Redox reaction (ncert)

• Redox reactions


  Reactions in which oxidation and reduction reactions occur simultaneously
• Chemistry deals with varieties of matter and change of one kind of matter into the other. Transformation of matter from one kind into another occurs through the various types of reactions. One important category of such reactions is Redox Reactions.
• Redox reactions


  • They find extensive use in pharmaceutical, biological, industrial, metallurgical and agricultural areas
  • They are concerned with burning of different types of fuels for obtaining energy, electrochemical processes for extraction of highly reactive metals and non-metals, manufacturing of chemical compounds like caustic soda, operation of dry and wet batteries and corrosion of metals
  • Environmental issues like Hydrogen Economy and development of 'Ozone Hole' have started figuring under redox phenomenon
• Oxidation


  Addition of oxygen to an element or a compound, or removal of hydrogen from a substance
• Oxidation reactions


  • 2 Mg (s) + O2 (g) → 2 MgO (s)

  S (s) + O2 (g) → SO2 (g)

  CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (l)
• Reduction


  Removal of oxygen from a compound, or addition of hydrogen/electropositive element to a substance, or removal of electronegative element from a substance
• Reduction reactions


  • 2 HgO (s) → 2 Hg (l) + O2 (g)

  2 FeCl3 (aq) + H2 (g) → 2 FeCl2 (aq) + 2 HCl(aq)

  CH2 = CH2 (g) + H2 (g) → H3C – CH3 (g)

  2HgCl2 (aq) + SnCl2 (aq) → Hg2Cl2 (s)+SnCl4 (aq)
• Where there is oxidation, there is always reduction – Chemistry is essentially a study of redox systems.
• Oxidation (in terms of electron transfer)


  Loss of electron(s) by any species
• Reduction (in terms of electron transfer)


  Gain of electron(s) by any species
• Oxidising agent


  Acceptor of electron(s)
• Reducing agent


  Donor of electron(s)
• Redox reactions in terms of electron transfer

  • 2 Na(s) → 2 Na+(g) + 2e-

  Cl2(g) + 2e- → 2 Cl-(g)
• Zinc strip placed in aqueous copper nitrate solution

  Zinc is oxidised, copper ions are reduced
• Copper metal placed in aqueous silver nitrate solution

  Copper is oxidised, silver ions are reduced
• Cobalt metal placed in aqueous nickel sulphate solution

  Cobalt is oxidised, nickel ions are reduced
• CuS has such a low solubility that this is an extremely sensitive test; yet the amount of Cu2+ formed cannot be detected
• The state of equilibrium for the reaction (7.15) greatly favours the products over the reactants
• Redox reaction between copper and aqueous solution of silver nitrate

  1. Cu(s) is oxidised to Cu2+(aq)

  2. Ag+(aq) is reduced to Ag(s)

  3. Equilibrium greatly favours the products Cu2+ (aq) and Ag(s)
• Redox reaction between metallic cobalt and nickel sulphate solution

  1. Co(s) is oxidised to Co2+(aq)

  2. Ni2+(aq) is reduced to Ni (s)

  3. At equilibrium, both Ni2+(aq) and Co2+(aq) are present at moderate concentrations
• Neither the reactants [Co(s) and Ni2+(aq)] nor the products [Co2+(aq) and Ni (s)] are greatly favoured in the reaction between metallic cobalt and nickel sulphate solution
• The competition for release of electrons incidently reminds us of the competition for release of protons among acids
• We have a metal activity series or electrochemical series based on the tendency of metals to release electrons
• The competition for electrons between various metals helps us to design a class of cells, named as Galvanic cells in which the chemical reactions become the source of electrical energy
• Hydrogen combines with oxygen to form water


  1. H atom goes from a neutral (zero) state in H2 to a positive state in H2O

  2. O atom goes from a zero state in O2 to a dinegative state in H2O

  3. There is an electron transfer from H to O
• The charge transfer is only partial and is perhaps better described as an electron shift rather than a complete loss of electron by H and gain by O
• Reactions involving formation of covalent compounds

  1. H2(s) + Cl2(g) → 2HCl(g)

  2. CH 4(g) + 4Cl2(g) → CCl4(l) + 4HCl(g)
• Halogen displacement series

  F>Cl>Br>I
• In halogen displacement reactions, the more reactive halogen replaces the less reactive one.
• A redox reaction involves both oxidation and reduction processes taking place simultaneously
• Reduction refers to the process whereby atoms lose electrons while oxidation refers to the process whereby atoms gain electrons
• A redox reaction involves both oxidation and reduction processes taking place simultaneously at different places within the same system.
• EXCEPTION OF HYDROGEN

  Oxidation number of hydrogen is +1 but with binary reaction with metals hydrogen oxidation number becomes -1