Lab 1

docx

School

Queens University *

*We aren’t endorsed by this school

Course

282

Subject

Chemistry

Date

Apr 3, 2024

Type

docx

Pages

Uploaded by MatePelicanPerson713

CHEM 282 Experiment #1: Crystallization and Melting Point Sophie Wolkoff (20107258) & Kaelen Partridge (20127197) TA: Bily Deng January 23, 2020 Experimental :

The lab began with part 2 which involved performing four tests of solubility of acetanilide in the solvents water, ethanol, acetone and hexanes placed in four separate test tubes. Acetanilide was added to the test tubes and its solubility within the liquids was observed for each liquid at room temperature and an elevated temperature when placed in a steam bath. Part 3 involved a melting point analysis of three substances; pure reference acetanilide, pure reference benzoic acid, and a mixture of the two substances. Each substance was transferred into a melting point tube and positioned in a melting point apparatus where their temperature ranges over which they melted was observed and recorded. Single-solvent crystallization was carried out in part 4 with impure acetanilide and a water solvent. The acetanilide was dissolved in a minimum amount of hot water, subsequently cooled to room temperature and then placed in an ice-water bath to form crystals. Vacuum filtration was used to collect the solid crystals and remove impurities, and the crystals were dried in an oven to obtain an accurate measurement of mass. This process was repeated in step 5, first dissolving the acetanilide in hot ethanol before the solvent of water was added. The same process was performed to recover the crystals and record a measurement of mass. The melting point procedure was repeated in step 6, placing pure reference acetanilide, the crystallized acetanilide sample obtained from the experiment, and a mixture of these two substances into the melting point apparatus and recording a temperature range for each substance over which they melted. The procedures from the lab manual were changed in step 4 and step 5. Instead of heating the flasks directly in a hot water bath on the hot plate and then adding water

as the lab manual procedure outlined for both steps, the procedure was modified to heat the water separately from the flask and add the warm water dropwise to the flasks. This way, the flasks were not heated directly and the process resulted in more accurate measurements of the minimum amount of hot water needed to dissolve the solids. Results: Table One: Solubility chart for acetanilide in various solvents Solvent Observations Water Room temp: some of the solvent dissolves, but a large amount of white, opaque precipitate remained in large clumps Elevated temp: a bit more of the precipitate dissolved, but a lot of precipitate remained in large clumps Ethanol Room temp: almost all of the solvent dissolved immediately, with only a few small granules of solvent remaining undissolved Elevated temp: all of the precipitate completely dissolved Acetone Room temp: all of the precipitate dissolved immediately Elevated temp: no further change occured Hexanes Room temp: when shaken, the solution was quite cloudy/murky with white precipitate, but when held still all the precipitate sunk to the bottom in a large pile Elevated temp: heating did not improve the solubility; the solution remained very cloudy with a lot of white precipitate Table Two: Recrystallization Chart (Sophie Wolkoff’s data) Solvent chosen for single-solvent recrystallization: water

Your preview ends here

Eager to read complete document? Join bartleby learn and gain access to the full version

Access to all documents
Unlimited textbook solutions
24/7 expert homework help

Single-Solvent Solvent-Pair Impure Weight (before recrystallization) 0.154 g 0.157 g Pure Weight (after recrystallization) 0.139 g 0.130 g % Recovery 90.3% 82.8% Appearance of Crystals White, very fine, opaque, shiny, snow-like appearance White, slightly clumpy, opaque, shiny Melting Point 115.4 - 116.6 ℃ N/A (not used for melting point analysis) Pure weight calculations Mass of watch glass and recrystallized product (g) - mass of watch glass (g) = pure weight (g) Single Solvent: 47.112 g - 46.973 g = 0.139 g Solvent Pair: 47.002 g - 46.892 g = 0.130 g Percent recovery calculation: Mass of pure M ass of impure × 100% = % recovery 0.134 g Single Solvent: 0.154 g × 100% = 90.3% 0.130 g Solvent Pair: 0.157 g × 100% = 82.8%

Table three: Recrystallization Chart (Kaelen Partridge’s data) Solvent chosen for single-solvent recrystallization: water Single-Solvent Solvent-Pair Impure Weight (before recrystallization) 0.149 g 0.151 g Pure Weight (after recrystallization) 0.091 g 0.045 g % Recovery 61.1% 29.8% Appearance of Crystals White, very thin, lustrous, flakes in solid state White, large clumps, flaky, lustrous Melting Point N/A (not used for melting point analysis) N/A (not used for melting point analysis) Pure weight calculations Mass of watch glass and recrystallized product (g) - mass of watch glass (g) = pure weight (g) Single Solvent: 31.470 g - 31.379 g = 0.091 g Solvent Pair: 29.151 g - 29.106 g = 0.045 g Percent recovery calculation: Mass of pure M ass of impure × 100% = % recovery 0.091 g Single Solvent: 0.149 g × 100% = 61.1% 0.045 g Solvent Pair: 0.151 g × 100% = 29.8%

Table 4 Compound Experimental Melting Point (°C) Literature Melting Point (°C) Pure reference acetanilide (left) 114.6 - 116.5 114.3 Pure reference benzoic acid (right) 123.2 - 124.4 122.4 Mixture of pure reference benzoic acid and pure reference acetanilide (middle) 78.9 - 89.21 N/A Pure reference acetanilide (left) 115.1 - 116.4 114.3 Crystallized acetanilide sample (right) 115.4 - 116.6 114.3 Ground mixture of pure reference acetanilide and crystallized acetanilide sample (middle) 114.8 - 115.8 114.3 Discussion: Part A Solubility is a chemical property of compounds that describes the degree to which a substance is able to dissolve in a solvent to form a solution.³ Solubilities of solids in liquids are generally known to increase with temperature. This topic is covered in the lab as the solubility of acetanilide is evaluated in a number of solvents, including water, ethanol, acetone and hexanes, at room temperature and elevated temperature. Crystallization is a technique used in the lab to purify substances existing as solids around room temperature by dissolving them in a hot solvent and slowly cooling

Your preview ends here

Eager to read complete document? Join bartleby learn and gain access to the full version

Access to all documents
Unlimited textbook solutions
24/7 expert homework help

them.¹ Recrystallization is a process to purify a crystallized compound once it is dissolved within a solvent.² The process of crystallization is performed by dissolving a solid in the least possible amount of hot solvent in order to increase the solubility of the compound, and the solution is subsequently cooled.¹ The solid crystals formed in the solution are obtained by filtration. Recrystallization keeps the potential presence of impurities in the solvent that is separated as the compound crystallizes before the impurities within the solution.² This process depends on the higher solubility of solids in hot solvents rather than cold solvents, and how the amount of solute that can be dissolved in a solvent increases as temperature increases. Solvents are chosen based on their ability to promote crystallization during cooling.¹ This process is used multiple times in the lab to purify a sample of acetanilide, first using a single solvent and then using a solvent pair. This was performed to evaluate the efficiency of the solvent(s) used to recover a pure solute product. A melting point analysis is performed to qualitatively determine the physical properties of melting point of specific substances. ⁴ This can be achieved by using various melting point apparatuses to heat the substances and observe their melting through a magnifying lens in order to record accurate and precise recordings of temperature from the thermometer. A melting point analysis is conducted twice during the lab in step 3 and 6, placing several substances at once into the apparatus to observe and record their respective melting temperatures and evaluate the purity of the substances used.

Part B Overall, the majority of the results that we obtained during this experiment were consistent with the theory, and we were quite successful at accurately carrying out this experiment to contain suitable results. When testing various solubilities of acetanilide in part one, we found that it was able to dissolve quite easily in ethanol and acetone, could dissolve partially in water, and could not dissolve in hexanes. This corresponds quite well with the theory of “like dissolves like,” in which compounds of the same polarity are able to dissolve in each other, whereas those with opposite polarity are unable to. In this experiment, acetanilide is a polar molecule, and can therefore dissolve in ethanol, acetone and water which are all also polar, but cannot dissolve in the non-polar hexanes. For the recrystallization portion of this experiment, our results also were consistent with the theories of recrystallization and melting point. For the most part we were successful in obtaining good crystals and accurate recrystallization data. When weighing our crystals after the recrystallization process, the mass decreased, indicating that impurities had been removed. Furthermore, when we measured the melting points of one of our recrystallized samples, we noticed that the experimental melting point was very similar to the literature value, indicating that our results were quite accurate and consistent with what was to be expected. However, some of our data reflected a lower percent yield of crystals than what we expected, which likely was due to adding too much water when dissolving the solvent Questions:

1. There are several errors that could have occurred during this experiment. One error is that we added more water to dissolve the solvent than we should have, which would have interrupted the formation of crystals, resulting in a lower amount of yield and a smaller percent recovery. Another error that could have resulted in a lower yield, is not scraping all the crystals off of the filter paper to measure them, which could result in an inaccurate depiction of our total yield. Also, not properly using the vacuum filtration to separate our crystals is an error that could have resulted in less yield than expected. Finally, it is possible that some of the tools we used in the lab were contaminated, contributing to any impurities in our final yield of our crystals. 2. The examined solvent that was the best choice for single-solvent recrystallization of acetanilide was water. The solubility test performed in step 2 of the lab analyzed the solubility of acetanilide in each of four solvents, water, ethanol, acetone and hexanes, at room temperature and elevated temperature. This revealed that all of the solvents at elevated temperature caused the solute to dissolve completely except for hexanes, and the only solvent that kept acetanilide solid in large quantities at room temperature was water. The other three solvents caused acetanilide to dissolve almost completely within them at room temperature. The ability of a solute to be relatively insoluble in a solvent at room temperature is extremely important in recrystallization in order to easily initiate crystallization when the solution is at lower temperatures. Therefore water was the best choice for single-solvent recrystallization.

Your preview ends here

Eager to read complete document? Join bartleby learn and gain access to the full version

Access to all documents
Unlimited textbook solutions
24/7 expert homework help

3. When comparing the outcomes of using single-solvent recrystallization and solvent-pair recrystallization in the lab, the single-solvent method was found to be better as it produced better experimental results. Although the observation and samples from the single-solvent and solvent-pair procedures were very similar, both producing white and shiny crystals, the measurement data collected from both procedures differed. The single-solvent recrystallization appeared to render a higher yield of the product after recrystallization than solvent-pair recrystallization as it consistently had a higher percent recovery than solvent-pair in both sets of data. The product from single-solvent recrystallization was also assumed to be almost pure as its melting point was close to the literature value melting point and the experimental value melting point of the pure reference acetanilide in the lab. 4. An impure compound will have a depressed melting point due to any impurities that will interfere with the intermolecular forces of the crystal lattice structure. The weakening of the intermolecular forces will result in the bonds requiring less energy to break them apart, thus requiring less heat in order to melt the compound.

5. The procedure to identify the unknown (X) occurs by performing a melting point analysis. The unknown (X) is first mixed with each of the three known compounds, A, B, and C, and is placed in a melting point apparatus to perform a melting point analysis for each compound. If the unknown (X) was mixed with itself (i.e., the same compound), the same melting point of 80.1°C would be observed and recorded, therefore now identifying the unknown compound. If the unknown (X) was mixed with the wrong compound (i.e., chemically different), the melting point observed and recorded would be different as it would be lower than

80.1°C. This is because the result of mixing two different compounds would introduce impurities into the substance and alter the intermolecular forces present within the compound, decreasing the stability of the chemical. This is observed in the melting point analysis from step 3 in the lab, as the mixture of pure reference benzoic acid and pure reference acetanilide produced a much lower melting temperature range than the pure reference benzoic acid and pure reference acetanilide compounds on their own. 6. Before filtration, filter paper is typically wetted with the same solvent that is used in the solution that is to be filtered. For instance, the filter paper used in the filtration in step 4 was wetted with water, which was the same solvent used to dissolve the acetanilide. Using the same solvent from the solution ensures that the solution to be filtered will not interact chemically with another solvent. If a solution does interact chemically with a different solvent, it could cause a reaction to occur or could alter the recrystallization process.

Your preview ends here

Eager to read complete document? Join bartleby learn and gain access to the full version

Access to all documents
Unlimited textbook solutions
24/7 expert homework help

References: 1. Vlahakis, J. Queen’s Chemistry 2020 Laboratory Manual: CHEM 282 General Organic Chemistry II ; Queen’s University: Kingston, ON, 2020; p 11-12. 2. Barich, A. Recrystallization. https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Te xtbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Ph ysical_Properties_of_Matter/Solutions_and_Mixtures/Case_Studies/RECRYSTA LLIZATION (accessed Jan 22, 2020). 3. The Editors of Encyclopaedia Britannica. Solubility. https://www.britannica.com/science/solubility-chemistry (accessed Jan 22, 2020). 4. Raja, P. M. V.; Barron, A. R. 2.1: Melting Point Analysis. https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Book:_Physical_M ethods_in_Chemistry_and_Nano_Science_(Barron)/02:_Physical_and_Thermal_ Analysis/2.01:_Melting_Point_Analysis (accessed Jan 22, 2020).