12: Solar Energy Storage
- Page ID
- 509307
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This lab was adapted from:
Thompson, S. (1989). Thermochemistry and solar energy storage. In ChemTrek: Small-scale experiments for general chemistry (pp. 97-113). Prentice Hall. https://www.cns-eoc.colostate.edu/smallscalechemistry/CHEMTREK_Fulltext.pdf
- Students will...
- Students will...
Section A. Calibration of a Calorimeter
The heat capacity measurement is accomplished by using a cooling curve analysis of an experiment involving the addition of a known volume of hot water to a known volume of room temperature water in the microcalorimeter. The temperature changes can be monitored by a LabQuest and temperature probe. The heat given by the hot water will need to heat both the cold water and the calorimeter, and so can be used to find the heat capacity of the calorimeter you created:
1. Insulate a 50 ml beaker by wrapping it with a paper towel and tucking it into a 100 ml beaker. The 50 mL beaker should be easily removed and replaced without disturbing the paper towel insulation. Turn on the LabQuest and connect the temperature probe to it. Change the time duration for the experiment to 10 minutes (600 seconds). Change the temperature range of the graph to display from 20 °C to 50 °C.
2. Measure 5.0 mL of DI water with a graduated cylinder and place in a hot bath. Take note of the bath temperature using the LabQuest temperature probe and record it in a local copy of the linked Excel spreadsheet as the initial temperature of the hot water.
3. Transfer a 15.0 mL DI water (measured with a 50 mL graduated cylinder) to the calorimeter. Start the run by hitting “play”. After 5 minutes, add all of the hot water to the hot water bath. Pipette 3 times up and down with a beral pipette to mix. Continue the experiment until the temperature after mixing has stabilized for one minute.
4. After stopping the experiment, pour out all water and rinse twice with room temperature DI water. Dry the container and refill with 15.0 mL of DI water.
5. Your graph should look similar to the graph below. Estimate the initial temperature of the room temperature water, Ti, from the initial 5 minutes of the graph, extending any line to half-way through the temperature increase. Estimate the final temperature from the final portion of the graph, extended backward to the same point. Record those temperatures in your local copy of the linked Excel spreadsheet.
6. Calculate the heat lost by the hot water using the density and specific heat given in the spreadsheet. Calculate the heat gained by the cold water in the same way. The difference between these is the heat gained by the calorimeter. The temperature change for the calorimeter is the same as the temperature change for the room temperature water. Use the heat gained by the calorimeter and the temperature change to solve for the heat capacity in units of J / °C.
Section B. Determination of the Specific Heat of Granite Rock Used for Solar Energy Storage
1. Begin a new ten-minute run with the temperature sensor in the water you added after rinsing in the last experiment. Make sure you have 1 minute of stable data.
2. Go to the oven and record the temperature shown on the thermometer. Record this on the “Granite” sheet as the initial temperature for the granite. Perform the calculations for this section here.
3. Quickly remove 3 rocks from the oven and place them directly into your calorimeter. Please make sure to use gloves or tongs.
4. Pipette 3 times up and down with a beral pipette to mix. Continue the experiment until the temperature after mixing has stabilized for one minute.
5. After stopping the experiment, carefully pour off the water and remove the rocks. Dry them with a paper towel.
6. Rinse the calorimeter twice with room temperature DI water. Dry the container and refill with 15.0 mL of DI water.
7. When the rocks are dry, weigh them and record the weight.
8. Now the heat balance is the heat lost by the rocks is equal to the heat gained by the room temperature water and calorimeter. Use the heat capacity of the calorimeter you calculated in Section A to find the heat lost by the rocks, then find the heat capacity of the rocks using the heat lost, mass, and temperature change.
Section C. Determination of the Heat of Crystallization of Sodium Thiosulfate Pentahydrate, Na2S2O3•5H2O
1. Cut 5 cm off the end of a straw and discard the 5 cm piece. You now have a piece about 15 cm long.
2. Insert a plastic cap tightly into one end of the 15 cm piece.
3. Weigh the capped straw and record the weight.
4. Make a mark on the straw 6 cm from the cap.
5. Using a thin-stem pipet, transfer liquid salt from the bath of molten sodium thiosulfate pentahydrate into the straw until it reaches the 6 cm mark. Set aside for a few minutes to allow the salt to cool to the crystallizing temperature (~48 °C). This is the initial temperature for the salt. Record it on your local copy of the linked Excel spreadsheet.
6. Begin a new ten-minute run with the temperature sensor in the water you added after rinsing in the last experiment. Make sure you have 1 minute of stable data.
7. Check on the salt. It should still be liquid.
8. Quickly add a seed crystal and make sure that the salt begins to crystallize.
9. Bend the straw at the center of the liquid salt and place doubled up into the water in the calorimeter, ensuring that the molten salt is completely covered, as shown above.
10. Pipette up and down slowly with a beral pipette to mix. Continue pipetting gently up and down to mix – the heat exchange is slower. Continue the experiment until the temperature after mixing has stabilized for one minute.
11. After stopping the experiment, remove the bent straw from the water and dry it with a paper towel.
12. Record the weight of the straw + crystallized salt.
13. Now the heat balance is the heat lost by the salt is equal to the heat gained by the room temperature water and calorimeter. Use the heat capacity of the calorimeter you calculated in Section A to find the heat lost by the salt, then find the heat capacity of the rocks using the heat lost, mass, and temperature change.
Post-Lab Questions:
1. Do you think that a classical mercury thermometer could have been used to carry out the microcalorimeter experiments? Give reasons for your answer.
2. Do you think that a heat-insulating top for the microcalorimeter would have improved the results?
3. Straws happen to be excellent containers for the determination of the heat of crystallization of molten salts. Why?
4. Calculate and compare the amount of heat energy stored by 1 pound of water, 1 pound of granite, and 1 pound of sodium thiosulfate pentahydrate. Assume a ΔT of 40 ° C for the temperature change of the water and the rock.
5. If you could not afford to pay elves (or students) to add seed crystals at the appropriate time to the solar energy storage tank to make the molten salt crystallize, how could you insure crystallization?