NCSU – Dept. of Chemistry – Lecture Demonstrations
Thermochemistry
Supersaturated Sodium Acetate
Description: A supersaturated solution of sodium acetate trihydrate is
crystallized upon initiation by a seed crystal..
Materials:
Sodium acetate trihydrate
Beaker or E. flask
Deionized water
Watch glass
Procedure:
For large lecture halls, perform demonstration using a document camera.
1. Dissolve 50 g of sodium acetate trihydrate in 5 mL of water with gentle
heating. Use a very small amount of water to rinse the sides of the
container to avoid initiating unwanted crystallization. Let solution cool
slowly.
2. Demonstration can be performed one of two ways. First, a seed crystal
can be added to the beaker (or flask) and crystallization can be initiated in
this manner. Secondly, seed crystals can be placed onto a watch glass.
Slowly pour the supersaturated solution on top of the seed crystals. This
will initiate crystallization of the solution. Continue pouring slowly to
produce a rising column of crystallized NaOAc.
3. A hot pack is also available in Dab114 which works by the same principle.
Crystallization is initiated upon bending the metal disk inside the pouch.
Once crystallized, the pouch can be reused by dissolving the sodium
acetate inside the pouch in boiling water for several minutes and allowing
to cool.
4. In an alternative demonstration, large crystals can be grown from the
evaporation of a supersaturated CuSO4 solution. Prepare this by
dissolving 30 g of CuSO4 in 50 mL of water with gentle heat. Allow this to
slowly evaporate over several days.
NCSU – Dept. of Chemistry – Lecture Demonstrations
Thermochemistry
Discussion:
At temperatures above 58 oC, NaOAc•3H2O begins to lose its water of hydration
and begins to dissolve in that water. The solution, once completely dissolved,
can be cooled to give a solution which is supersaturated in the anhydrous and
trihydrate form of sodium acetate. In the presence of a seed crystal, this solution
will “freeze” but in doing so must reach its freezing point which is 58 oC and thus
the crystallizing solution warms up. The heat of solution of NaOAc•3H2O is 19.7
kJ/mol, therefore the dissolution process is endothermic and the crystallization is
exothermic.
Safety: Wear proper protective equipment including gloves and safety glasses
when preparing and performing this demonstration. Avoid prolonged contact with
the flask as solutions are hot enough to cause burns. Crystallized sodium
acetate is an irritant to the skin.
Disposal: The materials in this demonstration can be reused. If materials are to
be disposed of solid should be dissolved in water and placed in an appropriate
aqueous waste container.
References:
Shakhashiri, B. Z. In Chemical Demonstrations: A Handbook for Teachers of
Chemistry; The University of Wisconsin Press: 1983; Vol. 1, p 27-30.
Ahmad, J. J. Chem. Educ. 2000, 77, 1446.
Pergler, J. A.; Ragsdale, R. O.; Richmond, T. G. J. Chem. Educ. 1995, 72, 1027.
McAfee, L. V.; Jumper, C. F. J. Chem. Educ. 1991, 68, 780.
Video:
http://genchem.chem.wisc.edu/demonstrations/Gen_Chem_Pages/11solutionspa
ge/solutionsmain.htm
http://www.youtube.com/watch?v=HnSg2cl09PI
http://www.youtube.com/watch?v=0wifFbGDv4I
http://www.youtube.com/watch?v=QIC0ATIELgI