Slide 1 of 26 Chemistry 20.2. © Copyright Pearson Prentice Hall Slide 2 of 26 Oxidation Numbers...

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Chemistry 20.2

Oxidation Numbers

When aluminum or magnesium powder in fireworks are heated to high temperatures in the explosion, they burn with an intense white light. As elements burn, their oxidation numbers change. You will learn about how oxidation and reduction are defined in terms of a change in oxidation number.

Oxidation Numbers >

Assigning Oxidation Numbers

What is the general rule for assigning oxidation numbers?

Oxidation Numbers > Assigning Oxidation Numbers

An oxidation number is a positive or negative number assigned to an atom to indicate its degree of oxidation or reduction.

As a general rule, a bonded atom’s oxidation number is the charge that it would have if the electrons in the bond were assigned to the atom of the more electronegative element.

The oxidation number of any element in the free or uncombined state is zero.

Oxidation Numbers > Assigning Oxidation Numbers20.2

Oxidation numbers are often written above the chemical symbols in a formula.

K2CrO4

Cr oxidation number = +6

Cr oxidation number = +3

Practice Problems for Conceptual Problem 20.2

Problem Solving 20.9 Solve Problem 9 with the help of an interactive guided tutorial.

Oxidation Numbers >

Oxidation-Number Changes in Chemical Reactions

How are oxidation and reduction defined in terms of a change in oxidation number?

Oxidation Numbers > Oxidation-Number Changes in Chemical Reactions

An increase in the oxidation number of an atom or ion indicates oxidation. A decrease in the oxidation number of an atom or ion indicates reduction.

When an iron nail is placed in a copper(II) sulfate solution, the iron reduces Cu2+ ions in solution and is simultaneously oxidized to Fe2+ The iron becomes coated with metallic copper.

Practice Problems for Conceptual Problem 20.3

Problem Solving 20.11 Solve Problem 11 with the help of an interactive guided tutorial.

Section Quiz

-or-Continue to: Launch:

Assess students’ understanding of the concepts in Section

20.2 Section Quiz.

1. Which of the following oxidation numbers is INCORRECT?

a. ClO3– ; Cl +5, O –2

b. SO32– ; S +4, O –2

c. H2O2; H +1, O –1

d. Ca(ClO)2; Ca +2, Cl +2, O –2

20.2 Section Quiz.

2. What is the oxidation number of Co in CoCl63–?

a. –3

b. –6

20.2 Section Quiz.

3. In the reaction PbO2(aq) + 4HBr(aq) PbBr2(aq) + Br2(l) + 2H2O(l) which element has the greatest change in oxidation number?

a. hydrogen

b. oxygen

c. bromine

d. lead

END OF SHOW

Slide 1 of 26 Chemistry 20.2. © Copyright Pearson Prentice Hall Slide 2 of 26 Oxidation Numbers...

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