 132. In which compound chlorine has highest oxidation number? (A) KCl (B) HClO (C) HClO2 (D) HClO4 133. What is the oxidation number of sulphur in Na2S4O6? [AIIMS 1998; DCE 1999] 2 3 (B) (A) 3 2 3 5 (D) (C) 5 2 134. When CuSO4 reacts with KI, the oxidation number of Cu changes by _______. [BHU 1997] (A) 0 (B) – 1 (C) 1 (D) 2 135. What is the oxidation number of Co in [Co(NH3)4ClNO2] _______. [BHU 1999] (A) + 2 (B) + 3 (C) + 4 (D) + 5 136. The oxidation number of nickel in K4[Ni(CN)4] is _______. [JIPMER 1999] (A) – 2 (B) – 1 (C) + 2 (D) 0 137. Oxidation number of nitrogen in NaNO2 is _______. [Pb. CET 2000] (A) + 2 (B) + 3 (C) + 4 (D) – 3 138. In which of the following compounds, the oxidation number of carbon is not zero? (A) C12H22O11 (B) HCHO (D) CH3COOH (C) CH3CHO 21 Chemistry Vol ‐ 1.2 (Med. and Engg.) 139. In the substance Mg(HXO3), the oxidation number of X is _______. (A) 0 (B) +2 (C) +3 (D) +5 140. What will be the oxidation state of copper in YBa2Cu3O7, if oxidation state of (Y) is +3 ? (A) 7/3 (B) 7 (C) 3 and 5 (D) 3/7 141. The oxidation number of P in SO 24 and that of respectively _______. (A) 3, +6 and +6 (B) +5, +6 and +6 (C) +3, +6 and +5 (D) +5, +3 and +6 PO34 , of S in Cr in Cr2 O72 are [Assam CEE 2015] 142. The oxidation state of chromium in the final product formed by the reaction between KI and acidified potassium dichromate solution is which of the following? [AIEEE 2005] (A) +4 (B) +6 (C) +2 (D) +3 143. Oxidation number of S in H2S2O7 is _______. (A) +4 (B) –6 (C) –5 (D) +6 144. The oxidation number of sulphur in H2S is _______. (A) –2 (B) +3 (C) +2 (D) –3 145. The oxidation state of S in H2SO4 is _______. (A) +8 (B) +6 (C) +5 (D) +4 146. The oxidation state of sulphur in SO 24 is _______. [Bihar MEE 1996] (A) +4 (B) 2 (C) +6 (D) –6 147. 3CuO + 2NH3 3Cu + N2 + 3H2O In the above conversion, the oxidation number of nitrogen is changing from _______. (A) +5 to 0 (B) 0 to +2 (C) +2 to 0 (D) –3 to –5 148. Oxidation number of _______. (A) From + 5 to – 3 (B) From – 5 to – 3 (C) From – 5 to + 3 (D) From+10 to + 6 22 nitrogen can be 149. Oxidation state of nitrogen is CORRECTLY given for _______. (A) (B) (C) (D) Compound HNO3 NH2OH (N2H5)2SO4 Mg3N2 Oxidation state –5 +1 +2 3 150. In which of the compounds does manganese exhibit highest oxidation number? (A) MnO2 (B) Mn3O4 (D) MnSO4 (C) K2MnO4 151. In which of the following compounds, the oxidation number of iodine is fractional? (B) I3 (A) IF7 (D) IF3 (C) IF5 152. The element, which shows minimum oxidation number in its compound is _______. [RPET 1992] (A) Fe (B) Mn (C) Ca (D) K 153. Oxidation number of N in HNO3 is _______. [BHU 1997] (A) –3.5 (B) +3.5 (C) –3, +5 (D) +5 154. Oxidation number of P in Mg2P2O7 is _______. [CPMT 1989; MP PMT 1995] (A) +3 (B) +2 (C) +5 (D) –3 155. The oxidation number of hydrogen in MgH2 is _______. [CPMT 1976] (A) +1 (B) –1 (C) +2 (D) –2 156. The oxidation number of carbon in CH2O is _______. [IIT 1982; EAMCET 1985; MNR 1990; UPSEAT 2001; CPMT 1997] (A) –2 (B) +2 (C) 0 (D) +4 157. The oxidation number of carbon in CH2Cl2 is _______. [CPMT 1976; Pb. PET 1999; AFMC 2004] (A) 0 (B) + 2 (C) –2 (D) + 4 158. Which one of the following oxidation number of iodine? (A) KI3 (B) (D) (C) IF5 has the highest [CPMT 1982] KI KIO4 Chapter 08: Redox Reactions 159. The oxidation state of Mo in Mo 2 Cl84 ion is [EAMCET 1994] _______. (A) –4 (B) –2 (C) +6 (D) +2 160. The oxidation number of Cr in K2Cr2O7 is _______. [CPMT 1981, 85, 90, 93, 99; KCET 1992; BHU 1988, 98; AFMC 1991, 99; EAMCET 1986; MP PMT 1996, 99, 2002; MP PET/PMT 1998; Bihar CEE 1995; RPET 2000] (A) +6 (B) –7 (C) +2 (D) –2 161. The valency of Cr in the complex [Cr(H2O)4Cl2]+ is _______. [MP PMT 2000] (A) 1 (B) 3 (C) 5 (D) 6 162. The oxidation number of Ba in barium [Pb. PMT 2002] peroxide is _______. (A) +6 (B) +2 (C) 1 (D) +4 163. If HNO3 changes into N2O, the oxidation number is changed by _______. [BHU 1997; AFMC 2001] (A) + 2 (B) –1 (C) 0 (D) +4 164. The oxidation states of the most electronegative element in the products of the reaction of BaO2 with dilute H2SO4 are _______. [IIT 1991; CBSE PMT 1992; BHU 2000] (A) 0 and –1 (B) –1 and –2 (C) –2 and 0 (D) –2 and +1 165. Thiosulphate reacts differently with iodine and bromine in the reactions given below: 2S2 O32 I 2 S4 O62 2I S2 O32 2Br2 5H 2 O 2SO 24 2Br 10H Which of the following statements justifies the above dual behaviour of thiosulphate? [NCERT Exemplar] (A) Bromine is a stronger oxidant than iodine. (B) Bromine is a weaker oxidant than iodine. (C) Thiosulphate undergoes oxidation by bromine and reduction by iodine in these reactions. (D) Bromine undergoes oxidation and iodine undergoes reduction in these reactions. 166. The most common oxidation state of an element is –2. The number of electrons present in its outermost shell is _______. [BHU 1983; NCERT 1974; CPMT 1977] (A) 4 (B) 2 (C) 6 (D) 8 167. When KMnO4 is reduced with oxalic acid in acidic solution, the oxidation number of Mn changes from _______. [MNR 1987; MP PET 2000; CBSE PMT 2000; UPSEAT 2000, 02; BHU 2003; AMU 2002] (A) 7 to 4 (B) 6 to 4 (C) 7 to 2 (D) 4 to 2 168. In the chemical reaction Cl2 + H2S 2HCl + S, the oxidation number of sulphur changes from _______. [MP PMT 1999] (A) 0 to 2 (B) 2 to 0 (C) – 2 to 0 (D) – 2 to – 1 169. When KMnO4 acts as an oxidising agent and ultimately forms [MnO4]–2, MnO2, Mn2O3, Mn+2 then the number of electrons transferred in each case respectively are _______. [AIEEE 2002] (A) 4, 3, 1, 5 (B) 1, 5, 3, 7 (C) 1, 3, 4, 5 (D) 3, 5, 7, 1 8.5 Balancing redox reactions in terms of loss and gain of electrons 170. In acidic medium, reaction : MnO 4 Mn2+ [MP PMT 1989] is an example of : (A) Oxidation by three electrons (B) Reduction by three electrons (C) Oxidation by five electrons (D) Reduction by five electrons 171. Which of the following equation is a balanced [EAMCET 1980] one ? (A) 5BiO3 + 22H+ + Mn2+ 5Bi3+ + 7H2O + MnO 4 (B) 5BiO3 + 14H+ + 2Mn2+ 5Bi3+ + 7H2O + 2 MnO 4 (C) 2BiO3 + 4H+ + Mn2+ 2Bi3+ + 2H2O + MnO 4 (D) 6BiO3 + 12H+ + 3Mn2+ 6Bi3+ + 6H2O + 3 MnO 4 23 Chemistry Vol ‐ 1.2 (Med. and Engg.) 172. Which of the following equation is a balanced equation? (A) FeS2 + 4O2 8SO2 + 2Fe2O3 (B) 4FeS2 + 6O2 2Fe2O3 + 8SO2 (C) 4FeS2 + 11O2 2Fe2O3 + 8SO2 (D) 2FeS2 + 11O2 Fe2O3 + 6SO2 173. In a balanced equation, H2SO4 + x HI H2S + y I2 + zH2O, the values of x, y, z are _______. [EAMCET 2003] (A) x = 3, y = 5, z = 2 (B) x = 4, y = 8, z = 5 (C) x = 8, y = 4, z = 4 (D) x = 5, y = 3, z = 4 174. In the balanced equation, 5H2O2 + XClO2 + 2OH– XCl– + YO2 + 6H2O The reaction is balanced if _______. [CPMT–2000] (A) X = 5, Y = 2 (B) X = 2, Y = 5 (C) X = 4, Y = 10 (D) X = 5, Y = 5 175. In the equation, 4M + 8CN– + 2H2O + O2 4[M(CN)2]– + 4OH– Identify the metal M. [AFMC 1998] (A) Cobalt (B) Iron (C) Gold (D) Zinc 176. The number of moles of KMnO4 reduced by one mole of KI in alkaline medium is: [CBSE PMT 2005] (A) One fifth (B) five (C) One (D) Two 177. The product of oxidation of I– with MnO 4 in alkaline medium is _______. [IIT-JEE Screening 2004] (A) IO3 (B) I2 (C) IO– (D) IO 4 178. In the following equation ClO3 + 6H+ + X Cl- + 3H2O, X is _______. (A) O (B) 6e– (D) 5e– (C) O2 24 179. The number of electrons involved in the reduction of Cr2 O72 in acidic solution to Cr3+ [EAMCET 1983] is _______. (A) 0 (B) 2 (C) 3 (D) 5 180. C2H6 (g) + nO2 CO2 (g) + H2O(l) In this equation, the ratio of the coefficients of CO2 and H2O is _______. [KCET 1992] (A) 1 : 1 (B) 2 : 3 (C) 3 : 2 (D) 1 : 3 181. In the following reaction, Cr2 O72 +14H+ + 6I 2Cr3+ + 3H2O + 3I2. Which element is reduced? [CPMT 1976] (A) Cr (B) H (C) O (D) I 182. 2MnO 4 +5H2O2+ 6H+ 2Z + 5O2 + 8H2O. In this reaction Z is _______. [Rajasthan PMT 2002] (A) Mn+2 (B) Mn+4 (D) Mn (C) MnO2 183. For the redox reaction, MnO 4 + C2 O 42 + H+ Mn2+ + CO2 + H2O the CORRECT coefficients of the reactants for the balanced reaction are _______. [IIT 1988, 92; BHU 1995; CPMT 1997; RPMT 1999; DCE 2000; MP PET 2003] (A) (B) (C) (D) MnO 4 C2O 42 H+ 2 16 5 2 5 5 16 16 16 2 2 5 184. In which reaction there is a change in valency? [NCERT 1971; CPMT 1971] (A) 2NO2 N2O4 (B) 2NO2 + H2O HNO2 + HNO3 (C) NH4OH NH4+ + OH (D) CaCO3 CaO + CO2 185. In which of the following reactions there is no change in valency? [NCERT 1974; CPMT 1978] (A) 4KClO3 3KClO4 + KCl (B) SO2 + 2H2S 2H2O + 3S (C) BaO2 + H2SO4 BaSO4 + H2O2 (D) 2BaO + O2 2BaO2 Chapter 08: Redox Reactions 186. 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. E values: Fe3+/Fe2+ = + 0.77; I2(s)/I– = + 0.54; Cu2+/Cu = + 0.34; Ag+/Ag = + 0.80 V [NCERT Exemplar] 3+ (B) I2(s) (A) Fe (C) Cu2+ (D) Ag+ Applications of redox reactions: 187. Which of the following involves the redox reaction? (A) Corrosion of metals (B) Extraction of metals from their ores (C) Respiration (D) All of these 188. Sulphide minerals are converted to oxides by _______. (A) cracking (B) coke (C) roasting (D) bleaching 189. Bleaching is _______. (A) decolourization of coloured material (B) extraction and purification of metals (C) destruction of metals by oxidation (D) using oxygen for biological redox reactions 190. The reaction involved in household bleaching is _______. (A) acid-base neutralisation (B) redox (C) precipitation reaction (D) complex reaction 191. ______ is used as an oxidizing agent to bleach dark hair by a redox reaction. (B) HO2 (A) H2O (D) HO (C) H2O2 192. To remove stains from clothes ______ is used. (A) NaCl (B) NaOH (C) Na2O2 (D) NaOCl 193. The burning of methane in air by oxidation is an example of _______. (A) bleaching (B) metallurgy (C) combustion (D) respiration 194. ________ is the destruction of metals by oxidation. (A) Combustion (B) Bleaching (C) Roasting (D) Corrosion 195. In the rusting of iron, iron has been _______. (A) Oxidized (B) Reduced (C) Vapourised (D) Decomposed 196. Corroded iron is _______. (A) FeO3.H2O (B) (C) 2Fe2O3.H2O (D) 2FeO3. H2O Fe2O3.2H2O 197. The metal used in galvanizing of iron is _______. [MP PET 1985, 96] (A) Pb (B) Zn (C) Al (D) Sn 198. Prevention of corrosion of iron by zinc coating is called _______. [MP PMT 1993; CPMT 2002] (A) Galvanization (B) Cathodic protection (C) Electrolysis (D) Photo–electrolysis 199. Respiration is _______. (A) Oxidation (B) (C) Both (A) and (B) (D) Reduction None of these 200. The overall effect of respiration is similar to which of the following? (A) Photosynthesis (B) Corrosion (C) Combustion (D) Bleaching Miscellaneous 201. The process in which oxidation number increases is known as _______. [CPMT 1976] (A) Oxidation (B) Reduction (C) Auto-oxidation (D) None of the above 202. Following reaction describes the rusting of iron 4Fe + 3O2 4Fe3+ + 6O2. Which one of the following statement is INCORRECT ? [NCERT 1981; MNR 1991; AIIMS 1998] (A) This is an example of a redox reaction. (B) Metallic iron is reduced to Fe3+. (C) Fe3+ is an oxidising agent. (D) Metallic iron is a reducing agent. 25 Chemistry Vol ‐ 1.2 (Med. and Engg.) 203. The process of conversion of SnCl4 to SnCl2 is _______. (A) oxidation (B) reduction (C) neither reduction nor oxidation (D) both oxidation and reduction 204. When iron or zinc is added to CuSO4 solution, copper is precipitated. It is due to which of the following? [CPMT 1974, 79] +2 (A) Oxidation of Cu (B) Reduction of Cu+2 (C) Hydrolysis of CuSO4 (D) Ionisation of CuSO4 205. SnCl2 gives a precipitate with a solution of HgCl2.In this process, HgCl2 is _______. [CPMT 1983] (A) Reduced (B) Oxidised (C) Converted into a complex compound containing both Sn and Hg (D) Converted into a chloro complex of Hg 206. Hot concentrated sulphuric acid is a moderately strong oxidizing agent. Which of the following reactions does NOT show [NEET P-II 2016] oxidizing behaviour? (A) CaF2 + H2SO4 CaSO4 + 2HF (B) Cu + 2H2SO4 CuSO4 + SO2 +2H2O (C) 2S + 2H2SO4 2SO2 + 2H2O (D) C + 2H2SO4 CO2 + 2SO2 + 2H2O 207. Which of the following acts both as an oxidizing as well as reducing agent? (A) HNO3 (B) HNO2 (C) HI (D) H2SO4 208. In which of the following reactions oxidation and reduction is occurring? (A) AgNO3 + HCl AgCl + HNO3 (B) H2 + Cl2 2HCl (C) BaCl2 + H2SO4 BaSO4 + 2HCl (D) KI + HCl KCl + HI 209. Which one of the following reactions does NOT involve either oxidation or reduction? [EAMCET 1982] (A) VO2 V2O3 (B) 26 Na Na+ (C) CrO24 Cr2 O72 (D) Zn2+ Zn 210. The atomic number of an element which shows the oxidation state of +3 is: [CPMT 1989, 94] (A) 13 (B) 32 (C) 33 (D) 17 211. Most common oxidation number of an element is 1. The number of electrons in its outermost shell is _______. (A) 7 (B) 1 (C) 8 (D) 4 212. Calculate the oxidation number of sulphur in S2 O72 . (A) +6 (B) 6 (C) +7 (D) +4 213. Oxidation number of sulphur in S2 O32 is _______. (A) – 2 (C) + 6 (B) (D) [CPMT 1979] +2 0 214. Oxidation state of Fe in Fe3O8 is ________. [CBSE 1999] (A) + 3/2 (B) + 4/5 (C) + 15/4 (D) + 16/3 215. In ferrous ammonium sulphate oxidation number of Fe is _______. [CPMT 1988] (A) +3 (B) +2 (C) +1 (D) –2 216. Oxidation number of carbon in carbon suboxide (C3O2) is _______. (A) +2/3 (B) +4/3 (C) +4 (D) 4/3 217. E values of some redox couples are given below. On the basis of these values, choose the CORRECT option. E values: Br2/Br– = + 1.09; Ag+ /Ag(s) = + 0.80 Cu2+/Cu(s) = + 0.34; I2(s)/I– = + 0.54 [NCERT Exemplar] (A) Cu will reduce Br– (B) Cu will reduce Ag (C) Cu will reduce I– (D) Cu will reduce Br2 Chapter 08: Redox Reactions 218. In the following reaction, which is the species being oxidized? 3 2 2Fe(aq) + 2I(aq) I2(aq) + 2Fe(aq) (A) (C) Fe3+ I2 (B) (D) I Fe2+ 219. Oxidation number of carbon in diamond is _______. (A) – 4 (B) + 4 (C) 0 (D) + 2 220. 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? [NCERT Exemplar] 1 2 (B) 3d3 4s2 (A) 3d 4s 5 1 (C) 3d 4s (D) 3d5 4s2 221. In which one of the following compounds the oxidation number of carbon is zero? (B) C6H6 (A) C2H6 (C) CH3Cl (D) C12H22O11 222. Which one of the following nitrates will leave behind a metal on strong heating? [AIEEE 2003] (A) Ferric nitrate (B) Copper nitrate (C) Manganese nitrate (D) Silver nitrate 223. In which one of the following changes there are transfer of five electrons? [NCERT 1982] (A) MnO 4 Mn2+ (B) CrO 2-4 Cr3+ (C) MnO 4 MnO2 (D) Cr2 O72 2Cr3+ 224. How many moles of K2Cr2O7 are reduced by 1 mole of formic acid ? (A) 1/3Mole (B) 1Mole (C) 2/3Mole (D) 2Mole 225. Which halide is not oxidised by MnO2? [MNR 1985; JIPMER 2000] (A) F (B) Cl (C) Br (D) I 226. In the reaction, 8Al + 3Fe3O4 4Al2O3 + 9Fe, the number of electrons transferred from reductant to oxidant is _______. (A) 8 (B) 4 (C) 16 (D) 24 227. Oxidation number of S in S2Cl2 is _______. [CPMT 1979] (A) +1 (B) – 1 (C) + 6 (D) 0 228. To prevent rancidification of food material, which of the following is added? [CPMT 1996] (A) Reducing agent (B) Anti-oxidant (C) Oxidizing agent (D) None of these 229. The ultimate products of oxidation of most of hydrogen and carbon in food stuffs are _______. [DCE 2001] (A) H2O alone (B) CO2 alone (C) H2O and CO2 (D) None of these 230. A particular gas bleaches the colour of flowers by reduction while the other by oxidation. Name the gases. [EAMCET 1980] (A) CO and Cl2 (B) SO2 and Cl2 (C) H2S and Br2 (D) NH3 and SO2 231. The oxidation number and covalency of sulphur in the sulphur molecule (S8) are respectively _______. [NCERT 1977] (A) 0 and 2 (B) 6 and 8 (C) 0 and 8 (D) 6 and 2 232. Using the standard electrode potential, find out the pair between which redox reaction is NOT feasible. E values: Fe3+/Fe2+ = + 0.77; I2/I– = + 0.54; Cu2+/Cu = + 0.34; Ag+/Ag = + 0.80 V [NCERT Exemplar] 3+ – (A) Fe and I (B) Ag+ and Cu (C) Fe3+ and Cu (D) Ag and Fe3+ 27 Chemistry Vol ‐ 1.2 (Med. and Engg.) Answers to MCQs 1. (C) 11. (B) 21. (C) 31. (D) 41. (A) 51. (A) 61. (C) 71. (B) 81. (A) 91. (C) 101. (A) 111. (C) 121. (A) 131. (B) 141. (B) 151. (B) 161. (B) 171. (B) 181. (A) 191. (C) 201. (A) 211. (A) 221. (D) 231. (A) 2. (D) 12. (C) 22. (A) 32. (C) 42. (A) 52. (C) 62. (D) 72. (C) 82. (B) 92. (C) 102. (B) 112. (B) 122. (B) 132. (D) 142. (D) 152. (D) 162. (B) 172. (C) 182. (A) 192. (D) 202. (B) 212. (A) 222. (D) 232. (D) 3. (B) 13. (C) 23. (D) 33. (C) 43. (A) 53. (D) 63. (C) 73. (B) 83. (B) 93. (A) 103. (A) 113. (A) 123. (A) 133. (D) 143. (D) 153. (D) 163. (D) 173. (C) 183. (A) 193. (C) 203. (B) 213. (B) 223. (A) 4. (A) 14. (D) 24. (D) 34. (B) 44. (B) 54. (D) 64. (C) 74. (D) 84. (B) 94. (D) 104. (C) 114. (B) 124. (B) 134. (C) 144. (A) 154. (C) 164. (B) 174. (B) 184. (B) 194. (D) 204. (B) 214. (D) 224. (A) 5. (D) 15. (B) 25. (D) 35. (C) 45. (B) 55. (C) 65. (C) 75. (D) 85. (D) 95. (C) 105. (D) 115. (A) 125. (B) 135. (A) 145. (B) 155. (D) 165. (A) 175. (C) 185. (C) 195. (A) 205. (A) 215. (B) 225. (A) 6. (A) 16 (B) 26. (B) 36. (A) 46. (A) 56. (D) 66. (C) 76. (B) 86. (A) 96. (B) 106. (D) 116. (C) 126. (A) 136. (D) 146. (C) 156. (C) 166. (C) 176. (D) 186. (D) 196. (C) 206. (A) 216. (B) 226. (D) 7. (B) 17. (B) 27. (D) 37. (B) 47. (C) 57. (A) 67. (D) 77. (D) 87. (D) 97. (A) 107. (C) 117. (D) 127. (B) 137. (B) 147. (C) 157. (A) 167. (C) 177. (A) 187. (D) 197. (B) 207. (B) 217. (D) 227. (A) 8. (A) 18. (D) 28. (A) 38. (D) 48. (D) 58. (D) 68. (D) 78. (A) 88. (B) 98. (C) 108. (C) 118. (C) 128. (C) 138. (C) 148. (A) 158. (D) 168. (C) 178. (B) 188. (C) 198. (A) 208. (B) 218. (B) 228 (B) 9. (A) 19. (B) 29. (B) 39. (B) 49. (C) 59. (B) 69. (A) 79. (D) 89. (D) 99. (A) 109. (B) 119. (B) 129. (B) 139. (C) 149. (D) 159. (C) 169. (C) 179. (C) 189. (A) 199. (C) 209. (C) 219. (C) 229. (C) 10. (C) 20. (C) 30. (A) 40. (B) 50. (A) 60. (C) 70. (A) 80. (B) 90. (B) 100. (D) 110. (D) 120. (A) 130. (C) 140. (A) 150. (C) 160. (A) 170. (D) 180. (B) 190. (B) 200. (C) 210. (A) 220. (D) 230. (B) Hints to MCQs 4. It is the process in which electrons are lost (de-electronation). 6. Sn2+ Sn4+ + 2e. This is an oxidation reaction. 7. Zn + Cu2+ Zn2+ + Cu Here Zn is oxidized to Zn2+. 8. The change of hydrogen into proton involves the loss of a hydrogen atom. Hence it is an oxidation reaction. 10. 16. 2MnO4 +5H2O2+6H+ Mn2++5O2+8H2O 17. S + 2e S2 20. (A) (B) (C) (D) Oxidation 0 0 Zn + I2 ZnI2 In this reaction, Zn atom gets oxidised to Zn2+ ion and iodine gets reduced to I. 11. 2Ag+ + Cu Cu++ + 2Ag. 12. O.N. of Mn is already oxidized with change in oxidation number from +2 to +7, whereas all other ions can get oxidized. 28 CO CO2 +2 +2 +1 0 CuO CuCl2 H2O H2 0 +4 C CO2 Only in option (C), ON. of hydrogen decreases from + 1 to 0 and hence H2O gets reduced to H2. +2 –1 Reduction +4 +2 21. +5 3 NO 3 NH 4 Here, O.N. of N decreases from +5 in NO 3 to 3 in NH 4 and hence NO 3 gets reduced to NH 4 . 23. Ag2O + H2O + 2e 2Ag + 2OH Here the O.N. of Ag decreases from +1 in Ag2O to 0 in Ag. Therefore, silver is reduced. Chapter 08: Redox Reactions 24. (A) (B) +4 +3 45. Hydrogen peroxide (H2O2) acts as an oxidising as well as reducing agent. +5 +2 46. All redox reactions are exothermic. 48. BaCl2 + H2SO4 BaSO4 + 2HCl This is an example of double displacement reaction. Here, the oxidation number of all the interacting species remains the same after the reaction. Hence, it is not a redox reaction. NO2 NO2 , N is reduced. NO3 NO, N is reduced. +5 (C) (D) 29. 0 –3 3 4 NO NH , N is reduced. –3 0 0 +1 1 NH 4 N2 , N is oxidised. 2Li + H2 2Li H Here, H2 acts as an oxidizing agent since the O.N. of Li increases from 0 to + 1 and that of H2 decreases from 0 to 1. +3 2 0 +2 Reduction 49. 3 0 1 P +NaOH P H 3 NaH 2 P O 2 30. 2FeCl3 + H2S 2 FeCl2 + 2 HCl + S Here, FeCl3 acts as an oxidizing agent and oxidizes H2S to S. Hence option (A) is correct. 50. The oxidation number of P changes from 0 to +1 (in KH2PO2) and 0 to –3 (in PH3). Hence P is oxidised as well as reduced. 31. HClO1 is the strongest oxidizing agent. The correct order of oxidizing power is 53. H2O + Br2 HOBr + HBr +1 +3 0 +7 +5 HClO1 > HClO2 > HClO3 > HClO4 32. Because I2 is a reducing agent. 33. Reducing agent donates electrons. 37. 2NO + 3I2 + 4 H2O 2 NO3 + 6 I + 8 H+ +5 +2 Here, NO acts as a reducing agent and reduces I2 to I. Since the O.N. of nitrogen changes from + 2 in NO to + 5 in NO 3 , it undergoes oxidation whereas in reaction (C) and (D), the oxidation number of nitrogen changes from +2 in NO to +1 in N2O, thereby undergoing reduction. Hence NO acts as a reducing agent in reaction (B). 38. 54. The oxidation number of Ni changes from 0 to +2. Hence, Ni acts as a reducing agent in the given equation. 40. H2O2 acts as a reducing agent in the reaction between O3 and H2O2. 41. Reduction (oxidizing agent) Ag2O + H2O2 2Ag + H2O + O2 Oxidation (reducing agent) –1 Only in reaction (D), the O.N. of element changes. 0 +1 0 +2 Zn + 2HCl ZnCl2 + H2 55. (A) H2SO4 + 2NaOH Na2SO4 + 2H2O is a neutralization reaction. 0 (B) Iodine (I) is the least electronegative among the given set of ions. Therefore, electrons are more easily given away thereby resulting in Ibeing the strongest reducing agent. 39. +1 In the above reaction, the oxidation number of Br2 increases from zero (in Br2) to +1 (in HOBr) and decreases from zero (in Br2) to –1 (in HBr). Thus Br2 is oxidised as well as reduced and hence it is a redox reaction. 0 2O3 3O2 O.N. of O in both O3 and O2 is zero, therefore, it is not a redox reaction but is actually a decomposition reaction. 0 0 +2 2 2N2 + O2 2N O Here O.N. of N increases from 0 in N2 to + 2 in NO and that of O decreases from 0 in O2 to 2 in NO therefore, it is a redox reaction. (D) H2O(l) H2O(g) Only state of matter changes and hence there is no change in O.N. Thus, option (C) is correct. (C) 56. Only in reaction (D), the O.N. of the elements undergo a change +1 2 +2 0 0 +4 Cu2S + 2 FeO 2 Cu + 2 Fe + SO2 29 Chemistry Vol ‐ 1.2 (Med. and Engg.) i.e., O.N. of Cu decreases from + 1 to 0 and that of Fe decreases from + 2 to 0 while that of S increases from 2 to + 4. Therefore, it is a redox reaction. 57. 58. 67. –4 +1 O. N. of Ag changes from +2 to 0 (undergoing reduction) whereas that of Zn changes from 0 to +4 (undergoing oxidation). Oxidation 0 +2 0 H2 0 None of the element in reaction (C) undergoes a change in O.N., therefore, (C) is not a redox reaction. +1 1 +1 +5 2 +1 1 0 +1 +1 0 2 Na + 2H2O 2NaOH + H2 It is a redox reaction since here Na is oxidised to NaOH while H2O is reduced to H2. 0 65. 66. 30 +5 Disproportionation reactions are those reactions in which the same substance gets oxidised as well as reduced. Fluorine is the most electronegative element of the entire periodic table. In its compounds, it always has an oxidation number of –1. Hence, F can only be reduced and so, it does not show disproportionation tendency. +3 – 2 +2 –1 +1 –2 +5 – 2 +1 – 2 The element with atomic number 9 is Fluorine and the oxidation number of fluorine is always 1 in all of its compounds. Fluorine has the electronic configuration 1s22s22p5. Hence, it can exhibit oxidation state of –1. 77. 1 in metal hydrides, zero in H2 and + 1 in halogen acids. 79. (A) (B) (C) –1 3Cl2+6NaOH NaClO3 + 5 NaCl + 3H2O. Here, O.N. of Cl increases from 0 in Cl2 to + 5 in NaClO3 and decreases to 1 in NaCl, therefore, Cl gets both oxidised and reduced. –2 +1 –1 72. +1 +5 2 Oxidation Reduction 2+ + Zn Zn + 2e , Ag + e Ag –2 +1 +1 –1 Oxidation number of hydrogen is different in different compounds. Hydrogen exhibits + 1 oxidation number in hydrogen halides, –1 in hydrides and zero in H2 molecule. In fact, it is a double decomposition reaction. 63. 2F2 + 2 OH– 2F– + OF2 + H2O +4 –1 70. AgNO3 + NaCl AgCl + NaNO3 62. (C) 0 In the reaction (A) Cl2, reaction (B) Cu and reaction (C) P, undergo disproportionation reaction. reduction 61. CH4 + 4Cl2 CCl4 + 4HCl 68. +1 –1 0 + Br2 2HBr –2 (B) +4 –2 Oxidation +1 (D) 2NO2 +2OH– N O 2 + NO3– + H2O. In option (D) nitrogen is both oxidized i.e. (O.N. increases from +4 to +5) as well as reduced i.e., (O.N. decreases from + 4 to + 3). Hence, option (D) is an example of disproportionation reaction. 2Fe3+ + Sn2+ 2Fe2+ + Sn4+ 60. +4 –2 CH4 + 2O2 CO2 + 2H2O. 0 Cu + 2AgNO3 Cu(NO3)2 + 2Ag This is a redox reaction. 59. 0 (A) –4 +1 Reduction +1 Disproportionation reactions are those reactions in which the same substance gets oxidised as well as reduced. (D) 80. O.N. of Cl in Cl = 1 O.N. of Cl in ClO = x 2 = 1 or x = +1 O.N. of Cl in ClO 2 = x + 2 ( 2) = 1 or x = + 3 O.N. of Cl in ClO 3 = x + 3 ( 2) = 1 or x = + 5 Nitrogen exhibits two different oxidation states in NH4NO3. NH4NO3 NH 4 + NO3 i. Let the oxidation number of nitrogen be ‘x’ in NH 4 x + 4 = +1 x = –3 the oxidation state of nitrogen = –3 in NH 4 Chapter 08: Redox Reactions ii. Let the oxidation number of nitrogen be ‘y’ in NO3 y + (–2 3) = –1 y = +5 the oxidation state of nitrogen = +5 in NO3 88. 82. In metal carbonyls, the O.N. of metal is always zero. (A) O.N. of Fe in Fe (CO)5 is x + 5 0 = 0 or x = 0 (B) O.N. of Fe in Fe2O3 is 2x + 3 ( 2) = 0 or x = + 3 (C) O.N. of Fe in K4[Fe(CN)6] is 4 (+ 1) + x + 6 ( 1) = 0 or x = + 2 (D) O.N. of Fe in FeSO4 of FeSO4.(NH4)2SO4.6H2O = x + 6 + 4 ( 2) = 0 x2=0 x=+2 Thus, option (A) is correct. 1 x H P 2 (+ 1) + (2x) + ( 2 7) = 1 or x = + 6. 84. 1 x Fe (CN)6 x + (1 6) = 3 or x=3+6 x=+3 +1 x K Mn O4 1 + x + (2 4) = 0 or x = +7. 86. O.N. of Cr in K2Cr2O7 is 2 (+ 1) + 2 x + (2 7) = 0 or x = + 6 Similarly, O.N. of Cr in K2CrO4 is 2 (+1) + x + (2 4) = 0 or x = + 6. 87. x 1 2 C12 H 22 O11 12x + 22 (+1) + 11 ( 2) = 0 or x = 0. 1 92. The oxidation no. of chromium in K2Cr2O7 is +6 and in Cr2(SO4)3 is +3. 0 +1 1 +1 2 93. S8, S2F2, H2S Here O.N. of S in S8, S2F2 and H2S respectively are 0, + 1 and 2. 94. O.N. of Cr in CrO2Cl2 is x + 2 ( 2) + 2 ( 1) = 0 or x 4 2 = 0 or x = + 6 95. The structure of CrO5 is O O Cr O O O i.e., it has four peroxide bonds each having an O.N. of 1 and one double bond in which O.N. of O is 2. Therefore, the O.N. of Cr in CrO5 is x + 4 (1) + 1 (2) = 0 or x = + 6 96. Ba2XeO6 = (2 2) + x + (–2 6) = 4 + x – 12 = 0, x=8 97. CH3OH: x + 3 + (–2) + 1 = 0 x + 3 –2 + 1 = 0 x+2=0 x = –2 2 85. +2 1 O.N of S = x 2x + (–2 8) = –2 2x – 16 = –2 2x = –2 + 16 = 14 14 x= = +7 2 2 O 7 3 +12 1 89. M3+ M6+ + 3e. Thus, the O.N. of metal = + 6. 83. 1 PbO2 + (Cl) (ClO) + Pb (OH) 3 Change in O.N. of Pb = + 2 4 = 2 Change in O.N. of Cl = + 1 ( 1) = + 2 Hence option (B) is correct. Thus, nitrogen exhibits two different oxidation states in NH4NO3. 81. +4 2 CH2O: x + 2 + (–2) = 0 x = 0. HCOOH: 1 + x + (–2) + (–2) + 1 = 0 x–2=0 x=2 C2H2: 2x + 2 = 0 2x = –2 x = –1 Oxidation number of C in CH3OH, CH2O, HCOOH and C2H2 is –2, 0, 2, and –1 respectively. 31 Chemistry Vol ‐ 1.2 (Med. and Engg.) 99. The oxidation states of each central atom in the given species are as follows: +3 +5 +6 +7 CrO2–, ClO3–, Cr O 24 , Mn O 4 The arrangement represented in option (A) is the correct answer. 101. The molecular formula of orthophosphorous acid is H3PO3. Oxidation state of P is 3(1) + x + 3(2) = 0. therefore, x = +3 102. In KMnO4 : 1 + x + (–2 4) = 0 1+x–8=0 x=7 Oxidation no. of Mn = +7. In K2Cr2O7 : +2 + 2x + (–2 7) = 0 2x – 12 = 0 x = 6. Oxidation no. of Cr = +6 Manganese has the highest oxidation state. 105. H 2SO3 4 ; SO2 4 H 2SO4 6 ; H 2S 2 . 108. In [Co(CN)6]3– complex, Co shows +3 oxidation state. 2 +1 1 109. [Pt(C2H4) Cl3]– x + 2(– 2) + 4(+ 1) + 3(– 1) = –1 x – 4 + 4 – 3 = – 1 or x = – 1 + 3, x = + 2 110. In C6H5CHO : 6x + 5 + x + 1 + (–2) = 0 7x + 4 = 0 4 x= 7 Sum of the oxidation numbers of all the carbons in 4 4 C6H5CHO = 6x + x = 6 + 7 7 28 = =–4 7 111. K4P2O7 4(+1) + 2x + 7(–2) = 0 2x – 10 = 0 2x = +10 x = +5 112. The oxidation state of N in NO, N2O, NH2OH and N2H4 are +2, +1, –1, –2 respectively. 32 114. S2 O 24 SO 32 S2 O 62 Oxi. state of sulphur in S2 O 24 = + 3 Oxi. state of sulphur in SO 32 = + 4 Oxi. state of sulphur in S2 O 62 = + 5 115. In H2S, sulphur shows –2 oxidation state and in SO2 it shows +4 oxidation state. Hence, SO2 shows both oxidising and reducing properties. 5 0 BrO3 , in this reaction oxidation 116. Br2 state changes from 0 to + 5. 117. HClO4 1 + x 2 4 = 0; 1 + x 8 = 0 x = 8 1 = +7 oxidation state. 118. Sn2+ Sn4+ + 2e– 119. H 2 C2 O 4 2 + 2x – 2 4 = 0; 2x = 8 – 2 = 6 6 x= = + 3. 2 120. [Cr(NH3)4Cl2]+ x + 4 (0) 2 = 1 x + 0 2 = 1. x = 1 + 2 = + 3. 121. The molecular formula of iron pyrites is FeS2 FeS2 x4=0 x = +4 FeS2 4 + 2x = 0 2x = 4 4 x = = 2. 2 124. In this complex, iron is a central metal atom showing + 2 oxidation state. 2 2 1 128. XeOF2 x + (– 2) + 2(– 1) = 0 or x – 2 – 2 = 0 or x = +4 129. The oxidation state of Xe in both XeO3 and XeF6 is + 6 XeO3 XeF6 x–23=0 x = +6 x–6=0 x = +6 Chapter 08: Redox Reactions 130. NH4(NO3)2 comprises of NH +4 and NO3 ions. In NO3 , x + 3 (–2) = –1 x – 6 = –1 x=5 In NH4(NO3)2, x + 4 + 5 – 6 = 0 x = –3 The oxidation numbers of the two nitrogen atoms present in NH4(NO3)2 are –3 and +5 respectively. 131. Two Cl atoms show +1 and –1 oxidation state. 10 5 = . 4 2 +2 K2SO4 + CuI2 134. CuSO4 + 2KI +1 2CuI2 Cu 2 I 2 +I2 135. [Co(NH3 ) 4 ClNO 2 ] x + 4(0) + 1(–1) + 1(–1) = 0 x+0–1–1=0 x – 2 = 0; x = +2 136. K 4 [Ni(CN)4 ] = 4 (+1) + x + 4 (–1) = 0 = +4 + x – 4 = 0 x = 0 137. Let the oxidation number of N in NaNO2 be x. 1 + x + (–2) 2 = 0 1 + x – 4 = 0; x = +3 138. (A) C12H22O11 : 12(x) + 22(+1) + 11(– 2) = 0 12x + 22 – 22 = 0 or x = 0 (B) HCHO : x + 2(+1) + (– 2) = 0 x + 2 – 2 = 0 or x = 0 (C) CH3CHO : 2x + 4(+ 1) – 2 = 0 2x + 4 – 2 = 0 2x + 2 = 0 or x = 1 (D) CH3COOH : 2x + 4(+1) + 2(– 2) = 0 2x + 4 – 4 = 0 or 2x = 0 or x = 0 Hence option (C) is correct. +2 +3 + 4 + 3x – 14 = 0 3x – 7 = 0 x = 7/3 141. PO34 SO24 Cr2 O72 x – 8 = 3 x – 8 = 2 2x – 14 = 2 x = 3 + 8 x = 2 + 8 2x = 2 + 14 x = +5 x = +6 2x = 12 x = +6 142. K2Cr2O7 + 6KI + 7H2SO4 4K2SO4 + Cr2(SO4)3 + 7H2O + 3I2 133. Na 2 S4 O6 2 + 4x –12 = 0 4x = 10 x = 140. YBa2Cu3O7 +1 2 139. Mg(HXO3) (+ 2) + (+ 1) + x + 3(– 2) = 0 or +3 + x – 6 = 0 x = + 3. * +3 Cr2 (SO4 )3 2Cr +3SO2-4 144. H2S [O.N. of H = +1] (+1) 2 + x = 0 2 + x = 0; x = – 2 +1 2 145. H2SO4 2(+ 1) + x + 4(– 2) = 0 + 2 + x – 8 = 0 or x = + 6 146. SO24 x + 4(–2) = – 2 x–8=–2 x = – 2 + 8 or x = + 6 147. In CuO, x –2 = 0, x = 2 In Cu, the oxidation no. is 0 Change in the oxidation no is +2 to 0. 149. (A) (B) (C) O.N. of N in HNO3 = +1 + x+ 3 (2) = 0 or x = + 5 O.N. of N in NH2OH= x +2 1 2+1= 0 or x = 1 O.N. of N in N 2 H5 2 =2(2x + 5)= + 2 2 or x = 2 O.N. of N in Mg3N2 = 3 (+ 2) + 2x = 0 or x = 3 Thus, option (D) is correct. (D) 150. Highest O.N. of Mn in K2MnO4 is 2+ x 8= 0 or x = + 6 while in all other compounds O.N. of Mn is lower than 6, i.e., +4 +8/3 +2 MnO2, Mn3O4, MnSO4 151. In I3 , the O.N. of I = 3x = 1 or x = 1/3. 33 Chemistry Vol ‐ 1.2 (Med. and Engg.) x +1 –2 –1 152. O. N. of Fe : + 2, + 3 Mn : + 3 Ca : + 2 K : + 1. Hence option (D) is correct. 161. [Cr(H2O)4Cl2]+ x + 8(+ 1) + 4 (– 2) + 2(– 1) = + 1 x+8–8–2=+1 x=+3 +1 x –2 153. H N O3 1 + (x) +3(– 2) = 0 or x – 5 = 0 or x = + 5 +2 x –2 154. Mg2P2O7 2(+ 2) + 2x + 7(– 2) = 0 + 4 + 2x – 14 = 0 +2x = + 10 x=+5 –1 165. 2S+2 O2 + 0I +2.5 S4 O62 + 2I– 2 3 2 x +1 –2 156. CH2O x + 2(+ 1) – 2 = 0 x+2–2=0 or x = 0 +2 x +1 1 CH2Cl2 x + 2(+1) + 2(1) = 0 or x = 0 +1 x (A) KI3 = (+ 1) + (3x) = 0 3x = – 1; x = 1 3 +1 x (B) KI = (+ 1) + (x) = 0 x=–1 x –1 (C) IF5 = x + 5(– 1) = 0 x–5=0 x=+5 +1 x –2 (D) x KIO4 = (+ 1) + x + 4(– 2) = 0 x–8+1=0 x=+7 –2 159. Mo 2 Cl84 2x + 8(– 2) = – 4 2x – 16 = – 4 2x = + 12 x=+6 160. K2Cr2O7 2 + 2x – 2 7 = 0; 2x – 14 + 2 = 0 12 =+6 2x = 12; x = 2 34 N2O 2x – 2 = 0 2x = 2 2 x = = +1. 2 The change in oxidation number = 5 1 = +4. 164. BaO2 + H2SO4 H2O2 + BaSO4 In H2O2, oxygen shows –1 oxidation state and in BaSO4, oxygen shows –2 oxidation state. +2 x 158. 163. HNO3 1 + x – 6 = 0; x = +5 155. MgH2 x + 2(+ 1) = 0 or x = – 2 157. 162. +2, it is a second group element. 0 +6 –1 S2 O32 + 2Br2 + 5H2O 2S O24 + 4Br– + 10H+ In the first reaction, the oxidation number of sulphur changes from + 2 to + 2.5 when iodine is used as an oxidising agent. In the second reaction, the oxidation number of sulphur changes from +2 to +6 when bromine is used as an oxidising agent. Hence, bromine is a stronger oxidant than iodine. 166. Oxygen has 6 electrons in the outer most shell and shows common oxidation state –2. COOH 7 167. 5 | + 2KMnO 4 + 3H2SO4 COOH 2 K2SO4 + 2MnSO 4 + 10CO2 + 8H2O In this reaction, oxidation state of Mn changes from +7 to +2. 169. In KMnO4, the oxidation number is given by : 1 + x – (2 4) = 0 1+x–8=0 x=7 In [MnO4]–2 : x + (–2 4) = –2 x=6 In MnO2 : x + (–2 2) = 0 x=4 In Mn2O3 : 2x + (–2 3) = 0 2x = 6, x = 3. In Mn2+ : oxidation number = 2. Chapter 08: Redox Reactions The number of electrons transferred in the case of [MnO4]–2, MnO2, Mn2O3, M2+ is therefore 1, 3, 4, 5 respectively. 170. MnO 4 + 5e– Mn2+ 173. The values of x,y,z are 8, 4, 4 respectively hence the reaction is H2SO4 + 8HI H2S + 4I2 + 4H2O 175. In the above given equation the change in the oxidation no. is from 0 to +1 (i.e. in M, it is 0 and in [M(CN)2]–, it is +1) Among the given options, Gold is the only metal existing in +1 oxidation state. 176. In alkaline medium, 2KMnO4 + KI + H2O 2MnO2 + 2KOH + KIO3 +6 207. In HNO2, the O.N. of N is + 3 which is less than the maximum possible O.N. of + 5 and more than the minimum O.N. of 3, therefore, it can act both as an oxidising as well as a reducing agent. 209. (A) (B) (C) (D) 177. 6MnO4 + I– + 6OH– 6MnO24 + IO3 + 3H2O x –2 Reduction In this reaction, three electrons are required for the reduction of Cr2 O72 into Cr3+. 180. The balanced equation is 2C2H6 + 7O2 4CO2 +6H2O. Ratio of the coefficients of CO2 and H2O is 4 : 6 or 2 : 3. 181. O. N. of Cr changes from + 6 to + 3, undergoing reduction. 183. MnO4 + 8H+ + 5e Mn2+ + 4H2O 2 C2 O 4 2 4 2CO2 + 2e 5 2MnO + 5C2 O +16H 2Mn2+ +10CO2 + 8H2O + Thus the coefficient of MnO4 , C2 O24 and H+ in the above balanced equation respectively are 2, 5, 16. 186. The E values of the redox couple Ag+/Ag is most positive amongst the four given redox couples. Ag+ has greater tendency to get reduced and is the strongest oxidising agent. 202. The metallic iron is oxidised to Fe3+. 2 2 1 O. N. of ‘V’ changes from + 5 to + 3. O. N. of ‘Na’ changes from 0 to 1. O. N. of ‘Cr’ does not change (i.e. it is same + 6) O. N. of ‘Zn’ changes from + 2 to 0. Hence option (C) is correct. 212. S2 O72 2 Cr 3 179. Cr2 O7 3e 2 4 +6 206. CaF2 + H2SO4 CaSO4 + 2HF The oxidation number of S remains unchanged. Hence, reaction given in option (A) is the one that does not show oxidizing behaviour of H2SO4. 1 SnCl4 Hg 2 Cl2 In this 205. SnCl2 2HgCl2 reaction, HgCl2 is reduced. O. N. of S is 2(x) + 7(– 2) = –2 2x – 14 = –2 2x = + 12 x=+6 x –2 214. Fe3O8 3x + 8(– 2) = 0 or 3x – 16 = 0 or x = 16 3 215. O.N. of Fe in FeSO4.(NH4)2SO4.6H2O = x + 6 + 4 ( 2) = 0 =x2=0 x=+2 216. O.N. of C in C3O2 = 3x + 2 ( 2) = 0 or x = + 4/3. 217. Copper will reduce Br2, if the E of the following redox reaction is positive. Cu + Br2 CuBr2 Cu Cu2+ + 2e–; E = – 0.34 V Br2 + 2e– 2Br–; E = + 1.09 V Cu + Br2 CuBr2; E = + 0.75 V E of the reaction is positive. Hence, Cu can reduce Br2. 35 Chemistry Vol ‐ 1.2 (Med. and Engg.) 3+ 1 218. 2Fe(aq) + 2I 0 (aq) I2(aq) + 2Fe(aq) Here O.N. of I increases from 1 in I to 0 in I2, therefore, I ions are oxidized to I2. 220. The highest oxidation number (O.N.) of any transition element is given by: O.N. = (n – 1)d electrons + ns electrons. (A) 3d14s2: O.N. = 1 + 2 = 3 (B) 3d34s2: O.N. = 3 + 2 = 5 (C) 3d54s1 : O.N. = 5 + 1 = 6 (D) 3d54s2 : O.N. = 5 + 2 = 7 Hence, element having outer electronic configuration 3d54s2 exhibits largest oxidation number. 221. In C12H22O11, O.N of C is given by 12x + 22 – (2 11) = 0 12x + 22 – 22 = 0 12x = 0 x = 0. 222. 2AgNO3 2Ag + 2NO2 + O2 223. MnO4 + 5e Mn2+ 224. Equation is Cr2 O72 + 8H+ + 3HCOOH 2Cr3+ + 3CO2 + 7H2O 3 moles of formic acid reduces 1 mole K2Cr2O7 1 mole of formic acid reduces 1/3 mole K2Cr2O7. 225. Fluorine has highest E0 value and is more reactive than MnO2. 228. To prevent rancidification of food material anti-oxidants are added which are called oxidation inhibitor. 230. SO2 bleaches by reduction while chlorine bleaches colour of flowers by oxidation. 232. For a redox reaction to be feasible, Ecell should be a positive value. Now consider, (A) 2Fe3+ + 2e– 2Fe2+; E = + 0.77 V 2I– I2 + 2e–; E = – 0.54 V 2Fe3+ + 2I– 2Fe2+ + I2; Ecell = + 0.23 V. the given redox reaction is feasible. 36 Cu Cu2+ + 2e– ; Ecell = – 0.34 V 2Ag+ + 2e– 2Ag; Ecell = +0.80 V Cu + 2Ag+ Cu2+ + 2Ag; Ecell = + 0.46 V. the given redox reaction is feasible. (B) 2+ 2Fe3+ + 2e– 2Fe2+; Ecell = + 0.77 V. Cu Cu2+ + 2e–; Ecell = – 0.34V. 2Fe3+ + Cu 2Fe2+ + Cu2+; Ecell = + 0.43 V the given redox reaction is feasible. (C) Ag Ag+ + e–; Ecell = – 0.80 V Fe3+ + e– Fe2+; Ecell = + 0.77 V Ag + Fe3+ Ag+ + Fe2+ Ecell = – 0.03 V the given redox reaction is not feasible. (D) Chapter 08: Redox Reactions Topic Test 1. 2. 3. 4. 5. 6. 7. 8. The conversion of PbO2 to Pb(NO3)2 is _______. (A) Oxidation (B) Reduction (C) Neither oxidation nor reduction (D) Both oxidation and reduction Which of the following statement is NOT CORRECT? (A) Oxidant is a substance which increases the oxidation number of other substance. (B) In oxidation, there is increase in oxidation number. (C) The oxidation number of an oxidant decreases. (D) In oxidation, there is decrease in oxidation number. LiAlH4 is used as a/an _______. (A) oxidizing agent (B) reducing agent (C) bleaching agent (D) water softener In the reaction, 2Na + Cl2 2NaCl, which of the following is TRUE? (A) Na has been oxidized while Cl2 has been reduced. (B) Na has been reduced while Cl2 has been oxidized. (C) No substance has been oxidized or reduced as it is not a redox reaction. (D) NaCl cannot be obtained by this reaction. Which of the following is NOT a redox reaction? (A) 2Rb + 2H2O 2RbOH + H2 (B) 2CuI2 2CuI + I2 (C) 2H2O2 2H2O + O2 (D) 4KCN + Fe(CN)2 K4Fe(CN)6 Oxidation state of hydrogen in CaH2 is _______. (A) +1 (B) –1 (C) + 2 (D) 0 In which one of the following compounds, the oxidation number of oxygen is positive? (B) Na2O2 (A) H2O2 (D) H2O (C) OF2 Which of the following arrangements represent increasing oxidation number of S? (A) H2SO3, H2S, H2SO4, H2S2O3 (B) H2S2O3, H2SO3, H2S, H2SO4 (C) (D) H2S, H2SO3, H2SO4, H2S2O3 H2S, H2S2O3, H2SO3, H2SO4 9. The value of x in the partial redox equation Mn2+ + 4H2O is MnO 4 + 8H+ + xe– _______. (A) 5 (B) 3 (C) 1 (D) 0 10. The oxidation number of an element in its elementary state is _______. (A) zero (B) one (C) half its atomic number (D) always negative 11. In which of the following compounds, oxidation state of the metal atom is +3? (A) AuCl3 (B) Mg3N2 (D) KClO3 (C) V2O5 12. Identify the reaction in which the oxidation number of zinc decreases from +2 to 0? (A) Zn + 2HCl ZnCl2 + H2 (B) 2ZnS + 3O2 2ZnO + 2SO2 (C) Zn + CuSO4 ZnSO4 + Cu (D) ZnO + C Zn + CO 13. During respiration, the oxidation number of carbon _______. (A) decreases from 0 to –2 (B) increases from 0 to +2 (C) decreases from +6 to +4 (D) increases from 0 to +4 14. An element (Z = 20) loses electrons to achieve noble gas electronic configuration. The outer electronic configuration of the ion formed is 3s2 3p6. The number of electrons (x) lost and the ion formed is _______ respectively. (A) 1, Ca+ (B) 2, Ca2+ 3+ (D) 4, Ti4+ (C) 3, Sc 15. The oxidation number of iron in Fe3O4 is _______. (A) + 2 (B) + 3 (C) 8/3 (D) 2/3 Answers to Topic Test 1. 5. 9. 13. (B) (D) (A) (D) 2. 6. 10. 14. (D) (B) (A) (B) 3. 7. 11. 15. (B) (C) (A) (C) 4. (A) 8. (D) 12. (D) 37