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REDOX REACTIONS 263 UNIT 8 REDOX REACTIONS Where there is oxidation, there is always reduction – Chemistry is essentially a study of redox systems. After studying this unit you will be able to Chemistry deals with varieties of matter and change of one kind of matter into the other. Transformation of matter from ••• identify redox reactions as a class of reactions in which oxidation one kind into another occurs through the various types of and reduction reactions occur reactions. One important category of such reactions is simultaneously; Redox Reactions. A number of phenomena, both physical ••• define the terms oxidation, as well as biological, are concerned with redox reactions. reduction, oxidant (oxidising These reactions find extensive use in pharmaceutical, agent) and reductant (reducing biological, industrial, metallurgical and agricultural areas. agent); The importance of these reactions is apparent from the fact ••• explain mechanism of redox that burning of different types of fuels for obtaining energy reactions by electron transfer for domestic, transport and other commercial purposes, process; electrochemical processes for extraction of highly reactive ••• use the concept of oxidation metals and non-metals, manufacturing of chemical number to identify oxidant and compounds like caustic soda, operation of dry and wet reductant in a reaction; batteries and corrosion of metals fall within the purview of ••• classify redox reaction into redox processes. Of late, environmental issues like combination (synthesis), Hydrogen Economy (use of liquid hydrogen as fuel) and decomposition, displacement and disproportionation development of ‘Ozone Hole’ have started figuring under reactions; redox phenomenon. ••• suggest a comparative order 8.1 CLASSICAL IDEA OF REDOX REACTIONS – among various reductants and oxidants; OXIDATION AND REDUCTION REACTIONS ••• balance chemical equations Originally, the term oxidation was used to describe the using (i) oxidation number addition of oxygen to an element or a compound. Because (ii) half reaction method; of the presence of dioxygen in the atmosphere (~20%), ••• learn the concept of redox many elements combine with it and this is the principal reactions in terms of electrode reason why they commonly occur on the earth in the processes. form of their oxides. The following reactions represent oxidation processes according to the limited definition of oxidation: → 2 Mg (s) + O2 (g) 2 MgO (s) (8.1) → S (s) + O2 (g) SO2 (g) (8.2) 2019-20 264 CHEMISTRY In reactions (8.1) and (8.2), the elements been broadened these days to include removal magnesium and sulphur are oxidised on of oxygen/electronegative element from a account of addition of oxygen to them. substance or addition of hydrogen/ Similarly, methane is oxidised owing to the electropositive element to a substance. addition of oxygen to it. According to the definition given above, the → following are the examples of reduction CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (l) (8.3) processes: A careful examination of reaction (8.3) in which hydrogen has been replaced by oxygen 2 HgO (s) 2 Hg (l) + O2 (g) (8.8) prompted chemists to reinterpret oxidation in (removal of oxygen from mercuric oxide ) terms of removal of hydrogen from it and, 2 FeCl (aq) + H (g) →2 FeCl (aq) + 2 HCl(aq) therefore, the scope of term oxidation was 3 2 2 (8.9) broadened to include the removal of hydrogen from a substance. The following illustration is (removal of electronegative element, chlorine another reaction where removal of hydrogen from ferric chloride) → can also be cited as an oxidation reaction. CH2 = CH2 (g) + H2 (g) H3C – CH3 (g) (8.10) → (addition of hydrogen) 2 H2S(g) + O2 (g) 2 S (s) + 2 H2O (l) (8.4) → As knowledge of chemists grew, it was 2HgCl2 (aq) + SnCl2 (aq) Hg2Cl2 (s)+SnCl4 (aq) natural to extend the term oxidation for (8.11) reactions similar to (8.1 to 8.4), which do not (addition of mercury to mercuric chloride) involve oxygen but other electronegative In reaction (8.11) simultaneous oxidation elements. The oxidation of magnesium with of stannous chloride to stannic chloride is also fluorine, chlorine and sulphur etc. occurs occurring because of the addition of according to the following reactions : electronegative element chlorine to it. It was soon realised that oxidation and reduction → Mg (s) + F2 (g) MgF2 (s) (8.5) always occur simultaneously (as will be apparent by re-examining all the equations Mg (s) + Cl (g) → MgCl (s) (8.6) 2 2 given above), hence, the word “redox” was Mg (s) + S (s) → MgS (s) (8.7) coined for this class of chemical reactions. Incorporating the reactions (8.5 to 8.7) Problem 8.1 within the fold of oxidation reactions In the reactions given below, identify the encouraged chemists to consider not only the species undergoing oxidation and removal of hydrogen as oxidation, but also the reduction: removal of electropositive elements as (i) H S (g) + Cl (g) → 2 HCl (g) + S (s) oxidation. Thus the reaction : 2 2 → → (ii) 3Fe3O4 (s) + 8 Al (s) 9 Fe (s) 2K4 [Fe(CN)6](aq) + H2O2 (aq) 2K3[Fe(CN)6](aq) + 2 KOH (aq) + 24AlO3 (s) → is interpreted as oxidation due to the removal (iii) 2 Na (s) + H2 (g) 2 NaH (s) of electropositive element potassium from Solution potassium ferrocyanide before it changes to (i) H S is oxidised because a more potassium ferricyanide. To summarise, the 2 electronegative element, chlorine is added term “oxidation” is defined as the addition to hydrogen (or a more electropositive of oxygen/electronegative element to a element, hydrogen has been removed substance or removal of hydrogen/ from S). Chlorine is reduced due to electropositive element from a substance. addition of hydrogen to it. In the beginning, reduction was (ii) Aluminium is oxidised because considered as removal of oxygen from a oxygen is added to it. Ferrous ferric oxide compound. However, the term reduction has 2019-20 REDOX REACTIONS 265 For convenience, each of the above (Fe3O4) is reduced because oxygen has been removed from it. processes can be considered as two separate steps, one involving the loss of electrons and (iii) With the careful application of the the other the gain of electrons. As an concept of electronegativity only we may illustration, we may further elaborate one of infer that sodium is oxidised and these, say, the formation of sodium chloride. hydrogen is reduced. + – Reaction (iii) chosen here prompts us to 2 Na(s) → 2 Na (g) + 2e think in terms of another way to define – → – Cl2(g) + 2e 2 Cl (g) redox reactions. Each of the above steps is called a half reaction, which explicitly shows involvement 8.2 REDOX REACTIONS IN TERMS OF of electrons. Sum of the half reactions gives ELECTRON TRANSFER REACTIONS the overall reaction : We have already learnt that the reactions → + – → 2 Na(s) + Cl2 (g) 2 Na Cl (s) or 2 NaCl (s) 2Na(s) + Cl2(g) 2NaCl (s) (8.12) Reactions 8.12 to 8.14 4Na(s) + O (g) → 2Na O(s) (8.13) suggest that half 2 2 reactions that involve loss of electrons are → 2Na(s) + S(s) Na2S(s) (8.14) called oxidation reactions. Similarly, the are redox reactions because in each of these half reactions that involve gain of electrons reactions sodium is oxidised due to the are called reduction reactions. It may not addition of either oxygen or more be out of context to mention here that the new electronegative element to sodium. way of defining oxidation and reduction has Simultaneously, chlorine, oxygen and sulphur been achieved only by establishing a are reduced because to each of these, the correlation between the behaviour of species electropositive element sodium has been as per the classical idea and their interplay in added. From our knowledge of chemical electron-transfer change. In reactions (8.12 to bonding we also know that sodium chloride, 8.14) sodium, which is oxidised, acts as sodium oxide and sodium sulphide are ionic a reducing agent because it donates electron compounds and perhaps better written as to each of the elements interacting with it and + – + 2– + 2– Na Cl (s), (Na )2O (s), and (Na )2 S (s). thus helps in reducing them. Chlorine, oxygen Development of charges on the species and sulphur are reduced and act as oxidising produced suggests us to rewrite the reactions agents because these accept electrons from (8.12 to 8.14) in the following manner : sodium. To summarise, we may mention that Oxidation: Loss of electron(s) by any species. Reduction: Gain of electron(s) by any species. Oxidising agent : Acceptor of electron(s). Reducing agent : Donor of electron(s). Problem 8.2 Justify that the reaction : → 2 Na(s) + H2(g) 2 NaH (s) is a redox change. Solution Since in the above reaction the compound formed is an ionic compound, which may also be represented as Na+H– (s), this suggests that one half reaction in this process is : 2 Na (s) → 2 Na+(g) + 2e– 2019-20 266 CHEMISTRY and the other half reaction is: At this stage we may investigate the state H (g) + 2e– → 2 H–(g) of equilibrium for the reaction represented by 2 equation (8.15). For this purpose, let us place This splitting of the reaction under a strip of metallic copper in a zinc sulphate examination into two half reactions solution. No visible reaction is noticed and automatically reveals that here sodium is attempt to detect the presence of Cu2+ ions by oxidised and hydrogen is reduced, passing H2S gas through the solution to therefore, the complete reaction is a redox produce the black colour of cupric sulphide, change. CuS, does not succeed. Cupric sulphide has such a low solubility that this is an extremely 8.2.1 Competitive Electron Transfer 2+ sensitive test; yet the amount of Cu formed Reactions cannot be detected.
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