Balancing Redox Equations

1. 20.3 Balancing Redox Equations • When concentrated hydrochloric acid (HCl) and nitric acid (HNO3) are mixed with gold, oxidation and reduction reactions produce gaseous nitrogen monoxide (NO) and soluble, stable AuCl4– ions. You will learn how to write and balance chemical equations for redox reactions such as this one.

2. 20.3 Identifying Redox Reactions • Identifying Redox Reactions • How are oxidation numbers used to identify redox reactions?

3. 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.

4. 20.3 Identifying Redox Reactions K + H2O KOH + H2 2Zn + HCl ZnCl2 + H2

5. 20.3 Identifying Redox Reactions • A redox reaction takes place between nitrogen and oxygen when lightning bolts heat the air.

6. 20.3 Identifying Redox Reactions • A color change can signal a redox reaction.

10. for Conceptual Problem 20.4 Problem Solving 20.18 Solve Problem 18 with the help of an interactive guided tutorial.

11. 20.3 Two Ways to Balance Redox Equations • Two Ways to Balance Redox Equations • How are changes in oxidation numbers used to balance a redox equation? • What is the procedure for balancing a redox equation using half-reactions?

12. 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.

13. 20.3 Two Ways to Balance Redox Equations • To use the oxidation-number-change method, start with the skeleton equation for the redox reaction.

14. 20.3 Two Ways to Balance Redox Equations • Step 1Assign oxidation numbers to all the atoms in the equation.

15. 20.3 Two Ways to Balance Redox Equations • Step 2Identify which atoms are oxidized and which are reduced. • Step 3Use one bracketing line to connect the atoms that undergo oxidation and another such line to connect those that undergo reduction.

16. 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.

17. 20.3 Two Ways to Balance Redox Equations • Step 4Make the total increase in oxidation number equal to the total decrease in oxidation number by using appropriate coefficients.

18. 20.3 Two Ways to Balance Redox Equations • Step 5 Finally, make sure that the equation is balanced for both atoms and charge.

23. for Conceptual Problem 20.5 Problem Solving 20.20 Solve Problem 20 with the help of an interactive guided tutorial.

24. 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.

25. 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.

26. 20.3 Two Ways to Balance Redox Equations

27. 20.3 Two Ways to Balance Redox Equations

28. 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.

29. 20.3 Two Ways to Balance Redox Equations • Steps 1 and 2

30. 20.3 Two Ways to Balance Redox Equations • Step 3a

31. 20.3 Two Ways to Balance Redox Equations • Step 3b

32. 20.3 Two Ways to Balance Redox Equations • Steps 4 and 5

33. 20.3 Two Ways to Balance Redox Equations • Steps 6 and 7

40. for Conceptual Problem 20.6 Problem Solving 20.21 Solve Problem 21 with the help of an interactive guided tutorial.

41. 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.

42. 20.3 Section Quiz. • 20.3.

43. 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

44. 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

45. 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