This is an in class exercise that I use to emphasize the need for metal ion transport and storage in biochemistry. Applying the Van't Hoff equation to the Ksp value at 25°C for ferric hydroxide, students calculate the iron concentration at which ferric hydroxide would begin to precipitate out in the blood. It' s an interesting problem that requires very little math beyond that used in gen chem, and the answer is in stark contrast to the amount of iron that we actually store in our bodies.
Student should recognize that the balanced chemical equation to which the Ksp can be applied is the precipitation reaction.
Student should be able to write a balanced chemical equation for the precipitation of Ferric hydroxide.
Student should be able to apply Hess’ Law to calculate the enthalpy of reaction for a balanced chemical equation (given DH°f for relevant species).
Student should be able to apply the van’t Hoff equation to predict the equilibrium constant at a temperature other than 25°C (in this case, 37°C).
Student should be able to calculate the [OH-] at a given pH
Student should be able to write the solubility product expression for Fe(OH)3 and use that to calculate the maximum amount of ferric ion that can exist in aqueous solution at a given pH before precipitation occurs.
I use this at the beginning of class before I start covering Metal transport and storage. The idea is to draw from the students’ previous knowledge to help them understand the need for transport and storage to avoid accumulation of solid precipitates. It can also lead, as discussed in the key, to a good discuss of bioavailability.
I generally evaluate success based on the quality of discussion that comes out of the in class activity, since the actual math problem is completed collaboratively.
Since I provide the van’t Hoff equation, students can typically manage the math of the equation; an understanding of why the calculation is necessary (a reminder that K is temperature dependent) can prove useful. Students sometimes need some prodding to remember how to do a Qsp vs Ksp problem ( or to remember that this is what they need to do to answer the question) in the second half.