Summary

In this lesson students will review oxidation states, half-reactions, balancing reactions and understand how to complete calculations and perform a redox titration.

Grade Level

High School (AP Chemistry)

NGSS Alignment

This lesson will help prepare your students to meet the performance expectations in the following standards:

• HS-PS1-2: Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.
• HS-PS1-7: Use mathematical representation to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.
• Scientific and Engineering Practices:
  • Using Mathematics and Computational Thinking
  • Analyzing and Interpreting Data
  • Engaging in Argument from Evidence

AP Chemistry Curriculum Framework

This lesson supports the following unit, topics and learning objectives:

• Unit 4: Chemical Reactions
  • Topic 4.6: Introduction to Titration
    • 4.6.A: Identify the equivalence point in a titration based on the amounts of the titrant and analyte, assuming the titration reaction goes to completion.
  • Topic 4.7: Types of Chemical Reactions
    • 4.7.A: Identify a reaction as acid-base, oxidation-reduction, or precipitation.
  • Topic 4.9: Oxidation-Reduction (Redox)Reactions
    • 4.9.A: Represent a balanced redox reaction equation using half-reactions.

Objectives

By the end of this lesson, students should be able to

• Understand oxidation and reduction reactions
• Assign oxidation states
• Be able to balance redox reactions for charge and mass
• Understand redox titration
• Perform redox titration calculations
• Perform a redox titration experiment

Chemistry Topics

This lesson supports students’ understanding of

• Redox Reaction
• Oxidation and reduction process
• Half Reactions
• Stoichiometric calculations
• Titrations
• End point

Time

Teacher Preparation: 60 minutes

Lesson: 120-150 minutes (approximate times)

• Demo-15 minutes
• Inquiry-45 minutes
• Lab-60 minutes

Materials

• Demonstration: Chemical Chameleon
  • Potassium Permanganate (2mg) dissolved in 500 mL distilled water
  • Sodium Hydroxide (10 g)
  • Sugar (6g)
  • 3 Erlenmeyer flasks
  • Scale
• Lab: Analysis of Hydrogen Peroxide (listed as enough for each lab group)
  • Distilled water (100 ml)
  • Commercial Hydrogen Peroxide (3%) (3.0 mL)
  • 0.025 M Potassium Permanganate (75 mL)
  • 6 M Sulfuric Acid (15mL)
  • Titration set up, ring stand with buret clamp and 50 mL buret
  • 1.00 mL Volumetric pipet
  • 125 mL Erlenmeyer flask
  • 100 mL beaker
  • 10 mL graduated cylinder
  • 250 mL waste beaker

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Students should wash their hands thoroughly before leaving the lab.
• When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
• When working with acids, if any solution gets on students’ skin, they should immediately alert you and thoroughly flush their skin with water.
• When diluting acids, always add acid to water.
• Students should wear proper safety gear during chemistry demonstrations. Safety goggles and lab apron are required.
• The demonstration results may be flushed down the sink with plenty of water as the permanganate is reduced to manganese dioxide.
• Sulfuric acid solution is severely corrosive to eyes, skin, and other body tissues.
• Potassium permanganate solution is a skin and eye irritant and a strong stain-it will stain skin and clothing.

Teacher Notes

• This lesson assumes that the students have some background knowledge of redox reactions although the Guided Inquiry activity reviews important information. Students should have prior experience with acid-base titrations before completing this lesson.

Lesson Outline:

• Introduction/Engagement: To review oxidation states with students, begin the lesson with a quick demo. Students are not expected to know the color changes for manganese oxidation states. Always practice demonstrations before performing for students. Check out this video if you have never done this demonstration.
• “Chemical Chameleon” Demo Procedure (5-10 minutes): Various Colors of Manganese Oxidation States
  • Create two different solutions to be used during the demonstration. (prep time: 10-15 minutes)
  • Solution A: dissolve 2 mg KMnO ₄ with 500 mL of distilled water in an Erlenmeyer flask.
  • Solution B: warm (do NOT allow to come to a boil) 500 mL of water on hot plate in an Erlenmeyer Flask , add 10g NaOH and 6g of sugar and stir to dissolve. Allow the solution to cool before performing the experiment.
  • In an empty Erlenmeyer add about half of the sodium hydroxide/sugar solution. Then add a small amount of the potassium permanganate solution, start with 10-20ml. Swirl the solution and watch the color change as the permanganate ion is reduced. Play around with the amounts and swirling to change the speed of the color change.
  • The color changes are due to the oxidation state changes of the permanganate ion. The students do not need to understand the chemistry involved in the color change.
  • Tell students which reactions are taking place for each color change and have them determine the oxidation state of the manganese.
• The permanganate ion appears purple but soon reduces to the hypomanganate ion which is green.
  • Purple: MnO₄^- + e^- → MnO₄^-2
  • Blue: Due to gradual colour transition from MnO₄^- (purple) toMnO₄^-2 (green)
  • Green: MnO₄^-2
  • The hypomanganate reduces to manganese dioxide which is a yellowish color
  • Orange/Yellow: MnO₄^-2 + 2H₂O + 2e^- → MnO₂ + 4OH^-
• Activity: Redox titration guided inquiry (30-40 minutes)
  • For the guided inquiry activity, randomly divide students into teams of 2-4. Students should work through this activity together and divide the work among the members as noted in the activity.
  • After the groups have finished, discuss the answers and be certain all questions have been addressed.
• Lab: Analysis of Hydrogen Peroxide with Potassium Permanganate
  • The lab activity should be finished in one 50 minute period with students working in pairs.
  • Students should have performed an acid/base titration prior to this lab so that they understand the titration process and outcome.
  • If materials are not available, there is also a virtual lab activity at Learn Chemistry (Royal Society of Chemistry) which is free for teachers and students.
  • Discuss pre-lab questions prior to allowing students to participate in the activity.
  • It is important that they understand how to observe the endpoint. The endpoint should be a very faint pink color. Use white paper under the flask to help determine the endpoint. The color should remain at least 30 seconds. If the color is not faint pink, discard this sample.
  • Students should have practice balancing redox reactions in acidic environments prior to the lab. Try this free animated practice balancer.
  • The teacher should prepare all solutions prior to the lab period.