Ochem lab 3 - Recrystallization and Melting point PDF

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Recrystallization and Melting point...

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Will Davidson Chem 253-TA: Erome Hankore Lab 3: Recrystallization and Melting Point 2/04/15

Purpose: The purpose of this experiment is to use the recrystallization method to purify different compounds (benzoic acid and naphthalene). This lab will also explore what happens to the melting points of compounds when they are mixed, opposed to when they are pure.

Theory: There are different types of intermolecular forces, with varying levels of strength. Van der Waals interactions are the weakest of these interactions and occur because of temporary displacement of electrons around the molecules. The strength of Van der Waals interactions depends on the number of interactions, the amount of surface area on a molecule that can efficiently come into contact with a neighboring molecule, and how easily the electron cloud can be deformed. The easier it is to deform the electron cloud, the stronger the interactions will be. Dipole-dipole interactions are the next strongest and occur because a molecule has a dipole moment. A dipole moment occurs when two atoms in a molecule have a large difference in electronegativity. This results in one side of the molecule having a negative charge and another with a positive charge. These negative and positive charges then interact with one another, creating an intermolecular force relatively stronger than Van der Waals interactions. Hydrogen bonding is the strongest of the intermolecular forces and occurs when a hydrogen is bonded to an oxygen. nitrogen, or a fluorine. Hydrogen bonding is what gives water its unusual properties. These intermolecular forces play a large role in melting and boiling points. Molecules that have hydrogen bonds such as water will have much higher boiling points than molecules with only Van der Waals interactions. Recrystallization is a process used to purify substances by taking advantage of the substances solubility. The substance is usually dissolved in a warm solvent and then brought down to a low temperature to super saturate the mixture. The substance can then recrystallize in a purified form. It is important to super saturate the solvent because the compound will not crystalize when the temperature of the mixture is lowered without being super saturated initially. A compound must be recrystallized in a cool/cold environment to reduce the solubility of the solvent. A hot solvent will able to dissolve more of a compound than a cold one. Using a hot solvent initially enables us to supersaturate it. Once it is cooled down the compound will crystalize until the solvents new saturation limit, related to the colder temperature, is reached. If recrystallization does not happen on its own it can be induced by scratching the side of the beaker. A seed crystal can also be added to induce the recrystallization. Crystals begin to form at

nucleation sites that resemble the structure of the crystal lattice. To know how much volume of a solvent you need to dissolve a certain mass of solute you must know the solubility of the compound in the solvent being used. For example if the solubility of a compound in boiling water is 18 grams per 100 milliliters of water you can use train tracks to find out the exact amount of water needed. This method can be observed in the “exercises” section of this lab report under number three. Melting point is another property that changes depending on many molecular factors. A mixture melting point is the melting point of a mixture of 2 or more substances. A mixture of compounds often melts at a lower temperature and over a wider range. A pure substance will usually have a much “sharper” melting point, meaning it melts faster within a narrower temperature range. You can use this technique to find the identity of a compound by mixing an unknown with a known compound. If the mixture melts very sharply at a certain temperature then it can be assumed the unknown is the same as the known. Reaction Structure of Benzoic Acid:

Structure of Naphthalene:

Percent Yield Initial mass of benzoic acid: 0.52 grams Final mass of benzoic acid: .31 grams Percent recovery:

final mass initial mass

=

0.31 g 0.52 g

= 0.596 = 60%

=

0..34 g 0.52 g

= 0.65 = 65%

Initial mass of naphthalene: 0.52 Final mass of naphthalene: 0.34 Percent recovery:

final mass initial mass

Solubility Calculations Benzoic acid: Solubility in 100 degree Celsius water is 6.8 grams per 100 milliliters. (0.52g x 100ml)/6.8 grams= 7.65 mL required to dissolve 0.52 grams

Naphthalene: Solubility in 100 degree Celsius methanol is 10 grams per 100 milliliters. (0.52 x 100ml)/10 grams= 5.2 mL required to dissolve 0.52 grams

Methods and Procedure: This experiment was followed verbatim from the textbook as found on pages 20 through 22. No exceptions were made save that the filter paper was not weighed due to the fact that we would not be able to recover it after the filtration had occurred. To do this lab as safely as possible one must be aware that benzoic acid is a mild irritant. One must avoid contact with eyes, skin, or clothing. One should not ingest this compound and should avoid inhalation. Naphthalene is also hazardous when ingested, so the same safety protocol should be followed with this material. Naphthalene is also flammable, so it should be kept away from flames. Cinnamic acid and urea are irritants to the skin, eye, and lungs as well. Goggles, gloves, and eye protection should be worn throughout the duration of this lab.

Observations/Results Amount of water needed to dissolve 0.52 grams of benzoic acid: 7.65 mL Percent recovery benzoic acid: 60% Amount of methanol needed to dissolve 0.52 grams of naphthalene: 5.2 mL Percent recovery for naphthalene: 65%

Melting Ranges

Benzoic Acid Naphthalene Cinnamic Acid Urea Mixture of Cinnamic Acid and Urea

Book Value (degrees Celsius) 122.12 128.17 148.16 60.06 N/A

Starting point (degrees Celsius) 109 68 128 128 92

Ending point (degrees Celsius) 119 76 131 134 114

Discussion/Conclusion The two compounds which are recrystallized are benzoic acid and naphthalene. Naphthalene has no dipole moment or hydrogen bonds, but is able to form a very solid lattice due to its shape. Benzoic acid has a hydrogen bong but does not fit very efficiently with other molecules.

The low percent recoveries on these compounds is due to several issues. One of the issues is that some of the material was lost during transfers from beaker to beaker. These losses due to transfers add up the more times the material is moved. A lot of the compound was also left in the solvent after recrystallization had occurred. Only enough of the compound crystalizes to return the solvent to a saturated state, so there is a known loss when using this method of purification. The benefit of recrystallization is that it is a relatively simple way to purify a compound. Recrystallization is a method used to make sugar because it is so effective and efficient. The evidence present in this lab proves that when two different compounds are mixed the melting range becomes very large. This is due to the change in forces between the two different types of interacting molecules. Pure substances melt within a much smaller range.

Exercises #3) Phthalic acid has a solubility of approximately 18 g per 100 ml of water at 100 degrees Celsius and 0.62 g per 100 mL of water at 25 degrees Celsius. What is the minimum volume of water needed to recrystallize 50 mg of phthalic acid? How much phthalic acid will remain in solution after recrystallization? mg∗1 g ∗100 mL 1000 mg =0.278 mL 50 18 g acid

0.278

ml∗0.62 =0.00172 g 100 ml

Minimum volume to recrystallize 50 mg of phthalic

Would remain in solution after recrystallization

#7) In some cases, mixed solvents are used for recrystallization. What would be the purpose of using mixed solvent? The purpose of using a mixed solvent is to recrystallize a compound that cannot be recrystallized from any single solvent. #8) a)The melting point of phthalic acid is higher than that of naphthalene even though they have almost the same molecular weight because phthalic acid exhibits hydrogen bonding, a very strong intermolecular force. Naphthalene exhibits only Van der Waals interactions which is much weaker than hydrogen. b) Cyclohexane has a higher melting point because its molecular shape allows it to create a tighter lattice than regular hexane....