OCHEM 2 Qualitative Analysis 1

OCHEM 2 Qualitative Analysis 1 - Qualitative Analysis Lindy...

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Qualitative Analysis Lindy Hilpert Chemistry 0340 Aaron Crandall February 6, 2008
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Introduction The purpose of this experiment was to isolate a solid and liquid unknown from an ether solution, recrystallize the solid, and identify both the solid and liquid based on boiling point, melting point, IR, H-NMR, and C-NMR spectra. Theory Extraction is a way of separating two immiscible compounds through their differences in polarity. Polarity is a measure of how electrons in a bond are shared. The more electronegative and atom is, the more the electrons are attracted to that atom. When on atom in a molecule is more electronegative than the other, the electrons spend more time close to that atom than the other. This results in dipoles, which are partial charges. The more electronegative atom obtains a partial negative charge and the less electronegative atom obtains a partial positive charge. Unequal sharing of electrons creates a polar molecule. If two atoms in a molecule have similar electronegativities, there is an equal sharing of the electrons which means there are no dipoles, and the molecule is non-polar. In extraction, the two immiscible substances are mixed together. When the solution settles, two layers form, one contains the polar molecules (the aqueous layer), and one contains the non polar molecules (the organic layer). They do not combine however, because polar substances cannot dissolve non polar substances and vice versa. One of the layers can then be drained off, separating the polar and non polar components. Kd is the distribution coefficient. It is the ratio of the solubilities of the compound being extracted in the two layers. The higher the Kd, the more organic the mixture. Kd = [organic] / [aqueous]. Another important concept in this experiment is acid base reactions. An acid is a substance that donates and proton, and a base is a substance that accepts a proton. When an acid and base are mixed, the acid donates a proton to the base. The products of an acid base reaction are a salt and water. Recrystallization is a way of purifying non-volatile organic substances. It consists of dissolving the substance to be purified in an appropriate hot solvent. An appropriate solvent will dissolve the substance when it is hot, but not when it is cool. Also, when the mixture of the solvent and substance are cooled, the crystals should reform quickly and neatly. It must also dissolve impurities readily so that they do not become trapped in the purified substance when it is cooling and recrystallizing. The solvent cannot react with the solute by breaking bonds. It cannot chemically alter the solute, only dissolve it, by breaking the intermolecular forces. Its boiling point must be lower than the solute’s melting point so that the solute does not melt and oil out in the solvent. Lastly, it should be volatile so that it can be easily evaporated off of the newly formed crystals. Once the solvent is chosen, it is slowly added to the solvent until it has completely dissolved. Only
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This note was uploaded on 04/07/2008 for the course CHEM 0340 taught by Professor Crandall during the Fall '07 term at Pittsburgh.

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OCHEM 2 Qualitative Analysis 1 - Qualitative Analysis Lindy...

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