Submitted by Adam Johnson / Harvey Mudd College on Sun, 05/15/2016 - 13:57
Categories
Prerequisites
Corequisites
Course Level
Topics Covered
Subdiscipline
Description

Equilibrium reactions are those that are dynamic: the reaction can shift to form more reactants or more products depending on the physical or chemical conditions present. They were discovered and described empirically, but have a thermodynamic basis in the Gibbs Energy of the reaction. A reaction at equilibrium has both reactants and products present, and the rate of formation of products is equal to the rate of formation of reactants. A common application of equilibrium is the chemistry of aqueous acids. Acid strength is measured by the pH scale.

It costs approximately \$10,000 per pound to ship supplies to the international space station in orbit around the earth. One way to minimize costs and provide additional life support options for the astronauts is to recycle wastewater back to drinkable water. Russia developed a dehumidifier type device that reclaimed moisture from the air from sweat and breathing, and this was used on the Mir space station in the 1990s. Scientists and Engineers at NASA developed a water reclamation device that improves overall water efficiency on the ISS by reclaiming water from urine. This unit was installed in 2009. However, the device is not working up to specifications and it is your job to figure out what is going wrong and make recommendations to improve it.

This activity was inspired by a conversation I had with Anne Jones from Arizona State at the VIPEr faculty development workshop at Northwestern in 2014.

Attachment Size
35 KB
34.63 KB
89.13 KB
Learning Goals

1.    Read and interpret tabular data
2.    Determine which precipitates might form from a complex mixture of ions
3.    Calculate the maximum solubility of a species in solution
4.    Determine the effect of acid/base chemistry on solubility

Equipment needs

none

Related activities
Implementation Notes

This was done as a 2 day activity. The first day, students examine the speciation of various polyprotic ions in soluction and predict a precipitate. The second day, students look more closely at the chemical reactions carried out to purify water on the ISS and make a recommendation for an improved procedure.

There was originally going to be additional work involving EDTA complexation and formation constants, but the exercise was too long and this material was cut. However, consideration of the addition of EDTA would allow for a more sophisticated treatment.

This exercise is designed to show the effects of calcium leaching from bones by astronauts. This process is not completely understood, but blood concentrations of calcium are higher for astronauts. This led to a problem with the water purification system developed by NASA.

The keys for this exercise involve the solution of multiple simultaneous equations. This can be done using mathcad, mathmatica, or wolfram alpha. A link to a sample wolfram alpha solution is provided below.

Time Required
1 50 minute class period

Evaluation

Evaluation Methods

the exercises were evaluated according to the attached answer keys.

Evaluation Results

this ended up being a very difficult exercise, possibly more suited for sophomore or junior students. The students required some handholding to get through the exercises, but in the end, they seemed to understand that even seemingly simple aqueous systems are actually quite complicated. For Fall 2016, I intend to reframe and better stage this activity and possibly add in a discussion of EDTA as a complexing agent.