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Computer Simulations of Salt Solubility provides
an animated, visual interpretation of the different
solubilities of related salts based on simple
entropy changes associated with dissolution:
configurational disorder and thermal disorder.
This animation can also help improve students’ conceptual
understanding of chemical equilibrium before
any quantitative interpretation of equilibrium
constants is attempted.
The animation compares pairs of compounds
chosen according to the following criteria.
Paired together are:
- Familiar salts of very different solubility
- Salts of similar packing structures
- Salts whose solubility is either not
significantly affected by ion reaction
with water (such as acid–base and
complex formation reactions), or affected
in a way that opposes the observed solubility
differences
- Salts that essentially require discussion
of only configurational disorder or only
thermal disorder
The pairs chosen for these animations are
NaCl/CaCO3 and MgCO3/MgSO4.
In the former case, both dissolution phenomena
are almost athermic, hence the major differences
are in configurational disorder and solvation
effects. In the latter pair, the major difference
lies in thermal disorder, as the dissolution
of magnesium sulfate is quite exothermic.
The program simulates the “before” and
the “after” states for the dissolution
of equal amounts of the two salts of each
pair in identical amounts of water. Changes
in total entropy and in its components are
represented qualitatively in bar graphs.
In the case of NaCl versus CaCO3,
the animation illustrates that the small
solubility of CaCO3 in water is
due to the decreased mobility of the water
molecules associated with the small, dipositive
calcium ion. This decrease results in a decrease
in total entropy upon dissolution of calcium
carbonate, and thus the amount dissolved
is very small. On the other hand, the dissolution
of NaCl is substantial because it yields
a gain in total entropy since there is not
a similar decrease in motility.
Figure
1. A screenshot from Computer Simulations
of Salt Solubility comparing the dissolution
of NaCl and CaCO3.
For the pair MgCO3/MgSO4,
where the solvation effects are similar,
the increased thermal disorder of dissolved
MgSO4 is illustrated by greater
particle agitation. The dissolution of MgSO4 yields
greater thermal entropy and particle movement,
and thus it is much more soluble than MgCO3.
Figure
2. A still image from Computer Simulations
of Salt Solubility of the animation comparing
the dissolution of MgCO3 and
MgSO4.
A word of reservation is due. First, the
simulations are qualitative in nature, and
no rigorous calculations of the contributions
to entropy changes are illustrated. Also,
when using these pictorial simulations, students
should be alerted specially to the unrealistically
small number of particles represented and
the unrealistically large spaces between
them, to the symbolic representation of ions
and molecules, and to qualitative illustrations
of only some molecular motions. In addition,
exchange processes involving water molecules
of the hydration sphere are not shown.
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