Chemistry 20.2

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.

Assigning Oxidation Numbers 20.2 Assigning Oxidation Numbers Assigning Oxidation Numbers What is the general rule for assigning oxidation numbers?

Assigning Oxidation Numbers 20.2 Assigning Oxidation Numbers An oxidation number is a positive or negative number assigned to an atom to indicate its degree of oxidation or reduction.

Assigning Oxidation Numbers 20.2 Assigning Oxidation Numbers 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.

Assigning Oxidation Numbers 20.2 Assigning Oxidation Numbers The oxidation number of any element in the free or uncombined state is zero. The oxidation number of any element in the free or uncombined state is zero. The elements shown here (left to right) are white phosphorus (stored under water), sulfur, potassium (stored under oil), carbon, and bromine (liquid and vapor). The potassium and phosphorus are stored under a liquid to prevent them from reacting with oxygen in the air.

Assigning Oxidation Numbers 20.2 Assigning Oxidation Numbers

Assigning Oxidation Numbers 20.2 Assigning Oxidation Numbers Oxidation numbers are often written above the chemical symbols in a formula.

Assigning Oxidation Numbers 20.2 Assigning Oxidation Numbers Yellow potassium chromate (K2CrO4) and green chromium (III) oxide (Cr2O3) each have a chromium-containing polyatomic ion. Inferring What is the oxidation number of chromium in each compound? K2CrO4 Cr oxidation number = +6 Cr2O3 Cr oxidation number = +3

for Conceptual Problem 20.2 Problem Solving 20.9 Solve Problem 9 with the help of an interactive guided tutorial.

Oxidation-Number Changes in Chemical Reactions 20.2 Oxidation-Number Changes in Chemical Reactions Oxidation-Number Changes in Chemical Reactions How are oxidation and reduction defined in terms of a change in oxidation number?

Oxidation-Number Changes in Chemical Reactions 20.2 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.

Oxidation-Number Changes in Chemical Reactions 20.2 Oxidation-Number Changes in Chemical Reactions Copper reacts with silver nitrate. a) A copper wire is placed in a silver nitrate solution. b) Crystals of silver coat the wire and the solution slowly turns blue as a result of the formation of copper(II) nitrate. Drawing Conclusions What change occurs in the oxidation number of silver? How does the oxidation number of copper change?

Oxidation-Number Changes in Chemical Reactions 20.2 Oxidation-Number Changes in Chemical Reactions 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. A redox reaction occurs between iron and copper. a) An iron nail is placed in a copper(II) sulfate solution. b) The iron reduces Cu2+ ions in solution and is simultaneously oxidized to Fe2+. The iron becomes coated with metallic copper.

for Conceptual Problem 20.3 Problem Solving 20.11 Solve Problem 11 with the help of an interactive guided tutorial.

20.2 Section Quiz. 20.2.

20.2 Section Quiz. 1. Which of the following oxidation numbers is INCORRECT? ClO3– ; Cl +5, O –2 SO32– ; S +4, O –2 H2O2; H +1, O –1 Ca(ClO)2; Ca +2, Cl +2, O –2

20.2 Section Quiz. 2. What is the oxidation number of Co in CoCl63– ? –3 –6 +9 +3

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? hydrogen oxygen bromine lead

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