26 Jun 2011

Hard-Soft Acid-Base Theory in Action: A New Ion-Exchange Material for Sequestering Heavy Metals

Literature Discussion

Submitted by Maggie Geselbracht, Reed College

This paper from Chemistry: A European Journal by Manolis Manos and Mercouri Kanatzidis (link provided below in Web Resources) describes the ion-exchange chemistry of a layered sulfide (KMS-1) that exhibits an enhanced preference for soft metal cations (Cd2+, Pb2+, and Hg2+) replacing K+ in between the metal sulfide layers of KMS-1.  Not only does this paper provide a practical application of hard-soft acid-base theory (HSAB), but it provides an accessible introduction to the technical literature for undergraduates, particularly at the first or second-year level.  This learning object was used to illustrate hard-soft acid-base theory, the structures of extended solids, solid solution formation, ion-exchange chemistry, and the analysis of structural changes by X-ray powder diffraction.

Students in a second-year inorganic chemistry course were asked to read this paper in preparation for a class discussion (see further details in implementation notes).  For many students, this was a first or second exposure to reading the primary literature.


Learning Goals: 

After reading and discussing this paper, a student will be able to:

• Articulate the underlying problem and motivations driving the research presented in the paper
• Explain the collection of experimental data that lead to figures and tables presented in the paper and describe the interpretation of these results by the authors
• Discuss the relative importance of various thermodynamic factors predicting the favorability of ion-exchange reactions
• Gain confidence reading a paper from the primary literature in inorganic chemistry


Implementation Notes: 

Students in a second-year inorganic chemistry course read the paper "Sequestration of Heavy Metals from Water with Layered Metal Sulfides" prior to a class discussion.  The pdf file was posted to the course Moodle, and students were asked to “Annotate” their copy (either in printed or electronic form) to highlight terms or sections of text that they did not understand.  At the end of the class discussion, they turned in either a printed or an electronic copy of the annotated paper.  

A series of questions that I used to guide the discussion are attached below as a Word document.  While I typically give out these types of discussion questions in advance and ask students to answer them prior to the discussion, in this case I did not.  The timing of this literature discussion immediately followed an exam, so I was just wanted to ensure that students had read the paper prior to our class discussion.  Making them turn in the annotated copy of the article was generally successful in achieving that outcome.

A second Word document is attached below containing the list of unfamiliar terms that were marked as unfamiliar by my students.  Most of these terms were highlighted by the majority of the class.  In some cases, I explained the meaning of the term in the course of our discussion, for example energy dispersive spectroscopy, but in other cases, the concepts were not central for a basic understanding of the paper and we never addressed them.  


Time Required: 
50 minutes
Evaluation Methods: 

I was most interested in assessing if this was an accessible paper for students at this level (2nd year inorganic, post general chemistry).  I collected their annotated copies of the paper and gave them a “participation” score for reading the paper.  I looked through the annotated copies, comparing what terms different students highlighted as being unfamiliar and collecting them in the attached document.

Ultimately, students were responsible for the larger “take-home” content message as there was a question on the final exam relating directly to this paper discussion.  This assessment question is also posted on VIPEr (see related activities link above).


Evaluation Results: 

Several students mentioned anecdotally that this paper was very understandable.  Furthermore, by highlighting what they did not know, they realized that they actually understood quite a bit! A Word file is attached that lists the terms that one or more students marked as “unfamiliar” on their annotated copy of the pdf file for this article.

While I did not collect or evaluate written responses to the discussion questions, students were better prepared to answer some questions more than others.  In particular, they were all stumped (long silence!) when I asked question 5.

Creative Commons License: 
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