Dissolution and Precipitation: Can We Transfer Ideas?
Lofty ideas always look good on paper. When one gets into the details, things can get messy pretty quick. There are numerous notions and prejudices with regard to learning. An example is a distaste for spoon feeding. Thus, one objective of instructors is to help students develop the skill of transferring a concept they have learned before to a problem or phenomenon on which that they have not received specific instruction. The challenge is that this skill is quite difficult to accomplish inside the classroom. And, of course, this is likewise not easy in an online environment. To illustrate this difficulty, the simple phenomenon of dissolution and precipitation of ionic solids in aqueous environment serves as a good example. Kelly and Jones have specifically looked at this specific topic and explored the challenges students face in transferring concepts they have learned from dissolution to the phenomenon of precipitation in an article published in the Journal of Chemical Education. The article, "Investigating Students’ Ability To Transfer Ideas Learned from Molecular Animations of the Dissolution Process", examines in depth how students who have just viewed animations on how a crystal of sodium chloride dissolves in water, apply what they have just learned to explain why a precipitate of silver chloride is formed when clear aqueous solutions of silver nitrate and sodium chloride are mixed. There are various videos available on the internet that show how chemists imagine the dissolution of sodium chloride in water. Here is one example:
The advantage of an online content is evidently seen here since it provides a dynamic picture of the dissolution process. Sodium and chloride ions are being taken out from the crystal lattice by water molecules and the two ions are separated from each other. The video clearly shows that the particle that is inside the solution is not a sodium chloride molecule but separated sodium and chloride ions.
After viewing a video similar to the one shown above, the students are asked to make drawings that explain the dissolution process of sodium chloride in water. And most of the drawings correctly depict the concepts shown in the video. Ions are separated and there are water molecules surrounding each separated ion. The next material shown to the student is a demonstration of how silver chloride precipitates out upon mixing two clear solutions, one containing an aqueous solution of sodium chloride and another containing an aqueous solution of silver nitrate. This video is limited to a macroscopic demonstration, no simulation of how this process occurs at the molecular or ionic level was presented. An example of such video is shown below.
After seeing this video, the students are then asked to explain by making drawings how silver chloride precipitates out of solution. Not one of the students in the study is able to explain correctly how the precipitation occurs. The failure as noted by the authors comes mainly from the students' inability to keep the image of separated sodium and chloride ions when sodium chloride is in an aqueous solution. Thus, some of the explanations even make the claim that a silver ion tears away a chloride ion from a sodium chloride molecule or sodium chloride lattice. To explore further why students fail to transfer a concept from one phenomenon, dissolution, to another, precipitation, it may be useful to imagine a student's point of view. The following are sections from the ChemWiki web page of University of California at Davis:
Precipitation and Double Replacement Reactions
It is possible that students in this study did not immediately connect the aqueous sodium chloride solution with the salt dissolution activity because the clear, colorless sodium chloride solution in a test tube depicted in the video looked like many solutions the students experienced in the laboratory course. As a result, perhaps students paid more attention to the fact that a precipitate was formed, which triggered their thoughts of double displacement reactions and solubility rules, concepts that were mentioned when students orally explained their written and drawn explanations.The ChemWiki page at UCSD continues the lesson on precipitation with the following:
Net Ionic Equations
The findings of Kelly and Jones are highly informative regarding the challenges of helping students develop the skill of transferring knowledge;
...According to Crotty, a constructivist views meaning as not being discovered, but rather as a mental process in which understanding undergoes restructuring. When people gain knowledge they are trying to make sense of new thoughts that coincide with their previous ideas. Schwandt explains that learning does not occur in isolation; rather it is constructed against a backdrop of shared understandings, practices, and language. Thus, a given person’s knowledge must “fit” with reality; people’s life experiences are constantly testing the viability of their knowledge...
...The wide variety in what students reported is consistent with the distinctive way people construct their unique mental models when they are from diverse backgrounds and have different prior knowledge into which to fit their understanding. It also suggests that students have difficulty interpreting what they see in the animations, even when the viewing is followed by a reinforcing discussion...
The results of this study suggest that students learn to incorporate some features seen in animations into their own explanations, although they ultimately have difficulty transferring their understanding to new situations. When student responses were further probed we discovered that students were able to recall the process of salt dissolution, yet they did not relate that process to the same solution used as a reactant, an aqueous sodium chloride solution involved in a precipitation reaction. The teaching implication is that students need frequent reinforcement of the meaning of scientific terms such as aqueous and water soluble. Students also need help connecting concepts learned previously with new material. When showing animations to a class, an instructor can help students process the new information and make meaningful connections to other chemical systems and processes by asking them at the time to describe similar systems, and by varying the substances involved....To this, one must wonder how students would jump to another topic, the world of coordination complexes, which I introduce in one of my lectures:
Nobel Prize in Chemistry 1913
"in recognition of his work on the
linkage of atoms in molecules by which he has thrown new light on earlier
investigations and opened up new fields of research especially in
Alfred Werner was first to explain the existence and properties of a variety of cobalt(III) chloride compounds with ammonia:
Compound Formula Color 1 CoCl3 . 6NH3 orange-yellow 2 CoCl3 . 5 NH3 purple 3 CoCl3 . 4 NH3 green 4 CoCl3 . 3 NH3 green
When an aqueous solution of HCl is added to either compounds 1, 2, 3 or 4, NH3 is not removed.Werner's explanation
Compound 1, when treated with an aqueous solution of Ag(NO3) precipitates all the chloride as AgCl.
Compound 2, when treated with an aqueous solution of Ag(NO3) precipitates only 2/3 of the Cl.
Compound 3, when treated with an aqueous solution of Ag(NO3) precipitates only 1/3 of the Cl.
Compound 4, when treated with an aqueous solution of Ag(NO3) precipitates no AgCl.
There are two kinds of chemical bonds present in these cobalt(III) compounds. One is ionic (similar to the bond between Na+ and Cl- in solid NaCl) and the other is a coordination bond. An example of a coordination bond that you may have seen before is the following:
The bond between B and N is a coordinate bond.
Coordinate covalent bond
A single covalent bond in which both electrons in the shared pair come from the same atom. The molecule or ion which contains the donor atom is called a ligand.Werner's complexes (Compound 1-4 are therefore described in the following manner)
Compound Formula 1 [Co(NH3)6]Cl3 2 [Co(NH3)5Cl]Cl2 3 [Co(NH3)4Cl2]Cl 4 [Co(NH3)3Cl3]
_________________________________________________________________________________By convention, the ligands are enclosed inside the brackets with the metal symbol. These ligands are coordinately bound to Co(III).
Do we wait for students to transfer, or do we guide them? And what should one do in an online course?