2.6.1 Oxidation Numbers 4/28/2019

Introduction Oxidation is Loss of Electrons Reduction is Gain of Electrons OIL RIG Substances that cause oxidation are called oxidising agents E.g. O2, Cl2 [halogens], MnO41-, Cr2O72- Substances that cause reduction are called reducing agents E.g. C, CO, H2

4/28/2019

The number of electrons an atom appears to have gained or lost. Man made system so anomalous results can be obtained.

Rule 1 In free elements the oxidation number is 0 P = 0, H2 = 0, e.g. Na = 0, Cu = 0, P = 0, H2 = 0, S8 = 0 Cl2 = 0 A free element is an element on its own It is neutral so it will have neither lost nor gained electrons

Rule 2 The sum of all the oxidation numbers in a molecule is 0. peter jackson Rule 2 The sum of all the oxidation numbers in a molecule is 0. E.g CaCO3 = 0 (NH4)2SO4 = 0

Rule 3 The oxidation number of a simple ion is the charge on that ion. E.g. Cl- is -1, Na+ is +1 O2- is -2 Ca2+ is +2 Al3+ is +3 N3- is -3 S2- is -2

Rule 4 The sum of oxidation numbers in a complex ion is the charge on the ion SO42- = -2 [total] PO43- = -3 NH4+ = +1 NO3- = -1

Rule 5 Except in metal hydrides where it is -1 In compounds containing H the Oxidation Number of H is +1 e.g. H2O, HCl, NH4+, CH3COOH Except in metal hydrides where it is -1 e.g. NaH, KH, CaH2 where it is -1

Rule 6 In compounds containing oxygen the Oxidation Number of oxygen is -2 Except in (i) peroxides where it is -1 e.g. H2O2, (Na2O2 and BaO2) (ii) When bonded to F when it is +2

Rule 7 i.e Na, K, Li, Rb, Cs, Fr The Alkali Metals [Group I] are always +1 in a compound i.e Na, K, Li, Rb, Cs, Fr

Rule 8 The Alkaline Earth Metals [Group II] are all +2 in compounds Be, Mg, Ca, Sr, Ba, Ra

Rule 9 The Halogens [Group VII] are all -1 F, Cl, Br, I Except when bonded to a more electronegative element e.g in Cl2O the Cl is +1 and in ClO2 the Cl is +4

Rule 10 Oxidation is a increase in oxidation number Reduction is a decrease of oxidation number Mg + Cu2+ = Mg2+ + Cu 0 +2 +2 0 Mg has been oxidised Ox. No. zero to +2 Loss of 2 electrons Cu has been reduced Ox. No. +2 to zero Gain of 2 electrons

Examples K MnO4 K Mn O4 individual combined +1 +7 -2 +1 +7 -8 Overall Sodium Oxide Na2 O Overall = 0 +1 -2 individual +2 -2 combined K MnO4 K Mn O4 individual combined Overall = 0 +1 +7 -2 +1 +7 -8

Examples #2 Na2 S2 O4 +1 -2 +2 -8 Na2 S2 O3 +1 ? -2 +2 -6 +2 +3 ? +4 +1 ? -2 +2 -6 +2 +3 ? +4 +6 C12 H22 O11 ? +1 -2 +22 -22

Work out the oxidation number of Mn in each of the following compounds MnCl2 = +2 Manganese (II) chloride MnO2 Manganese (IV) oxide = +4 MnO4- = +7 Manganate (VII) Mn can have different oxidation numbers depending on the elements it is combined with. These are called Oxidation States When naming transition metal compounds it is normal to include the oxidation state in the name

Transition Elements Have variable valency Variable oxidation numbers [oxidation states] Definite colours associated with oxidation states Fe2+ iron (II) = Green [+2] FeCl2 Fe3+ iron (III) = Yellow [+3] FeCl3 Manganese (VII) = Pink [+7] MnO4- Manganese(IV) = Brown [+4] MnO2 Manganese(II) = Colourless [+2] MnCl2 Catalytic properties [as do their compounds.]

Anomalies C6 H12 O6 +1 -2 +12 -12 C appears to have gained or lost no electrons Calculate the oxidation number of S in Na2S4O6 S is + 2.5 Clearly it can’t lose half an electron so this is an anomaly.

Oxidation Numbers and Nomenclature Compounds containing two elements end in ide if in a compound an element has more than one oxidation state then the oxidation state of the least electronegative element is stated. e.g. Cu2O is copper(I) oxide CuO is copper (II) oxide PbO is Lead(II) oxide PbO2 is lead(IV) oxide

Oxidation numbers and Nomenclature MnO2 is manganese(IV) oxide MnO41- is manganate(VII)