21 Sep 2011

How does changing solvent affect redox potential?

In-Class Activity

Submitted by Sheila Smith, University of Michigan- Dearborn
Categories
Description: 

There are three ways to modulate the redox potential of a metalloenzyme:  Changing ligands, changing geometry, and changing solvent. When I introduce this topic in Bioinorganic, I try to give my students concrete examples of each.  I love this one because it applies what they learned in Gen Chem about the Nernst Equation to a biological problem.  Granted, I don't use a metalloenzyme as my example, but I do pull the biological chemistry into it at the end, by referrring to the cytochrome oxidase/O2 couple.  

In a Gen Chem class this could be used as an in class exercise or a (challenging) test question. 

Learning Goals: 

General Chemistry

  • A student should be able to balance a redox half-reaction in either acidic or basic conditions.
  • A student should know the conditions defined as standard conditions in aqueous electrochemical cells.
  • A student should understand the connection between the Nernst equation and potentials for reactions at other than standard conditions.
  • A student should be able to calculate the cell potential from two half-reaction potentials. 
  • A student should be able to explain the connection between redox potential and strength as an oxidizing (or reducing) agent.
  • A student should be able to calculate the free energy derived (or cost) from a given redox couple. 

 Advanced

  • A student should be able to relate these calculations to the affect of changing solvent (either pH, or concentrations of substrate or even availability of water in a microenvironment around the active site of a metalloenzyme) on the redox potential (and therefore the free energy of a biological redox couple).
Equipment needs: 

calculators

Time Required: 
30 minutes, including discussion
Evaluation
Evaluation Methods: 

Students are evaluated based on their participation in the in class discussion. Students can be asked a similar test question.

Evaluation Results: 

I suspect that my gen chem students would do better on this exercise (since they are still fully immersed in the topic) than my senior level bioinorganic students, many of whom have forgotten how to balance redox equations. Students did well with the application of the Nernst equation; they seem to retain more about math than concepts from redox. Students seemed interested by the discussion of how changing pressures of O2 in venous vs arterial blood, could shift potentials further. I may incorporate this into the exercise in the future

Creative Commons License: 
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