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CH 236 Study Guide - CH 236 Final Exam Study Guide This...

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CH 236 Final Exam Study Guide This guide is a list of major points that you should know from each experiment. You should read through all of the technique sections and the experiment sections in your lab manual that are listed in the syllabus. Also use your quizzes as a guide. RECRYSTALLIZATION: An impure solid is dissolved in a solvent at elevated temperatures and then slowly crystallizes out as the solution cools. This crystal framework excludes any impurities, so that a pure solid is obtained. Know the steps in recrystallizing a compound 1. Find a suitable solvent for the recrystallization 2. Dissolve the impure solid in a minimum volume of hot solvent 3. Remove any insoluble impurities by filtration 4. Slowly cool the hot solution to crystallize the desired compound from the solution 5. Filter the solution to isolate the purified solid compound Know the properties of a good solvent for recrystallization. 1. The compound should be very soluble at the boiling point of the solvent and only sparingly soluble in the solvent at room temperature. 2. The unwanted impurities should be either very soluble in the solvent at room temperature or insoluble in the hot solvent. 3. The solvent should not react with the compound being purified. 4. The solvent should be volatile enough to be easily removed from the solvent after the compound has crystallized. When do you need to add decolorizing carbon? -- Used to remove the colored impurities from the sample (decolorizing carbon is very finely divided carbon that provides high surface area to absorb the colored impurities.) What are some methods to induce crystallization if the compound fails to crystallize from the solution? -- 1. Scratching the inner wall of the flask with a stirring rod 2 (removes nuclei-like particles of glass for crystal growth). 2. Add a small crystal of the desired compound – seed crystal – acts as a template Know the difference between crystallization and precipitation. -- Crystallization involves a molecule changing from one physical state to another. Precipitation is different atoms combining into a new molecule that falls out of solution. MELTING POINT: The melting point is the temperature at which the solid state is in equilibrium with the liquid state (a phase transition from solid to liquid). It is a range from the point the solid begins to melt to the point the solid has completely melted and is a clear liquid. To obtain accurate results, the solid must be heated slowly . The melting point is a characteristic physical property of a compound, so it can be used in identification The melting point can establish the purity of a compound —know the characteristics of the melting point of a pure compound and be able to explain the effects an impurity has on the melting point 1. A pure compound has a sharp melting point (range less than 1 ° C) – an impure compound (a) depress the melting
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point and (b) broaden the melting range. (A mixture of two compounds will
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