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chemistry Slide 1 of 47 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. Slide 2 of 47 © Copyright Pearson Prentice Hall 20.3 Balancing Redox Equations > Identifying Redox Reactions Identifying Redox Reactions How are oxidation numbers used to identify redox reactions? Slide 3 of 47 © Copyright Pearson Prentice Hall 20.3 Balancing Redox Equations > 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. Slide 4 of 47 © Copyright Pearson Prentice Hall 20.3 Balancing Redox Equations K + H2O KOH + H2 > Identifying Redox Reactions 2Zn + HCl ZnCl2 + H2 Slide 5 of 47 © Copyright Pearson Prentice Hall 20.3 Balancing Redox Equations > Identifying Redox Reactions A redox reaction takes place between nitrogen and oxygen when lightning bolts heat the air. Slide 6 of 47 © Copyright Pearson Prentice Hall 20.3 Balancing Redox Equations > Identifying Redox Reactions A color change can signal a redox reaction. Slide 7 of 47 © Copyright Pearson Prentice Hall Slide 8 of 47 © Copyright Pearson Prentice Hall Slide 9 of 47 © Copyright Pearson Prentice Hall Slide 10 of 47 © Copyright Pearson Prentice Hall Practice Problems for Conceptual Problem 20.4 Problem Solving 20.18 Solve Problem 18 with the help of an interactive guided tutorial. Slide 11 of 47 © Copyright Pearson Prentice Hall 20.3 Balancing Redox Equations > 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? Slide 12 of 47 © Copyright Pearson Prentice Hall 20.3 Balancing Redox Equations > 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. Slide 13 of 47 © Copyright Pearson Prentice Hall 20.3 Balancing Redox Equations > Two Ways to Balance Redox Equations To use the oxidation-number-change method, start with the skeleton equation for the redox reaction. Slide 14 of 47 © Copyright Pearson Prentice Hall 20.3 Balancing Redox Equations > Two Ways to Balance Redox Equations Step 1 Assign oxidation numbers to all the atoms in the equation. Slide 15 of 47 © Copyright Pearson Prentice Hall 20.3 Balancing Redox Equations > 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. Slide 16 of 47 © Copyright Pearson Prentice Hall 20.3 Balancing Redox Equations > 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. Slide 17 of 47 © Copyright Pearson Prentice Hall 20.3 Balancing Redox Equations > 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. Slide 18 of 47 © Copyright Pearson Prentice Hall 20.3 Balancing Redox Equations > Two Ways to Balance Redox Equations Step 5 Finally, make sure that the equation is balanced for both atoms and charge. Slide 19 of 47 © Copyright Pearson Prentice Hall Slide 20 of 47 © Copyright Pearson Prentice Hall Slide 21 of 47 © Copyright Pearson Prentice Hall Slide 22 of 47 © Copyright Pearson Prentice Hall Slide 23 of 47 © Copyright Pearson Prentice Hall Practice Problems for Conceptual Problem 20.5 Problem Solving 20.20 Solve Problem 20 with the help of an interactive guided tutorial. Slide 24 of 47 © Copyright Pearson Prentice Hall 20.3 Balancing Redox Equations > 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 halfreactions separately before combining them into a balanced redox equation. Slide 25 of 47 © Copyright Pearson Prentice Hall 20.3 Balancing Redox Equations > Two Ways to Balance Redox Equations To balance a redox reaction using halfreactions, 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. Slide 26 of 47 © Copyright Pearson Prentice Hall 20.3 Balancing Redox Equations > Two Ways to Balance Redox Equations Slide 27 of 47 © Copyright Pearson Prentice Hall 20.3 Balancing Redox Equations > Two Ways to Balance Redox Equations Slide 28 of 47 © Copyright Pearson Prentice Hall 20.3 Balancing Redox Equations > 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. Slide 29 of 47 © Copyright Pearson Prentice Hall 20.3 Balancing Redox Equations > Two Ways to Balance Redox Equations Steps 1 and 2 Slide 30 of 47 © Copyright Pearson Prentice Hall 20.3 Balancing Redox Equations > Two Ways to Balance Redox Equations Step 3a Slide 31 of 47 © Copyright Pearson Prentice Hall 20.3 Balancing Redox Equations > Two Ways to Balance Redox Equations Step 3b Slide 32 of 47 © Copyright Pearson Prentice Hall 20.3 Balancing Redox Equations > Two Ways to Balance Redox Equations Steps 4 and 5 Slide 33 of 47 © Copyright Pearson Prentice Hall 20.3 Balancing Redox Equations > Two Ways to Balance Redox Equations Steps 6 and 7 Slide 34 of 47 © Copyright Pearson Prentice Hall Slide 35 of 47 © Copyright Pearson Prentice Hall Slide 36 of 47 © Copyright Pearson Prentice Hall Slide 37 of 47 © Copyright Pearson Prentice Hall Slide 38 of 47 © Copyright Pearson Prentice Hall Slide 39 of 47 © Copyright Pearson Prentice Hall Slide 40 of 47 © Copyright Pearson Prentice Hall Practice Problems for Conceptual Problem 20.6 Problem Solving 20.21 Solve Problem 21 with the help of an interactive guided tutorial. Slide 41 of 47 © Copyright Pearson Prentice Hall 20.3 Balancing Redox Equations > 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. Slide 42 of 47 © Copyright Pearson Prentice Hall 20.3 Section Quiz. Assess students’ understanding of the concepts in Section 20.3. Continue to: -or- Launch: Section Quiz Slide 43 of 47 © Copyright Pearson Prentice Hall 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. a. oxidation, electrons b. oxidation, protons c. atomic, electrons d. mass, protons Slide 44 of 47 © Copyright Pearson Prentice Hall 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 _____. a. 14, 6 b. 14, 3 c. 28, 3 d. 7, 6 Slide 45 of 47 © Copyright Pearson Prentice Hall 20.3 Section Quiz 3. Balance the equation below with the lowest wholenumber 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 a. 1 b. 2 c. 3 d. 4 Slide 46 of 47 © Copyright Pearson Prentice Hall Balancing Redox Equations > Concept Map 20 Concept Map 20 Create your concept map using the computer. Slide 47 of 47 © Copyright Pearson Prentice Hall END OF SHOW