Using this rule ,the oxidation number of unknown atom of an element in a charged radical/complex ion can be determined as in the examples below; a) SO42- has- -one atom of S with oxidation number +6( refer to Rule 4) -four atoms of O each with oxidation number -2( refer to Rule 1) Sum of oxidation numbers of atoms in SO42- = ( +6 + (-2 x 4)) = -2 The chemical name of this radical is thus sulphate(VI) ion b) NO3- has- -one atom of N with oxidation number +4( refer to Rule 4) -three atoms of O each with oxidation number -2( refer to Rule 1) Sum of oxidation numbers of atoms in NO3- = ( +4 + (-2 x 3)) = -1 The chemical name of this radical is thus nitrate(IV) ion. Determine the oxidation number of: I.Nitrogen in; -NO2- => x + (-2 x2)= -1 thus x = -1 – (-4) = + 3 The chemical name of this compound/ion/radical is thus Nitrate(III)ion II. Sulphur in; -SO32- => x + (-2 x3)= -2 thus x = -2 – (-6) = + 4 The chemical name of this compound/ion/radical is thus Sulphate(IV)ion III. Carbon in; -CO32- = x + (-2 x 3) = -2 thus x = -2 – (-6) = + 4 The chemical name of this compound/ion/radical is thus Carbonate(IV)ion IV.Manganese in; -MnO4 - = x + (-2 x 4)= -1 thus x= -1-(-2 +-8) =+7 The chemical name of this compound/ion/radical is thus manganate(VII) ion V.Chromium in -Cr2O72- => 2x + (-2 x7)= -2 thus 2x = -2 – +2 +-14 = +12 / 2= +6 The chemical name of this compound/ion//radical is thus dichromate(VI) ion -CrO42- => x + (-2 x4)= -2 thus x = -2 + (-2 x 4) = +6 The chemical name of this compound/ion//radical is thus chromate(VI) ion (c)Using the concept/idea of oxidation numbers as increase and decrease in oxidation numbers , the oxidizing and reducing species/agents can be determined as in the following examples; (i) Cu2+ (aq) + Zn(s) -> Zn2+ (aq) + Cu(s) Oxidation numbers -> +2 0 +2 0 Oxidizing species/agents =>Cu2+ ;its oxidation number decrease from+2 to 0 in Cu(s)
8 Reducing species/agents => Zn2+ ;its oxidation number increase from 0 to +2 in Zn(s) (ii) 2Br- (aq) + Cl2(g) -> 2Cl- (aq) + Br2 (l) Oxidation numbers -> -1 0 -1 0 Oxidizing agent =>Cl2(g) ;its oxidation number decrease from 0 to-1 in 2Cl- (aq) Reducing agents => Zn2+ ;its oxidation number increase from -1 to 0 in Zn(s) (iii) Br2 (l) + Zn(s) -> Zn2+ (aq) + 2Br- (aq) Oxidation numbers -> 0 0 +2 -1 Oxidizing agent => Br2 (l) ;its oxidation number decrease from 0 to-1 in 2Br-(aq) Reducing agents => Zn(s) ;its oxidation number increase from 0 to +2 in Zn2+ (iv) 2HCl (aq) + Mg(s) -> MgCl2 (aq) + H2 (g) Oxidation numbers -> 2 (+1 -1) 0 +2 2(-1) 0 Oxidizing agent => H+ in HCl;its oxidation number decrease from +1to 0 in H2 (g) Reducing agents => Mg(s) ;its oxidation number increase from 0 to +2 in Mg2+ (v) 2H2O (l) + 2Na(s) -> 2NaOH (aq) + H2 (g) Oxidation numbers -> +1 -2 0 +1 -2 +1 0 Oxidizing agent => H+ in H2O;its oxidation number decrease from +1to 0 in H2 (g) Reducing agents => Na(s) ;its oxidation number increase from 0 to +1 in Na+ (vi) 5Fe2+ (aq) + 8H+ (aq) + MnO4- -> 5Fe3+ (aq) + Mn2+ (aq) + 4H2O (l) +2 +1 +7 -2 +3 +2 +1 -2 Oxidizing agent => Mn in MnO4- ;its oxidation number decrease from +7to+2 in Mn2+ Reducing agents => Fe2+ ;its oxidation number increase from +2 to +3 in Fe3+ (vii) 6Fe2+ (aq) + 14H+ (aq) + Cr2O72-(aq) -> 6Fe3+ (aq) + Cr3+ (aq) + 7H2O (l) +2 +1 +6 -2 +3 +3 +1 -2 Oxidizing agent: Cr in Cr2O72- ;its oxidation number decrease from +6 to+3 in Cr3+ Reducing agents => Fe2+ ;its oxidation number increase from +2 to +3 in Fe3+ (viii) 2Fe2+ (aq) + 2H+ (aq) + H2O2(aq) -> 2Fe3+ (aq) + 2H2O (l) +2 +1 +1 -1 +3 +1 -2 Oxidizing agent: O in H2O2;its oxidation number decrease from -1 to -2 in H2O Reducing agents => Fe2+ ;its oxidation number increase from +2 to +3 in Fe3+ (ix) Cr2O72-(aq) + 6H+ (aq) + 5H2O2(aq) -> 2Cr3+ (aq) + 2H2O (l) + 5O2(g) +6 -2 +1 +1 -1 +3 +1 -2 0 Oxidizing agents: O in H2O2;its oxidation number decrease from -1 to -2 in H2O Cr in Cr2O72- its oxidation number decrease from +6 to +3 in Cr3+ Reducing agents
9 O in H2O2;its oxidation number increase from -1 to O in O2(g) O in Cr2O72- its oxidation number increase from -2 to O in O2(g) (x) 2MnO4-(aq) + 6H+ (aq) + 5H2O2(aq) -> 2Mn2+ (aq) + 8H2O (l) + 5O2(g) +7 -2 +1 +1 -1 +2 +1 -2 0 Oxidizing agents: O in H2O2;its oxidation number decrease from -1 to -2 in H2O Mn in MnO4- its oxidation number decrease from +7 to +2 in Mn2+ Reducing agents O in H2O2;its oxidation number increase from -1 to O in O2(g) O in MnO4- its oxidation number increase from -2 to O in O2(g) (ii)ELECTROCHEMICAL (VOLTAIC) CELL
10 1.