Experiment 1 - Melting Points

 

Introduction

 

The melting point of a substance (the temperature at which a substance melts) is a physical property that can be used for its identification.  It is a measure of the amount of kinetic energy (heat) that must be supplied to the particles of the substance in order to overcome the intermolecular forces (such as Van der Waals, dipole-dipole, and H-bonding) that confine them to the solid state.  The determination of melting points is particular­ly important to organic chemists, since they generally work with solid molecular compounds that have low melting points (below 300°C) and which can be conveniently measured.  Organic com­pounds are used in this experiment for the same reasons.

Melting points are also used as an indication of purity.  Substances melt throughout a tempera­ture range in which both the solid and liquid phases of the substance coexist in a state of equilibri­um.  Above that range, the substance exists only as a liquid, and below it only the solid phase is present (no wetness observed).  The extent of this temperature range is a measure of the purity of the substance; that is, impure samples of compounds have lower and broader temperature ranges of melting.  If a pure sample of a compound melts from 110 to 111.5°C, the addition of substantial amounts of another compound might result in a new melting point range from 85 to 100ºC.

An identical or near identical temperature range of melting is not, in itself, proof of the sameness of two organic chemical samples.  There are thousands of solid organic compounds that melt within any relatively short temperature range; overlap of melting points is therefore inevitable. If an unknown solid sample is believed to be a certain known compound, it is a relatively simple task to prove or disprove this belief by mixing the known and unknown together in relatively equal quantities. A 50:50 mixture will either be a pure sample of the known compound or a highly impure sample of the known compound.  The melting point of the mixture will be identical to that of the known compound in the first instance or lowered and much broadened in the latter.  This identifica­tion/confirmation procedure is referred to as the determination of a "mixed" melting point.

 

The Determination of Melting Points

 

Melting points will be determined by using one of the Meltemp units (Figure 1-1 on the next page). The Meltemp units must always be kept upright.  A videotape on the use of the meltemp is available in room 207.  Do not remove the thermometers from the Meltemp units.  Place a small quantity (1/16 inch in tube) of the solid to be melted in a capillary tube (labeled melting point tubes).  Tap the closed end of the tube on the desk, clean the outside, and drop the tube (closed end down) down a section of glass tubing (see TA) to compact the solid in the bottom or closed end of the tube. Place the tube loaded with the sample into the sample holder of the Meltemp with the closed end down. The crystals can be ground up in a clean, dry mortar and pestle if they are too big to fit into the capillary tube.

 

If the melting point of the sample is unknown or unavailable, a fast run with the Meltemp set at 70 volts can be done to obtain an approximate melting range.  A more precise value can then be obtained by heating the Meltemp more slowly at a lower voltage (about 2-4ºC/min.) in the vicinity of the known melting temperature.


 

 Figure 1-1:   The Meltemp apparatus.  Melting point capillary tubes are placed (closed end down) in the slots directly in front of the thermometer and are viewed through the magnifying lens.  Up to three samples can be viewed at once.  The heating rate of the Meltemp is adjusted with the dial.  A dial setting of 70 volts causes a rapid temperature rise and a setting of 30 to 50 volts causes a slower rise.  The apparatus responds slowly to changes in the dial setting.  At the conclusion of a run, turn the dial to 0 volts so that it can cool.   DO NOT REMOVE THERMOMETER FROM MELTEMP FOR ANY REASON. 

 

Refer to the Meltemp heating curves at the end of the experiment to select the right voltage (Figures 1-2 and 1-3). Record the temperature the crystals begin to melt (crystals will look wet) and the temperature at which the substance becomes a clear liquid. This is the melting range. The thermometers in the Meltemp equipment are not calibrated and may be off as much as ±3ºC.  Consequently, do not expect the melting points obtained with the Meltemp apparatus to be identical to those listed in the Table shown on page 3. The calibration of the Meltemp thermometer will be checked using the melting range obtained for pure urea or pure cinnamic acid. Use the same Meltemp for all your measurements.

The Experiment

 

Prelab Work: Answer the prelab questions at the end of this write-up on a piece of loose leaf paper (not in lab notebook) after you have read the experiment. A quiz will be given at the start of the period covering the introduction and this experiment in the laboratory manual, any lab lecture material from last week or this week, and the prelab questions. View the videotape on using the Meltemp in room 221 or 207.

Laboratory Notebook: Be sure to read the section on the laboratory notebook in the introduction of the laboratory manual. All data, calculations, observations, and conclusions should be recorded directly in the laboratory notebook. Be sure to save the first two pages of the notebook for a table of contents. Results for unknowns are reported by filling a report sheet found at the end of the experiment and giving the sheet to your TA or stockroom (216).

 

CAUTION:  All the used chemicals for this experiment should be placed into the bottle marked “Waste Organic Acid Solids.”  Used capillary tubes should be thrown in the broken glass container.  Avoid contact with these chemicals; some are irritants or are toxic.  Wash your hands when fin­ished.

Exp. Notes: If possible, use the same thermometer/meltemp for all your work in this lab.

 

Melting Ranges of a Number of Organic Compounds

Compound

Description/Uses

Melting Point (ºC)**

Vanillin

Natural vanilla flavoring

81-82

Dibenzofuran

Minor constituent of coal tar

81-83

Acetamide

Solvent, plasticizer, stabilizer

79-81

Azelaic Acid

Rancidification product of fats

106-107

 

containing oleic acids

 

o-Toluic Acid*

Substituted benzoic acid

103-105

m-Toluic Acid*

Substituted benzoic acid

108-110

Resorcinol

Disinfectant

109-110

Benzoic Acid

Found naturally in berries

122-123

Urea

Used as fertilizer

132-133

Cinnamic Acid

Oxidation product of cinnamon oil

132-133

Acetylsalicylic Acid

Aspirin

135-136

Maleic Acid

Manufacture of resins

137-139

Benzilic Acid

A carboxylic acid

150-153

Adipic Acid

Used to manufacture nylon

152-153

Citric Acid

Sour taste of citrus fruits

153-154

Mannitol

Manufacture of radio condensers

167-170

Tartaric Acid

Used in soft drinks, cream of tartar

168-170

Itaconic Acid

A dicarboxylic acid

166-167

Succinic Acid

Manufacture of dyes, perfumes

187-190

Ascorbic Acid

Vitamin C

190-192

Cholic Acid

Emulsifies fats in the intestine

198-200

*           Note that these two compounds are isomers.

**         These melting points may vary according to supplier.  Be sure to run a "mixed melting" range in identifying unknowns.

Melting Points and Mixed Melting Points of Compounds Having Similar Melting Points

Determine the melting points of pure samples of cinnamic acid and urea as well as a 50-50 mixture of the two, and record the data in your lab notebook.  The three samples can be run simultaneously in the Meltemp (see prelab questions for voltage settings).  To prepare a 50:50 mixture, mix equal small portions of these compounds (estimate the amount of each, about 0.02-0.06g). Grind the mixture to a fine powder mix in a clean, dry mortar and pestle provided (Wash the mortar and pestle with soap and water, rinse with tap water, distilled water and acetone in hood to clean and dry).

Is the melting point of the mixture different? If it is different explain why. If the melting point of urea differs from the value listed by more than as ±3ºC see your TA. You may have to repeat the melting point of urea and then run succinic acid and check the temperature of ice water to check the calibration of your thermometer.

 

The Unknown

To identify your unknown you must first measure its melting range.   Tap a small amount of your unknown into two different capillary tubes.  Just a few crystals is adequate.  You may need to grind some of your unknown into a powder if it is too coarse to fit into the capillary tube.  Find the melting point range of the pure unknown substance by first quickly determining an approximate melting range and then doing a slow, careful melting range with the second capillary tube you prepared. Make sure the meltemp is below 70°C before starting the first melting range and 10-20ºC below the compound’s melting range before doing a slow careful melting range. Run cinnamic acid again with the unknown if the calibration was off.

Using the melting ranges listed in the table on the previous page determine which possible compounds are within ±8ºC of your unknown's melting range.   Make a 50:50 mixture of the unknown with each of these possible compounds and carefully grind each mixture to a uniform powder in a clean, dry mortar and pestle.  Take the melting range of each mixture.  Record all of your results on your report sheet.  The mixture where the melting point does not change indicates that the two compounds in the mixture are the same. Clean the mortar and pestle and return to the storage location in the lab. You should now be able to identify your unknown and complete the report sheet for this experiment. Record all results and answers to questions in your laboratory notebook.

 

Prelab Questions 

1.  Two samples have the exact same melting points.  Are they the same compound? How could you tell for sure?

2.  You have two samples of mannitol. One melted between 168º-169º and the other melted between 161º-168º. Which sample has the greater purity?  Why?

3. Which would be the best way to determine the melting point of a compound? Why?

     a.     Slowly run two very precise melting points.

     b.     Run a very precise melting point and then run a fast one to double check your work.

     c.     Run a quick melting point for an approximate melting range, then a slow precise one.

4.  Risk Assessment: What are the safety hazards and precautions for this experiment?

5.  How much sample needs to be placed in the capillary tube to determine a melting point?

6.  How do you place the capillary tube in the Meltemp?

7.  Explain how a "mixed" melting point can be used to confirm the identity of a compound.

8.   Use the second Meltemp voltage chart to determine the voltage which would give a 2°/min increase in temperature for pure urea or cinnamic acid.

9.   Name three intermolecular forces that hold organic molecules together as solids.

 

Figure 1-2  Heating Curve for Meltemp Temperature vs. Time

 

Figure 1-3  Meltemp Heating Curve Degrees vs. Volts

 

 

 

 

 

 

 

                                                                                                               


 

 

Name______________________________________________ Dana ID_________________

Course:  230L  235L  (circle one)   Section Letter____ Locker #______ Date_________

 

                                         CHM 230L/235L - Melting Points Experiment

                                                    Determination of Melting Points

 

Experimental:

A. Determination of the melting points of pure cinnamic acid, pure urea, and a 50:50 mix.

                                                                    Melting Range

Compound                                       Start                        Finish

            Cinnamic Acid             _____________ ºC_____________ ºC

            Urea                            _____________ ºC_____________ ºC

            50:50 Mixture               _____________ ºC_____________ ºC

 

 

B.    Determination of Identity of an Unknown

 

        Melting Range of Unknown __________________________ ºC

        Mixed melting points (50:50 mix of unknown with compounds of similar melting ranges from table on page 1-2):

                        Mixture                                                                        Melting Range

        Unknown    &  ________________________________  ____________ – ___________ ºC

        Unknown    &  ________________________________  ____________ – ___________ ºC

        Unknown    &  ________________________________  ____________ – ___________ ºC

        Unknown    &  ________________________________  ____________ – ___________ ºC

 

Identification of Unknown ___________________________________________________

for Unknown #____________

 

 

Turn in this completed report sheet to the box in the prep stockroom window (room 216) well before the next lab period.