Redox Reactions 176 05. Redox Reactions

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DGT MH –CET 11th CHEMISTRY Study Material176Redox Reactions05.1Redox Reactions5.O : Prominent Scientists :ScientistsArrhenius, Svante (1859 - 1927)(Swedish chemist)Contributionsi.ii.iii.Bronsted Johannes icolaus (1879 - 1947)(Danish chemist)iv.i.ii.Proposed theory of ionic dissociation of electrolytesin their aqueous solutions.Studied the temperature dependance of the ratesof chemical reactions.Worked on various subjects such as immunology,cosmic physics and 'greenhouse effect'.Received the Nobel Prize in chemistry in 1903.Worked on electrochemistry, reaction kinetics andapplied thermodynamics to chemical problems.Gained recognition for his definitions of acids andbases.5.1 : Introduction :Q.1. What is a chemical reaction?Ans: Chemical reaction is a process in which a single substance or many substances (reactants) interact witheach other to produce one or more substances (products).Q.2. State the types of chemical reaction.Ans: Chemical reactions are classified into three types:i.precipitation reactionsii. acid–base neutralization reactionsiii. oxidation–reduction or redox reactions.Q.3. What are the important aspects of redox reactions?Ans: Almost every element participate in redox reactions. The important aspects of redox reactions are asfollows:i.Large number of natural, biological and industrial processes involve redox reactions.ii. Combustion of fuels like gasoline, oil, natural gas and combustion of hydrocarbons involve redoxreactions.iii. In metallurgy, the extraction of the metallic elements from their ores involve redox reactions.iv. Household bleaching, functioning of batteries, corrosion of metals, etc., are some of the commonexamples which involve redox reactions.5.2 : Concept of oxidation and reduction :Q.4. Define oxidation in terms of classical concept. Give example.Ans: According to the classical concept, oxidation is defined as the combination of an element or a substancewith oxygen:eg. Addition of oxygen:C O2 CO2Carbon Oxygen Carbon dioxideMH - CET Chemistry - XIDGT Group - Tuitions (Feed Concepts) XIth – XIIth JEE CET NEET Call : 9920154035 / 8169861448

DGT MH –CET 11th CHEMISTRY Study MaterialRedox Reactions2177Note:Nowadays the term oxidation has a much broader meaning.The term oxidation is defined as the addition of oxygen/electronegative element to a substance or removalof hydrogen/electropositive element from a substance.eg. i.Addition of an electronegative element:ZnSZn S Zinc SulphurZinc sulphideii. Removal of hydrogen:H2S Br 2 2HBr S Hydrogen BromineHydrogenSulphursulphidebromideiii. Removal of an electropositive element/radical:2Kr Cl2 2KCl h PotassiumChlorinePotassiumIodineiodidechlorideQ.5. Define reduction in terms of classical concept. Give example.Ans: According to the classical concept, reduction is defined as the removal of oxygen from a compound.eg.Removal of oxygen:2Fe 2O 3 3C 4Fe 3CO2 Iron (III)CarbonIronCarbonoxidedioxideNote:Nowadays the term reduction has a much broader meaning.The term reduction is defined as the removal of oxygen/electronegative element from a substance oraddition of hydrogen/electropositive element to a substance.eg.i.Addition of hydrogen:2HClCl2 H 2 Chlorine HydrogenHydrogen chlorideii. Addition of an electropositive element:2HgCl2 SnCl2Hg2Cl2 SnC4 MercuryTin (II)MercuryTin (IV)(II) chloride chloride(I) chloridechlorideiii. Removal,of an electronegative element/radical:2FeCl2 2HCl2FeCl2 H 2 Iron (III)HydrogenIron (II)HydrochloricchloridechlorideacidQ.6. Define the terms oxidation and reduction in terms of electron transfer.Ans: i. Oxidation: Oxidation is a process which involves loss of one or more electrons from a substance (anelement or a compound or an ion).eg. Zn(s)Zn2 (aq) 2e– ZincZincElectrons(II) ionii. Reduction: Reduction is a process which involves gain of one or more electrons from a substance(an element or a compound or an ion).MH - CET Chemistry - XIDGT Group - Tuitions (Feed Concepts) XIth – XIIth JEE CET NEET Call : 9920154035 / 8169861448

DGT MH –CET 11th CHEMISTRY Study MaterialRedox Reactionseg. 2H (aq)Proton 2e – Electrons3178H 2(g)HydrogenQ.7. Explain oxidation and reduction on the basis of electronic concept with suitable example.Ans: Consider the reaction of magnesium with oxygen to form magnesium oxide.2Mg O 2 2MgO MagnesiumOxygenMagnesiumoxidei.Each magnesium atom loses two valence electrons to form magnesium ion (Mg2 ). According toelectronic concept, this is the oxidation process.2Mg2 4e –2Mg MagnesiumMagnesiumElectrons(II) ionii. Each oxygen atom (present in oxygen molecule) accepts two electrons (which are released bymagnesium atom) to form oxide ion (O2–). According to electronic concept, this is the reductionprocess.O2 4e – 2O 2– OxygenElectronsOxide ioniii. The overall reaction is2MgO2Mg2 2O 2– MagnesiumOxideMagnesium(II)oxideiv. Thus, Mg is oxidized to Mg2 by the loss of electrons and O2 is reduced to O2– by the gain ofelectrons.Q.8. Define the terms oxidant and reductant in terms of electron transfer.Ans: i. Oxidant: Oxidant or oxidizing agent is a species that accepts electron(s) and causes other substanceto lose electron(s) (that is to oxidize). OR Oxidant is an electron acceptor.ii. Reductant: Reductant or reducing agent is a species that donates electron(s) and causes othersubstance to accept electron(s) (that is to reduce). OR Reductant is an electron donor.Note:i.Oxidants (oxidizing agents) oxidizes other substances and themselves get reduced.ii. Reductants (reducing agents) reduces other substances and themselves get oxidized.Q.9. Explain oxidizing and reducing agents on the basis of electronic concept with suitable example.Ans: Oxidizing and reducing agents can be explained using the following reaction,Zn(s) 2HC1(aq) ZnCl2(aq) H 2(g) ZincHydrochloricZincHydrogenacidchloridei.Each Zn atom loses two electrons to form Zn2 ion. This is oxidation process.Zn2 (aq) 2e –Zn(s) ZincZincElectrons(II) ionii. In aqueous medium, the ionization of HCl gives Wand Cl Ions.2HCl2W (aq) 2Cl–(aq) HydrochloricProtonChlorideacidioniii. Each W ion accepts one electron (which is released by zinc) to form a H–atom. This is reductionprocess. Two H–atoms combine to form H2 molecule.2H (aq) 2e – H 2(g) MH - CET Chemistry - XIDGT Group - Tuitions (Feed Concepts) XIth – XIIth JEE CET NEET Call : 9920154035 / 8169861448

DGT MH –CET 11th CHEMISTRY Study MaterialRedox Reactionsiv.4179ProtonElectronsHydrogenThus, zinc atom reduces W ions i.e., zinc acts as a ,reducing agent (reductant) and itself is oxidizedwhereas W ion acts as an oxidizing agent and itself is reduced.2Mg O2MagnesiumOxygen5.3 : Redox reactions :Q.10. Define Redox reactions.Ans: The reactions in which both oxidation and reduction reactions occur simultaneously (together) are calledredox reactions.eg. Zn(s) Cu2 (aq)Zn2 (aq) Cu(s)ZincCopperZincCopper(II) ion(II) ionQ.11. Redox reactions are electron transfer reactions. Explain.Ans: i. In a redox reaction, one species gets oxidized by loss of electrons. The electrons thus released areaccepted by other species which inturn gets reduced.ii. Thus, redox reactions involve transfer of electrons from one species to other species. Hence, thesereactions are electron transfer reactions.eg.Zn2 (aq) Cn(s)Zn(s) Cu 2 (aq) ZincCopperZincCopper(II) ion(II) ionEach Zn atom loses two electrons and is oxidized to Zn2 ion.Zn2 (aq) 2e –.(Oxidation)Zn(s) ZincZincElectrons(II) ionThe electrons released by Zn atom are accepted by Cu2 ion. In this process, Cu2 ions get reduced tometallic Cu atoms.Cu 2 (aq) 2e – Cn(s).(Reduction) CopperElectronsCopper(II) ionThe net reaction is the simultaneous oxidation and reduction which involves transfer of electrons frommetallic zinc to cupric ions (Cu 2). Hence, it is an electron transfer (redox) reaction.Note:In this reaction, Zn donates electrons and acts as a reducing agent (i.e., it itself is oxidized). Cu 2 ionaccepts electrons and acts as an oxidizing agent (i.e., it itself is reduced).Q.12. Justify that the reaction:2Na(s) H2(g) (NCERT) 2NaH(s) is a redox reaction.Ans: i. Each sodium atom loses an electron to form sodium ion (Na ). This is oxidation process.Na (s)Na (g) e– SodiumSodiumElectronionii. Each H–atom (present in H2 molecule) accepts an electron to form Ir ion. This is reduction process.H 2(g) 2e – 2H –(g) HydrogenElectronsHydride ioniii. In the net reaction, sodium reacts with hydrogen to form sodium hydride (NaH) which is an ioniccompound and can be represented as Na H–.MH - CET Chemistry - XIDGT Group - Tuitions (Feed Concepts) XIth – XIIth JEE CET NEET Call : 9920154035 / 8169861448

DGT MH –CET 11th CHEMISTRY Study Material180Redox Reactionsiv.52Na(s) H 2(g) 2Na(s) SodiumHydrogenSodium hydrideThe net reaction involves simultaneous oxidation and reduction. Hence, it is a redox reaction.5.4 : Oxidation number or oxidation state :Q.13. What is the significance of oxidation number?Ans: i. The redox reactions leading to formation of ionic compounds can easily be explained by the conceptof electron transfer. However, the redox reactions leading to formation of the molecular compoundscannot be explained in terms of electron transfer, as in molecular compounds, no actual transfer ofelectrons take place.ii. Therefore, to explain the redox reactions involving formation of molecular compounds, the concept ofoxidation number was introduced. It has also proved to be very useful in balancing the redox reactions.Q.14. What is an oxidation number or oxidation state?Ans: The oxidation number or oxidation state of an atom in a molecule or ion is defined as the number ofcharges it would carry if electrons were completely transferred.Note:i.For monoatomic ions:Monoatomic ionCl Cu 2 S2 Ch arg e on the ion / oxidation number 1 2 2ii. For polyatomic ions or molecules oxidation number is assigned arbitrarily for all atoms.eg. Oxidation number of oxygen is –2 in CO. However, this does not mean that there is a charge of –2 onthe oxygen atom in CO.Rules to assign oxidation numbers or oxidation states:Q.15. Explain the rules to assign oxidation number to an atom in any molecule or ion.Ans: The rules to assign oxidation number to an atom in any molecule or ion are as follows:Rule–1: An atom in free uncombined elemental state has zero oxidation number.eg.The oxidation number of each atom in the elements (such as Ca, H 2, Cl2, O3, S8, P4, etc.) is zero.Rule–2: An atom in the monoatomic ion has oxidation number equal to its charge.eg.Monoatomic ionBa 2 Cr 3 K Br S2 Ionic ch arg e / oxidation number 2 3 1 1 2Rule–3: The oxidation number of H–atom is either 1 or –1. When H–atom is bonded to non–metals, itsoxidation number is 1.When it is bonded to metals, its oxidation number is –1.eg.[O – H]–H–O–H Li–HH–Ca–H –2 1 1 –2 1 1 –1–1 2 –1Rule–4: The oxidation number of oxygen is usually –2 in all of its compounds except in peroxides andperoxide ions where it has oxidation number of –1.eg.Ca–OH–O–O–H[O–O] 2– 2 –2 1 –1 –1 1 –1 –1In OF2, oxidation number of oxygen is 2.Rule–5: The oxidation number of F is always –1 in all of its compounds. The other halogens CI, Br andMH - CET Chemistry - XIDGT Group - Tuitions (Feed Concepts) XIth – XIIth JEE CET NEET Call : 9920154035 / 8169861448

DGT MH –CET 11th CHEMISTRY Study Material6181Redox ReactionsI usually have oxidation number of –1. in their compounds. The exception is the compounds inwhich these halogens (CI, Br and I) are bonded to oxygen. In such compounds, oxidation numberof CI, Br or I is 1.eg.H–FKBrCl–O–CIH–O–Cl 1 –1 1–1 1 –2 1 1 –2 1Rule–6: The algebraic sum of the oxidation numbers of all atoms in a neutral molecule is zero.eg. In CdS,Oxidation number of Cd Oxidation number of S 2 –2 0.Rule–7: The algebraic sum of the oxidation numbers of all the atoms of a polyatomic ion is equal to its netcharge.eg. In SO42– ion,Oxidation state of S 4 O xidation state of oxygen 6 4 (–2) 6 – 8 –2Rule–8: The alkali metals have 1 oxidation number and the alkaline earth metals have 2 oxidationnumber in their compounds.Notes:i.Oxidation numbers may be fractional. Electrons are never shared or transferred in fractions. Thisfractional oxidation number is the average' oxidation number of an element in a compound.eg.The oxidation number of S atom in S4O62– is 2.5.This is, its average oxidation number. The structureof S4O62– ion is given below:ii.iii.iv.The oxidation number of each S–atom present at end is 5 and the oxidation number of each S–atompresent at middle is 0. The total oxidation number of 4 S–atoms is 10. Consequently, the averageoxidation number is 10/4 2.5.The metallic elements exhibit only positive oxidation states and the non–metallic elements may exhibiteither positive or negative oxidation states.In the molecular formula, the oxidation number or oxidation state is also represented by Romannumbers in parenthesis after the chemical symbol of the metal.eg. Au(III)Cl3, Sn(II)Cl2, Hg(II)Cl2, etc.Oxidation numbers of some of the elements in their compounds are given in the following table:Symbolof theelementOxidation numberSymbolof theelementHLiBeBCN 1, –1 1 2 3 4, 2,–4–5, 4, 3 2, 1,–3,–2CoNiCuZnGaGeOxidation number 3, 2 2 2, 1 2 3 4, –4Symbolof theelementFeSnSbTeIXeOxidation number 3, 2 4, 2 5, 3,–3 6, 4,–2 7, 5, 1,–1 6, 4, 2MH - CET Chemistry - XIDGT Group - Tuitions (Feed Concepts) XIth – XIIth JEE CET NEET Call : 9920154035 / 8169861448

DGT MH –CET 11th CHEMISTRY Study Material7182Redox ReactionsSymbolof theelementOFNaMgAlSiPSClKCaScTiVCrMnOxidation number 2,–1/2,–1–1 1 2 3 4, –4 5, 3,–3 6, 4, 2,–2 7, 6, 5, 4, 3, 1,–1 1 2 3 4, 3, 2 5, 4, 3, 2 6, 5, 4, 3 7, 6, 4, 3, 2Symbolof theelementOxidation numberSymbolof theelementOxidation numberAsSeBrKrRbSrYZrNb 5, 3,–3 6, 4,–2 5, 3, 1,–1 4, 2 1 2 3 4 5, 4CsBaLaHfTaWReOsIr 1 2 3 4 5 6, 4 7, 6, 4 8, 4 4, 3MoTcRuPdAgCdIn 6, 4, 3 7, 6, 4 8, 6, 4, 3 4, 2 1 2 3PtAuHgTIPbBiPoAt 4, 2 3, 1 2, 1 3, 1 4, 2 5, 3 2–1Q.16. Suggest a list of the substances where carbon can exhibit oxidation states from –4 to 4 andnitrogen from –3 to 5.Ans: Carbon exhibits range of oxidation states from –4 to 4. These are listed in the following tableOxidation state of carbon atom–4–3–2–10 1 2 3 4Name of the substanceMethane (CH4)Ethane (H3C – CH3)Ethylene (H2C CH2)Acetylene (HC CH)Dichloromethane (CH2Cl2)Difluoroacetylene (FC CF)Carbon monoxide (CO)Potassium oxalate (K2C2O4)Carbon dioxide (CO2)Nitrogen exhibits range of oxidation states from –3 to 5. These are listed in the following table.Oxidation state of nitrogen atom–3–2–10 1 2 3 4 5Name of the substanceAmmonia (NH3)Hydrazine (H2N – NH2)Hydroxylamine (H2N – OH)Nitrogen (N2)Nitrous oxide (N2O)Nitric oxide (NO)Nitrous acid (HNO2)Nitrogen dioxide (NO2)Nitric acid (HNO3)MH - CET Chemistry - XIDGT Group - Tuitions (Feed Concepts) XIth – XIIth JEE CET NEET Call : 9920154035 / 8169861448

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ii. acid–base neutralization reactions iii. oxidation–reduction or redox reactions. Q.3. What are the important aspects of redox reactions? Ans: Almost every element participate in redox reactions. The important aspects of redox reactions are as follows: i. Large number of natural, biological and industrial processes involve redox reactions .

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