This experiment uses the product of the experiment where you reacted vinegar with sodium bicarbonate. It uses the left-over sodium acetate that was formed as a reaction product to make a handwarmer. When sodium acetate solution is evaporated it becomes supersaturated. That means there is more material dissolved than would normally occur. Any disturbance of the solution causes it to crystalize and heat is released in the process. This allows us to study the heat of crystallization, heat capacity and the concept of enthalpy.
My intent was for my students to use plastic lay-flat tubing as the case for the solution. I have a lot of it on hand. Unfortunately, I've had the tubing for so long that I think it has degraded. When we tried to demonstrate how to seal the tubing into bags, it leaked. Until I get some better plastic, we will have to finish the experiment another way. I still want you to learn about heat capacity so I have revised the experiment. The procedure follow this note. Please study it and follow the directions given:
1. Accurately weigh out some dry sodium acetate crystals. Use at least a gram or two in a small beaker.
2. Put the beaker in a hot water bath until the crystals melt.
3. Take the beaker out of the bath and immediately put a thermometer into the melt. Hold the thermometer so it doesn't fall out and shatter. Record the initial temperature and the time. Keep the thermometer in the melt until the crystals resolidify. Record the temperature at time intervals. You might try two minute intervals. If that is too long or too short, repeat with another time of your selection.
4. Prepare another beaker, larger than the one holding the crystals. Accurately weigh twice the weight of water as crystals and put it in the larger beaker.
5. Clean off crystals that may be sticking to the thermometer. Add those crystals back to the smaller beaker.
6. Put the thermometer in the larger beaker. (Mount it so it doesn't fall out.) Take the initial temperature of the water.
7. Return the beaker with the crystals to the hot water bath. As soon as the crystals melt, put it in the beaker with the water. Swirl the beaker to warm the water as quickly as possible.
8. Keep watching the thermometer. Record the temperature as it rises to a peak (and the time), then starts back down. Keep measuring time and temperature. When the sodium acetate in the smaller beaker starts to re crystallize, note it (time and temperature). You might see an increase in temperature at that point. That would be due to the heat of crystallization. If it does increase, great! Track it. It will hit a maximum, then start cooling. The difference between the maximum temperature and temperature when the crystals started to form is due to the heat released from crystallization.
9. We also need to know the heat capacity of the glass beaker. To find this, first weigh the beaker and record the weight. Then submerge the beaker in the hot water bath for a few moments. Meanwhile, fill another container with cool water. Take its temperature. Using tongs, remove the beaker from the hot water bath, pour out the hot water and immediately fill it with cool water. Insert the thermometer and watch as the temperature of the water rises. Record the maximum temperature. Weigh the beaker full of water. Subtract the weight of the beaker (tare weight) from the filled weight (gross weight). This tells you the weight of the water.
10. You know the heat capacity of water: It is 1 calorie per gram per degree C. (We can convert to joules, but it is more convenient to use calories for this experiment.) You can calculate the enthalpy of the glass beaker from the temperature rise and the amount of water. (Dividing this by the mass of the beaker yields the heat capacity of the glass per unit of mass.) You will subtract the enthalpy of the glass from the overall heat gain of the water in the larger beaker. With this information, the moles of water in the larger beaker and the temperature difference, you can calculate the heat released by the sodium acetate. That heat is the same needed by the sodium acetate to melt. If you get two temperature peaks, you can calculate the heat released during crystallization.
11. Here is the equation again; q = Cp x moles x (T1 - T2) where q = enthalpy, Cp = heat capacity per mole and (T1 - T2) is the temperature difference.
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