REDOX REACTIONS - NCERT

1 263 REDOX REACTIONSC hemistry deals with varieties of matter and change of onekind of matter into the other. Transformation of matter fromone kind into another occurs through the various types ofreactions. One important category of such REACTIONS isRedox REACTIONS . A number of phenomena, both physicalas well as biological, are concerned with REDOX REACTIONS find extensive use in pharmaceutical,biological, industrial, metallurgical and agricultural importance of these REACTIONS is apparent from the factthat burning of different types of fuels for obtaining energyfor domestic, transport and other commercial purposes,electrochemical processes for extraction of highly reactivemetals and non-metals, manufacturing of chemicalcompounds like caustic soda, operation of dry and wetbatteries and corrosion of metals fall within the purview ofredox processes. Of late, environmental issues likeHydrogen Economy (use of liquid hydrogen as fuel) anddevelopment of Ozone Hole have started figuring underredox IDEA OF REDOX REACTIONS OXIDATION AND REDUCTION REACTIONSO riginally, the term oxidation was used to describe theaddition of oxygen to an element or a compound.

2 Becauseof the presence of dioxygen in the atmosphere (~20%),many elements combine with it and this is the principalreason why they commonly occur on the earth in theform of their oxides. The following REACTIONS representoxidation processes according to the limited definition ofoxidation:2 Mg (s) + O2 (g) 2 MgO (s)( )S (s) + O2 (g) SO2 (g)( )After studying this unit you will beable to identify REDOX REACTIONS as a classof REACTIONS in which oxidationand reduction REACTIONS occursimultaneously; define the terms oxidation,reduction, oxidant (oxidisingagent) and reductant (reducingagent); explain mechanism of redoxreactions by electron transferprocess; use the concept of oxidationnumber to identify oxidant andreductant in a reaction; classify REDOX reaction intocombination (synthesis),decomposition, displacementand disproportionationreactions; suggest a comparative orderamong various reductants andoxidants; balance chemical equationsusing (i) oxidation number(ii) half reaction method.

3 Learn the concept of redoxreactions in terms of 8 REDOX REACTIONSW here there is oxidation, there is always reduction Chemistry is essentially a study of REDOX REACTIONS ( ) and ( ), the elementsmagnesium and sulphur are oxidised onaccount of addition of oxygen to , methane is oxidised owing to theaddition of oxygen to (g) + 2O2 (g) CO2 (g) + 2H2O (l)( )A careful examination of reaction ( ) inwhich hydrogen has been replaced by oxygenprompted chemists to reinterpret oxidation interms of removal of hydrogen from it and,therefore, the scope of term oxidation wasbroadened to include the removal of hydrogenfrom a substance. The following illustration isanother reaction where removal of hydrogencan also be cited as an oxidation H2S(g) + O2 (g) 2 S (s) + 2 H2O (l)( )As knowledge of chemists grew, it wasnatural to extend the term oxidation forreactions similar to ( to ), which do notinvolve oxygen but other electronegativeelements.

4 The oxidation of magnesium withfluorine, chlorine and sulphur etc. occursaccording to the following REACTIONS :Mg (s) + F2 (g) MgF2 (s)( )Mg (s) + Cl2 (g) MgCl2 (s)( )Mg (s) + S (s) MgS (s)( )Incorporating the REACTIONS ( to )within the fold of oxidation reactionsencouraged chemists to consider not only theremoval of hydrogen as oxidation, but also theremoval of electropositive elements asoxidation. Thus the reaction :2K4 [Fe(CN)6](aq) + H2O2 (aq) 2K3[Fe(CN)6](aq) + 2 KOH (aq)is interpreted as oxidation due to the removalof electropositive element potassium frompotassium ferrocyanide before it changes topotassium ferricyanide. To summarise, theterm oxidation is defined as the additionof oxygen/electronegative element to asubstance or removal of hydrogen/electropositive element from a the beginning, reduction wasconsidered as removal of oxygen from acompound.

5 However, the term reduction hasbeen broadened these days to include removalof oxygen/electronegative element from asubstance or addition of hydrogen/electropositive element to a to the definition given above, thefollowing are the examples of reductionprocesses:2 HgO (s) 2 Hg (l) + O2 (g)( )(removal of oxygen from mercuric oxide )2 FeCl3 (aq) + H2 (g) 2 FeCl2 (aq) + 2 HCl(aq)( )(removal of electronegative element, chlorinefrom ferric chloride)CH2 = CH2 (g) + H2 (g) H3C CH3 (g)( )(addition of hydrogen)2 HgCl2 (aq) + SnCl2 (aq) Hg2Cl2 (s)+SnCl4 (aq)( )(addition of mercury to mercuric chloride)In reaction ( ) simultaneous oxidationof stannous chloride to stannic chloride is alsooccurring because of the addition ofelectronegative element chlorine to it. It wassoon realised that oxidation and reductionalways occur simultaneously (as will beapparent by re-examining all the equationsgiven above), hence, the word REDOX wascoined for this class of chemical the REACTIONS given below, identify thespecies undergoing oxidation andreduction:(i) H2S (g) + Cl2 (g) 2 HCl (g) + S (s)(ii) 3Fe3O4 (s) + 8 Al (s) 9 Fe (s) + 4Al2O3 (s)(iii) 2 Na (s) + H2 (g) 2 NaH (s)Solution(i) H2S is oxidised because a moreelectronegative element, chlorine is addedto hydrogen (or a more electropositiveelement, hydrogen has been removedfrom S).

6 Chlorine is reduced due toaddition of hydrogen to it.(ii) Aluminium is oxidised becauseoxygen is added to it. Ferrous ferric oxide2022-23265 REDOX REACTIONS (Fe3O4) is reduced because oxygen hasbeen removed from it.(iii) With the careful application of theconcept of electronegativity only we mayinfer that sodium is oxidised andhydrogen is (iii) chosen here prompts us tothink in terms of another way to defineredox REACTIONS IN TERMS OFELECTRON TRANSFER REACTIONSWe have already learnt that the reactions2Na(s) + Cl2(g) 2 NaCl (s)( )4Na(s) + O2(g) 2Na2O(s)( )2Na(s) + S(s) Na2S(s)( )are REDOX REACTIONS because in each of thesereactions sodium is oxidised due to theaddition of either oxygen or moreelectronegative element to , chlorine, oxygen and sulphurare reduced because to each of these, theelectropositive element sodium has beenadded. From our knowledge of chemicalbonding we also know that sodium chloride,sodium oxide and sodium sulphide are ioniccompounds and perhaps better written asNa+Cl (s), (Na+)2O2 (s), and (Na+)2 S2 (s).

7 Development of charges on the speciesproduced suggests us to rewrite the REACTIONS ( to ) in the following manner :For convenience, each of the aboveprocesses can be considered as two separatesteps, one involving the loss of electrons andthe other the gain of electrons. As anillustration, we may further elaborate one ofthese, say, the formation of sodium Na(s) 2 Na+(g) + 2e Cl2(g) + 2e 2 Cl (g)Each of the above steps is called a halfreaction, which explicitly shows involvementof electrons. Sum of the half REACTIONS givesthe overall reaction :2 Na(s) + Cl2 (g) 2 Na+ Cl (s) or 2 NaCl (s) REACTIONS to suggest that halfreactions that involve loss of electrons arecalled oxidation REACTIONS . Similarly, thehalf REACTIONS that involve gain of electronsare called reduction REACTIONS . It may notbe out of context to mention here that the newway of defining oxidation and reduction hasbeen achieved only by establishing acorrelation between the behaviour of speciesas per the classical idea and their interplay inelectron-transfer change.

8 In REACTIONS ( ) sodium, which is oxidised, acts asa reducing agent because it donates electronto each of the elements interacting with it andthus helps in reducing them. Chlorine, oxygenand sulphur are reduced and act as oxidisingagents because these accept electrons fromsodium. To summarise, we may mention thatOxidation: Loss of electron(s) by any : Gain of electron(s) by any agent : Acceptor of electron(s).Reducing agent : Donor of electron(s).Problem that the reaction :2 Na(s) + H2(g) 2 NaH (s) is a in the above reaction the compoundformed is an ionic compound, which mayalso be represented as Na+H (s), thissuggests that one half reaction in thisprocess is :2 Na (s) 2 Na+(g) + 2e 2022-23266 CHEMISTRYand the other half reaction is:H2 (g) + 2e 2 H (g)This splitting of the reaction underexamination into two half reactionsautomatically reveals that here sodium isoxidised and hydrogen is reduced,therefore, the complete reaction is a Electron TransferReactionsPlace a strip of metallic zinc in an aqueoussolution of copper nitrate as shown in Fig.

9 ,for about one hour. You may notice that thestrip becomes coated with reddish metalliccopper and the blue colour of the solutiondisappears. Formation of Zn2+ ions among theproducts can easily be judged when the bluecolour of the solution due to Cu2+ hasdisappeared. If hydrogen sulphide gas ispassed through the colourless solutioncontaining Zn2+ ions, appearance of white zincsulphide, ZnS can be seen on making thesolution alkaline with reaction between metallic zinc and theaqueous solution of copper nitrate is :Zn(s) + Cu2+ (aq) Zn2+ (aq) + Cu(s)( )In reaction ( ), zinc has lost electronsto form Zn2+ and, therefore, zinc is , now if zinc is oxidised, releasingelectrons, something must be reduced,accepting the electrons lost by zinc. Copperion is reduced by gaining electrons from the ( ) may be rewritten as :At this stage we may investigate the stateof equilibrium for the reaction represented byequation ( ).

10 For this purpose, let us placea strip of metallic copper in a zinc sulphatesolution. No visible reaction is noticed andattempt to detect the presence of Cu2+ ions bypassing H2S gas through the solution toproduce the black colour of cupric sulphide,CuS, does not succeed. Cupric sulphide hassuch a low solubility that this is an extremelysensitive test; yet the amount of Cu2+ formedcannot be detected. We thus conclude that thestate of equilibrium for the reaction ( )greatly favours the products over the us extend electron transfer reaction nowto copper metal and silver nitrate solution inwater and arrange a set-up as shown inFig. The solution develops blue colour dueto the formation of Cu2+ ions on account of thereaction:Fig. REDOX reaction between zinc and aqueous solution of copper nitrate occurring in a beaker.( )Here, Cu(s) is oxidised to Cu2+(aq) andAg+(aq) is reduced to Ag(s). Equilibrium greatlyfavours the products Cu2+ (aq) and Ag(s).