HW From textbook “CHEMISTRY a Contemporary Approach”

• Read “Oxidation Numbers” p421&422

Oxidation numbers (states) can be assigned to atoms and ions.

Arbitrarily, all elements are considered to become ions and assigned a charge called the oxidation state or number, which can be found in the periodic table.

In monatomic ions, it is the charge of the ion.

Example

Cl^-  = 1-    Ca ²⁺ = 2+

In free elements it is 0.

Example

K = 0   Cl₂ = 0

In neutral compounds the sum of the oxidation numbers is 0 and the most electronegative element is negative.

Example

In CO₂  C = 4+ and O = 2- therefore (4+) + 2(2-) = 0

The oxidation number of H is 1+, except in hydrides where it is 1-

Example

lithium hydride, LiH

calcium hydride, CaH₂

The oxidation number of O is 2-, except in peroxides where it is 1-

Example

hydrogen peroxide, H₂O₂ and sodium peroxide, Na₂O₂

The sum of the charges of the atoms in a polyatomic ion is equal to the charge on the ion. See table E

Example

NH₄⁺

 PO₄^3-

Problem

Assign oxidation numbers to the atoms for all the polyatomic ions in table E. Notice that the periodic table of elements give only some selected oxidation states, therefore it is possible that others exist.

TEST YOUR UNDERSTANDING

1. The oxidation number of nitrogen in N₂ is (1) +1    (2) 0    (3) +3      (4) -3

2. What is the oxidation number of hydrogen in CaH₂?  (1) +1     (2) +2      (3) -1     (4) -2

3. What is the oxidation number of carbon in NaHCO₃?  (1) -2     (2) +2      (3) -4   (4) +4

4. What is the oxidation number of chlorine in HClO₄? (1) +1      (2) +5    (3) +3     (4) +7

5. What is the oxidation number of sulfur in H₂SO₄? (1) 0     (2) -2     (3) +6   (4) +4

1/07

45 What is the oxidation number of chromium in the chromate ion, CrO₄^2–?    (1) +6 (2) +2 (3) +3 (4) +8

1/06

1/03

 In which compound does chlorine have the highest oxidation number?  (1) NaClO   (2) NaClO₂  (3) NaClO₃       (4) NaClO₄

6/02

What is the oxidation number of chromium in K₂Cr₂O₇? (1) +12     (2) +2   (3) +3    (4) +6

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