Lab 

CHEMISTRY

Melting Points of Selected Organic Compounds

Objectives

1.  Determine Melting Point of selected known organic compounds

2.  Prepare a Thermometer Calibration Plot

3.  Determine the Melting Point and Identity an Organic Unknown Solid

4.  Use Mixed Melting Point to Confirm Identity of Unknown Solid

Identifying an unknown compound can be a tedious and exacting task. In identifying a compound, a chemist often measures several physical properties  and observes a few chemical properties of the compound. The reason for determining several chemical and physical properties of the compound is that it is quite possible for two different compounds to have a few physical and/or chemical properties in common; but it is unlikely for two compounds to have the same numerical values for all chemical and physical properties.

Useful physical properties that are often utilized by chemists in identifying an organic compound include color, odor, physical state, melting point (mp), boiling point (bp), and density (d).

Physical constants are numerical values measured at the time certain physical properties are observed.  As long as the physical constants are determined under standard conditions (temperature, pressure, etc.), they are constant, and therefore useful in determininh the identity of unknown substances.

Chemists regard a table of physical properties and physical constants to be extremely helpful in identifying unknown compounds. There are a number of reference works that contain tables of physical properties and physical constants of compounds. One of the most common is the Handbook of Chemistry and Physics published by the Chemical Rubber Company. 

Most Libraries have this in their Reference Section

 If the physical properties of an unknown compound are identical to the physical properties of a compound listed in the tables, the two compounds are probably the same.  Thus a colorless liquid compound with a melting point of 5.5 deg.C,  a boiling point  of 80.1 deg.C, and  a refractive index of 1.5011 is likely to be benzene, although we might want to make a few more observations to be sure.

Crystalline solids are composed of atoms, ions, or molecules in a highly ordered geometric pattern . The atoms, ions or molecules are held in their positions by intermolecular forces (dispersion, dipole-dipole, and/or H-bonds).  When a pure crystalline solid is heated, the atoms ions or molecules vibrate more and more rapidly until at a definite temperature the thermal motion of the particles becomes great enough to overcome the forces of attraction. Then the atoms ions or molecules enter a more random and mobile state, the liquid state. The melting point of a solid is defined as the temperature at which the liquid and solid phases are in equilibrium.  Useful information as to a compound's identity and its purity can often be obtained from its melting point.

The freezing point of a liquid is the same temperature as the melting point of its solid.

Most melting points are determined as capillary melting points, which can be done quickly with a small amount of sample. A capillary melting point is defined as the temperature range over which a small amount of solid in a thin walled capillary glass tube first visibly liquefies.  Review this Tutorial for  a demonstration of how to obtain a melting point using capillary glass tubes.

A solid is said to melt sharply if the melting point range is 1 - 2 deg. C. A pure solid will generally melt sharply because the forces of attraction between its particles are the same. However, the presence of a foreign particle in a crystal lattice interrupts its uniform structure and the forces of attraction are weakened.  The MP Range is recorded by taking the temperature at which the solid first begins to melt and the temperature when the solid has completely melted.  The Mid-Point of the Range is found by adding the begin melt and final melt temperatures together and dividing by two.  For example, suppose a solid begins to melt at 60 degrees C and is completely melted at 64 degrees C.  The Range and Mid-Point are...

Range  =  60 - 64

Mid-Point  = (60+64)/2 = 62

An impure solid melts at a lower temperature and over a wider range. Thus, a solid's melting point is useful not only as an aid in identification but also as an indication of purity.

Suppose two compounds A and B have identical melting points of 131 deg.C and appear to be identical. We can easily determine whether or not A and B really are the same compound by mixing a small amount of A with B and taking the melting point of the mixture. The melting point of a mixture is called the mixture melting point. If A and B are the same compound, the mixture melting point will be the same as the melting point of pure A or pure B.  If A and B are not the same compound, one will act as an impurity in the other and the mixture melting point will be lower and more spread out (perhaps 120-5 deg.C in this case) than the individual melting points of pure A or pure B. 

Some solids pass directly from the solid state to the gaseous state without first liquefying; this phenomenon is called sublimation . The temperature at which sublimation occurs is called the sublimation point. Other solids decompose rather than melt. The temperature at which a solid decomposes is the decomposition point. While both sublimation points and decomposition points are useful helping to identify compounds, neither is very helpful in establishing the purity of a compound.

 

Note 1: Physical properties are those properties that can be observed or measured without changing the composition of the substance.

Note 2: Chemical properties are those properties observed only as a substance is changed chemically into another substance.

Resources Needed

Links Other
Melting Point Experiment
Tutorial - this tutorial shows how melting points are obtained in the laboratory. Review before working through the simulation.

Part A - Determine Melting Points of Known Compounds

Determine the Melting Point of the following compounds and compare with accepted values.  You will need to look up the accepted or true melting point of each compound.  

Compound Observed MP Accepted MP
Benzoic Acid    
Napthalene    
Acetanilide    
4-Nitrotoluene    

The Melting Point apparatus consists of a hotplate, digital thermometer, beaker of vegetable oil, and capillary tube containing the organic sample solid.  In Figure 1., the capillary tube contains benzoic acid.  Notice Figures 2 and 3 below for views of the actual lab apparatus used in the laboratory for determining melting points.

You can select different compounds from the Compound Menu.    The HEAT button on the hotplate will begin heating the oil in the beaker at a constant rate.  You can monitor the temperature with the digital thermometer.  When the solid in the capillary tube begins to melt it will change color.  Record the temperature at which the solid begins to melt, and the temperature at which all of the solid has melted as indicated by a complete change of color.  Click the Off button to stop the heating.   Notice the temperature begins to decrease, and the liquid will eventually re-crystallize to a solid at its melting/freezing point.  Run the melting point determination 3 times for Benzoic Acid and calculate an Average Melting point.

Repeat this procedure for each of the 4 known organic compounds.  Show your average calculation for each.

Figure 1.

Figure 2 shows what the apparatus setup would like in the laboratory.

Figure 2.

And Figure 3 shows a close-up view of how the capillary is attached to an ordinary thermometer.

Figure 3.

Part B - Prepare a Thermometer Calibration Plot

Frequently thermometers are not equally accurate at all temperatures and must be calibrated to obtain accurate melting points.  To calibrate a thermometer, the melting point of pure solid samples are measured.  Next a graph is made of the difference between the observed and the expected (true) mp versus the measured (observed) mp. 

Difference = Observed MP - True MP

Here is an example of a Calibration Graph. This graph seems to show a linear relationship between the Difference and true MP, but in most cases the graph will not be linear.  In your graph you should connect the dots instead of trying to draw a straight line between all of the points.

When reporting an experimental mp, corrections should be made using the Calibration Graph.  The correction is made by adding the appropriate value from the graph to the observed mp.

Example:   An observed mp of 190 C would be reported as 188 C .

observed temp + correction factor = 190 + (-2) = 188.

In this experiment you should plot the true MP of each of the 4 solid unknowns on the X-Axis and the Difference between True and Observed MP on the Y-Axis.  Use the mid-point if a melting point range was recorded for the Solid known compounds.  

Melting points for the Unknown and the Mixed Melting Points should be corrected using the Calibration Graph.

Part C - Determine Melting Point of Unknown

Determine the MP of the unknown using the method in Part A.  Record the range (if any) and calculate the mid-point of the range. Use your Calibration Plot in Part B to determine the corrected melting point.  Report the observed and corrected melting point for the unknown.

Part D - Use Mixed Melting Point to Confirm Unknown

In this part of the experiment the melting point of the unknown mixed with a small amount of a known compound is determined.  If the melting point behavior of the mixture is the same as that of the unknown, then you have confirmed the identity of the unknown.  If the melting point of the mixture differs substantially from that of the pure compound or if the range is too wide, then the unknown is not identified.  Do a mixed melting point of the unknown with each of the known compounds by selecting from the menu at bottom right of apparatus.  One trial for each mixed melting point is sufficient.  Record the melting point range, mid-point, and corrected MP for each mixture.

Identify the unknown.

 

Michael W.  McClure
Professor, Chemistry
Hopkinsville Community College
Phone:  270-707-3863

 

Copyright 2007  M.W. McClure, All Rights Reserved

No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form, by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of the publisher.