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2016 topic 0 - oxidation and reduction (INTRODUCTION)

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introduction to oxidation and reduction

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2016 topic 0 - oxidation and reduction (INTRODUCTION)

  1. 1. IB Chemistry Power Points Topic 09 Oxidation and Reduction www.pedagogics.ca
  2. 2. REDOX A guide for A level students KNOCKHARDY PUBLISHING 2008 SPECIFICATIONS Much taken from
  3. 3. OXIDATION & REDUCTION – Simplified Definitions OXIDATION
  4. 4. OXIDATION & REDUCTION – Simplified Definitions OXIDATION is a GAIN OF OXYGEN 2Mg + O2 ——> 2MgO magnesium has been oxidised as it has gained oxygen
  5. 5. OXIDATION & REDUCTION – Simplified Definitions OXIDATION is a GAIN OF OXYGEN 2Mg + O2 ——> 2MgO magnesium has been oxidised as it has gained oxygen is the REMOVAL (LOSS) OF HYDROGEN C2H5OH ——> CH3CHO + H2 ethanol has been oxidised as it has ‘lost’ hydrogen
  6. 6. OXIDATION & REDUCTION – Simplified Definitions REDUCTION
  7. 7. OXIDATION & REDUCTION – Simplified Definitions REDUCTION is a GAIN OF HYDROGEN C2H4 + H2 ——> C2H6 ethene has been reduced as it has gained hydrogen
  8. 8. OXIDATION & REDUCTION – Simplified Definitions REDUCTION is a GAIN OF HYDROGEN C2H4 + H2 ——> C2H6 ethene has been reduced as it has gained hydrogen is the REMOVAL (LOSS) OF OXYGEN CuO + H2 ——> Cu + H2O copper(II) oxide has been reduced as it has ‘lost’ oxygen However as chemistry became more sophisticated, it was realised that another definition was required
  9. 9. ... OXIDATION AND REDUCTION IN TERMS OF ELECTRONS Oxidation and reduction are not only defined as changes in O and H OXIDATION REDUCTION REDOX OXIDATION & REDUCTION – Better Definitions
  10. 10. ... OXIDATION AND REDUCTION IN TERMS OF ELECTRONS Oxidation and reduction are not only defined as changes in O and H OXIDATION Removal (loss) of electrons ‘OIL’ species will get less negative or more positive REDUCTION REDOX OXIDATION & REDUCTION – Better Definitions
  11. 11. ... OXIDATION & REDUCTION – Better Definitions OXIDATION AND REDUCTION IN TERMS OF ELECTRONS Oxidation and reduction are not only defined as changes in O and H OXIDATION Removal (loss) of electrons ‘OIL’ species will get less negative or more positive REDUCTION Gain of electrons ‘RIG’ species will become more negative or less positive REDOX When reduction and oxidation take place
  12. 12. OIL - Oxidation Is the Loss of electrons RIG - Reduction Is the Gain of electrons
  13. 13. OXIDATION NUMBERS (STATES) Used to... tell if oxidation or reduction has taken place work out what has been oxidised and/or reduced construct half equations and balance redox equations FOR ATOMS AND SIMPLE IONS The number of electrons which must be added or removed to become neutral Atoms Na = 0 neutral already ... no need to add any electrons Cations Na+ = +1 need to add 1 electron to make Na+ neutral Anions Cl¯ = -1 need to take 1 electron away to make Cl¯ neutral Q. What are the oxidation states of the elements in the following? a) C b) Fe3+ c) Fe2+ (0) (+3) (+2) d) O2- e) He f) Al3+ (-2) (0) (+3)
  14. 14. OXIDATION STATES ELEMENTS H in H2 = 0 both are the same and must add up to Zero COMPOUNDS C in CO2 = +4 Explanation MOLECULES The SUM of the oxidation states adds up to ZERO O in CO2 = -2 1 x +4 and 2 x -2 = Zero • because CO2 is a neutral molecule, the sum of the oxidation states must be ? • for this, one element must have a positive OS and the other must be ? HOW DO YOU DETERMINE THE VALUE OF AN ELEMENT’S OXIDATION STATE? • from its position in the periodic table and/or • the other element(s) present in the formula (oxygen is almost always -2 etc) HOW DO YOU DETERMINE WHICH IS THE POSITIVE ONE? • the more electronegative species will have the negative value
  15. 15. OXIDATION STATES COMPLEX IONS The SUM of the oxidation states adds up to THE CHARGE - sum of the oxidation states = - 1 2- sum of the oxidation states = - 2 + sum of the oxidation states = +1 2- the oxidation state of S = +6 there is ONE S in SO4 O = -2 there are FOUR O’s +6 + 4(-2) = -2 so the ion has a 2- charge e.g. NO3 SO4 NH4 Example SO4 2-
  16. 16. OXIDATION STATES - sum of the oxidation states = - 1 2- sum of the oxidation states = - 2 + sum of the oxidation states = +1 What is the oxidation number of Mn in MnO4¯ ? • the oxidation state of oxygen in most compounds is - 2 • there are 4 O’s so the sum of its oxidation states - 8 • overall charge on the ion is - 1 • therefore the sum of all the oxidation states must add up to - 1 • the oxidation states of Mn four O’s must therefore equal - 1 • therefore the oxidation state of Mn in MnO4¯is +7 +7 + 4(-2) = - 1 COMPLEX IONS The SUM of the oxidation states adds up to THE CHARGE e.g. NO3 SO4 NH4 Example
  17. 17. OXIDATION STATES CALCULATING OXIDATION STATE - 1 Many elements can exist in more than one oxidation state In compounds, certain elements are used as benchmarks to work out other values HYDROGEN +1 except 0 atom (H) and molecule (H2) -1 hydride ion, H¯ in sodium hydride NaH OXYGEN -2 except 0 atom (O) and molecule (O2) -1 in hydrogen peroxide, H2O2 +2 in F2O HALOGENS -1 except 0 atom (X) and molecule (X2) Q. Give the oxidation state of the element other than O, H or F in... + IF7 Cl2O7 SO2 NH3 NO2 NH4 2- S2O3 NO3¯ NO2¯ SO3 2- S4O6 2- MnO4 2- 2- ? What is odd about the value of the oxidation state of S in S4O6
  18. 18. OXIDATION STATES A. The oxidation states of the elements other than O, H or F are SOO = -2 2 x -2 = - 4 overall neutral S = +4 2 NHH = +1 3 x +1 = +3 overall neutral N = - 3 3 NOO = -2 2 x -2 = - 4 overall neutral N = +4 2 NH+ H = +1 4 x +1 = +4 overall +1 N = - 3 4 IFF = -1 7 x -1 = - 7 overall neutral I = +7 7 ClOO = -2 7 x -2 = -14 overall neutral Cl = +7 27 NO¯ O = -2 3 x -2 = - 6 overall -1 N = +5 3NO¯ O = -2 2 x -2 = - 4 overall -1 N = +3 2SO2- O = -2 3 x -2 = - 6 overall -2 S = +4 3 2- O = -2 3 x -2 = - 6 overall -2 S = +2 S2O3 2- O = -2 4 x -2 = - 8 overall -2 Mn = +6 MnO4
  19. 19. OXIDATION STATES CALCULATING OXIDATION STATE - 2 The position of an element in the periodic table can act as a guide METALS • have positive values in compounds • value is usually that of the Group Number Al is +3 • where there are several possibilities the values go no higher than the Group No. Sn can be +2 or +4 Mn can be +2,+4,+6,+7 NON-METALS • mostly negative based on their usual ion Cl usually -1 • can have values up to their Group No. Cl +1 +3 +5 or +7
  20. 20. OXIDATION STATES CALCULATING OXIDATION STATE - 2 The position of an element in the periodic table can act as a guide METALS • have positive values in compounds • value is usually that of the Group Number Al is +3 • where there are several possibilities the values go no higher than the Group No. Sn can be +2 or +4 Mn can be +2,+4,+6,+7 NON-METALS • mostly negative based on their usual ion Cl usually -1 • can have values up to their Group No. Cl +1 +3 +5 or +7 Q. What is the theoretical maximum oxidation state of the following elements? Na P Ba Pb S Mn Cr What will be the usual and the maximum oxidation state in compounds of? Li Br Sr O B N +1
  21. 21. OXIDATION STATES CALCULATING OXIDATION STATE - 2 The position of an element in the periodic table can act as a guide A. What is the theoretical maximum oxidation state of the following elements? Na P Ba Pb S Mn Cr +1 +5 +2 +4 +6 +7 +6 What will be the usual and the maximum oxidation state in compounds of? Li Br Sr O B N USUAL +1 -1 +2 -2 +3 -3 or +5 MAXIMUM +1 +7 +2 +6 +3 +5
  22. 22. OXIDATION STATES THE ROLE OF OXIDATION STATE IN NAMING SPECIES To avoid ambiguity, the oxidation state is often included in the name of a species manganese(IV) oxide shows that Mn is in the +4 oxidation state in MnO2 sulphur(VI) oxide for SOS is in the +6 oxidation state 3 dichromate(VI) for CrO2- Cr is in the +6 oxidation state 27 phosphorus(V) chloride for PCl5 P is in the +5 oxidation state phosphorus(III) chloride for PCl3 P is in the +3 oxidation state Q. Name the following... PbO2 SnCl2 SbCl3 TiCl4 BrF5
  23. 23. OXIDATION STATES THE ROLE OF OXIDATION STATE IN NAMING SPECIES To avoid ambiguity, the oxidation state is often included in the name of a species manganese(IV) oxide shows that Mn is in the +4 oxidation state in MnO2 sulphur(VI) oxide for SOS is in the +6 oxidation state 3 dichromate(VI) for CrO2- Cr is in the +6 oxidation state 27 phosphorus(V) chloride for PCl5 P is in the +5 oxidation state phosphorus(III) chloride for PCl3 P is in the +3 oxidation state Q. Name the following... PbO2 lead(IV) oxide SnCl2 tin(II) chloride SbCl3 antimony(III) chloride TiCl4 titanium(IV) chloride BrF5 bromine(V) fluoride
  24. 24. REDOX REACTIONS OXIDATION AND REDUCTION IN TERMS OF ELECTRONS Oxidation and reduction are not only defined as changes in O and H REDOX When reduction and oxidation take place OXIDATION Removal (loss) of electrons ‘OIL’ species will get less negative or more positive REDUCTION Gain of electrons ‘RIG’ species will become more negative or less positive +7 +6 +5 +4 +3 +2 +1 0 -1 -2 -3 -4 R E D U C T I O N O X I D A T I O N
  25. 25. REDOX REACTIONS OXIDATION AND REDUCTION IN TERMS OF ELECTRONS Oxidation and reduction are not only defined as changes in O and H REDOX When reduction and oxidation take place OXIDATION Removal (loss) of electrons ‘OIL’ species will get less negative or more positive REDUCTION Gain of electrons ‘RIG’ species will become more negative or less positive REDUCTION in O.N. Species has been REDUCED e.g. Cl is reduced to Cl¯ (0 to -1) INCREASE in O.N. Species has been OXIDISED e.g. Na is oxidised to Na+ (0 to +1) +7 +6 +5 +4 +3 +2 +1 0 -1 -2 -3 -4 R E D U C T I O N O X I D A T I O N
  26. 26. REDOX REACTIONS OXIDATION AND REDUCTION IN TERMS OF ELECTRONS REDUCTION in O.S. INCREASE in O.S. Species has been REDUCED Species has been OXIDISED Q. State if the changes involve oxidation (O) or reduction (R) or neither (N) Fe2+ —> Fe3+ I—> I¯ 2 F—> FO 2 2CO2- —> CO24 2 HO—> O22 2 HO—> HO 22 2CrO2- —> Cr3+ 27 Cr2O7 2- —> CrO4 2- SO4 2- —> SO2
  27. 27. REDOX REACTIONS OXIDATION AND REDUCTION IN TERMS OF ELECTRONS REDUCTION in O.S. INCREASE in O.S. Species has been REDUCED Species has been OXIDISED Q. State if the changes involve oxidation (O) or reduction (R) or neither (N) Fe2+ —> Fe3+ O +2 to +3 I2 —> I¯ R 0 to -1 F2 —> F2O R 0 to -1 C2O4 2- —> CO2 O +3 to +4 H2O2 —> O2 O -1 to 0 H2O2 —> H2O R -1 to -2 Cr2O7 2- —> Cr3+ R +6 to +3 Cr2O7 2- —> CrO4 2- N +6 to +6 SO4 2- —> SO2 R +6 to +4
  28. 28. OXIDATION STATES - Review CALCULATING OXIDATION STATE – MOST IMPORTANT Q. What is the oxidation state of each element in the following compounds/ions ? CH4 PCl3 NCl3 CS2 ICl5 BrF3 PCl4 + H3PO4 NH4Cl H2SO4 MgCO3 SOCl2
  29. 29. OXIDATION STATES CALCULATING OXIDATION STATE - 2 Q. What is the oxidation state of each element in the following compounds/ions ? CHC = - 4 H = +1 4 PClP = +3 Cl = -1 3 NClN = +3 Cl = -1 3 CSC = +4 S = -2 2 IClI = +5 Cl = -1 5 BrFBr = +3 F = -1 3 PCl+ P = +5 Cl = -1 4 H3PO4 P = +5 H = +1 O = -2 NH4Cl N = -3 H = +1 Cl = -1 H2SO4 S = +6 H = +1 O = -2 MgCO3 Mg = +2 C = +4 O = -2 SOCl2 S = +4 Cl = -1 O = -2

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