Boiling Point Experiment

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Organic Chemistry: CHP 225
Thomas Hsieh

Experiment 2: Boiling Points

o Date experiment was performed: September 16, 2011

o Objective: To determine the boiling point of organic compounds such as tert-butyl alcohol, sec-butyl alcohol, n-butyl alcohol, and an unknown.

o Principle: Boiling point is the temperature at which gas and liquid phases coexist in equilibrium. At this temperature, the vaporization rate and the condensation rate are equal. The liquid and vapor reach a state of dynamic equilibrium: liquid molecules evaporate and vapor molecules condense at the same rate. The boiling point of a liquid is the temperature at which its vapor pressure equals atmospheric pressure. While a gas is forming, the vapor pressure within the gas bubbles equals the external pressure. However, if the vapor pressure does not equal the external pressure, the bubble may burst or corrupt. If the vapor pressure within a bubble is lower than the external pressure, the bubble will corrupt. If the inverse is true, the bubble will burst. Maintaining an equilibrium pressure between vapor pressure and external pressure allows the boiling point to be maintained at a constant temperature. However, vapor pressure and the external pressure vary slightly so the boiling point temperature will vary slightly as well. Due to the variations in pressure, the temperature observed during boiling is taken to be an average value. At this equilibrium temperature the heat added to the system goes into pulling the molecules further apart from each other. The strength of the attraction forces that hold a liquid together such as dispersion forces tends to increase with increased molecular weight. This is due to the fact that larger atoms have larger electron clouds which are easier to polarize. Polarization also increases the boiling temperature because polarity creates strong attractive forces within the molecules of the substance, requiring more energy to break those bonds. The shape of the molecule also affects the strength of dispersion forces: long, skinny molecules (like n-pentane tend to have stronger dispersion forces than short, fat ones (like neopentane). This is due to the increased surface area in n-pentane that allows the molecules to make contact over the entire surface area. With mass being equal, branching molecules have lower boiling points than chain molecules because again the surface area of branching molecules makes the breaking of bonds easier than long chain molecules. The temperature does not rise during the phase change. At this equilibrium state, the heat absorbed and the heat released are balanced, thus creating a constant temperature for boiling point. Boiling continues with no change in temperature until all liquid molecules have converted to gas phase. The boiling point is one of the characteristic properties of a substance and can be used to determine the identity of a substance. In this experiment, capillary method and reflux method are used to determine the boiling points of organic compounds.

o Procedure:

Part A: Capillary Method

1. Obtain a 150ml beaker half full and fill it with tap water. Place the beaker on top of wire gauze supported on a ring stand. Connect a Bunsen burner to the gas faucet on the bench top and light the Bunsen burner. Continue heating the water bath. 2. Obtain a capillary which has one end closed, cut a 2.5 piece from this end. Place the capillary in a small test tube, with the open end downward. Fill the test tube with tert-butyl alcohol to the level covering the whole capillary. 3. Place the test tube, in the water bath; the level of the water bath must be above the level of tert-butyl alcohol in the test tube so that it will heat all of the tert-butyl alcohol. Clamp a thermometer in the water bath. 4. Moderately heat the water bath with a small Bunsen burner. Occasionally, stir the water bath with a stirring rod, so that the water bath will heat...
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