Sublimation

=**Introduction**= Sublimation is a technique used to heat a solid directly to a gas phase, without going through the intermediate liquid stage. It is an efficient process which chemist use to purify a substance that has the ability to go through this process; however, only a few substances will sublimate. Most elements as they are heated will go through three stages; solid, liquid and gas. When a compound is heated with enough energy to pass through these different phases, such as water being heated to produce gas, many impurities are left behind. A common example of sublimation is carbon dioxide or "dry ice". Carbon dioxide is a solid when it reaches temperatures under -109 °F. In sublimation, the boiling point of a substance is also its freezing point at normal atmospheric pressure. When it is heated above -109 °F, it reaches a boiling point in which its intermolecular forces have enough energy to break the bonds between them and transpire directly to a gas. The Diagram below shows that a compound with a lower boiling point temperature does not pass through the liquid phase at all, like carbon dioxide. Compounds that have a boiling point higher then 205 K will go through the liquid phase at normal atmospheric pressure. The goal of sublimation is often to either purify substances or separate them utilizing differences in sublimation temperatures. Though this process requires compounds that are capable of sublimation, it has several advantages which outweigh this fact. Sublimation does not require the use of catalysts or solvents and it takes place within a closed system that leaves few places for vapor to be lost or impurities to enter.

The following experiment is to determine the identity of an unknown substance and purify it by sublimation. Nicely written introduction. Please cite the source of the cool figure you have immediately above, would you? =Procedure=

Higginbotham, Carol. "Week 6: Sublimation." //Central Oregon Community College//. Web. 5 Nov 2010. .

Williamson, Kenneth L. //Macroscale and Microscale Organic Experiments//. (pp.120-125). 4th. Boston, NY: Houghton Mifflin Company, 2003. Print. this looks good. =Data/Analysis= Unknown: 241-7-15

Starting weight: 0.051g

Yield of purified substance: 0.003g or 6%

Melting Range: 112.8-113.5⁰C

- Sublimating the unknown caused large, white needle crystals to form over 30 minutes that. typo? I think that "that" needs to be removed. When the melting range was measured it very closely matched the 114.3 melting point of acetanilide; indicating a near pure substance. - Although aluminum foil was placed around the sublimating apparatus to keep the glass warm so that condensation would not occur, it was not kept warm enough to keep crystals from forming on the side of the glass instead of solely on the cold finger. Adding an additional layer of aluminum foil and wrapping it around the filter flask as tightly as possible was of no use. Large crystals formed on the side of the glass that were hard to collect, which gave a low percent yield as some crystals were lost. Also, the impurities collected at the bottom of the glass created a large, solid puddle, which is where a large portion of the original weight came from. Your identification of the stuff is correct. Melting temperature data looks good. Yield is super-small! If it is at all wet inside your apparatus you can lose a lot of product because it simply dissolves--and in solution it won't sublime properly. Washing the glassware immediately before use can actually contribute to this problem, since small amounts of water get caught on the surface of the glass. It's not noticable as you handle the glass but once you warm up the flask it vaporizes and then recondenses on the coldfinger. =Conclusion= The experiment performed was the process of sublimating a substance in order to purify the substance by separating it from molecules that may contaminate the substance. The purity of the substance was determined by finding an experimental melting point and comparing it to a theoretical melting point. The experimental melting point of the unknown substance was between approximately 112.8°C-113.5°C. Since the theoretical melting point for Acetanalide is approximately 114.3°C, the data collected shows that the substance contained from the experiment was pure in regards to the melting point and shows that the unknown most closely matches the characteristics of Acetanilide. Percent yield, however, is very low for any experiment. Because the low value of yield, there were either a large amount of impurities in the original unknown or, many sources of error occurred as stated above. explain to me briefly how the melting point data indicates the purity.

You two seem to have missed several of the Post-lab Questions. How does this happen over and over?

This report earned the following scores for: format (2/2) style (1.5/2) data (3/3) quality of the result (0.5/1) quality of the reported data (1/1) conclusion (1.5/2) error analysis (1/1) post-lab Q (0/2) for a total of 10.5