1. chemistry

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

3. Oxidation-Number Changes in Chemical Reactions
20.2
Oxidation-Number Changes in Chemical Reactions
Bell Answer:
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.

4. 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?

6. Identifying Redox Reactions
20.3
Identifying Redox Reactions
Section 3
How are oxidation numbers used to identify redox reactions?

7. Identifying Redox Reactions
20.3
Identifying Redox Reactions
If the oxidation number of an element in a reacting species changes, then that element has undergone either oxidation or reduction. Therefore, the reaction as a whole must be a redox reaction.

8. Identifying Redox Reactions
20.3
Identifying Redox Reactions
A redox reaction takes place between nitrogen and oxygen when lightning bolts heat the air.
The redox reaction between nitrogen and oxygen that takes place when lightning bolts heat the air to extreme temperatures forms nitrogen monoxide.

9. Identifying Redox Reactions
20.3
Identifying Redox Reactions
A color change can signal a redox reaction.
A color change can signal a redox reaction. When a colorless solution containing bromide ions (Br-) is added to a solution containing permanganate ions (MnO4-), the distinctive purple color of the permanganate ion is replaced by the pale brown color of bromine.

13. Two Ways to Balance Redox Equations
20.3
Two Ways to Balance Redox Equations
Two Ways to Balance Redox Equations
Using changes in oxidation numbers
Using half-reactions

14. Two Ways to Balance Redox Equations
20.3
Two Ways to Balance Redox Equations
Using Oxidation-Number Changes
In the oxidation-number-change method, you balance a redox equation by comparing the increases and decreases in oxidation numbers.

15. Two Ways to Balance Redox Equations
20.3
Two Ways to Balance Redox Equations
To use the oxidation-number-change method, start with the skeleton equation for the redox reaction.
In a blast furnace like this one, air is blown through a combination of iron ore and coke. The carbon monoxide produced from the oxidation of coke reduces the Fe3+ ions to metallic iron.

16. Two Ways to Balance Redox Equations
20.3
Two Ways to Balance Redox Equations
Step 1 Assign oxidation numbers to all the atoms in the equation.

17. Two Ways to Balance Redox Equations
20.3
Two Ways to Balance Redox Equations
Step 2 Identify which atoms are oxidized and which are reduced.
Step 3 Use one bracketing line to connect the atoms that undergo oxidation and another such line to connect those that undergo reduction.

18. Two Ways to Balance Redox Equations
20.3
Two Ways to Balance Redox Equations
In a balanced redox equation, the total increase in oxidation number of the species oxidized must be balanced by the total decrease in the oxidation number of the species reduced.

19. Two Ways to Balance Redox Equations
20.3
Two Ways to Balance Redox Equations
Step 4 Make the total increase in oxidation number equal to the total decrease in oxidation number by using appropriate coefficients.

20. Two Ways to Balance Redox Equations
20.3
Two Ways to Balance Redox Equations
Step 5 Finally, make sure that the equation is balanced for both atoms and charge.

25. for Conceptual Problem 20.5
Problem Solving Solve Problem 20 with the help of an interactive guided tutorial.

26. Two Ways to Balance Redox Equations
20.3
Two Ways to Balance Redox Equations
Using Half-Reactions
A half-reaction is an equation showing just the oxidation or just the reduction that takes place in a redox reaction.
In the half-reaction method, you write and balance the oxidation and reduction half- reactions separately before combining them into a balanced redox equation.

27. Two Ways to Balance Redox Equations
20.3
Two Ways to Balance Redox Equations
To balance a redox reaction using half- reactions, write separate half-reactions for the oxidation and the reduction. After you balance atoms in each half-reaction, balance electrons gained in the reduction with electrons lost in the oxidation.

28. Two Ways to Balance Redox Equations
20.3
Two Ways to Balance Redox Equations

29. Two Ways to Balance Redox Equations
20.3
Two Ways to Balance Redox Equations

30. Two Ways to Balance Redox Equations
20.3
Two Ways to Balance Redox Equations
The oxidation of sulfur by nitric acid in aqueous solution is one example of a redox reaction that can be balanced by the half-reaction method.

31. Two Ways to Balance Redox Equations
20.3
Two Ways to Balance Redox Equations
Steps 1 and 2

32. Two Ways to Balance Redox Equations
20.3
Two Ways to Balance Redox Equations
Step 3a

33. Two Ways to Balance Redox Equations
20.3
Two Ways to Balance Redox Equations
Step 3b

34. Two Ways to Balance Redox Equations
20.3
Two Ways to Balance Redox Equations
Steps 4 and 5

35. Two Ways to Balance Redox Equations
20.3
Two Ways to Balance Redox Equations
Steps 6 and 7

38. Conceptual problem 20.6

42. for Conceptual Problem 20.6
Problem Solving Solve Problem 21 with the help of an interactive guided tutorial.

43. Choosing a Balancing Method
20.3
Choosing a Balancing Method
Choosing a Balancing Method
Balancing by oxidation number change usually works well if the oxidized and reduced species appear only once on each side of the equation.
Balancing by the half-reaction method works best in redox reactions where the same element is both oxidized and reduced and in reactions that take place in acidic or alkaline solution.

44. 20.3 Section Quiz.
20.3.

45. 20.3 Section Quiz
1. Choose the correct words for the spaces. One method used to balance redox reactions compares changes in ______ numbers. When an equation is balanced, the total number of _________ lost must be equal to the total number gained.
oxidation, electrons
oxidation, protons
atomic, electrons
mass, protons

46. 20.3 Section Quiz
2. Choose the correct numbers for the spaces. Balance the equation below with whole-number coefficients using the oxidation-number-change method.
NH3 + O2 NO2 + H2O
The least common multiple of the changes in oxidation numbers of atoms is _____, and the coefficient of water in the balanced equation is _____.
14, 6
14, 3
28, 3
7, 6

47. 20.3 Section Quiz
3. Balance the equation below with the lowest whole- number coefficients using the half-reaction method.
Ag+(aq) + SO2(g) + H2O(l)  Ag(s) + SO42–(g) + H+(aq)
The reduction half-reaction must be multiplied by 1 2 3 4

48. Concept Map 20 Create your concept map using the computer.

49. END OF SHOW