Ellie Tille - Lab 2: Crystallization

Objective Statement

In this lab, we're going to learn about crystallization and the techniques involved when crystallizing a substance. This process accounts for solids being more soluble when it's hot in a solvent rather than cold. With this in mind, we will take a contaminated product, dissolve it in a hot substance to remove the contaminants, and then cool the pure substance to room temperature. The pure product with re-crystallize while the contaminants will stay within the solution.

We are doing this lab to learn techniques of crystallization and how to acquire a pure substance and check its purity as well as develop essential lab skills that can be utilized later on. By doing this lab, I hope to learn essential lab etiquette and the process of crystallization as well as look into applications of melting and cooling substances.

Compounds Of Study

Phthalic Acid

Slightly Polar

BP: 289° C

Molar Mass: 166.14 g/mol

naphthalene

Non-Polar

BP: 218° C

Molar Mass: 128.17 g/mol

methanol

Very-Polar

BP: 64.7° C

Molar Mass: 32.04 g/mol

Water

Polar

BP: 100° C

Molar Mass: 18.02 g/mol

Pre-Lab Questions

While picking a solvent, you should test it by adding 5-10 drops of the trail solvent to the solute. If it dissolves immediately then it's too soluable for recrystallization to be effective. If no solubility is seen, heat the solvent to make it boil then wait to see if solubility occurs. If it does, let the solution cool and if re-crystallization happens within 10-20 minutes, it suggests a good re-crystallization solvent. If no solvent works, a two-solvent re-crystallization method can be used using miscible solvents.

The solute and solvent will be dis-similar in intermolecular force in order for the solute to dissolve. This is because polar substances can only dissolve other polar substances and using water and methanol as our solvents, our solutes must be polar for solubility. Since they're non-polar, solubility won't occur meaning they will crystallize outward as a pure substance.

A sand bath is a container filled with heated sand. Sand is put into a container and then put onto a heat source. Its purpose is to evenly heat another container.

The purpose of a boiling stone or stick is to ensure liquid will boil smoothly and calmer. If one isn't used, the solution can become overheated, or unnecessary bumping will occur.

This is done to ensure that re-crystallization doesn't occur before the charcoal is filtered out. If re-crystallization occurs too early, the substance will still be contaminated. Having the solution understated with the substance ensures the most product can be captured while still filtering out impurities.

It's essential the funnel stays hot by contact with solvent papers while filtering to avoid crystallization from happening too early otherwise the funnel will get clogged at the bottom. This requires the use of the fluted paper filter rather than a long stem funnel that allows for time for the crystal to cool and clog it preventing proper filtration.

Gravity filtration is used to filter out the charcoal because vacuum filtration creates low pressure causing the solution to boil. Using gravity filtration doesn't create a change in pressure so proper filtration can occur.

You should wait until it's cooled at room temperature to avoid impurities in the crystallization. When the solution is at high temperatures, the impurities and substance still remain and as it cools, they separate removing impurities and having the maximum amount of crystallized substance.

The appropriate size of filter paper changes based on the vacuum filtration devices in use. It should just cover the bottom of the funnel. Any bigger or smaller will affect the filtration and cause inaccurate results.

The purpose of re-filtering at the end of the procedure is to ensure as pure of a substance as possible. Re-filtering removes impurities that dissolved in the solvent and therefore attached themselves to the crystallized substance while drying.

a. The first solvent used would be the one most "like" and therefore the most soluble.

b. The second solvent is introduced when the solution is brought nearly to its boiling point and it starts to get cloudy.

If an oil is present, you could add more solvent so the substance doesn't get removed from the solution at so high of a temperature or switch to a solvent with a lower boiling point. Oils can also be crystallized by dissolving them in small amounts of diethyl ether or hexane and having the solvent evaporate.

13. The weight boats are disposed of and the sticks should be washed and dried for other groups.

Experimental

Experiment #1

-0.3 grams of phthalic acid were weighed.

-Put the Phthalic Acid into a 50 ml flask with approximately 10 ml of DI water added to it. With the DI water, none of the substance initially dissolved.

-Filled another 50 ml flask with DI water.

-Both flasks were placed upon the hot plate with the phthalic acid DI water mixture in the center and the DI water to the side. Boiling sticks were placed in both.

-The mixture was brought to a calm boil then stirred about every 30 seconds.

-Once the phthalic acid was dissolved it was removed from the heat and placed upon a cork ring to cool to room temperature.

-When the mixture reached room temperature, it was firmly tapped against the table to assist in the formation of crystals.

-After the crystals formed, the solution was ran though a vacuum filter to isolate the crystals.

-After filtration, the crystals on top of the filter paper were scrapped off into an empty beaker.

-A lid was placed onto the beaker and the crystals were left to dry for a week.

Experiment #2

-1.25 grams of naphthalene was weighed out.

-A 50 ml flask was filled with the 1.25 grams of naphthalene along with roughly 10 ml of methional

-Another 50 ml flask was filled with about 25 ml of methional.

-The flasks were placed upon the hot plate at a low temperature setting with the naphthalene and methional mixture in the center and the flask containing only methional on the corner.

-Once the naphthalene mixture was brought to a boil, around 10 ml of methanol was added.

-When the naphthalene mixture was fully dissolved, the flask was removed from the hot plate and placed upon the cork ring to cool.

-After 3 minutes of cooling, a heaping spoonful of charcoal was added until the solution turned black.

-The solution was put back onto the hot plate to boil for another 5 minutes while being stirred about every 30 seconds.

-Once the solution was taken off the heat, gravity filtration was used with a fluted paper filter to remove the charcoal.

-After filtration, the flask was boilied again for another 2 minutes.

-Once the solution has boiled for 2 minutes, it was removed from the heat when 10 ml of Di water was added causing crystals to form almost immediately.

-When crystals were formed, the solution was then left to cool on the cork ring until it reached room temperature.

-The crystals were then filtered by vacuum filtration.

-The crystals were then removed from the filter paper and put into a beaker with a lid on to dry for a week.

Results

Experiment 1:

-Mass of the re-crystallized anthracene was 0.28 grams

-The percent recovery = (re-crystallized mass/original mass) *100

= (0.28/0.30)*100 = 93.3% recovery

-The re-crystalized anthracene was a white, small, and powdery substance.

Experiment 2:

-Mass of the re-crystallized naphthalene was 0.81 grams

- The percent recovery = (re-crystallized mass/original mass)*100

= (0.81/1.25)*100 = 64.8%

-The re-crystallized naphthalene looked small, white, shiny, and less fine than the anthracene

-The literature melting point for naphthalene is 80.26°C

-The melting point for the re-crystallized naphthalene was 82.9° C

Discussion and Conclusion

Experiment 1:

Through the process of re-crystallization, we were able to recover 0.28 grams or 93.3% of the original mass of the anthracene. Using the "like dissolves like" ideology, it was mixed with water and then boiled to remove impurities. Since they are slightly miscible, the crystals were able to separate from the water, re-crystalize, and were then filtered out. This was a successful process as my group was able to recover most of the original mass of the substance in a purified form.

Although a lot of the original mass was recovered, about 7% was lost. This could've been due to the slight polarity of the anthracene making it so some of the substance did dissolve in the water. It also could've been due to not letting the substance cool down enough before filtration, not filtration for long enough, or crystals getting stuck to the filtration paper. When doing this process of purification in the future, I would give more time for the crystals to cool and do more thorough vacuum filtration.

Experiment 2:

When purifying naphthalene, we were able to recover 0.81 grams or 64.8% of the original mass. Again using the "like dissolves like" ideology, the naphthalene was mixed with water and methane to induce crystallization. The naphthalene was also contaminated with red dye so a charcoal filtering process was needed to purify the substance. This part of the lab was less successful as about 35% of the original mass was lost during the process. To test the purity of the substance, a melting point analysis was done with he literature value being 80.26°C. Our re-crystallized naphthalene had a melting point of 82.9° C. This makes me confident that our purification process was highly successful as it's within 3° C of the literature value melting point, making this the most successful aspect of the process of re-crystallizing naphthalene.

Although I'm confident in the purification of our substance, the mass recovery could've been improved. Various aspects could contributed to the lost mass such as too much charcoal, too high of heat on the burner, and improper vacuum filtration along with crystals getting stuck to the filter. There was even more room for error within this part of the lab as the solution had to go through two different filtration processes.

Reflection

I was able to learn a lot throughout this lab while focusing on the process of purification. I can see the usefulness of this laboratory technique as substances are more often contaminated than not. To conduct proper experiments, purification could be necessary. I felt as though this lab taught me a lot about this process along with patience and the significance of human error. This lab required some patience when waiting for crystals to form or keeping the solution on a lower heat to avoid rigorous boiling. My group was not as patient as we should've been and not as thorough with the charcoal filtration and with scrapping the crystals of the filter paper. Other than patience and more precision with each step, I feel as though this lab went well and taught me a lot about the purification of substances and different filtration methods.