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Oxidation Numbers & Redox Reactions

Oxidation Numbers & Redox Reactions. REDOX A guide for A level students. KNOCKHARDY PUBLISHING. REDOX. CONTENTS Definitions of oxidation and reduction Calculating oxidation state Use of H, O and F in calculating oxidation state Naming compounds Redox reactions

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Oxidation Numbers & Redox Reactions

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  1. Oxidation Numbers & Redox Reactions

  2. REDOX A guide for A level students KNOCKHARDY PUBLISHING

  3. REDOX • CONTENTS • Definitions of oxidation and reduction • Calculating oxidation state • Use of H, O and F in calculating oxidation state • Naming compounds • Redox reactions • Balancing ionic half equations • Combining half equations to form a redox equation • Revision check list

  4. OXIDATION & REDUCTION - Definitions OXIDATION GAIN OF OXYGEN 2Mg + O2——> 2MgO magnesium has been oxidised as it has gained oxygen REMOVAL (LOSS) OF HYDROGEN C2H5OH ——> CH3CHO + H2 ethanol has been oxidised as it has ‘lost’ hydrogen

  5. OXIDATION & REDUCTION - Definitions REDUCTION GAIN OF HYDROGEN C2H4 + H2 ——> C2H6 ethene has been reduced as it has gained hydrogen 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

  6. OXIDATION & REDUCTION - Definitions OXIDATION AND REDUCTION IN TERMS OF ELECTRONS Oxidation and reduction are not only defined as changes in O and H ... OXIDATIONRemoval (loss) of electrons ‘OIL’ species will get less negative or more positive REDUCTIONGain of electrons ‘RIG’ species will become more negative or less positive REDOXWhen reduction and oxidation take place

  7. OXIDATION & REDUCTION - Definitions OXIDATION AND REDUCTION IN TERMS OF ELECTRONS Oxidation and reduction are not only defined as changes in O and H ... OXIDATIONRemoval (loss) of electrons ‘OIL’ species will get less negative or more positive REDUCTIONGain of electrons ‘RIG’ species will become more negative or less positive REDOXWhen reduction and oxidation take place OIL -Oxidation Is the Loss of electrons RIG -Reduction Is the Gain of electrons

  8. OXIDATION STATES Used to... tell if oxidation or reduction has taken place work out what has been oxidised and/or reduced ATOMS AND SIMPLE IONS The number of electrons which must be added or removed to become neutral atomsNa in Na = 0 neutral already ... no need to add any electrons cationsNa in Na+ = +1 need to add 1 electron to make Na+ neutral anionsCl in Cl¯ = -1 need to take 1 electron away to make Cl¯ neutral

  9. OXIDATION 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 ATOMS AND SIMPLE IONS The number of electrons which must be added or removed to become neutral atomsNa in Na = 0 neutral already ... no need to add any electrons cationsNa in Na+ = +1 need to add 1 electron to make Na+ neutral anionsCl in 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+ d) O2- e) He f) Al3+

  10. OXIDATION STATES Used to... tell if oxidation or reduction has taken place work out what has been oxidised and/or reduced ATOMS AND SIMPLE IONS The number of electrons which must be added or removed to become neutral atomsNa in Na = 0 neutral already ... no need to add any electrons cationsNa in Na+ = +1 need to add 1 electron to make Na+ neutral anionsCl in 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 (0) b) Fe3+(+3) c) Fe2+ (+2) d) O2-(-2) e) He (0) f) Al3+ (+3)

  11. OXIDATION STATES MOLECULES The SUM of the oxidation states adds up to ZERO ELEMENTSH in H2 = 0 both are the same and must add up to Zero COMPOUNDSC in CO2 = +4 O in CO2 = -2 1 x +4 and 2 x +2 = Zero

  12. OXIDATION STATES MOLECULES The SUM of the oxidation states adds up to ZERO ELEMENTSH in H2 = 0 both are the same and must add up to Zero COMPOUNDSC in CO2 = +4 O in CO2 = -2 1 x +4 and 2 x -2 = Zero Explanation • because CO2 is a neutralmolecule, the sumoftheoxidationstatesmust be zero • for this, oneelementmusthave a positiveOS and the othermustbenegative

  13. OXIDATION STATES MOLECULES The SUM of the oxidation states adds up to ZERO ELEMENTS H in H2 = 0 both are the same and must add up to Zero COMPOUNDS C in CO2 = +4 O in CO2 = -2 1 x +4 and 2 x +2 = Zero • HOW DO YOU DETERMINE WHICH IS THE POSITIVE ONE? • the more electronegative species will have the negativevalue • electronegativity increases across a period and decreases down a group • O is further to the right than C in the periodic table so it has the negative value

  14. OXIDATION STATES MOLECULES The SUM of the oxidation states adds up to ZERO ELEMENTS H in H2 = 0 both are the same and must add up to Zero COMPOUNDS C in CO2 = +4 O in CO2 = -2 1 x +4 and 2 x +2 = Zero • HOW DO YOU DETERMINE THE VALUE OF • AN ELEMENT’S OXIDATION STATE? • from its positionintheperiodictable and/or • the other element(s) present in the formula

  15. OXIDATION STATES COMPLEX IONS The SUM of the oxidation states adds up to THE CHARGE e.g. NO3-sum of the oxidation states = - 1 SO42- sum of the oxidation states = - 2 NH4+ sum of the oxidation states = +1 Examples in SO42- the oxidation state of S = +6 there is ONE S O = -2 there are FOUR O’s +6 + 4(-2) = -2 so the ion has a 2- charge

  16. OXIDATION STATES COMPLEX IONS The SUM of the oxidation states adds up to THE CHARGE e.g. NO3-sum of the oxidation states = - 1 SO42- sum of the oxidation states = - 2 NH4+ sum of the oxidation states = +1 Examples • What is the oxidation state (OS) 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

  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 except0 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 FLUORINE -1 except 0 atom (F) and molecule (F2)

  18. 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 except0 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 FLUORINE -1 except 0 atom (F) and molecule (F2) Q.Give the oxidation state of the element other than O, H or F in... SO2NH3NO2NH4+IF7Cl2O7 NO3¯ NO2¯ SO32-S2O32-S4O62-MnO42- What is odd about the value of the oxidation state of S in S4O62- ?

  19. OXIDATION STATES A. The oxidation states of the elements other than O, H or F are SO2 O = -2 2 x -2 = - 4 overall neutral S = +4 NH3 H = +1 3 x +1 = +3 overall neutral N = - 3 NO2 O = -22 x -2 = - 4 overall neutral N = +4 NH4+ H = +1 4 x +1 = +4 overall +1 N = - 3 IF7 F = -1 7 x -1 = - 7 overall neutral I = +7 Cl2O7 O = -27 x -2 = -14 overall neutral Cl = +7 (14/2) NO3¯ O = -23 x -2 = - 6 overall -1 N = +5 NO2¯ O = -22 x -2 = - 4 overall -1 N = +3 SO32- O = -23 x -2 = - 6 overall -2 S = +4 S2O32- O = -23 x -2 = - 6 overall -2 S = +2 (4/2) S4O62- O = -26 x -2 = -12 overall -2 S = +2½ ! (10/4) MnO42- O = -24 x -2 = - 8 overall -2 Mn = +6 What is odd about the value of the oxidation state of S in S4O62- ? An oxidation state must be a whole number (+2½ is the average value)

  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

  21. 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 +5or+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

  22. 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

  23. OXIDATION STATES CALCULATING OXIDATION STATE - 2 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

  24. OXIDATION STATES CALCULATING OXIDATION STATE - 2 Q. What is the oxidation state of each element in the following compounds/ions ? CH4C = - 4 H = +1 PCl3P = +3 Cl = -1 NCl3N = +3 Cl = -1 CS2C = +4 S = -2 ICl5I = +5 Cl = -1 BrF3Br = +3 F = -1 PCl4+P = +4 Cl = -1 H3PO4P = +5 H = +1 O = -2 NH4Cl N = -3 H = +1 Cl = -1 H2SO4S = +6 H = +1 O = -2 MgCO3Mg = +2 H = +4 O = -2 SOCl2S = +4 Cl = -1 O = -2

  25. 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 SO3 S is in the +6 oxidation state dichromate(VI) for Cr2O72- Cr is in the +6 oxidation state 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

  26. 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 SO3 S is in the +6 oxidation state dichromate(VI) for Cr2O72- Cr is in the +6 oxidation state 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... PbO2lead(IV) oxide SnCl2tin(II) chloride SbCl3antimony(III) chloride TiCl4titanium(IV) chloride BrF5bromine(V) fluoride

  27. REDOX REACTIONS OXIDATION AND REDUCTION IN TERMS OF ELECTRONS Oxidation and reduction are not only defined as changes in O and H REDOXWhen reduction and oxidation take place OXIDATIONRemoval (loss) of electrons ‘OIL’ species will get less negative or more positive REDUCTIONGain of electrons ‘RIG’ species will become more negative or less positive

  28. REDOX REACTIONS OXIDATION AND REDUCTION IN TERMS OF ELECTRONS Oxidation and reduction are not only defined as changes in O and H REDOXWhen reduction and oxidation take place OXIDATIONRemoval (loss) of electrons ‘OIL’ species will get less negative or more positive REDUCTIONGain of electrons ‘RIG’ species will become more negative or less positive REDUCTION in O.S. Species has been REDUCED e.g. Cl is reduced to Cl¯ (0 to -1) INCREASE in O.S.Species has been OXIDISED e.g. Na is oxidised to Na+ (0 to +1)

  29. REDOX REACTIONS OXIDATION AND REDUCTION IN TERMS OF ELECTRONS REDUCTION in O.S. INCREASE in O.S. Species has been REDUCEDSpecies has been OXIDISED Q. State if the changes involve oxidation (O) or reduction (R) or neither (N) Fe2+ —> Fe3+ I2 —> I¯ F2 —> F2O C2O42- —> CO2 H2O2 —> O2 H2O2 —> H2O Cr2O72- —> Cr3+ Cr2O72- —> CrO42- SO42- —> SO2

  30. REDOX REACTIONS OXIDATION AND REDUCTION IN TERMS OF ELECTRONS REDUCTION in O.S. INCREASE in O.S. Species has been REDUCEDSpecies 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 C2O42- —> CO2 O +3 to +4 H2O2 —> O2 O -1 to 0 H2O2 —> H2O R -1 to -2 Cr2O72- —> Cr3+ R +6 to +3 Cr2O72- —> CrO42- N +6 to +6 SO42- —> SO2 R +6 to +4

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