Oxidation-reduction or Redox reactions

Transcription

1 08 Redox Reactions Chapter 08: Redox Reactions 8.0 Introduction 8.1 Concept of oxidation and reduction 8. Redox reactions (Oxidation reduction reactions) 8. Oxidation number 8. Change in oxidation number 8.5 Balancing redox reactions in terms of loss and gain of electrons 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 Oxidation-reduction or Redox reactions Acid-base Neutralization reactions Importance of redox reactions: i. The redox reactions are a very important group of reactions which occur with almost every element. ii. These reactions are involved in large number of processes in nature (biological and industrial). iii. They take place in burning of fuels such as gasoline, oil, natural gas, organic substances of carbon and hydrogen and so on. iv. The redox reactions are involved in household bleaching. v. The metallic elements are extracted from their ores by oxidation-reduction reactions. vi. The functioning of batteries is based on redox reactions. vii. The corrosion of metals involve redox reactions. 8.1 Concept of oxidation and reduction Oxidation: i. 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. ii. According to the modern concept, Oxidation is a process which involves loss of electrons. It is also called de-electronation. iii. Oxidation increases the oxidation number of the element in the given substance and it involves, a. Addition of oxygen. C + O CO b. Addition of an electronegative element. Zn + S ZnS c. Removal of hydrogen. H S + Br HBr + S d. Removal of an electropositive element/radical. KI + Cl KCl + I 1

2 Chemistry Vol 1. (Med. and Engg.) Reduction: i. 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 ii. According to the modern concept, Reduction is a process which involves gain of electrons. It is also called electronation. iii. Reduction decreases the oxidation number of the element in the given substance and it involves, a. Addition of hydrogen. Cl + H HCl b. Addition of an electropositive element. HgCl + SnCl Hg Cl + SnCl c. Removal of oxygen. ZnO + C Zn + CO d. Removal of an electronegative element/ radical. FeCl + H FeCl + HCl Oxidizing agent: i. The substance which gains one or more electrons (i.e.itself gets reduced) is called oxidizing agent or oxidant. ii. Oxidizing agent increases the oxidation number of an element in a given substance. Eg. O, Cl, Br, H O, HNO, CO, etc. Reducing agent: i. The substance which loses one or more electrons (i.e.,itself gets oxidized) is called reducing agent or reductant. ii. Reducing agent lowers the oxidation number of an element in a given substance. Eg. H, Fe, Cu, C, LiAIH, Sodium amalgam etc. 8. Redox reactions (Oxidation reduction reactions) Redox reactions: i. The reactions in which both oxidation and reduction reactions occur simultaneously (together) are called redox reactions. ii. 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 Cu + ions gain electrons and get reduced. loss of e : Oxidation Zn (s) + Cu Zn + Cu (s) iii. iv. gain of e : Reduction 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. v. In this case, zinc acts as a reducing agent or reductant, while Cu + ions act as an oxidizing agent or oxidant. Other examples of redox reactions are, Zn + HCl ZnCl + H 5Fe + + 8H + + MnO 5Fe + + Mn + + H O vi. Examples for substances which can act both as oxidising as well as reducing agents are SO, H O, HNO etc.

3 Chapter 08: Redox Reactions Half reactions: Every redox reaction according to the electronic concept consists of two reactions known as half-reactions. These are: i. Oxidation half reaction (loss of electrons). ii. Reduction half reaction (gain of electrons). Eg. The reaction involving zinc and dilute HCl. The two half reactions are: aq Zn (s) Zn + e aq H + e H (g) Zn (s) + H aq Zn aq + H (g) (oxidation half reaction) (reduction half reaction) (overall reaction) Types of redox reactions: Redox reactions Combination reactions Decomposition reactions Displacement reactions Disproportionation 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. Eg C (s) + O (g) CO ; (g) Mg (s) + N (g) Mg N (s) ii. iii. 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. Eg Δ H O (l) H (g) + O (g) ; Δ KClO (s) KCl (s) + O (g) Note: All decomposition reactions are not redox reactions. Eg. Decomposition of calcium carbonate is not a redox reaction Δ CaCO (s) CaO (s) + CO (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

4 Chemistry Vol 1. (Med. and Engg.) iv. c. These reactions are of two types: 1. Metal displacement: In these reactions, a metal in a compound can be displaced by another metal in the uncombined state. Eg. V O 5(s) + 5Ca (s) V (s) + 5CaO (s). Non-metal displacement: The non-metal displacement redox reactions include hydrogen displacement and a rarely occurring reaction involving oxygen displacement. i. All alkali metals and some alkaline earth metals (Ca, Sr and Ba) which are very good reductants, will displace hydrogen from cold water. Eg Na (s) + H O (l) NaOH + H (g) ii. Less active metals such as magnesium and iron react with steam to produce hydrogen gas. Eg Mg (s) + H O (l) Mg(OH) + H (g) iii. Many metals, including those which do not react with cold water, are capable of displacing hydrogen from acids. Eg Mg (s) + HCl MgCl + H (g) iv. Metals like cadmium and tin do not react with steam, but react with acids to displace hydrogen gas. Eg CuSO + Zn (s) Cu (s) + ZnSO Cd (s) + HCl CdCl + H (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 O (g) H O H O (l) Oxidized In this reaction, oxidation number of O decreases from 1 to (in H O) and increases from 1 to 0 (in O ). Note: 1. All redox reactions are exothermic.. Phosphorus, sulphur and chlorine undergo disproportionation in the alkaline medium as shown below: Reduced 0 P (s) + +1 OH + H O (l) PH (g) + HPO Oxidized In this reaction, oxidation number of P increases from 0 to + 1 (in HPO ) and decreases from 0 to (in PH ).

5 Chapter 08: Redox Reactions 8. Oxidation number Oxidation number: i. 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. ii. Metals invariably have positive oxidation states, while non-metals may have positive or negative oxidation states. iii. Transition metals usually display several oxidation states. iv. 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). v. 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 Na Mg Al Si P S Cl Compounds NaCl MgSO AlF SiCl IF 5, P O 10 SF 6, SO HClO, Cl O 7 Oxidation state of the underlined element in the compound Rules to assign oxidation number or oxidation states to an atom: i. 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.. 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. ii. If there is a co - ordinate bond between two atoms then, a. Oxidation number of acceptor atom will be. b. Oxidation number of donor atom will be +. iii. The oxidation number of an element in the free or elementary state or in any of its allotropic forms is always zero. iv. Eg. Oxidation numbers of helium in He, hydrogen in H, Oxygen in O or O 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 Ca + is +. v. 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. vi. vii. 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, MgH, etc., where hydrogen has the oxidation number of 1. 5

6 Chemistry Vol 1. (Med. and Engg.) viii. The oxidation number of oxygen is 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. H O, each oxygen atom is assigned an oxidation number of 1 and in superoxides i.e. KO, RbO etc. each oxygen atom is assigned an oxidation number of 1/. b. The second exception is found in compounds in which oxygen is bonded to fluorine. Eg. In OF (oxygen difluoride) the oxidation no. of O is + and in O F (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 IF 5, it is + 5 and in IF 7, it is + 7. x. For neutral molecule, the sum of the oxidation numbers of all the atoms is equal to zero. Eg. In NH, Nitrogen is in oxidation state, whereas hydrogen is in + 1 oxidation state and there are three hydrogen atoms present. Therefore the net charge is zero i.e. and + equals to zero. 8. Change in oxidation number i. The oxidation number is more oftenly termed as oxidation state. ii. 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. iii. 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. iv. 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 0 Zn HCl + 1 ZnCl + 0 H Reduced In this reaction, the oxidation number (O. N.) of zinc increases from 0 to + 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: i. Write the unbalanced net equation for the redox reaction. Balance the equation for all atoms except H and O. 6

7 Chapter 08: Redox Reactions ii. 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. iii. 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. iv. Determine the factor that will make the total increase and decrease in oxidation numbers equal. Insert these coefficients into the equation. v. Balance oxygen atoms by adding H O to the side containing less O atoms, one H O for one O atom. Balance H atoms by adding H + ions, to the side with less H atoms. vi. 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 H O molecules. vii. 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: + H O + CrO 7 O (g) + Cr 1. Balance Cr atom and assign oxidation number to each atom. H O + Cr O + O (g) Identify the atoms undergoing change in oxidation number. The oxidation number of Cr reduces from + 6 to +. 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. H O + CrO 7 O (g) + + Cr 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 : O ( 1) O (0). Increase per atom = + 1 Net increase = +. ii. Decrease in oxidation number : Cr (+ 6) Cr (+ ) Decrease per atom = Net decrease = 6.. Choose the factors that will make net increase and decrease equal. The net increase must be multiplied by. Hence, the coefficient is needed for the O species on both sides. H O + CrO + 7 O (g) + Cr 5. Balance the equation for O atoms by adding H O to the side with less O atoms. There are 1 (O) atoms on the left side and 6 (O) atoms on the right side. Hence, add 7 H O molecule to the right side. H O + Cr O + O (g) + Cr + 7H O (l) 7 Cr 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 1 (H) atoms on right side. Hence add 8H + to the left side. H O + CrO H O (g) + Cr + 7H O (l) Thus, the equation is balanced for both charge as well as atoms. 7

8 Chemistry Vol 1. (Med. and Engg.) b. Reaction occurring in basic medium: Zn (s) + NO NH (g) + 1. Assign oxidation number to all atoms. Zn (s) + Zn(OH) Identify the atoms undergoing changes in oxidation numbers. The oxidation number of N reduces from + 5 to and that of Zn increases from 0 to +. 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. Zn (s) + NO NH (g) + NO NH (g) Zn(OH) 6 Zn(OH) 6 8. 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 () Increase per atom = Net increase = + ii. Decrease in oxidation number : N (+5) N () 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. Hence, a coefficient is needed for both species of Zn. Zn (s) + NO NH (g) + Zn(OH) 6 5. Balance the O atoms by adding H O molecule to the side with less O atoms. The left side has oxygen atoms and right side has 1 oxygen atoms. Therefore, balance the equation for O atoms by adding 9H O to the left side. Zn (s) + NO + 9H O (l) NH (g) + Zn(OH) 6 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 H + ions to the right side to balance H atoms. Zn (s) + NO + 9H O (l) NH (g) + Zn(OH) H 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 OH ions on both sides. Zn (s) + NO + 9H O (l) + OH NH (g) + Zn(OH) H + OH 8. H + and OH on the right side will be neutralized to form H O. Zn (s) + NO + 9H O (l) + OH NH (g) + Zn(OH) 6 + H O (l) 9. Cancel H O molecules on both sides of equation. The final equation is Zn (s) + Gain of e Loss of e NO + 6H O (l) + OH NH (g) + The equation is balanced for both atoms and charges. Zn(OH) 6

9 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 H O 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. vii. Then add two half equations and cancel the number of electrons on both sides of the equation. 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 H O 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: + + SO (g) + Fe Fe + SO (acidic) 1. Write the unbalanced equation and assign oxidation number to all atoms. SO (g) + Fe Fe + SO Divide the equation into two half equations, an oxidation half equation and a reduction half equation. The oxidation number of S increases from + to + 6 and that of Fe decreases from + to +. Hence, S is oxidized and Fe is reduced. Oxidation half equation: SO (g) SO Reduction half equation: Fe Fe. Balance the half equations for O atoms by adding H O to the side with less O atoms. Hence, add H O to the left side of oxidation half equation whereas no addition of H O to reduction half equation. Oxidation: SO (g) + H O (l) SO Reduction: Fe Fe. Balance H atoms by adding H + ions to the side with less H atoms. Hence, add H + to the right side of oxidation half equation and no addition to reduction half equation. Oxidation: SO (g) + H O (l) SO + H Reduction: Fe Fe 5. Add e to the right side of oxidation half equation and le to the left side of reduction half equation to balance the charges. Oxidation: SO (g) + H O (l) SO + H +e Reduction: Gain of e Loss of e Fe + e Fe 9

10 Chemistry Vol 1. (Med. and Engg.) Multiply reduction half equation by to equalize the number of electrons in the two half equations and then add the two half equations. SO (g) + H O (l) SO + H + e Fe Fe + e Fe SO (g) + + H O (l) SO + Fe + H Hence, the equation is balanced in terms of number of atoms and charges. b. Reaction occurring in basic medium: SO - + I SO I (basic) 1. Balance S and I atoms and assign oxidation number to all atoms. SO - + I SO - 6(aq.) + I Loss of e Gain of e. Divide the equation into two half equations, an oxidation half equation and a reduction half equation. The oxidation number of S increases from + to +.5 and that of I decreases from 0 to 1. Therefore S is oxidized and I is reduced. i. Oxidation half equation: S O - SO - 6 ii. Reduction half equation: I I -. 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 electrons on the R.H.S. of the oxidation half equation and electrons on the L.H.S. of the reduction half equation. Oxidation: SO - SO e Reduction: I + e I - Adding two half equations we get, SO - + I SO I 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)Cl. Similarly, Stannous chloride and Stannic chloride are written as Sn(II)Cl and Sn(IV)Cl. 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. HAuCl + HAu(III)Cl ii. FeO + Fe(II)O iii. Fe O + Fe (III)O iv. CuI +1 Cu(I)I v. CuO + Cu(II)O vi. MnO + Mn(II)O vii MnO + Mn(IV)O

11 Difference between valency and oxidation number: iii. iv. Chapter 08: Redox Reactions No. Valency Oxidation number (O.N.) i. It is the combining capacity of the element. No (+) or () sign is attached to it. O.N. is the charge (real or imaginary) which an atom has or appears to have when all the atoms 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 noble gases. O.N. of the element may be zero. 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 L 1 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. Applications of redox reactions: i. 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. ZnS (s) + O (g) heat ¾¾ ZnO (s) + SO (g) The oxidation number of S increases from to + and that of O decreases from 0 to. b. ZnO is reduced by coke to prepare Zn. ZnO (s) + C (s) heat ¾¾ Zn (s) + CO (g) The oxidation number of Zn decreases from + to 0 and that of C increases from 0 to +. ii. 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 Cu + ions which are reduced. Thus, + + Zn (s) + Cu Zn + Cu (s) The oxidation number of Zn increases from 0 to + and that of Cu decreases from + 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. H O 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. CH burns by oxidation with oxygen. CH (g) + O (g) CO (g) + H O (l)

12 Chemistry Vol 1. (Med. and Engg.) The oxidation number of C increases from to + and that of O decreases from 0 to. Hence, it is a redox reaction. CH is a reducing agent and O is an oxidizing agent. v. Corrosion: Corrosion is the destruction of metals by oxidation. Eg. Rusting of iron is its oxidation by oxygen of air in presence of moisture. vi. Fe (s) + O (g) Fe O.H O (s) Moisture H O Corroded Fe 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 CO and H O releasing energy. The oxidation number of C increases from 0 to + and that of O decreases from 0 to. Hence, it is a redox reaction. Quick Review Oxidation: C 6 H 1 O 6 + 6O 6CO + 6H O + energy L E O Eg. Na (s) Na + e Reduction: Loss of Electrons is Oxidation G E R Oxidizing agent: (Oxidant) Gain of Electrons is Reduction O E A Eg. Cl (g) + e Cl Eg. F (g) + e F Oxidizing (F is an oxidizing agent) agent Electron Acceptor Reducing agent: (Reductant) R E D Eg. Fe Fe + e Reducing agent Electron Donor (Fe + is a reducing agent) 1

13 Chapter 08: Redox Reactions Oxidation numbers of some of the elements in their compounds: Element Oxidation Number Element Oxidation Number Element Oxidation Number H +1, 1 Co +, + Fe +, + Li +1 Ni + Sn +, + Be + Cu +, +1 Sb +5, +, B + Zn + Te +6, +, C +, +, Ga + I +7, +5, +1, 1 N +5, +, + +, +1,,, 1 Ge +, Xe +6, +, + O +1, +, 1/, 1, As +5, +, Cs +1 F 1 Se +6, +, Ba + Na +1 Br +5, +, +1, 1 La + Mg + Kr +, + Hf + Al + Rb +1 Ta +5 Si +, Sr + W +6, + P +5, +, Y + Re +7, +6, + S +6, +, +, Zr + Os +8, + Cl +7, +6, +5, +, +, +1, 1 Nb +5, + Ir +, + K +1 Mo +6, +, + Pt +, + Ca + Tc +7, +6, + Au +, +1 Sc +,+ Ru +8, +6, +, + Hg +, +1 Ti +, +, + Pd +, + Tl +, +1 V +5, +, +, + Ag +1 Pb +, + Cr +6, +5, +, + Cd + Bi +5, + Mn +7, +6, +, +, + In + Po + At 1 1

14 Chemistry Vol 1. (Med. and Engg.) 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. 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, 8, 8; MP PMT 198] (A) Loss of electrons (B) Gain of electrons (C) Increase in the valency of negative part (D) Decrease in the valency of positive part 5. 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 6. When Sn + changes to Sn + 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 7. Consider the reaction, Zn+Cu + Zn + + Cu With reference to the above, which one of the following is the CORRECT statement? (A) Cu is oxidized to Cu +. (B) Zn is oxidized to Zn +. (C) Zn + is oxidized to Zn. (D) Cu + is oxidized to Cu. 1 Multiple Choice Questions Introduction 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, H S+NO H O + NO + S, H S 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 AgNO 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 1. Which one of the following does NOT get oxidized by bromine water? (A) Fe + to Fe + (B) Cu + to Cu + (C) Mn + to MnO (D) Sn + to Sn + 1. In which of the following reactions, the underlined substance has been oxidized? (A) Br + H S HBr + S (B) HgCl + SnCl Hg Cl + SnCl (C) Cl + KI KCl + I (D) Cu + + I Cu I + I 1. 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. H O reduces MnO ion to [KCET (Med.) 000] (A) Mn + (B) Mn + (C) Mn + (D) Mn

15 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 18. The reaction of KMnO and HCl results in (A) Oxidation of Mn in KMnO and production of Cl (B) Reduction of Mn in KMnO and production of H (C) Oxidation of Mn in KMnO and production of H (D) Reduction of Mn in KMnO and production of Cl 19. The conversion of PbS to Pb is (A) dissociation (B) reduction (C) oxidation (D) electrolysis 0. In which of the following reactions, the underlined substance has been reduced? (A) CO + CuO CO + Cu (B) CuO + HCl CuCl + H O (C) H O (g) + Fe H (g) + Fe O (D) C + HNO CO + H O + NO 1. In the reaction, Zn+ NO +7H O Zn + + NH +10OH, the substance reduced is (A) Zn (B) H O (C) NO (D) NH. Which of the following is an example of reduction? (A) CuO Cu O (B) [Fe(CN) 6 ] [Fe(CN) 6 ] (C) KI I (D) H S S. In the chemical reaction, Ag O + H O + e Ag + OH (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) NO NO (B) NO NO (C) NO NH (D) NH N 5. 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 6. 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. 7. Which of the following is an oxidizing substance? [CPMT 1997] (A) C H O (B) CO (C) H S (D) CO 8. 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. 9. 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 0. In the reaction, FeCl + H S FeCl + HCl + S (A) FeCl acts as an oxidizing agent (B) Both H S and FeCl are oxidized (C) FeCl is oxidized while H S is reduced (D) H S acts as an oxidizing agent 1. Of the four oxyacids of chlorine the strongest oxidizing agent in dilute aqueous solution is (A) HClO (B) HClO (C) HClO (D) HClO 1. Which of the following CANNOT act as an oxidizing agent? [CPMT 1996] (A) O (B) KMnO (C) I (D) K Cr O 7. Reducing agent is a substance which can [CPMT 1971, 7, 76, 78, 80; NCERT 1976] (A) accept electrons (B) accept protons (C) donate electrons (D) donate protons 15

16 Chemistry Vol 1. (Med. and Engg.). Sodium amalgam is useful as which of the following? (A) Oxidizing agent (B) Reducing agent (C) Catalyst (D) Bleaching agent 5. Oxidation number of chlorine increases from 1 to 0. Hence HCl is agent. (A) redox (B) oxidizing (C) reducing (D) complex 6. 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 7. Nitric oxide acts as a reducing agent in which of the following reaction? (A) NH + 5O NO + 6H O (B) NO + I + H O NO + 6I + 8H + (C) NO + H SO N O + H SO (D) NO + H S N O + S + H O 8. Strongest reducing agent is [CPMT 1977; BHU 198, 96; MP PET 1990; AMU 1999] (A) F (B) Cl (C) Br (D) I 9. Which substance is serving as a reducing agent in the following reaction? 1H + + Cr O + NiCr + +7H O+ Ni + 7 [CBSE PMT 199; AFMC 000; DPMT 001] (A) H O (B) Ni (C) H + (D) Cr O 7 0. In the reaction between ozone and hydrogen peroxide, H O acts as [RPET 000] (A) Oxidizing agent (B) Reducing agent (C) Bleaching agent (D) Both oxidizing and bleaching agent 1. In the reaction, Ag O + H O Ag + H O + O, H O acts as [BHU 00] (A) Reducing agent (B) Oxidizing agent (C) Bleaching agent (D) None of the above 8.. In C + H O CO + H, H O acts as : [AFMC 1988] (A) Oxidizing agent (B) Reducing agent (C) Both oxidizing agent and reducing agent. (D) Neither oxidizing agent nor reducing agent. Redox reactions. A chemical reaction in which oxidation and reduction processes take place simultaneously is known as reaction. (A) redox (B) precipitation (C) complexometric (D) titration. Which of the following behaves as both oxidising and reducing agent? [AFMC 1995] (A) H SO (B) SO (C) H S (D) HNO 5. The compound that can work both as oxidising and reducing agent is [CPMT 1986; MP PET 000] (A) KMnO (B) H O (C) BaO (D) K Cr O 7 6. 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 7. 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. 8. Which of the following is NOT an example of redox reaction? [NCERT Exemplar] (A) CuO + H Cu + H O (B) Fe O + CO Fe + CO (C) K + F KF (D) BaCl + H SO BaSO + HCl 16

17 Chapter 08: Redox Reactions 9. In the reaction, P + NaOH PH + NaH PO [MP PET 00] (A) P is oxidised only (B) P is reduced only (C) P is oxidised as well as reduced (D) Na is reduced 50. In the following reaction, P + KOH + H O KH PO + PH, Which of the following is TRUE? [Pb. PMT 00] (A) P is oxidised as well as reduced (B) P is reduced only (C) P is oxidised only (D) None of these 51. CuI Cu + CuI, the reaction is [RPMT 1997] (A) Redox (B) Neutralisation (C) Oxidation (D) Reduction 5. Co (s) + Cu Co + 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 5. Which is the best description of the behaviour of bromine in the reaction given below? H O + Br HOBr + HBr [CBSE PMT 00] (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 5. Which of the following represents a redox reaction? (A) NaOH + HCl NaCl + H O (B) BaCl + H SO BaSO + HCl (C) CuSO + H O Cu(OH) + H SO (D) Zn + HCl ZnCl + H 55. Which of the following is a redox reaction? [CBSE PMT 1997] (A) H SO with NaOH. (B) In atmosphere, O from O by lightning. (C) Nitrogen oxides from nitrogen and oxygen by lightning. (D) Evaporation of H O. 56. Of the following reactions, only one is a redox reaction. Identify it (A) Ca(OH) + HCl CaCl + H O (B) BaCl + MgSO BaSO + MgCl (C) SO 7 + H O SO + H + (D) Cu S + FeO Cu + Fe + SO 57. Which of the following is a redox-reaction? [AIEEE 00] (A) Na[Ag(CN) ] + Zn Na [Zn(CN) ] + Ag (B) BaO + H SO BaSO + H O (C) N O 5 + H O HNO (D) AgNO + K Ag + + KNO 58. Which of the following is a redox reaction? [MP PMT 00] (A) P O 5 + H O H P O 7 (B) AgNO + BaCl AgCl + Ba(NO ) (C) BaCl + H SO BaSO + HCl (D) Cu + AgNO Ag + Cu(NO ) 59. What is A in the following reaction Fe Sn Fe A? [MP PET 00] (A) (B) (C) Sn Sn Sn (D) Sn 60. Which of the following reactions involves oxidation-reduction? [NCERT 197; AFMC 000; Pb. CET 00; CPMT 00] (A) NaBr + HCl NaCl + HBr (B) HBr + AgNO AgBr + HNO (C) H + Br HBr (D) NaOH + H SO Na SO + H O 61. Which of the following is NOT a redox reaction? (A) Na + Cl NaCl (B) C + O CO (C) AgNO + NaCl AgCl + NaNO (D) Zn + H SO ZnSO + H 6. Which of the following is a redox reaction? (A) NaCl + KNO NaNO + KCl (B) CaC O + NH Cl CaCl + H C O (C) Mg(OH) +NH ClMgCl +NH OH (D) Zn + AgCN Ag + Zn(CN) 17

18 Chemistry Vol 1. (Med. and Engg.) 6. The violent reaction between sodium and water is an example of (A) reduction (B) oxidation (C) redox reaction (D) neutralisation reaction 6. The reaction H O H O (l) + O (g) is an example for which of the following? (A) oxidation (B) reduction (C) disproportionation (D) neither oxidation nor reduction 65. In the reaction, Cl + 6NaOH NaClO + 5NaCl + H O 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) CH + O CO + H O (B) CH + Cl CCl + HCl (C) F + OH F + OF + H O (D) NO + OH NO NO H O 68. Which of the following change represents a disproportionation reaction? (A) Cl + OH ClO + Cl + H O (B) Cu O + H + Cu + Cu + + H O (C) P (s) + OH +H O (l) PH g + H P O (D) All of the above Oxidation number 69. Oxidation number of metals is always (A) positive (B) negative (C) zero (D) neutral 70. 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. (C) An element in the free or the uncombined state bears oxidation number zero. (D) In all its compounds, the oxidation number of fluorine is The oxidation number of oxygen in peroxide is (A) (B) 1 (C) +1 (D) + 7. The element with atomic number 9 can exhibit oxidation state of (A) + 1 (B) + (C) 1 (D) Which of the following halogens always show only one oxidation state? (A) Cl (B) F (C) Br (D) I 7. 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 number of atoms in free metals is [NCERT 1975] (A) Minus one (B) Two (C) One (D) Zero 76. The oxidation number of hydrogen in LiH is (A) + 1 (B) 1 (C) (D) Hydrogen can have oxidation number/s of (A) 1 only (B) + 1 only (C) 0 only (D) 1, 0, O.N. of hydrogen in KH, MgH and NaH respectively would be (A) 1, 1 and 1 (B) +1, + 1, and + 1 (C) +, +1 and (D), and In which of the following oxidation number of chlorine is + 5? (A) Cl (B) ClO (C) ClO (D) ClO 80. In which of the following compounds, an element exhibits two different oxidation states? [NCERT Exemplar] (A) NH OH (B) NH NO (C) N H (D) N H

19 Chapter 08: Redox Reactions 81. In which of the following compounds, iron has the lowest oxidation number? [MNR 198] (A) Fe(CO) 5 (B) Fe O (C) K [Fe(CN) 6 ] (D) FeSO.(NH ) SO.6H O 8. A metal ion M + loses electrons, its oxidation number will become [CPMT 00] (A) + (B) + 6 (C) 0 (D) 8. The oxidation number of P in HPO 7 (A) + 5 (B) + 6 (C) + 7 (D) + ion is 8. The oxidation number of Fe in [Fe(CN) 6 ] ion is (A) + (B) + (C) (D) 85. The oxidation number of Mn is + 7 in (A) manganese dioxide (B) manganese chloride (C) manganese sulphate (D) potassium permanganate 86. In the conversion of K Cr O 7 to K CrO, the oxidation number of chromium (A) remains same (B) increases (C) decreases (D) none of these 87. The oxidation number of C in sucrose (C 1 H O 11 ) is (A) + (B) + (C) + (D) zero 88. The change in oxidation number of Pb and Cl in the reaction, PbO +Cl ClO +Pb OH respectively are (A) +, + (B), + (C) +, 1 (D), The oxidation state of S in SO 8 is (A) + (B) + (C) + 6 (D) Oxidation number of iodine varies from [CPMT 198] (A) 1 to + 1 (B) 1 to + 7 (C) + to + 5 (D) 1 to 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) NO i. + 5 (B) HNO ii. (C) NH iii. + (D) N O 5 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. 9. When SO is passed through acidic solution of potassium dichromate, then chromium sulphate is formed. Change in valency of chromium is [CPMT 1979] (A) + to + (B) + 5 to + (C) + 6 to + (D) + 7 to + 9. The oxidation number of sulphur in S 8, S F, H S respectively are [IIT 1999] (A) 0, + 1 and (B) +, + 1 and (C) 0, + 1 and + (D), + 1 and 9. Among the following, identify the species with an atom in + 6 oxidation state. [IIT Screening 000] (A) MnO (C) (B) Cr(CN) NiF (D) CrO Cl The oxidation number of Cr in CrO 5 is (A) + (B) +5 (C) +6 (D) Calculate the oxidation number of the underlined element Ba XeO 6. (A) + (B) + 8 (C) + 5 (D) Oxidation number of C in CH OH, CH O, HCOOH and C H is respectively (A), 0, +, 1 (B) +, 0, +, (C), 0, +, 0 (D),, +, 98. The oxidation state of oxygen in H O is (A) 1 (B) + (C) (D)

20 Chemistry Vol 1. (Med. and Engg.) 99. Which of the following arrangements represent increasing oxidation number of the central atom? [NCERT Exemplar] (A) CrO, ClO, CrO,MnO (B) ClO,CrO,MnO,CrO (C) CrO, ClO, MnO, CrO (D) CrO, MnO, CrO, ClO The compound in which oxidation state of metal is zero is (A) Fe (CO) 9 (B) Ni(CO) (C) Fe (CO) 9 (D) All of the above 101. The oxidation state of phosphorus is + in which of the following? (A) Orthophosphorous acid (B) Orthophosphoric acid (C) Pyrophosphoric acid (D) Metaphosphoric acid 10. In the compounds KMnO and K Cr O 7, the highest oxidation state is of which of the following element? (A) Potassium (B) Manganese (C) Chromium (D) Oxygen 10. Oxidation number of nitrogen in NH is : [CPMT 1979; Pb CET 00] (A) (B) + (C) 0 (D) The oxidation number of chlorine in HOCl is (A) 1 (B) 0 (C) + 1 (D) Sulphur has lowest oxidation number in which of the following? [EAMCET 199] (A) H SO (B) SO (C) H SO (D) H S 106. Oxidation number of oxygen in ozone (O ) is [MP PET 000; MP PMT 001] (A) + (B) (C) (D) Oxidation number of cobalt in K[Co(CO) ] is [KCET 1996] (A) + 1 (B) + (C) 1 (D) 108. The charge on cobalt in [Co(CN) 6 ] is [CPMT 1985, 9] (A) 6 (B) (C) + (D) Oxidation number of Pt in [Pt(C H )Cl ] is [MLNR 199] (A) +1 (B) + (C) + (D) The sum of the oxidation numbers of all the carbons in C 6 H 5 CHO is [EAMCET 1986] (A) + (B) 0 (C) + (D) 111. Calculate the oxidation number of the underlined element K P O 7 (A) + (B) 5 (C) +5 (D) In which compound, oxidation state of nitrogen is +1? [MP PMT 1989] (A) NO (B) N O (C) NH OH (D) N H 11. Carbon is in the lowest oxidation state in [NCERT 1979; MH CET 1999] (A) CH (B) CCl (C) CF (D) CO 11. The oxidation states of sulphur in the anions SO,SO and SO 6 follows the order: [CBSE PMT 00] (A) SO 6 SO SO (B) SO SO SO 6 (C) SO SO SO 6 (D) SO SO SO H S acts only as a reducing agent while SO can act both as a reducing and oxidising agent because [AMU 1999] (A) S in H S has oxidation state and S in SO has oxidation state + (B) Hydrogen in H S is more +ve than oxygen (C) Oxygen is more ve in SO (D) None of the above 116. In the conversion Br BrO, the oxidation state of bromine changes from [EAMCET 1990; AMU 1999; RPMT 00] (A) 1 to 0 (B) 0 to 1 (C) 0 to + 5 (D) 0 to Oxidation state of chlorine in perchloric acid is [EAMCET 1989] (A) 1 (B) 0 (C) 7 (D) + 7

21 Chapter 08: Redox Reactions 118. Sn ++ loses two electrons in a reaction. What will be the oxidation number of tin after the reaction? (A) + (B) Zero (C) + (D) 119. Oxidation number of carbon in H C O is [CPMT 198] (A) + (B) + (C) + (D) 10. The oxidation state of Cr in [Cr(NH ) Cl ] + is [AIEEE 005] (A) + (B) + (C) +1 (D) The oxidation number of Fe and S in iron pyrites are [RPMT 1997] (A), (B), 1 (C), 1.5 (D), 1 1. The oxidation number of + N in N H 5 is [Pb. PMT 001] (A) (B) (C) 1 (D) + 1. The oxidation state of I in HIO 6 is [CBSE PMT 199] (A) + 7 (B) + 5 (C) + 1 (D) 1 1. The brown ring complex compound is formulated as [Fe(H O) 5 NO]SO. The oxidation state of iron is [EAMCET 1987; IIT 1987; MP PMT 199; AIIMS 1997; DCE 000] (A) 1 (B) (C) (D) Oxidation state of oxygen in F O is [BHU 198; UPSEAT 001; MH CET 00] (A) + 1 (B) + (C) 1 (D) 16. Oxidation number of P in KH PO is which of the following? [CPMT 1987; MH CET 1999] (A) + 1 (B) + (C) + 5 (D) 17. The oxidation number of phosphorus in Ba(H PO ) is [Kurukshetra CEE 1998; DCE 00] (A) 1 (B) + 1 (C) + (D) Oxidation number of Xenon in XeOF is (A) 0 (B) (C) (D) 19. In XeO and XeF 6 the oxidation state of Xe is [MP PET 00] (A) + (B) + 6 (C) + 1 (D) Oxidation numbers of the two nitrogen atoms present in ammonium nitrate are respectively? (A) + and + (B) 0 and 0 (C) and +5 (D) 1 and Oxidation numbers of two Cl atoms in bleaching powder, CaOCl are (A) 1, 1 (B) + 1, 1 (C) + 1, + 1 (D) 0, 1 1. In which compound chlorine has highest oxidation number? (A) KCl (B) HClO (C) HClO (D) HClO 1. What is the oxidation number of sulphur in Na S O 6? [AIIMS 1998; DCE 1999] (A) (B) (C) 5 (D) 5 1. When CuSO reacts with KI, the oxidation number of Cu changes by [BHU 1997] (A) 0 (B) 1 (C) 1 (D) 15. What is the oxidation number of Co in [Co(NH ) ClNO ] [BHU 1999] (A) + (B) + (C) + (D) The oxidation number of nickel in K [Ni(CN) ] is [JIPMER 1999] (A) (B) 1 (C) + (D) Oxidation number of nitrogen in NaNO is [Pb. CET 000] (A) + (B) + (C) + (D) 18. In which of the following compounds, the oxidation number of carbon is not zero? (A) C 1 H O 11 (B) HCHO (C) CH CHO (D) CH COOH 1

22 Chemistry Vol 1. (Med. and Engg.) 19. In the substance Mg(HXO ), the oxidation number of X is (A) 0 (B) + (C) + (D) What will be the oxidation state of copper in YBa Cu O 7, if oxidation state of (Y) is +? (A) 7/ (B) 7 (C) and 5 (D) /7 11. The oxidation number of P in PO, of S in SO and that of Cr in CrO7 are respectively [Assam CEE 015] (A), +6 and +6 (B) +5, +6 and +6 (C) +, +6 and +5 (D) +5, + and 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 005] (A) + (B) +6 (C) + (D) + 1. Oxidation number of S in H S O 7 is (A) + (B) 6 (C) 5 (D) The oxidation number of sulphur in H S is (A) (B) + (C) + (D) 15. The oxidation state of S in H SO is (A) +8 (B) +6 (C) +5 (D) The oxidation state of sulphur in SO is [Bihar MEE 1996] (A) + (B) (C) +6 (D) CuO + NH Cu + N + H O In the above conversion, the oxidation number of nitrogen is changing from (A) +5 to 0 (B) 0 to + (C) + to 0 (D) to Oxidation number of nitrogen can be (A) From + 5 to (B) From 5 to (C) From 5 to + (D) From+10 to Oxidation state of nitrogen is CORRECTLY given for Compound Oxidation state (A) HNO 5 (B) NH OH + 1 (C) (N H 5 ) SO + (D) Mg N 150. In which of the compounds does manganese exhibit highest oxidation number? (A) MnO (B) Mn O (C) K MnO (D) MnSO 151. In which of the following compounds, the oxidation number of iodine is fractional? (A) IF 7 (B) I (C) IF 5 (D) IF 15. The element, which shows minimum oxidation number in its compound is [RPET 199] (A) Fe (B) Mn (C) Ca (D) K 15. Oxidation number of N in HNO is [BHU 1997] (A).5 (B) +.5 (C), +5 (D) Oxidation number of P in Mg P O 7 is [CPMT 1989; MP PMT 1995] (A) + (B) + (C) +5 (D) 155. The oxidation number of hydrogen in MgH is [CPMT 1976] (A) +1 (B) 1 (C) + (D) 156. The oxidation number of carbon in CH O is [IIT 198; EAMCET 1985; MNR 1990; UPSEAT 001; CPMT 1997] (A) (B) + (C) 0 (D) The oxidation number of carbon in CH Cl is [CPMT 1976; Pb. PET 1999; AFMC 00] (A) 0 (B) + (C) (D) Which one of the following has the highest oxidation number of iodine? [CPMT 198] (A) KI (B) KI (C) IF 5 (D) KIO