In particular, we learned about making a supersaturated sodium acetate solution. (This post segues into the discussion of "enthalpy.") The chemical equation for the reaction between acetic acid (which is 5% in the vinegar that we used) and sodium bicarbonate produced sodium acetate, a salt, in addition to the carbon dioxide that we studied last week, and water.
The reason we are using sodium acetate is because it stays fluid, even though you would expect it to crystalize when its solubility limit is reached. Crystallization will occur if a "seed crystal" is dropped into the solution, or if the system is disturbed in some other way. Then, heat is released as the crystals grow. The heat release is enough to allow sodium acetate to be used in hand warmers. (They can be "recharged" by placing in boiling water for a few moments.)
The hand warmers are made by sealing the supersaturated sodium acetate in pouches made from "lay-flat tubing." I brought an Impulse sealer for this purpose, and tubing. To activate the change from fluid to solid, a small strip of stainless steel sheet is sealed in the pouch with the solution. When the strip is flexed, it creates "nucleation sites" for crystallization to occur.
The following information was provided to the students as a note:
"A reusable hand warmer can be made of a supersaturated solution of sodium acetate, which releases heat on crystallization. Sodium acetate trihydrate crystals melt at 58 °C, dissolving in their water of crystallization. When they are heated to around 100 °C, and subsequently allowed to cool, the aqueous solution becomes supersaturated. This solution is capable of supercooling to room temperature without forming crystals. By clicking on a metal disc in the heating pad, a nucleation center is formed which causes the solution to crystallize into solid sodium acetate trihydrate again. The bond-forming process of crystallization is exothermic, hence heat is emitted. The latent heat of fusion is about 264–289 kJ/kg. Unlike some other types of heat packs that depend on irreversible chemical reactions, sodium acetate heat packs can be easily recharged by boiling until all crystals are dissolved. Therefore they can be recycled indefinitely.
"Sodium acetate is inexpensive, and is usually purchased from chemical suppliers, instead of being synthesized in the laboratory. It is sometimes produced in a laboratory experiment by the reaction of acetic acid with sodium carbonate, sodium bicarbonate, or sodium hydroxide. These reactions produce aqueous sodium acetate, and water. Carbon dioxide is produced in the reaction with sodium carbonate and bicarbonate, and it leaves the reaction vessel as a gas (unless the reaction vessel is pressurized):
CH3–COOH + Na+[HCO3]– _ CH3–COO– Na+ + H2O + CO2
"This is the basis for the well-known "volcano" reaction between baking soda and vinegar. 84 grams of sodium bicarbonate (baking soda) react with 750 ml of 8% vinegar to make 82 g sodium acetate in water. By boiling off most of the water, one can refine either a concentrated solution of sodium acetate or crystals."
I also gave my students the following question:
Given that the solubility of sodium acetate (Molar mass=82g/mol) is 76 grams per 100 grams of water.?
What is "super saturated?"
a) 8.5 moles of sodium acetate dissolved in 1L of water
b)1.8 moles of sodium acetate dissolved in 300 ml of water
c)5.5 moles of sodium acetate dissolved in 500 ml of water
d) 1.2 moles of sodium acetate dissolved in 200 ml of water
e) all of them
The reason for this question is to help students think logically to a conclusion. To answer the question, you need to calculate how many moles of sodium acetate will saturate 1000 grams of water. (Since 1 gram of water occupies a volume of 1 ml, we are determining how many moles of sodium acetate it takes to saturate a liter of water.) Then, you need to see which answer results in MORE sodium acetate in the water (in other words, "supersaturated").
How do you know the amount of sodium acetate to make? Based on experiments shown in class, students should have about 25 ml of supersaturated solution in a pouch about 4 inches long. Using the answer to the problem, above, my students are guided to determine how much sodium acetate is needed for 25 ml. Set up a "proportion" equation as shown in class. Then, use the concept of stoichiometry (i.e. -- mole ratio) to determine how much bicarbonate of soda is needed to produce it, and how much acetic acid is needed to react with the bicarb. Remember, there is only 5% acetic acid in vinegar, so the amount of vinegar to be used must be calculated.
Once the acetic acid and bicarb have reacted, a clear solution results. If it is cloudy, then not all of the bicarb has reacted and the acid was the "limiting reagent." Remember though that bicarb is somewhat soluble in water so you need to add enough acetic acid to react with that amount, as well as with the undissolved bicarb. You can't just look for a clear solution.
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