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Boiling Point Determination and Simple Distillation
Experiment Description & Background

Part I
The influence of molecular weight and structure on the boiling point of an organic compound will be determined. Each lab bench will be assigned to one of the four groups of compounds listed in Table 1.1 below.  Each student at that bench will be responsible for determining the boiling point of one of  the compounds in that group using a small scale boiling point apparatus.  After obtaining the experimental data, students at each bench will discuss the experimental boiling point results, and establish any boiling point trends associated with this group of compounds that could be related to molecular structure.  A description of the experimental procedure will be provided.
 

Part II
The percent ethanol content in commercial mouthwashes will be determined.  Students will work individually to distill ethanol from commercial mouthwash using a simple distillation apparatus. Each bench will be provided a specific mouthwash with an unknown concentration of ethanol.  All four students at the bench will work individually (n = 4) to distill the ethanol and determine the experimental percent concentration of ethanol in that mouthwash. The students at each bench will compare their results with each other, calculate a mean and standard deviation and compare it with the ethanol content reported by the manufacturer.  A description of the experimental procedure will be provided.

The molecules of compounds that exist in the liquid state are relatively close together, compared with molecules of gaseous compounds.  The close proximity of molecules in the liquid state allow these molecules to interact via non-covalent interactions ( dipole-dipole, H-bonding, van der Waals forces).  In general, these interactions are favorable and help to hold the molecules together in a defined volume, but still allow free motion or "flow".   Conversely, molecules of a gaseous compound are much farther away from each other and are not confined to a specific volume by non-covalent interactions.  If enough energy (often in the form of heat) is provided to the liquid, the molecules begin to move away from each other by "breaking" the non-covalent forces that hold the compound in the liquid state.  Thus, the boiling point is the temperature range over which enough energy is provided to a liquid compound so that its molecules can separate sufficiently to  transform to a gaseous state by breaking non-covalent interactions. No covalent bonds are broken during a change from the liquid phase to the gas phase.

Figure 1.4:  Simple distillation apparatus (adapted from Aikens et. al, p.147)

The distillation process can be explained in simple terms.  When a compound  in the distilling flask is heated to its boiling point temperature, a phase change from the liquid state to the gas state is induced.  The compound, in the gas phase, moves out of the distilling flask up into the other parts of the distilling apparatus.  When the gas vapors encounter the cold condenser tube (below the boiling point temperature) of the distilling apparatus, the gaseous compound reverts back to the liquid phase and drips into the collection flask. One might assume that distillation of a 50:50  mixture of  two components, say ethanol (bp 78°C) and water (bp 100°C),  would follow a similar phenomenon, where the ethanol would distill off first at 78°C, and the water would distill at100°C.   However, this is not what is observed.  The H-bonding interactions between the ethanol molecules and the water molecules prevent a "clean" distillation from occuring.  Instead, what occurs is the formation of an azeotrope, a mixture of the compounds that co-distills in a specific molar ratio at a temperature different from the boiling point of either of the individual components. Boiling point-composition curves, which plot the specific molar ratio against azetrope temperatures,  have been constructed from experimental data for a variety of compound mixtures.  The boiling point-composition curve for ethanol-water mixtures is shown in Figure 1.5.

Interpretation of the graph in Figure 1.5 reveals that the azeotrope boiling point temperature is approximately 88°C when the molar ratio of ethanol and  water in the gas phase is 50:50. At 75% water and  25% ethanol, the azeotrope temperature is closer to 95°C.  Conversely, at 95% ethanol, 5% water, the azeotrope temperature is approximately 78°C.  It is clear from the experimental data represented in the boiling point-composition curve in Figure 1.5 that complete separation of ethanol from water is not possible using simple distillation.  However, mixtures concentrated in one component can be acheived using this technique.  Consider commercial mouthwash, the ethanol-water mixture that will be used in this experiment.  Heating commercial mouthwash to ~78°C will result in an azeotrope that has an ethanol-water molar ratio of 95%:5%. A distillate collected at or around this temperature, will contain mostly (95%) ethanol.

Figure 1.5:  Boiling Point-Composition Curve for  Ethanol-Water Mixtures (taken from Feiser & Williamson, pg. 70)