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Balancing Redox Reactions Chapter 20: Day 2 1 2 Review of Terminology for Redox Reactions • OXIDATION—loss of electron(s) by a species; increase in oxidation number. • REDUCTION—gain of electron(s); decrease in oxidation number. • OXIDIZING AGENT—electron acceptor; species is reduced. • REDUCING AGENT—electron donor; species is oxidized. 20.3 Describing Redox Equations > CHEMISTRY & YOU Why does cut fruit turn brown? Some fruits, including apples, turn brown when you cut them. What is happening on the surface of the fruit? Oxygen in air reacts with chemicals on the surface of the cut fruit. The oxygen oxidizes the chemicals in the fruit, causing a redox reaction and therefore the color change. 3 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. If changes in oxidation number occur, the reaction is a redox reaction. 20.3 Describing Redox Equations > • The element whose oxidation number increases is oxidized • The element whose oxidation number decreases is reduced 4 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 20.3 Describing Redox Equations > Identifying Redox Reactions Use the change in oxidation number to identify whether each reaction is a redox reaction Cl2(g) + 2NaBr(aq) → 2NaCl(aq) + Br2(aq) 2NaOH(aq) + H2SO4(aq) → Na2SO4(aq) + 2H2O(l) 5 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 20.3 Describing Redox Equations > a. 0 Sample Problem 20.5 Assign oxidation numbers. +1 –1 +1 –1 0 Cl2(g) + 2NaBr(aq) → 2NaCl(aq) + Br2(aq) The chlorine is reduced; The bromide ion is oxidized; 6 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 20.3 Describing Redox Equations > 2 Solve +1 –2 +1 Sample Problem 20.5 ASSIGN OXIDATION NUMBERS. +1 +6 –2 +1 +6 –1 +1 –2 2NaOH(aq) + H2SO4(aq) → Na2SO4(aq) + 2H2O(l) • NO change in oxidation number. • This is not a redox reaction. This is an acid-base (neutralization) reaction. 7 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 20.3 Describing Redox Equations > Which of the following are redox reactions? A. NH3 + HCl → NH4Cl B. SO3 + H2O → H2SO4 C. NaOH + HCl → NaCl + H2O D. H2S + NHO3 → H H2SO4 + NO2 + H2O 8 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. TRANSFER REACTIONS Atom/Group transfer (not) HCl + H2O ---> Cl- + H3O+ Redox: Electron transfer Cu(s) + 2 Ag+(aq) ---> Cu2+(aq) + 2 Ag(s) 9 OXIDATION-REDUCTION REACTIONS Cu(s) + 2 Ag+(aq) ---> Cu2+(aq) + 2 Ag(s) Why 2? 10 Balancing Equations Cu + Ag+ --give--> Cu2+ + Ag Need to Balance BOTH mass and CHARGE Step 1: Divide into half-reactions: one for oxidation and the other for reduction. Ox Red Cu ---> Cu2+ Ag+ ---> Ag 11 Balancing Equations Step 2: Step 3: Balance each for mass. Already done in this case. Balance each half-reaction for charge by adding electrons. Ox Cu ---> Cu2+ + 2eRed Ag+ + e- ---> Ag 12 Balancing Equations Multiply each half-reaction by a factor to have the electrons lost equal to number gained Step 4: Cu ---> Cu2+ + 2e2 Ag+ + 2 e- ---> 2 Ag Step 5: Add to give the overall equation. Cu + 2 Ag+ ---> Cu2+ + 2Ag The equation is now balanced for BOTH charge and mass. 13 14 Use the half-reaction method to balance the following redox equation. FeCl3 + H2S → FeCl2 + HCl + S Oxidation: H2S → 2H+ + S + 2e– Reduction: 2Fe3+ + 2e– → 2Fe2+ 2FeCl3 + H2S → 2FeCl2 + 2HCl + S Copyright © Pearson Education, Inc., or 20.3 Describing Redox Equations > Balancing Redox Equations What are two different methods for balancing a redox equation? Two different methods for balancing redox equations are the oxidation-number-change method and the half-reaction method. 15 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 20.3 Describing Redox Equations > Using Oxidation-Number Changes In the oxidation-numberchange method, you balance a redox equation by comparing the increases and decreases in oxidation numbers. 16 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 20.3 Describing Redox Equations > Using Oxidation-Number Changes Step 1: Assign oxidation numbers to all the atoms in the equation. • Write the numbers above the atoms. • The oxidation number is stated per atom. +3 –2 +2 –2 0 +4 –2 Fe2O3(s) + CO(g) → Fe(s) + CO2(g) 17 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 20.3 Describing Redox Equations > Using Oxidation-Number Changes Step 2: Identify which atoms are oxidized and which are reduced. • Iron is reduced. +3 to 0 • Carbon is oxidized. +2 to +4 +3 –2 +2 –2 0 +4 –2 Fe2O3(s) + CO(g) → Fe(s) + CO2(g) 18 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 20.3 Describing Redox Equations > Using Oxidation-Number Changes Step 3: Use one line to connect the atoms that undergo oxidation and another such line to connect those that undergo reduction. +2 (oxidation) +3 –2 +2 –2 0 +4 –2 Fe2O3(s) + CO(g) → Fe(s) + CO2(g) –3 (reduction) • Write the oxidation-number change at the midpoint of each line. 19 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 20.3 Describing Redox Equations > Using Oxidation-Number Changes Step 4: Make the total increase in oxidation number equal to the total decrease in oxidation number by using appropriate coefficients. 3 × (+2) = +6 +3 –2 +2 –2 0 +4 –2 Fe2O3(s) + 3CO(g) → 2Fe(s) + 3CO2(g) 2 × (–3) = –6 • The oxidation-number increase should be multiplied by 3 and the decrease by 2. 20 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 20.3 Describing Redox Equations > Using Oxidation-Number Changes Step 5: Finally, make sure the equation is balanced for both atoms and charge. Fe2O3(s) + 3CO(g) → 2Fe(s) + 3CO2(g) • If necessary, finish balancing the equation by inspection. 21 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Balancing Redox Equations by Oxidation-Number Change 20.3 Describing Redox Equations > Balance this redox equation by using the oxidation-number-change method. 2 × (-1) = -2 0 0 +1 –1 Cl2(g) + 2 K(s) → 2 KCl(s) 2 × (+1) = +2 22 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Ready for more complex reactions? 23 YES! Acid / Base Redox Reactions Some redox reactions have equations that must be balanced by special techniques. If reactions occur in the “presence of” acid or base MnO4- + 5 Fe2+ + 8 H+ ---> Mn2+ + 5 Fe3+ + 4 H2O 24 Permanganate and iron(II) ions are reacted in an acidic solution 25 MnO4- + Fe2+ ---> Mn2+ + Fe3+ Step 1: Divide into half-reactions Ox Fe2+ ---> Fe3+ Red MnO4- ---> Mn2+ Need to balance MASS Step 2: Balance each for mass. Already done for Iron Fe2+ ---> Fe3+ Need to have “O” on both sided: Add water MnO4- ---> Mn2+ + H2O Never add O2, O atoms, or O2- to balance oxygen. MnO4- ---> Mn2+ + 4H2O 26 27 Step 2: Balance each for mass. MnO4- ---> Mn2+ + 4H2O Need to have “H” on both sided. Told in an acidic solution: need H+ 8 H+ + MnO4- ---> Mn2+ + 4H2O Never add H2 or H atoms to balance hydrogen. Step 3: Balance each half-reaction for charge by adding electrons. 8 H+ + 5 e- +MnO4- ---> Mn2+ + 4H2O 5Fe2+ ---> 5Fe3+ + 5eMultiply each half-reaction by a factor to have the electrons lost equal to number gained 28 Acid / Base Redox Reactions Step 5: Add to obtain the overall equation MnO4- + 5 Fe2+ + 8 H+ ---> Mn2+ + 5 Fe3+ + 4 H2O Check by adding charges on both sides and by counting atoms The equation is now balanced for BOTH charge and mass. 29 30 Balancing Equations • Never add O2, O atoms, or O2- to balance oxygen. • Never add H2 or H atoms to balance hydrogen. • Be sure to write the correct charges on all the ions. • Check your work at the end to make sure mass and charge are balanced. • PRACTICE! Reduction of VO2+ with Zn 31 Balancing Equations Balance the following in acid solution— VO2+ + Zn ---> VO2+ + Zn2+ Step 1: Write the half-reactions Ox Zn ---> Zn2+ Red VO2+ ---> VO2+ Balance each for mass. Step 2: 2 H+ + ---> VO2+ + H O VO + 2 2 Add H2O on O-deficient side and add H+ on other side for H-balance. 32 Balancing Equations 33 Step 3: Add electrons to half reaction. Zn ---> Zn2+ + 2ee- + 2 H+ + VO2+ ---> VO2+ + H2O Step 4: Multiply by an appropriate factor. 2e- + 4 H+ + 2 VO2+ ---> 2 VO2+ + 2 H2O Zn ---> Zn2+ + 2e- Balancing Equations Step 5: Add balanced half-reactions Zn + 4 H+ + 2 VO2+ ---> Zn2+ + 2 VO2+ + 2 H2O Check by adding charges on both sides and by counting atoms 20.3 Describing Redox Equations > Balancing Redox Equations by Half-Reactions Balance this redox equation using the half-reaction method. KMnO4(aq) + HCl(l) → MnCl2(aq) + Cl2(g) + KCl(aq) 35 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 20.3 Describing Redox Equations > Sample Problem 20.7 Step 1:Write the equation in ionic form. K+(aq) + MnO4–(aq) + H+(aq) + Cl–(aq) → Mn2+(a) + 2Cl–(a) + Cl2(g) + H2O + K+(a) + Cl–(a) 36 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 20.3 Describing Redox Equations > Step 2: Write half-reactions. Determine the oxidation and reduction process. Oxidation half-reaction: –1 0 Cl– → Cl2 Reduction half-reaction: +7 +2 MnO4– → Mn2+ 37 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 20.3 Describing Redox Equations Step 3: Balance the>atoms in each half-reaction. • The solution is acidic, so use H2O and H+ to balance the oxygen and hydrogen. Oxidation: 2Cl–(aq) → Cl2(g) (atoms balanced) Reduction: MnO4–(aq) + 8H+(aq) → Mn2+(aq) + 4H2O(l) (atoms balanced) 38 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 20.3 Describing Redox Equations > Step 4: Balance the charges by adding electrons. Oxidation: 2Cl–(aq) → Cl2(g) + 2e– (charges balanced) Reduction: MnO4–(aq) + 8H+(aq) + 5e– → Mn2+(aq) + 4H2O(l) (charges balanced) 39 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 20.3 Describing Redox Equations > Step 5: Make the numbers of electrons equal. • Multiply the oxidation half-reaction by 5 and the reduction half-reaction by 2. Oxidation: 10Cl–(aq) → 5Cl2(g) + 10e– Reduction: 2MnO4–(aq) + 16H+(aq) + 10e– → 2Mn2+(aq) + 8H2O(l) 40 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 20.3 Describing Redox Equations > Step 6: Add the half-reactions. Then, subtract the terms that appear on both sides. 10Cl–(aq) + 2MnO4–(aq) + 16H+(aq) + 10e– → 5Cl2(g) + 10e– + 2Mn2+(aq) + 8H2O(l) Step 7: Add the spectator ions, making sure the charges and atoms are balanced. 10Cl– + 2MnO4– + 2K+ + 16H+ + 6Cl– → 5Cl2 + 2Mn2+ + 4Cl– + 8H2O + 2K+ + 2Cl– 41 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Combine the spectator and nonspectator Cl– on each side. 20.3 Describing Redox Equations > 16Cl–(a) + 2MnO4–(a) + 2K+(a) + 16H+(a) → 5Cl2(g) + 2Mn2+(a) + 6Cl–(a) + 8H2O(l) + 2K+(a) 42 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 20.3 Describing Redox Equations > Show the balanced equation for the substances given in the question (rather than for ions). 2KMnO4(aq) + 16HCl(aq) → 2MnCl2(aq) + 5Cl2(g) + 8H2O(l) + 2KCl(aq) 43 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 20.3 Describing Redox Equations > Use the half-reaction method to balance the following redox equation. FeCl3 + H2S → FeCl2 + HCl + S 44 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 20.3 Describing Redox Equations > Glossary Terms • oxidation-number-change method: a method of balancing a redox equation by comparing the increases and decreases in oxidation numbers • half-reaction: an equation showing either the oxidation or the reduction that takes place in a redox reaction • half-reaction method: a method of balancing a redox equation by balancing the oxidation and reduction half-reactions separately before combining them into a balanced redox equation 45 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 20.3 Describing Redox Equations > BIG IDEA Reactions Redox equations can be balanced by two methods, the oxidation-number-change method and balancing the oxidation and reduction half-reactions. 46 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.