Rules for assigning Oxidation numbers:

1. All elements in their free state (uncombined with other elements) have an oxidation number of zero

(e.g. Na, Cu, Mg, H2, O2, Cl2, N2)

2. H is +1 except in metal hydrides, where it is -1 (e.g. NaH, CaH2)

3. O is -2, except in peroxides, where it is -1, and in OF2, where it is +2.

4. The metallic element in an ionic compound has a positive oxidation number

5. In covalent compounds the negative oxidation number is assigned to the most electronegative atom. 6. The algebraic sum of the oxidation numbers of the elements in a compound is zero. 7. The algebraic sum of the oxidation numbers of the elements in a polyatomic ion is equal to the charge of the ion.

Determine the oxidation number of (a) S in Na2SO4, (b) As in K3AsO4, and (c) C in CaCO3

Na2 (SO4)2- 2- 6+1+

K3 (AsO4)3-

1+5+2-Ca (CO3)2-

4+2+ 2-

Determine the oxidation numbers of (a) N in NH4

+, (b) Cr in (Cr2O7)2- and (c) P in (PO4)3-

(NH4)+

1+3-(Cr2O7)2-

2-6+ (PO4)3-

2-5+

Determine the oxidation number of each element in these species:

BaCl2

2+ 1-

H2O2 (peroxide)1+ 1-

(BrO3)-

2-5+

H(ClO)1+ 1+ 2-

Oxidation: Increase in oxidation number (loss of e-)

 

Reduction: Decrease in oxidation number (gain of e-)

OIL RIG LEO GER

 Oxidizing agent (is reduced, gains e-)– the substance that causes an increase in the oxidation state of another substance

 

Reducing agent (is oxidized, loses e-)– the substance that causes a decrease in

the oxidation state of another substance

Cu + 2 AgNO3 → Cu(NO3)2 + 2 Ag

(Ox) Cu → Cu2++ 2e-

0

“Half Reaction”- shows only one half of the redox

Remember:-only reactants are oxidized or reduced

2e- + 2Ag+ → 2 Ag (Red)

2+1+ 1-0 1-

RA OA