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Oxidation and Reduction

Chapter 14. Oxidation and Reduction. Oxidation and Reduction. Oxidation - addition of Oxygen. Reduction - removal of Oxygen. In terms of electrons Oxidation is the loss of electrons. ( OILRIG) Reduction is the gain of electrons.

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Oxidation and Reduction

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  1. Chapter 14 Oxidation and Reduction

  2. Oxidation and Reduction Oxidation - addition of Oxygen. Reduction - removal of Oxygen. In terms of electrons Oxidation is the loss of electrons. (OILRIG) Reduction is the gain of electrons

  3. Oxidising and reducing agents An oxidising agent causes another substance to become oxidised and, in the process, the oxidising agent is reduced. Common oxidising agents include O2, Cl2, ClO-, KMnO4 and H2O2 A reducing agent causes another substance to become reduced and, in the process, the reducing agent is oxidised. Common reducing agents include CO, SO2, SO32- Oxidising agents are useful in Sterilising and Bleaching.

  4. Experiments: (a) Halogens as Oxidising agents (b) Displacement reactions of Metals.(See Book Pages 184-186)

  5. Oxidation Numbers The oxidation number of an atom is the charge that the atom has or appears to have when electrons are distributed according to certain rule. Rules The ON of an element is 0. The ON of an ion is the charge that it has. The ON of F is always -1. The ON of O IS -2 except OF2 (+2) and H2O2 (-1). The ON of H is +1 except with metal hydrides (-1)

  6. Oxidation and reduction in terms of Oxidation numbers Oxidation is an increase in Oxidation number. Reduction is a decrease in Oxidation number. • Balancing Redox reactions (See Book Page 187-188) + HANDOUT

  7. 2.6.2Balancing equations using Oxidation Numbers

  8. Chemical reactions are all about the movement of electrons • When an equation is balanced the electron movements must balance • Sometimes it is possible to balance an equation in more than one way • It is therefore possible to make the numbers of atoms balance but not the electron movement • Using oxidation numbers prevents this

  9. Sequence of events • Write out the formulae of all reactants and products • Assign oxidation numbers to all atoms • Pick out the atoms whose oxidation numbers change • Write down the electron movements • Balance the electron movements • Enter the numbers from balanced electron movement into the overall equation • Balance equation by inspection - [do H last]

  10. 1. Assign oxidation numbers (a) Assign Oxidation – increase in Ox. No (b) Assign Reduction – decrease in Ox. No 2. Select those which change 5 8 5 4 MnO4- + Fe2+ + H+ = Mn2+ + Fe3+ + H2O +7 -2 +2 +1 +2 +3 +1 -2 +7 +2 +2 +3 Oxidation Reduction 5. Write electron gains and losses 6. Balance electron transfers Fe2+ = Fe3+ +2 +3 5 - 1 e- 5 5 + 5 e- Mn = Mn2+ +7 +2 7. Put numbers up into the equation 8. Balance equation by inspection - leave H+ till last

  11. Sequence of events • Write down reactants and products • Assign oxidation numbers • Identify oxidation and reduction • Write down changes and electrons involves • Balance e- change • Put changes into equation • Balance equation by inspection [H last]

  12. 1. Assign oxidation numbers (a) Assign Oxidation – increase in Ox. No (b) Assign Reduction – decrease in Ox. No 2. Select those which change 3 2 16 3 4 11 C2H5OH + Cr2O72-+ H+ = CH3COOH + Cr3++ H2O -2+1-2+1 +6 -2 +1 0 +1 0 -2-2+1+3 +1-2 -2 +3 +6 0 0 Reduction Oxidation 5. Write electron gains and losses 6. Balance electron transfers 3 12 3 x3 4 C2 = C2 -2 0 - 2 e- x2 4 2 Cr2O72- = Cr3+ +6 +3 2 + 3 e- 12 6 7. Put numbers up into the equation • Balance equation by inspection - leave H+ till last :- O 3+(2*7) = (3*2) + x  x=11 • H  3(6*1) + X = (3*4) + 11(2*1)  x= 16

  13. Examples • C2H5OH + MnO41- + H+ = CH3CHO + Mn2+ + H2O • C2H5OH + MnO41- + H+ = CH3COOH + Mn2+ + H2O • Cr2O72- + Fe2+ + H+ = Cr3+ + Fe3+ + H2O • MnO41- + C2O42- + H+ = CO2 + Mn2+ + H2O • Fe2+ + Cl2 = Fe3+ + Cl- • MnO41- + H2O2 + H+ = Mn2+ + O2 +H2O • Cr2O72- + CH3OH+ H+ = Cr3+ + HCHO+ H2O • Cr2O72- + CH3CHO+ H+ = Cr3+ + CH3COOH+ H2O

  14. Answers • 5C2H5OH + 2MnO41- + 6H+ = 5CH3CHO + 2Mn2+ + 8H2O • 5C2H5OH + 4MnO41- + 12H+ = 5CH3COOH + 4Mn2+ + 11H2O • Cr2O72- + 6Fe2+ + 14H+ = 2Cr3+ + 6Fe3+ + 7H2O • 2MnO41- + 5C2O42- + 16H+ = 10CO2 + 2Mn2+ + 8H2O • 2Fe2+ + Cl2 = 2Fe3+ + 2Cl- • 2MnO41- + 5H2O2 + 8H+ = 2Mn2+ + 5O2 +8H2O • Cr2O72- + CH3OH+ H+ = Cr3+ + HCHO+ H2O • Cr2O72- + CH3CHO+ H+ = Cr3+ + CH3COOH+ H2O

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