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REDOX A guide for A level students O X I D A T I O N +7 +6 +5 +4 +3 +2 +1 0 -1 -2 -3 -4 R E D U C T I O N 2015 SPECIFICATIONS SPECIFICATION POINTS REDOX LEARNING OBJECTIVES: • Definitions of oxidation and reduction • Calculating oxidation state • Use of H, O and F in calculating oxidation state • Naming compounds REDOX Before you start it is necessary to… Recall the layout of the periodic table Know the charges on common ions Be able to write formulae from memory OXIDATION & REDUCTION – Definition first met at KS3 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 OXIDATION & REDUCTION – Definition first met at KS3 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 OXIDATION & REDUCTION - Definition first met at GCSE 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 OIL - Oxidation Is the Loss of electrons RIG - Reduction Is the Gain of electrons OXIDATION STATES What are they? Oxidation states are assigned to atoms in molecules or ions to show how many electrons the atom has used in bonding. This method has the advantage that it can also be applied to covalent substances where complete transfer of electrons does not occur. OXIDATION STATES Reduction • Look at the equation • Write out the oxidation numbers of the atoms involved • When the oxidation number of an atom increases (gets more positive) oxidation has occurred • When the oxidation number of an atom decreases (gets more negative) reduction has occurred. Oxidation Recognizing oxidation and reduction in reactions +7 +6 +5 +4 +3 +2 +1 0 -1 -2 -3 -4 -5 -6 -7 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 atoms Na in Na = 0 neutral already ... no need to add any electrons cations Na in Na+= +1 need to add 1 electron to make Na+ neutral anions Cl in Cl¯ = need to take 1 electron away to make Cl¯ neutral -1 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)f) Al3+ OXIDATION STATES MOLECULES The SUM of the oxidation states adds up to ZERO ELEMENTS COMPOUNDS H in H2 = 0 both are the same and must add up to Zero C in CO2= +4 O in CO2= -2 OVERALL (1 x +4) + (2 x -2) = Zero EXPLANATION: • because CO2 is a neutral molecule, the sum of the oxidation states must be zero • for this, one element must have a positive OS and the other must be negative OXIDATION STATES MOLECULES The SUM of the oxidation states adds up to ZERO ELEMENTS COMPOUNDS H in H2 = 0 both are the same and must add up to Zero C in CO2= +4 O in CO2= -2 OVERALL (1 x +4) + (2 x -2) = Zero EXPLANATION: 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 OXIDATION STATES MOLECULES The SUM of the oxidation states adds up to ZERO ELEMENTS COMPOUNDS H in H2 = 0 both are the same and must add up to Zero C in CO2= +4 O in CO2= -2 OVERALL (1 x +4) + (2 x -2) = Zero EXPLANATION: HOW DO YOU DETERMINE WHICH IS THE POSITIVE ONE? • the more electronegative species will have the negative value - electronegativity increases across a period and decreases down a group e.g. O is further to the right than C in the periodic table so it has the negative value OXIDATION STATES COMPLEX IONS The SUM of the oxidation states adds up to THE CHARGE e.g. NO3- sum of the oxidation states = SO42- sum of the oxidation states = NH4+ sum of the oxidation states = 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 -1 -2 +1 WORKED EXAMPLE 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: OS of Mn + 4(-2) = - 1 OS of Mn + -8 = - 1 OS of Mn = - 1 + 8 OS of Mn = +7 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 -1 atom (H) and molecule (H2) hydride ion, H¯ in sodium hydride NaH OXYGEN -2 except 0 -1 +2 atom (O) and molecule (O2) in hydrogen peroxide, H2O2 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... SO2 NH3 NO2 NH4+ IF7 Cl2O7 NO3¯ NO2¯ SO32- S2O32- S4O62- MnO42- What is odd about the value of the oxidation state of S in S4O62- ? OXIDATION STATES RULE 1: for simple ions, the oxidation state is the charge on the ion e.g. Br- & Cl- oxidation state 1 RULE 2: for atoms in elements, the oxidation state is zero e.g. Cl & Br oxidation state 0 RULE 3: for compounds, the sum of the oxidation states is zero e.g. KBr (+1) + (-1) = 0 RULE 4: Covalent molecules are assumed to be ionic e.g. H2O : H+ and O2- OXIDATION STATES A. The oxidation states of the elements other than O, H or F are SO2 NH3 O = -2 H = +1 2 x -2 = - 4 3 x +1 = +3 overall neutral overall neutral S = +4 N=-3 NO2 O = -2 2 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 = -2 7 x -2 = -14 overall neutral Cl = +7 NO3¯ O = -2 3 x -2 = - 6 overall -1 N = +5 NO2¯ O = -2 2 x -2 = - 4 overall -1 N = +3 SO32- O = -2 3 x -2 = - 6 overall -2 S = +4 S2O32- O = -2 3 x -2 = - 6 overall -2 S = +2 S4O62- O = -2 6 x -2 = -12 overall -2 S = +2½ ! (10/4) MnO42- O = -2 4 x -2 = - 8 overall -2 Mn = +6 (14/2) (4/2) 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) 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 • can have values up to their Group No. Cl Cl usually -1 +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 Answers: 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 USUAL +1 MAXIMUM +1 Br -1 +7 Sr +2 +2 O -2 +6 B +3 +3 N -3 or +5 +5 OXIDATION STATES Q. What is the oxidation state of each element in the following compounds/ions ? 1. CH4 2. PCl3 3. NCl3 4. CS2 5. ICl5 6. BrF3 7. PCl4+ 8. H3PO4 9. NH4Cl 10. H2SO4 11. MgCO3 12. SOCl2 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 lead(IV) oxide SnCl2 tin(II) chloride SbCl3 antimony(III) chloride TiCl4 titanium(IV) chloride BrF5 bromine(V) fluoride REVISION CHECK What should you be able to do? Recall the definitions for oxidation and reduction in terms of oxygen, hydrogen and electrons Write correct formulae given the relevant oxidation states and vice versa Use the trend in electronegativity across periods to work out likely oxidation numbers Predict the oxidation state of elements in atoms, simple ions, compounds and complex ions - QUIZ Name compounds using roman numeral notation to indicate key oxidation numbers as part of the name HOMEWORK Complete Qus 1 & 2 on pg 33 of textbook Complete Qu 18 on pg 35 of textbook Complete Qu 5 a)ii & b)v on pg 37 of textbook Work through PA sheet ensuring you are secure in naming the compounds