Redox Reaction - Spontaneous Color Change (Richard Hanley)

Author

Richard Hanley

Principle(s) Illustrated

1. Oxidization

2. Reduction

3. Analagous to Molecular Motion

Standards

8th Grade Physical Science:

3d. Students know the states of matter (solid, liquid, gas) depend on molecular motion.

5d. Students know physical processes include freezing and boiling, in which a

material changes form with no chemical reaction.

9a. Plan and conduct a scientific investigation to test a hypothesis.

Chemistry:

1d. Students know how to use the periodic table to determine the number of electrons

available for bonding.

3g. Students know how to identify reactions that involve oxidation and reduction and

how to balance oxidation-reduction reactions.

Questioning Script

Materials

- potassium hydroxide (KOH)

- methylene blue indicator solution (or approx. 0.5g of methelene blue powder)

- dextrose (glucose)

- water

- Erlenmeyer flask with a stopper

- goggles and gloves when mixing KOH

Prior knowledge & experience:

Students are used to seeing a dye change a clear substance to a colored substance. They are not used to seeing a color change which quickly returns to a clear substance. Students must have some knowledge of the fact that particles move. An understanding of indicator dyes may help as well. If this is being used in a chemistry class, an understanding of electrochemistry, oxidization and reduction are useful.

Root question:

What is causing this reaction? What does the indicator tell us about the electrochemistry of the reaction?

Target response:

If this demonstration is being used as an analogy of molecular motion, seeing the motion may help them understand other causes of molecular motion (heating things up) and/or other ways of slowing particle movement (cooling or freezing things). If this is being used as a chemistry demo, students will come to see the neutrality of KOH & C₆H₁₂O₆ and the reduction of oxygen in the flask (followed by oxidation).

Common Misconceptions:

Students may believe that once a substance has undergone a physical change, like a change of state, that it is not reversible (confusing chemical and physical changes). Students are confused by oxidization and reduction.

By adding 200mL of 0.5 M potassium hydroxide (5.7g KOH in 200mL water; allow to cool before adding other substances), 7g of dextrose (glucose), and a few drops of methylene blue indicator solution (or approx. 0.5g of methylene blue powder) to an Erlenmeyer flask, you can get a color change from clear to blue while shaking it, which turns back to clear when you allow it to stand! The video at the bottom of the page shows the reaction back to back over three cycles. While this reaction most clearly demonstrates oxidization and reduction (see the video below), it can also be used to show how molecular motion of particles causes changes to a solution and thereby use it to talk about changes of state. I've found it difficult to explain Cal. State Standard 3d. "Students know the states of matter (solid, liquid, gas) depend on molecular motion." This is especially true if when you heat something up, you can't physically see the particles moving. In this demonstration, one can clearly see a change caused by motion.

The lab itself and the graphic above it all come from Norm Herr's book "The Sourcebook for Teaching Science: Strategies, Activities and Instructional Resources."