experimental part of org.chem.
physical properties of chemicals: melting point, and boiling point
background
if you were asked to describe a friend, most likely you would start by identifying particular physical characteristics. you might begin by giving your friend’s height, weight, hair color, eye color, or facial features. these characteristics would allow you to single out the individual from a group.
chemicals also possess distinguishing physical properties which enable their identification. in many circumstances, a thorough determination of the physical properties of a given chemical can be used for its identification. if faced with an unknown sample, a chemist may compare the physical properties of the unknown to properties of known substances that are tabulated in the chemical literature if a match can be made, an identification can be assumed (unless chemical evidence suggests otherwise).
the physical properties most commonly listed in handbooks of chemical data are color, crystal form (if a solid), refractive index (if a liquid), density , solubility in various solvents , melting point, sublimation characteristics, and boiling point. when a new compound is isolated or synthesized, these properties almost always accompany the report in the literature.
the transition of a substance from a solid to a liquid to a gas, and the reversal, represent physical changes. this means that there is a change in the form or the state of the substance without any alteration in the chemical composition. water undergoes state changes from ice to liquid water to steam however, the composition of molecules in all three states remains h2o.
the melting or freezing point of a substance refers to the temperature at which the solid and liquid states are in equilibrium. the terms are interchangeable and correspond to the same temperature. the melting point is the temperature at equilibrium when starting in the solid state and going to the liquid state. the freezing point is the temperature at equilibrium when starting in the liquid state and going to the solid state.
melting points of pure substances occur over a very narrow range and are usually quite sharp. the criteria for purity of a solid is the narrowness of the melting point range and the correspondence to the value found in the literature.
impurities will lower the melting point and cause a broadening of the range. for example, pure benzoic acid has a reported melting point of 122.13°c benzoic acid with a melting point range of 121-122°c is considered to be quite pure.
the normal boiling point of the liquid will be the temperature when the pressure of the vapor is equal to the atmospheric pressure (1 atm or 760 mm hg).
if the external pressure vary, will the boiling point vary?
a liquid will boil at a higher temperature when the external pressure is higher and will boil at a lower temperature when the external pressure is reduced.
the change in state from a gas to a liquid represents condensation and is the reverse of boiling. the temperature for this change of state is the same as the boiling temperature but is concerned with the approach from the gas phase.
just as a solid has a characteristic melting point, a liquid has a characteristic boiling point. at one atmosphere, pure water boils at 100°c, pure ethanol (ethyl alcohol) boils at 78.5°c, and pure diethyl ether boils at 34.6°c.
the vapor pressure curves shown in fig. .1 illustrate the variation of the vapor pressure of these liquids with temperature. one can use these curves to predict the boiling point at a reduced pressure.
for example, diethyl ether has a vapor pressure of 422 mm hg at 20°c. if the external pressure were reduced to 422 mm hg, diethyl ether would boil at 20°c.
fig (1) diethyl ether, ethyl alcohol (ethanol), and water vapor pressure curves.
objectives
1. to use melting points and boiling points in identifying substances.
2. to use sublimation as a means of purification.
melting points are determined for three reasons.
1. if the compound is a known one the melting point will help to characterize the sample in hand.
2. if the compound is new then the melting point is recorded in order to allow future characterization by others.
3. the range of the melting point is indicative of the purity of the compound, i.e an impure compound will melt over a wide range of temperatures. recrystallization of the compound will purify it and the melting point range will decrease.
melting point determination
procedure
1. 0.1 g of unknown solid and place it on a small watch glass. record the code number of the unknown on the report sheet (1). (the instructor will weigh out a 0.1-g sample as a demonstration take approximately that amount with your spatula.) carefully crush the solid on a watch glass into a powder with the flat portion of a spatula.
2. obtain a melting point capillary tube. one end of the tube will be sealed. the tube is packed with solid in the following way:
step a press the open end of the capillary tube vertically into the solid sample (fig. 15.2 a). a small amount of sample will be forced into the open end of the capillary tube.
step b invert the capillary tube so that the closed end is pointing toward the bench top. gently tap the end of the tube against the lab bench top (fig. 15.2 b). continue tapping until the solid is forced down to the closed end. a sample depth of 5–10 mm is sufficient.
step c an alternative method for bringing the solid sample to the closed end uses a piece of glass tubing of approximately 20 to 30 cm. hold the capillary tube, closed end down, at the top of the glass tubing, held vertically let the capillary tube droping through the tubing so that it hits the lab bench top. the capillary tube will bounce and bring the solid down. (fig. 2).
melting point apparatus:
fig(3) test tube melting
point apparatus
fig(4) thomas-hoover uni-melt melting point apparatus
fig( 5) apparatus for melting fig(6) thiele apparatus.
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table 15.1
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melting points of selected solids
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solid
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melting point (°c)
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use
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acetamide
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82
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plasticizer stabilizer
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acetanilide
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114
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manufacture of other medicinals and dyes
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adipic acid
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152
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manufacture of nylon
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benzophenone
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48
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manufacture of antihistamines, hypnotics, insecticides
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benzoic acid
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122
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preserving foods antifungal agent
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p-dichlorobenzene
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54
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moth repellent insecticidal fumigant
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naphthalene
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80
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moth repellent insecticide
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stearic acid
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70
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suppositories ointments
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boiling point determination
caution!
the chemicals used for boiling point determinations are flammable. be sure all bunsen burner flames are extinguished before starting this part of the experiment.
1. obtain an unknown liquid from your instructor and record its code number on the report sheet (7).
clamp a clean, dry test tube onto a ring stand. add to the test tube approximately 3 ml of the unknown liquid and two small boiling chips. lower the test tube into a 250-ml beaker which contains 100 ml of water and two boiling chips. adjust the depth of the test tube so that the unknown liquid is below the water level of the water bath (fig 8).
fig(8)for determining
the boiling point.