The melting point is defined as the temperature at which the solid is in equilibrium with its liquid, and this characteristic is very unique, so a substance can be determined by the melting point. Determination of the melting point is very important technique in many areas of chemistry especially, in organic chemistry area because the melting point is really significant in order to identify the purity and the identity of a substance. Measuring the melting point is a fast and cost-effective technique which remains a strong link to the vast pre-instrumental chemistry literature, so the technique is still used for measuring purity of organic and pharmaceutical compounds. For that reason, determination of the melting point is more than just a classroom exercise in the organic chemistry lab. * Objective
1- In this experiment we will familiarize ourselves with the technique of melting point determination and its relevance to determining the purity of the crystalline solid.
2- We will determine the melting points of two different pure solids, urea and trans-cinamic acid, two compounds with similar melting points.
3- We will then determine the melting point ranges of onemixtures of these two substances, and plot the eutectic point diagram to generate a melting point pattern for the two solids.
* Experimental procedure
1- Melting point apparatus
2- Glass tubing
3- Capillary melting point tube
4- Very small, finely powdered pure urea, pure cinnamic acid and three mixtures .
1- Take about 50-100 mg of thin, finely powdered sample and fill a capillary melting point tube with the sample by thrusting the open end of the tube into the powder several time .
2- Vigorously tap the seal end of the tube on the table or drop the melting tube, closed end down, through the length of a glass tubing onto the bench top .
3- Insert the tube into the melting point apparatus and set up the melting temperature for the heating apparatus .( follow the steps of how to use the melting point apparatus ).
4- Make sure the apparatus’s electricity is “ON” first, and then push the “SET” button firmly. While holding the button pressed, press the increase “ “ several times to set primary temperature to the desire one.
5- Press the “Start” button, and wait for the yellow “plateau” light to turn on. Then push “Start” again for Ramping. Keep your eyes on the sample to note the start and end of the melting rang.
6- Dispose of the sample in the capillary tube in its designated waste container.
* Results and analysis
1- Fill out the information in the table below.
Substance| Initial Melting Temp.°C| Final Melting Temp.°C| Melting range| Melting Point| Mole Fraction| Pure Urea| 123| 133| 10| 128| 1|
Pure Cinamic Acaid| 125| 135| 10| 130| 0|
50urea-50c.a%| 92| 102| 10| 97| 0.7117|
2- Calculate the mole fraction of urea and cinamic acid in the three samples ( show all calculations ). * Number of moles of Urea= wt / Molar wt=5060.055 = 0.832 mol (x) * Number of moles of C.A = wt / Molar wt=50148.1660.337 mol (y) * Mole of fraction of Urea=(xx+y)=0.8320.832+0.337=0.7117
3- Sketch a graph of the eutectic point diagram using the mole of fraction calculated above as X-axis and the melting point as Y-axis.
4- Urea has theoretical melting point of 132.7°C, and C.A has a theoretical melting point of 133 °C. Compare your experimental results to these value and calculate the % error in your experimental melting point values. % error=|theorical-Exprementaltheorical|*100
% error of Urea=|132.7-128132.7|*100=3.54%
% error of C.A=|133-130133|*100=2.26%
* Discussion and calculations
I found out that when pure Urea and pure Cinamic Acid melts , the pure substances have specific melting point because of theirs high purity. whereas when we mixed 50%Urea- 50 C.A% the impurities increased between these compounds because of mixing of their molecules, The higher presence of impurities in substances cause to decrease the bonds to become weaker between their molecules . As a result the melting point decreased .(If the impurities increase the melting point will decrease )see the graph up . we are sure we might did some errors in the experiment maybe when we press the wrong button or we did not keep our eyes accurately on the sample to note the start and the end of the melting range but from %error I believe our experiment was good .