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Chemistry 30 Oxidation Numbers NG

Read your textbook pages 583586 and use the following information to complete this

notes guide.

Q1. Is an oxidation state an actual electric charge? Describe your answer.

Q2. Define oxidation number.

Q3. How do you distinguish oxidation numbers form electric charges?

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Examples of how to assign oxidation numbers:

1. The oxidation number of an atom (element) iszero.

e.g.,

Na Cl2 P4 Au

(each is assigned zero)

2.

The oxidation number ofhydrogen is +1, except when it is bonded to a less

electronegative element, where it is 1 (as in the hydrides).

e.g.,

CH4 NH3 H2O NaH

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3. The oxidation number ofoxygen is 2, except in peroxides, then oxygen is 1

e.g.,

H2O Fe2O3 H2O2

4. The oxidation number of a monoatomic ion is equal to the charge on the ion.

e.g.,

Na+ Cl in HCl Mg in MgBr2

Yuibmb,

5. In binary compounds of non-metals, the more electronegative atom is assigned the

negative oxidation number.

e.g.,

OF2 NO2 CCl4 NH3

6. Any unknown oxidation number is determined algebraically (using simple math)

from the sum of the known oxidation numbers and the net charge on the entity.The sum of the oxidation numbers for all the atoms in a compound or ion must equal the

total charge.h

for neutral molecules, the sum of the oxidation numbers is zero

for polyatomic ions, the sum of the oxidation numbers must equal the

charge on the ion

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e.g.,

MnO4 HCO3

ClO3

o in polyatomic ions or molecules that have two or more of the same atoms,

remember that each one carries the same (single) oxidation number

e.g.,

CH3COOH

o carbon is a special case you will have to determine it algebraically

Practice Problems

1. Determine the oxidation number of

(a) S in SO2 (c) S in SO42 (e) I in MgI2

(b) Cl in HClO4 (d) Cr in Cr2O72 (f) H in CaH2

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2. Determine the oxidation number of nitrogen in

(a) N2O(g) (d) NH3(g) (g) N2(g)

(b) NO(g) (e) N2H4(g) (h) NH4Cl

(c) NO2 (g) (f) NaNO3(s)

3. Determine the oxidation number of carbon in

(a) graphite (elemental carbon) (c) sodium carbonate

(b) glucose (d) carbon monoxide

4. Determine the oxidation number of every atom or ion in the following chemical

equation.

2 ClO3(aq) + 2 Cl(aq) + 4 H+(aq) 2 ClO2(g) + Cl2(g) + 2 H2O(l)

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5. Determine the oxidation number of carbon in each of the compounds in the

following series of oxidations.

methane methanol methanal methanoic acid carbon dioxide

CH4 CH3OH CH2O HCOOH CO2

6. Assign oxidation numbers to chlorine in each of the following chemicals.

HCl(aq), Cl2(g), NaClO(s), Cl(aq), HClO3(aq),

ClO3(aq), KClO2(s), ClO2(g), HClO4(aq)

7. Assign oxidation numbers to manganese in each of the following chemicals.

MnO2(s), KMnO4(s), Mn(s), MnO42(aq), MnCl2(s),

Mn2O7(s), Mn2+(aq)