If after reheating the aspirin/ethanol/water mixture, the crystals do not appear upon cooling, why would boiling off some solvent make the crystals appear?

Chemistry
10th Edition
ISBN:9781305957404
Author:Steven S. Zumdahl, Susan A. Zumdahl, Donald J. DeCoste
Publisher:Steven S. Zumdahl, Susan A. Zumdahl, Donald J. DeCoste
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Please answer the following question about a lab experiment about synthesizing aspirin. The procedure for this experiment is included in the photos.

If after reheating the aspirin/ethanol/water mixture, the crystals do not appear upon cooling, why would boiling off some solvent make the crystals appear? 

Procedure
Step 1: Synthesis.
A hot water bath needed for the reaction can be prepared in the fume hood. A water bath can be a
100 mL beaker filled with 70 mL water. Use the heating mantle and transformer to heat the beaker.
• Weigh about 2 g of salicylic acid. Record the mass to the nearest milligram. Transfer the solid to a
large test tube. Set the test tube in your test tube rack or a large beaker so it doesn't spill.
Carefully add about 4 mL of acetic anhydride to the test tube containing salicylic acid using a
10 mL graduated cylinder. Gently mix the contents, avoiding any spilling of the liquid. Most of the
solid will not dissolve at room temperature.
• Carefully add about 5 drops of concentrated sulfuric acid to the mixture. Gently swirl to mix the
sulfuric acid.
•
•
CAUTION
CAUTION: Acetic anhydride and conc. sulfuric acid are reactive and corrosive.
Work with these compounds in the fume hood.
Clamp the test tube to the support bars. Use the lab jack to raise the hot water bath to your test tube.
Continue heating the water. After about 1 minute, swirl the contents of the large test tube using a test
tube holder (wooden clothespin type) to dissolve any salicylic acid that was not dissolved initially.
Leave the test tube in the hot water bath for 20 min. Do not start time until water is boiling.
While the sample is being heated, fill one-fourth of a 250 mL beaker with ice (no more).
After the reaction is complete, remove the test tube from hot water bath carefully using the test tube
holder. Turn off the heat. Add the contents of the test tube slowly into a water/ice mixture taken in the
250 mL beaker while stirring continuously. Swirl the mixture to decompose all the acetic anhydride.
Rinse the test tube with deionized water several times using a wash bottle, adding the rinse water to
the water/ice beaker after swirling the water to transfer all the material to the beaker.
•
Let the sample cool to room temperature. A white precipitate will form. BE PATIENT!
If the product doesn't precipitate, try the following:
o
Wait for 2-3 minutes and then "scratch" the mixture. That is, use your glass stirring rod and gently
rub it along the side of the beaker. If done properly, that will provide a nucleation site (a
2
Transcribed Image Text:Procedure Step 1: Synthesis. A hot water bath needed for the reaction can be prepared in the fume hood. A water bath can be a 100 mL beaker filled with 70 mL water. Use the heating mantle and transformer to heat the beaker. • Weigh about 2 g of salicylic acid. Record the mass to the nearest milligram. Transfer the solid to a large test tube. Set the test tube in your test tube rack or a large beaker so it doesn't spill. Carefully add about 4 mL of acetic anhydride to the test tube containing salicylic acid using a 10 mL graduated cylinder. Gently mix the contents, avoiding any spilling of the liquid. Most of the solid will not dissolve at room temperature. • Carefully add about 5 drops of concentrated sulfuric acid to the mixture. Gently swirl to mix the sulfuric acid. • • CAUTION CAUTION: Acetic anhydride and conc. sulfuric acid are reactive and corrosive. Work with these compounds in the fume hood. Clamp the test tube to the support bars. Use the lab jack to raise the hot water bath to your test tube. Continue heating the water. After about 1 minute, swirl the contents of the large test tube using a test tube holder (wooden clothespin type) to dissolve any salicylic acid that was not dissolved initially. Leave the test tube in the hot water bath for 20 min. Do not start time until water is boiling. While the sample is being heated, fill one-fourth of a 250 mL beaker with ice (no more). After the reaction is complete, remove the test tube from hot water bath carefully using the test tube holder. Turn off the heat. Add the contents of the test tube slowly into a water/ice mixture taken in the 250 mL beaker while stirring continuously. Swirl the mixture to decompose all the acetic anhydride. Rinse the test tube with deionized water several times using a wash bottle, adding the rinse water to the water/ice beaker after swirling the water to transfer all the material to the beaker. • Let the sample cool to room temperature. A white precipitate will form. BE PATIENT! If the product doesn't precipitate, try the following: o Wait for 2-3 minutes and then "scratch" the mixture. That is, use your glass stirring rod and gently rub it along the side of the beaker. If done properly, that will provide a nucleation site (a 2
microscopic, angular fragment of glass) on which crystals can begin to grow.
o Alternatively, you can obtain a small "seed" crystal of aspirin (just big enough to see) from another
student or TA and add it to your mixture. Stir the solution with a stirring rod and wait for 1-2
minutes to see any precipitation.
o If none of these techniques work, repeat the synthesis.
Step 2: Filtering the reaction product.
• Set up the vacuum filtration apparatus as shown
in Figure 1. Clamp the filter flask to the support
bars. Connect the filter flask to the trap flask
and not directly to the vacuum line. Use the
thick vacuum tubing. Place a Buchner
funnel with filter paper on the flask.
Rubber bung/cork
• Using a stirring rod, break up any large Rubber tubing
particles of solid in the
precipitate.
moisten the filter
• Turn on the vacuum line knob,
paper with deionized water, and then filter the reaction mixture by slowly pouring the mixture into
the Buchner funnel. The filter flask will be getting full, so empty Figure 1: Filtration setup to be used for
it into a large beaker and set it in the corner of your hood, then
continue with the filtration.
isolating aspirin from the reaction mixture.
Buchner funnel
containing moistened
filter paper
Filtrate containing impurities, acetic acid,
water. Should be emptied periodically.
Vacuum line
•
Now that much of the solid has been transferred to the filtration apparatus, use deionized water
and a glass stirring rod or spatula to transfer the remaining solid.
•
Rinse the beaker with 10-15 mL of deionized water, and then transfer the water and the remaining
solid to the funnel. Keep the vacuum on to remove the water. Dry the solid as much as possible by
drawing air through the filter for about 1-2 min. Then, turn off the vacuum.
•
Transfer the solid with or without the filter paper to a dry watch glass, being careful not to tear the
filter paper. The filtrate from the flask and the beaker can be disposed of in the waste container.
Let the sample air dry for few minutes before transferring it to a 50 mL Erlenmeyer flask for
recrystallization in the next step. Weigh the crude aspirin.
•
Step 3: Recrystallization
• Set aside a small amount (spatula tip full) of the crude aspirin you obtained to test it in a future
lab. Keep it in a small labeled test tube in your drawer.
•
Transfer the rest of the crude aspirin to a 50-mL Erlenmeyer flask. Take about 1-2 mL ethanol and
heat the flask on the heating mantle. Add ethanol dropwise until all the solid dissolves.
Remove the flask from the heat and slowly add water dropwise while stirring until a solid appears.
Now heat the flask again to dissolve the solid and get a clear solution. You can add a few more
drops of ethanol if you are not getting a clear solution upon boiling.
• Once it is clear let it cool to room temperature.
•
Crystals should form. Chill this flask in an ice-water bath, and collect the crystals using vacuum
filtration. (If crystals don't appear at room temperature, you may have to boil again to get rid of
some solvent.)
• Carefully lift the filter paper with the crystals on it and place it on a clean watch glass. Weigh the
recrystallized aspirin. Leave this aspirin in your drawer to dry until the next laboratory period.
Transcribed Image Text:microscopic, angular fragment of glass) on which crystals can begin to grow. o Alternatively, you can obtain a small "seed" crystal of aspirin (just big enough to see) from another student or TA and add it to your mixture. Stir the solution with a stirring rod and wait for 1-2 minutes to see any precipitation. o If none of these techniques work, repeat the synthesis. Step 2: Filtering the reaction product. • Set up the vacuum filtration apparatus as shown in Figure 1. Clamp the filter flask to the support bars. Connect the filter flask to the trap flask and not directly to the vacuum line. Use the thick vacuum tubing. Place a Buchner funnel with filter paper on the flask. Rubber bung/cork • Using a stirring rod, break up any large Rubber tubing particles of solid in the precipitate. moisten the filter • Turn on the vacuum line knob, paper with deionized water, and then filter the reaction mixture by slowly pouring the mixture into the Buchner funnel. The filter flask will be getting full, so empty Figure 1: Filtration setup to be used for it into a large beaker and set it in the corner of your hood, then continue with the filtration. isolating aspirin from the reaction mixture. Buchner funnel containing moistened filter paper Filtrate containing impurities, acetic acid, water. Should be emptied periodically. Vacuum line • Now that much of the solid has been transferred to the filtration apparatus, use deionized water and a glass stirring rod or spatula to transfer the remaining solid. • Rinse the beaker with 10-15 mL of deionized water, and then transfer the water and the remaining solid to the funnel. Keep the vacuum on to remove the water. Dry the solid as much as possible by drawing air through the filter for about 1-2 min. Then, turn off the vacuum. • Transfer the solid with or without the filter paper to a dry watch glass, being careful not to tear the filter paper. The filtrate from the flask and the beaker can be disposed of in the waste container. Let the sample air dry for few minutes before transferring it to a 50 mL Erlenmeyer flask for recrystallization in the next step. Weigh the crude aspirin. • Step 3: Recrystallization • Set aside a small amount (spatula tip full) of the crude aspirin you obtained to test it in a future lab. Keep it in a small labeled test tube in your drawer. • Transfer the rest of the crude aspirin to a 50-mL Erlenmeyer flask. Take about 1-2 mL ethanol and heat the flask on the heating mantle. Add ethanol dropwise until all the solid dissolves. Remove the flask from the heat and slowly add water dropwise while stirring until a solid appears. Now heat the flask again to dissolve the solid and get a clear solution. You can add a few more drops of ethanol if you are not getting a clear solution upon boiling. • Once it is clear let it cool to room temperature. • Crystals should form. Chill this flask in an ice-water bath, and collect the crystals using vacuum filtration. (If crystals don't appear at room temperature, you may have to boil again to get rid of some solvent.) • Carefully lift the filter paper with the crystals on it and place it on a clean watch glass. Weigh the recrystallized aspirin. Leave this aspirin in your drawer to dry until the next laboratory period.
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