Define oxidation number. find the oxidation number of a. cl in hclo 4 b. sulfur in h 2 so 4 . c. phosphorus in po 4 3- d. manganese in mno 4 2-

(a) Cl = +7; (b) S = +6; (c) P = +5; (d) Mn = +6

Oxidation number is the charge that an atom appears to have when we count its valence electrons according to the following rules:

1.Lone pair electrons belong entirely to the atom.

2.Identical atoms share electrons equally

3.Electrons shared between two different atoms belong entirely to the more electronegative atom.

4.The sum of the oxidation numbers of all the atoms must add up to the charge on the species.

The most common oxidation numbers are: H = +1 and O = -2.

(a) HClO₄

H + Cl + 4O = 0

1 + Cl – 8 =0

Cl = 8 – 1 = +7

(b) H₂SO₄

2H + S + 4O = 0

2 + S – 8 = 0

S = 8 – 2 = +6

(c) PO₄³⁻

P + 4O = -3

P – 8 = -3

P = +5

(d) MnO₄²⁻

Mn + 4O = -2

Mn – 8 = -2

Mn = +6

your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.