Lab+IV+Sublimation


 * __ Sublimation __**

__** ﻿Introduction **__ Sublimation is a phase change; upon heating of a solid the solid vaporizes without passing through the liquid phase. Furthermore, deposition is condensation of a gas to a solid without--once again--passing through the liquid phase. Sublimation is used in the chemistry lab setting as a helpful technique for separation of two substances. This makes sublimation fairly analogous to distillation as we are determining purity through a phase change.

An impurity in a solid substance can be removed by sublimation as long as there is a sizable vapor pressure difference between the two compounds. The solid that is being sublimated is referred to as the sublimand while the pure product obtained after sublimation is designated as the sublimate. However, If there is little difference in vapor pressures then another approach should be implemented as sublimation could be inconclusive. OK.

__** Procedure **__ **__ ﻿ __** The procedure for this lab can be found in Williamson’s __Macroscale and Microscale Organic Experiments__, 4th edition, Ch. 7, all experiments.

The compound used in this lab was an Unknown solid. [//See conclusion section for this information.//] OK.

__** Data **__ > >
 * Overview**
 * Unknown #: 241-7-6
 * Initial mass: .050 grams
 * Initial appearance: Small, granular powder, off-white in color (the substance can be seen at the bottom of the flask contrasting with the sand).


 * Sublimation**
 * Timeline:
 * 1310: The unknown was measured out and put into a dry filtering flask. The flask was then inserted into a sandbath with the energy set at 12% of the maximum output voltage.
 * 1328: No changes observed 18 minutes into heating. Heat was increased to 25% maximum output voltage.
 * 1336: Increased heat to 30% maximum output voltage.
 * 1343: Increased heat to 35% maximum output voltage.
 * 1347: Increased heat to 45% maximum output voltage
 * 1352: Substance began to sublimate. Thin, filamentous crystals observed forming on the flask and the cold finger, they stretched the expanse of the flask . The flask was removed from the sandbath and placed on the counter top. All of the crystals in the flask were attached to the cold finger. There were no remaining particles of the unknown substance at the bottom of the flask. The flask was then scraped with a metal spatula and a rubber policeman; the filaments fragmented and the remaining compound proved difficult to scrape from the sides.
 * Final mass: .045 grams
 * Final appearance: Suspended crystal matrix. Many clear, branching filaments. The highest density of crystal formation was medial on the cold finger extending to the walls of the flask. The tip of the cold finger, inferior to the medial band, was also high in crystal density but in this location crystal formation was limited to the finger and did not extend to the walls of the flask.

beautiful pics, nicely reported data, sig figs etc. all looks good.
 * Melting Point**
 * Melting started: 159.8°C
 * Melting ended: 161.3°C

>

__** Analysis **__
 * //Percent recovery://

calculation OK, sig figs OK.


 * //Melting point://

In the lab instructions, a list of compounds was given as potential candidates for the unknown.



The observed melting point range of 159.8°C-161.3°C corresponds with the given melting point of Salicyclic Acid.

**__ ﻿Conclusion/discussion __**

The experiment was conducted slowly, and with caution to the amount of heat applied, so as not to sublimate the substance too quickly. Where the flask was submerged in the heated sand bath the crystals did not extend to the walls of the flask. The remaining portion of the flask, which was above the sand bath, created crystals that stretched to the walls of the flask, despite the foil tent surrounding it. As the unknown solid sublimed to the gaseous phase, the cooler air present in the superior portion of the flask facilitated deposition near the cold finger. In the portion submerged in the sand bath, the air was constantly above the deposition temperature, leaving only the surface of the cold finger on which to deposit.

Potential sources of error would include excess heat, which would affect the accuracy of the melting point range to assess the identity of the newly-deposited compound. Another source of error would be the innate difficulty of extracting all of the crystals, which would affect the percent recovery. The crystals structure was attached to both the cold finger and the inside of the flask, making it impossible to extract without sundering the formation, and having to sweep the remaining fragments out of the bottom. Nicely done.

For any solid compound suspected of being impure, sublimation becomes a leading procedure to undergo in the effort to improve purity levels. After the sublimation process is complete, the melting point of the new solid will confirm the purity of an already-known, or assist in revealing the identity of an unknown, like in the case of Salicylic acid presented here. identification is correct.
 * The unknown:**
 * Name: Salicylic acid
 * IUPAC Name: 2-hydroxybenzoic acid
 * CAS Number: 69-72-7
 * Molecular Formula: C 6 H 4 (OH)COOH
 * Compound Formula: C 7 H 6 O 3
 * Melting Point: 159.0*C
 * Skeletal Structure:

__** Post Lab Question **__ **Q:** //What is lyophilization? Why is it done?//

**A:** Lyophilization (cryodesiccation) is better known as "freeze-drying," where a perishable product is dehydrated for more effective preservation and storage. It occurs by freezing the desired object and reducing the pressure of its surroundings. Then enough heat is added--slowly--to the frozen product to initiate sublimation. A partial vacuum combined with a cold surface for water molecules to re-solidify on ends up removing most of the water molecules from the solid product. Lyophilization is a widely-used and important part of any industry that uses materials or substances that can easily go bad: food, pharmaceutical, etc. yep. OK.

This lab earned the following scores for: format (2/2) style (2/2) data (3/3) quality of the result (1/1) quality of the reported data (1/1) conclusion (2/2) error analysis (1/1) post-lab Q (2/2) for a total of 14/14. Well done--you seem to have a well functioning group and I am impressed with the quality of your reports!

**__ NOTES __** The chemical structures, formulas, and vital statistics of each compound was researched using [|Wolfram Alpha computational knowledge engine] and confirmed using the [|CRC Handbook of Chemistry and Physics [90th Edition.]]