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MELTING POINT OF ORGANIC COMPOUNDS

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Introduction

NAME : THAMARAI A/P RAJENDRAN ID NUMBER : 09ALB07214 LABORATORY 1A : ATOMIC STRUCTURE, BONDING AND PERIODICITY COURSE : BIOTECHNOLOGY (YEAR 1 SEM 1) EXPERIMENT 7: MELTING POINT OF ORGANIC COMPOUNDS. TITLE: Melting Point of Organic Compounds. OBJECTIVES OF EXPERIMENT: The objective of this experiment is to identify unknown organic compound by melting point depression method. Practice use of melting point apparatus by measuring melting point of some pure organic compound. THEORY AND BACKGROUND: The melting point of organic solid can be determined by introducing a tiny amount of the substance into a small capillary tube, attaching this to the stem of a thermometer centerd in heating bath, heating the bath slowly, and observing the temperatures at which melting begins and is complete. Pure samples usually have sharp melting points, for example 149.5°C-150°C or 189°C-190°C; impure samples of the same compounds melt at lower temperatures and over a wide range, for example; 145°C-148°C or 187°C-189°C. The contaminant that depresses the melting point and extends the melting range may be an indefinitely characterized resinous material or it may be a trace of a second chemical entity of melting point either higher or lower than that of the major component. Under equilibrium conditions (no super cooling) the temperature at which a pure solid melts is identical with that at which the molten substance solidifies or freezes.

Middle

The most efficient procedure is to range all the samples in order of increasing melting point and determine the apparent melting point of each, beginning with the sample of lowest melting point. The apparent melting point of ice was determined by inserting the bulb of the thermometer into ice slurry. The observed melting points and corrected melting points of the standard compounds as listed earlier were recorded. A correction chart was constructed by plotting the observed melting points against the corrected melting points. The resulting points were connected, and that is the calibration curve of your thermometer. Part 2: Melting points of known compounds and mixture a) Benzoic acid b) Urea c) 10 mol per cent benzoic acid and 90 mol per cent urea d) 90 mol per cent benzoic acid and 10 mol per cent urea Part 3: Identification of an unknown compound by the mixed melting point method Six of the compounds listed below were obtained from the instructor, which will be designated by number only. Their melting points were determined and the values found were compared with those given in the table. The probable identities of the unknowns were decided and were tested in the following way: a small sample of the reference compound was secured from the shelf and its melting point was determined.

Conclusion

In a sample that contains a mixture of two compounds, each component usually depresses the melting point of the other, giving an observed melting point range that is lower and broader than the melting point of either component. CONCLUSION: When a solid substance is heated, typically it will melt; that is to say, at some temperature the solid will begin to liquefy and by some slightly higher temperature all of the solid will have become liquid. The melting point (actually melting point range) of a compound is then defined as the temperature at which an observer can first see liquid forming from the solid to the temperature where the last particle of solid has become liquid. For example, the melting point of pure sucrose (table sugar) is 185o-186oC. This means that as a small sample of sucrose is slowly heated some of the crystals begin to liquefy at 185°C and all of the crystals have become liquid by 186oC. Sometimes only the second number (completely melted) is reported as the melting point. In general, this is not a good idea and should be avoided. Melting points are usually determined by placing one or two milligrams of the material to be tested into a melting point capillary, and heating the capillary and a thermometer together, and observing over what temperature range the material melts. The melting point capillary is a thin-walled glass tube, about 100 mm in length and not more than 2 mm in outside diameter, sealed at one end.

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