Oxidation-Reduction Reactions

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Presentation transcript:

Oxidation-Reduction Reactions Balancing Redox reactions in Basic solutions.

Learn how to balance redox reactions in acidic solutions. There are nine guidelines to balancing redox reactions in acidic solutions. Basic Solutions have just two more steps. The steps comes in right after balancing hydrogen with the adding of hydrogen ions. It is not difficult. Actually, it is fun.

Balance KMnO4 (aq) + Na2SO3 (s)  MnO2 (s) + Na2SO4 (aq) There are eleven guidelines to balancing redox reactions in basic solutions

Guideline #1: Separate reaction into half-reactions. Identify what is being oxidized and reduced and write the half-reactions. Ex. KMnO4 (aq) + Na2SO3 (s)  MnO2 (s) + Na2SO4 (aq) Mn: reduced, goes from +7 to +4 O: Neither, it remains -2 S: oxidized, goes from +4 to +6 K and Na: Neither, they remain +1 KMnO4  MnO2 Na2SO3  Na2SO4

Guideline #2: Balance all elements except hydrogen and oxygen. Balance elements, except hydrogen and oxygen, in each half reaction. KMnO4  MnO2 KMnO4  MnO2 + K+ Na2SO3  Na2SO4

Add water to balance oxygen in each half reaction. Guideline #3: Add Water. Add water to balance oxygen in each half reaction. KMnO4  MnO2 KMnO4  MnO2 + K+ KMnO4  MnO2 + K+ + 2H2O Na2SO3  Na2SO4 H2O + Na2SO3  Na2SO4

Guideline #4: Add Hydrogen ions. Add hydrogen ions to balance hydrogens in each half reaction. KMnO4  MnO2 KMnO4  MnO2 + K+ KMnO4  MnO2 + K+ + 2H2O 4H+ + KMnO4  MnO2 + K+ + 2H2O Na2SO3  Na2SO4 H2O + Na2SO3  Na2SO4 H2O + Na2SO3  Na2SO4 + 2H+

Guideline #5: Add Hydroxide ions. Add the same number of OH- ions as H+ ions. Do this to both sides of half-rxn. Important! KMnO4  MnO2 KMnO4  MnO2 + K+ KMnO4  MnO2 + K+ + 2H2O 4H+ + KMnO4  MnO2 + K+ + 2H2O 4OH- + 4H+ + KMnO4  MnO2 + K+ + 2H2O + 4OH- Na2SO3  Na2SO4 H2O + Na2SO3  Na2SO4 H2O + Na2SO3  Na2SO4 + 2H+ 2OH- + H2O+ Na2SO3  Na2SO4 + 2H+ +2e- +2OH-

Guideline #6: Reduce Water . After forming water from H+ and OH- reduce the water from both sides of the half-reactions. KMnO4  MnO2 KMnO4  MnO2 + K+ KMnO4  MnO2 + K+ + 2H2O 4H+ + KMnO4  MnO2 + K+ + 2H2O 4OH- + 4H+ + KMnO4  MnO2 + K+ + 2H2O + 4OH- 4 H2O + KMnO4  MnO2 + K+ + 2H2O + 4OH- Na2SO3  Na2SO4 H2O + Na2SO3  Na2SO4 H2O + Na2SO3  Na2SO4 + 2H+ 2OH- + H2O+ Na2SO3  Na2SO4 + 2H+ +2e- +2OH- 2OH- + H2O+ Na2SO3  Na2SO4 + 2 H2O

Guideline #7: Add electrons. Add electrons to balance charges in the half-reactions. KMnO4  MnO2 KMnO4  MnO2 + K+ KMnO4  MnO2 + K+ + 2H2O 4H+ + KMnO4  MnO2 + K+ + 2H2O 4OH- + 4H+ + KMnO4  MnO2 + K+ + 2H2O + 4OH- 4 H2O + KMnO4  MnO2 + K+ + 2H2O + 4OH- 3e- + 2H2O + KMnO4  MnO2 + K+ + 4OH- Na2SO3  Na2SO4 H2O + Na2SO3  Na2SO4 H2O + Na2SO3  Na2SO4 + 2H+ 2OH- + H2O+ Na2SO3  Na2SO4 + 2H+ +2e- +2OH- 2OH- + H2O+ Na2SO3  Na2SO4 + 2 H2O 2OH- + Na2SO3  Na2SO4 + H2O+2e-

Guideline #8: Equal out electrons. Multiply half-reactions by whole numbers to equal electrons. KMnO4  MnO2 KMnO4  MnO2 + K+ KMnO4  MnO2 + K+ + 2H2O 4H+ + KMnO4  MnO2 + K+ + 2H2O 4OH- + 4H+ + KMnO4  MnO2 + K+ + 2H2O + 4OH- 4 H2O + KMnO4  MnO2 + K+ + 2H2O + 4OH- 3e- + 2H2O + KMnO4  MnO2 + K+ + 4OH- 6e- + 4H2O + 2KMnO4  2MnO2 + 2K+ +8OH- Na2SO3  Na2SO4 H2O + Na2SO3  Na2SO4 H2O + Na2SO3  Na2SO4 + 2H+ 2OH- + H2O+ Na2SO3  Na2SO4 + 2H+ +2e- +2OH- 2OH- + H2O+ Na2SO3  Na2SO4 + 2 H2O 2OH- + Na2SO3  Na2SO4 + H2O+2e- 6OH- + 3Na2SO3  3Na2SO4 + 3H2O+6e-

Guideline #9: Add the two half-reactions Add reactants with reactants, products with products. 6e- + 4 H2O + 2KMnO4 + 6OH- + 3Na2SO3  2MnO2 + 2K+ +8 OH- + 3Na2SO4 + 3H2O +6e- KMnO4  MnO2 KMnO4  MnO2 + K+ KMnO4  MnO2 + K+ + 2H2O 4H+ + KMnO4  MnO2 + K+ + 2H2O 4OH- + 4H+ + KMnO4  MnO2 + K+ + 2H2O + 4OH- 4 H2O + KMnO4  MnO2 + K+ + 2H2O + 4OH- 3e- + 2H2O + KMnO4  MnO2 + K+ + 4OH- 6e- + 4H2O + 2KMnO4  2MnO2 + 2K+ +8OH- Na2SO3  Na2SO4 H2O + Na2SO3  Na2SO4 H2O + Na2SO3  Na2SO4 + 2H+ 2OH- + H2O+ Na2SO3  Na2SO4 + 2H+ +2e- +2OH- 2OH- + H2O+ Na2SO3  Na2SO4 + 2 H2O 2OH- + Na2SO3  Na2SO4 + H2O+2e- 6OH- + 3Na2SO3  3Na2SO4 + 3H2O+6e-

Guideline #10: Cancel common reactants and products. Cancel reactants and products that show up on both sides of the yield sign. 6e- + 4 H2O + 2KMnO4 + 6OH- + 3Na2SO3  2MnO2 + 2K+ +8 OH- + 3Na2SO4 + 3H2O +6e- KMnO4  MnO2 KMnO4  MnO2 + K+ KMnO4  MnO2 + K+ + 2H2O 4H+ + KMnO4  MnO2 + K+ + 2H2O 4OH- + 4H+ + KMnO4  MnO2 + K+ + 2H2O + 4OH- 4 H2O + KMnO4  MnO2 + K+ + 2H2O + 4OH- 3e- + 2H2O + KMnO4  MnO2 + K+ + 4OH- 6e- + 4H2O + 2KMnO4  2MnO2 + 2K+ +8OH- Na2SO3  Na2SO4 H2O + Na2SO3  Na2SO4 H2O + Na2SO3  Na2SO4 + 2H+ 2OH- + H2O+ Na2SO3  Na2SO4 + 2H+ +2e- +2OH- 2OH- + H2O+ Na2SO3  Na2SO4 + 2 H2O 2OH- + Na2SO3  Na2SO4 + H2O+2e- 6OH- + 3Na2SO3  3Na2SO4 + 3H2O+6e-

Guideline #11: Rewrite Balanced Equation. Rewrite the Balanced Chemical Equation and box in the answer. 6e- + 4 H2O + 2KMnO4 + 6OH- + 3Na2SO3  2MnO2 + 2K+ +8 OH- + 3Na2SO4 + 3H2O +6e- H2O (l) + 2KMnO4 (aq) + 3Na2SO3 (aq) 2MnO2 (s)+ 2KOH (aq) + 3Na2SO4 (aq) KMnO4  MnO2 KMnO4  MnO2 + K+ KMnO4  MnO2 + K+ + 2H2O 4H+ + KMnO4  MnO2 + K+ + 2H2O 4OH- + 4H+ + KMnO4  MnO2 + K+ + 2H2O + 4OH- 4 H2O + KMnO4  MnO2 + K+ + 2H2O + 4OH- 3e- + 2H2O + KMnO4  MnO2 + K+ + 4OH- 6e- + 4H2O + 2KMnO4  2MnO2 + 2K+ +8OH- Na2SO3  Na2SO4 H2O + Na2SO3  Na2SO4 H2O + Na2SO3  Na2SO4 + 2H+ 2OH- + H2O+ Na2SO3  Na2SO4 + 2H+ +2e- +2OH- 2OH- + H2O+ Na2SO3  Na2SO4 + 2 H2O 2OH- + Na2SO3  Na2SO4 + H2O+2e- 6OH- + 3Na2SO3  3Na2SO4 + 3H2O+6e-

Practice Follow the steps. Realize that the practice is to help with the steps and sometimes demonstrate when steps may not be needed. Practice is to reinforce the guidelines, and may be easier than problems on a quiz or assessment.

Practice #1: S-2 (aq) + I2 (s)  SO4-2 (aq) + I-1 (aq)

Practice #1: S-2 (aq) + I2 (s)  SO4-2 (aq) + I-1 (aq) 8e- + 4I2 + 8 OH- + S-2  SO4-2 + 4H2O +8e- + 8I-1 4I2 + 8 OH- + S-2  SO4-2 + 4H2O + 8I-1 S-2  SO4-2 4H2O +S-2  SO4-2 4H2O + S-2  SO4-2 + 8H+ 8OH- + 4H2O + S-2  SO4-2 + 8H+ + 8OH- 8OH- +4H2O + S-2  SO4-2 + 8H2O 8 OH- + S-2  SO4-2 + 4H2O +8e- I2  I-1 I2  2I-1 2e- + I2  2I-1 8e- + 4I2  8I-1

Practice #2: H2O2 (s) + MnO4- (aq)  O2 (g) + MnO2 (s)

Practice #2: H2O2 (s) + MnO4- (aq)  O2 (g) + MnO2 (s) 6e- + 4H2O + 2MnO4-1 + 6OH- + 3H2O2  3O2 + 6H2O +6e- + 2MnO2 +8OH- 2MnO4-1 (aq) + 3H2O2 (aq) 3O2 (g)+ 2H2O (l)+ 2MnO2 (aq) +2OH- (aq) H2O2  O2 H2O2  O2 + 2H+ 2OH- + H2O2  O2 + 2H++ 2OH- 2OH- + H2O2  O2 + 2H2O 2OH- + H2O2  O2 + 2H2O +2e- 6OH- + 3H2O2  3O2 + 6H2O +6e- MnO4-1  MnO2 MnO4-1  MnO2 +2H2O 4H+ + MnO4-1  MnO2 +2H2O 4OH- + 4H+ + MnO4-1  MnO2 +2H2O +4OH- 4H2O + MnO4-1  MnO2 +2H2O +4OH- 3e- + 2H2O + MnO4-1  MnO2 +4OH- 6e- + 4H2O + 2MnO4-1  2MnO2 +8OH- S should actually be S+6

Practice #3: CN-1 (aq) + MnO4-1 (aq)  CNO-1 (aq) + MnO2 (s)

Practice #3: CN-1 (aq) + MnO4-1 (aq)  CNO-1 (aq) + MnO2 (s) 6e- + 4H2O + 2MnO4-1 + 6OH- + 3CN-1  2MnO2 +8OH- + 3CNO-1 + 3H2O +6e- H2O + 2MnO4-1 + 3CN-1  2MnO2 +2OH- + 3CNO-1 CN-1  CNO-1 H2O + CN-1  CNO-1 H2O + CN-1  CNO-1 + 2 H+ 2OH- + H2O + CN-1  CNO-1 + 2 H+ + 2OH- 2OH- + H2O + CN-1  CNO-1 + 2H2O 2OH- + CN-1  CNO-1 + H2O +2e- 6OH- + 3CN-1  3CNO-1 + 3H2O +6e- MnO4-1  MnO2 MnO4-1  MnO2 + 2H2O 4H+ + MnO4-1  MnO2 + 2H2O 4OH- + 4H+ + MnO4-1  MnO2 +2H2O +4OH- 4H2O + MnO4-1  MnO2 +2H2O +4OH- 3e- + 2H2O + MnO4-1  MnO2 +4OH- 6e- + 4H2O + 2MnO4-1  2MnO2 +8OH-