Exceptional Cases of Oxidation Numbers, Assignment Help, Redox Reaction Homework Help

25/6/2014 Exceptional cases of oxidation numbers, Assignment Help, Redox Reaction Homework Help

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Exceptional cases of oxidation numbers, Redox Reaction Assignment Help

Oxidation number or Oxidation state - Exceptional cases of oxidation numbers, Redox Reaction

Exceptional cases of evaluation of oxidation numbers : The rules described earlier are usually helpful indetermination of the oxidation number of a specific atom in simple molecules but these rules fail in followingcases. In this type of cases, oxidation numbers are evaluated using the concepts of chemical bonding involved.

Type I. In the molecules containing peroxide linkage in an addition to element-oxygen bonds. For example,

(i) Oxidation number of S in H2SO5

(Permonosulphuric acid or the Caro's acid)

By usual method; H2SO5

2*1+x+5*(-2)=0 or x = -8

But this cannot be factual as maximum oxidation number for S cannot exceed + 6. Since S has only 6 electronsin its valence shell. This exceptional value is due to the fact that two oxygen atoms in shows peroxide linkage asshown below,

Thus the evaluation of o.n. of sulphur here should be made as follows,

2 (+1) + x + 3 (-2) + 2 (-1)

(H) (S) (O) (O-O)

or 2 + x - 6 - 2 = 0 or x = + 6.

(ii) Oxidation number of S in H2S2O8

(Peroxidisulphuric acid or the Marshall's acid)

By usual method; H2S2O8

1 2 + 2x + 8 (-2) = 0

2x = + 16 - 2 = 14 or x = + 7

Type II. In molecules containing covalent and coordinate bonds, the below given rules are used for evaluating theoxidation numbers of atoms.

(i) For each of the covalent bond between dissimilar atoms the less electronegative element is assigned theoxidation number of + 1 while the atom of the more electronegative element is assigned the oxidation number of-1.

(ii) In case of the coordinate-covalent bond between similar or dissimilar atoms but donor atom is lesselectronegative than the acceptor atom, the oxidation number of +2 is assigned to the donor atom and anoxidation number of -2 is assigned to the acceptor atom.

Conversely, if the donor atom is more electronegative than the acceptor atom, contribution of the coordinate bondis neglected. The examples are as follows

(a) Oxidation number of C in

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Evaluation of the oxidation number of C cannot be made directly by usual rules since no standard rule exists foroxidation numbers of N and C.

In such type of cases, evaluation of oxidation number should be made using indirect concept or by the originalconcepts of chemical bonding.

(b) Oxidation number of the carbon in molecule

Contribution of the coordinate bond is neglected since the bond is directed from a more electronegative N atom(donor) to a less electronegative carbon atom (acceptor).

Therefore the oxidation number of N in remains - 3 as it has three covalent bonds.

1 (+ 1) + 1 (- 3) + x = 0

H) (N) (C)

or 1 + x - 3 = 0 or x = + 2.

(c) Oxidation number of carbon in

In , N is more electronegative than carbon, each bond is gives an oxidation number of -1 to N. There

are basically three covalent bonds; the oxidation number of N in is taken as - 3

Now +1 + x - 3 = 0 x = + 2

Type III. In a molecule containing two or more atoms of same or different elements in different oxidationstates.

(i) Oxidation number of S in Na2S2O3

By usual method Na2S2O3

2 (+1) + 2 x + 3 (-2) = 0 or 2 + 2x - 6 = 0

or x = 2.

But this is unacceptable as the two sulphur atoms in Na2S2O3 cannot have the same oxidation number because

on treatment with dil. H2SO4, one sulphur atom is precipitated while the other is oxidised to SO2.

In this case, the oxidation number of sulphur is evaluated from concepts of chemical bonding. The chemicalstructure of Na2S2O3 is

Because of the presence of a co-ordinate bond between two sulphur atoms, the acceptor sulphur atom hasoxidation number of - 2 whereas the other S atom gets oxidation number of + 2.

2 (+1) + 3 (-2) + x 1 + 1 (- 2) = 0

( Na) (O) (S) (coordinated S)

or + 2 - 6 + x - 2 = 0 or x = + 6

Thus two sulphur atoms in Na2S2O3 have oxidation number of - 2 and +6.

(ii) Oxidation number of chlorine in CaOCl2

(bleaching powder)

In bleaching powder, Ca(OCl)Cl, the two Cl atoms are in different oxidation states i.e., one Cl- having oxidation

number of -1 and the other as OCl- having oxidation number of +1.

(iii) Oxidation number of N in NH4NO3

By usual method N2H4O3 ; 2x + 4 (+1) + 3 (-1) = 0

2x + 4 - 3 = 0 or 2x = + 1 (wrong)

No doubt NH4NO3 has two nitrogen atoms but one N has negative oxidation number (attached to H) and the other

has positive oxidation number (attached to O). Thus the evaluation should be made separately for NH4+ and NO3

-

NH4+ x + 4 (+1) = +1 or x = - 3

NO3- x + 3 (- 2) = -1 or x = + 5.

(iv) Oxidation number of Fe in Fe3O4

In Fe3O4, Fe atoms are in two different oxidation states. Fe3O4 can be considered as an equimolar mixture of

FeO [iron (II) oxide] and Fe2O3 [iron (III) oxide]. Thus in one molecule of Fe3O4, two Fe atoms are in + 3 oxidation



state and one Fe atom is in + 2 oxidation state.

(v) The Oxidation number of S in the sodium tetrathionate (Na2S4O6)

Its structure can be represented as follows,

The two S-atoms which are linked to each other have oxidation number zero. Oxidation number of the other S-atoms can be calculated as shown below

Let oxidation number of S = x.

2 x + 2 0 + 6 ( - 2) = - 2

(For S) (For S-S) (For O)

x = + 5.

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Exceptional Cases of Oxidation Numbers, Assignment Help, Redox Reaction Homework Help

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