10 Jan 2018

What happened to my green solution?

In-Class Activity

Submitted by Anthony L. Fernandez, Merrimack College
Categories
Description: 

Students in inorganic chemistry courses are often interested in the colors of transition metal complexes. This in-class activity serves an introduction to reactions of coordination complexes and pushes students to think about the relationship between the color of a complex cation and its structure. Students are provided with pictures of aqueous solutions of two chromium(III) salts [CrCl3*6 H2O and Cr(NO3)3*9 H2O] at two different times and are then asked to explain the changes observed in the solutions. This activity was inspired by a laboratory experiment which was done as part of the inorganic laboratory course for many years ("Determination of Delta_oct in Cr(III) Complexes" from Szafran, Z., Pike, R.M., and Singh, M.M "Microscale Inorganic Chemistry: A Comprehensive Laboratory Experience" Wiley, New York, (c)1991) .

Learning Goals: 

After completing this exercise, students should be able to:

  • describe how the color of a solution is related to the composition of the coordination complex present in solution,
  • explain how the change in color of a solution indicates that a reaction has occured, and
  • determine the identities of the products and reactants of a reaction that has taken place in solution.

If the UV-visible data are also provided, students should also be able to relate the shifts in the peaks observed in the UV-visible spectra to the position of the ligands in the spectrochemical series.

Equipment needs: 

No equipment is needed for this in-class activity. 

Implementation Notes: 

I usually use this activity to introduce reactions of coordination complexes in lecture, which falls just after a section in my text on the colors of coordination complexes. While my students have seen many transformations in lab, I use this to connect the two portions of the course. For added empahsis you could make the aqueous solutions and bring them to class.

I usually project the pictures on a screen at the front of the class and I therefore need a device to project it from and a projector.

I break up my class into groups and let them work on this activity collaboratively. I usually let them discuss the problem for about 5-10 minutes and I check in with each group individually. If they are having trouble determining the composition of the coordination complexes, I remind them that they need to write out the formulas in the current way that we represent coordiantion complexes. This usually gets them thinking about primary vs. secondary coordination spheres and waters of hydration. I then let them work for another 10 minutes so that they can write the reactions. I then bring the class together to discuss the results. If time allows, I share the UV-visible data with the entire class and as them to explain the observed changes.

Time Required: 
20-30 minutes
Evaluation
Evaluation Methods: 

I do not do any formal assessment of student learning for this activity, but instead I judge understanding by the quality of the in-class dicussion.

I have also used similar questions on exams in the past to see if the students can apply these ideas to different reactions.

Evaluation Results: 

I have experienced mixed results with this exercise over the three years I have used it. I find that my students have no trouble identifying that a reaction has occurred and they readily recognize that the color change is a consqeuence of the reaction.

My students tend to struggle with the composition of the complex ions in solution. For the CrCl3 solution, students provide many possible compositions of the coordination complex including the neutral complex, [CrCl3(OH2)3], and the hexaaqua complex, [Cr(OH2)6]3+.  More than 2/3 of the students suggest one of the two predominant complex ions that are present in solution. For the Cr(NO3)3 solution, students often want to use the nitrate as a ligand on the chromium center.

All of my students are usually able to write the balanced reactions and explain the changes in the UV-visible spectra once they identify the composition of the complex cations.

Creative Commons License: 
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