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Title : Melting Points of Urea, Cinnamic Acid, and an Unknown by Tarika Arjune Experiment performed: September 16th, 2021 Report Due: September 30th, 2021 Abstract : We performed an experiment to determine the melting point of Urea, Cinnamic Acid, and an unknown to find how pure the substances were as well as identify unknown substances that were provided to us. We also investigated the melting point of mixtures of Urea and cinnamic acid in concentrations of 25:75, 50:50 and 75:25. Objective: The purpose of this lab is to find the melting point of known and unknown substances to determine the purity and identity of the substances, Urea and Cinnamic Acid. Introduction: One of the physical properties of an organic compound is its melting point which is determined by intermolecular attractive forces. Melting points are important to determine because that will help to characterize the sample. If the sample is unknown, then it is recorded to allow for future study of the substance. The range of the melting point for a substance indicates how pure the compound is. An impure substance will melt at a wide range of temperatures while recrystallization of the compound will purify it and cause the melting point range to decrease. After recrystallization, the sample is determined as pure if the melting point does not increase. A critical factor in determining the melting point is that the temperature increase should not be
greater than 1C per minute. To find the melting point, we will be using the Mel-Temp melting point apparatus by crushing our samples and pushing it into the capillary tube and determining when the sample first starts making drops of liquids till all the sample is all gone. The purity of the sample will be determined based on the range of the melting point, if the range is smaller than the substance is mostly pure. The melting point changes a substance from its solid state to its liquid state and is used to identify compounds or determine the purity of the compound. So, by conducting this melting point experiment, we can classify/ identify known and unknown substances as well as determine the pure nature of the substance which in our experiment will be Urea and Cinnamic Acid. Reactions/Schemes/Mechanisms: No Reactions- N/A Materials: Mel-Temp Apparatus: use capillary tubes along with a thermometer to determine melting point Metal spatula: used to crush the sample into a fine powder Sample of Urea: sample used to determine the melting point Sample of Cinnamic Acid: sample used to determine the melting point Unknown Sample: sample used to determine the melting point Watch Glass: used to grind mixtures of Urea: Cinnamic acid on with a spatula Capillary Tubes: pushed open end into the powder, to contain the sample
Table of Physical Constants: Name Structure MW (g/mol) MF bp/mp (C) Density (g/cm3) Solubility (g/L) Cinnamic Acid
g/mol C9H8O2 bp :300 °C (572 °F; 573 K) mp : 133 °C (271 °F; 406 K)
g/cm In water: 500 mg/L Urea 60. g/mol CH4N2O bp: N/A decomposes before it boils mp :132.5- 133
g/cm In water: 1079 g/L (20 °C) 1670 g/L (40 °C) 2510 g/L (60 °C) 4000 g/L (80 °C) Williamson, Kenneth L., and Katherine M. Masters. (2017). Cinnamic Acid. Williamson, Kenneth L., and Katherine M. Masters. (2017). Urea. Procedure: Procedure Observations
- Determined the melting point of a standard substance to calibrate the thermometer *Repeat if does not agree within 1C
- Determine the melting point of pure urea and pure cinnamic acid: Use a metal spatula to crush Urea into a fine powder then push the open end of the melting point capillary into the powder and force the powder down by tapping on the capillary *The solid should be no more than 2-
mm
- If melting temperature is known, then the bath is heated to the temperature 20C below that point *Then slow heat so the rate of heating at the melting point is not greater than 1C per min while the urea is melting Urea : 117 ℃- 129 ℃
- Determine the melting point when the first drops of liquid appear and all solid disappear
- Repeat steps 2-4 but used cinnamic acid instead *If not within 1C then repeat Cinnamic Acid: 105 ℃-118 ℃
- Determine the melting point of Urea and Cinnamic Acid mixture: Make mixtures of urea and cinnamic acid In quantities of 25:75, 50:50, 75: Label them correctly and then mix them respectively Urea: Cinnamic Acid 25:75 => 95 ℃- 112 ℃ 50:50 => 64 ℃- 77 ℃ 75:25 => 70 ℃-117 ℃
- Grind each mixture thoroughly for 1 min on a watch glass using a metal spatula
- Determine the range of melting for three mixtures
- Determine the melting point of 1 or more unknown: Prepare two capillaries of each unknown Unknown # 55 ℃- 85 ℃
- Find the approximate melting point of the first sample by running a fast
- Cool melting point bath just below the melting point and make a slow careful determination with the second sample
Results & Discussion:
% Composition Temperature (Celsius) Mean/Average of Temperature
100% Urea 117-129 123 75:25 70-117 93. 50:50 64-77 70. 25:75 95-112 103. 100% Cinnamic Acid 105-118 111. The results of this experiment showed that substances that are 100% pure have a higher melting point than substances that are mixed. The chart shows that pure Urea has a melting point range of 117-129 C and pure Cinnamic Acid has a melting point range of 105-118 C. The 50: mixture of Urea to Cinnamic acid has a melting point range of 64-77 C. We find the mean or average of the melting point ranges and plot them on to a graph where we see the highest points on the graph correspond to the substances with the purest composition (100%) while the lowest point on the graph is the 50:50 mixture of Urea to Cinnamic Acid.
Conclusion: In conclusion, the melting point experiment was successful in determining the melting points of Urea and Cinnamic Acid as well as determining the purity of the compounds. From the results of the experiment, we have determined that compounds that are 100% pure will have a higher melting point while compounds that have a mixed composition will have a low melting point. We see that a pure substance will have a narrower range of temperatures compared to substances that are not completely pure. A substance with impurities melts at a wide range of lower temperatures than the pure compounds. The impurities in a solids structure will disrupt the intermolecular forces that hold the solid together so a small amount of energy will cause it to change its state. We see this in the results we obtain as pure Cinnamic Acid and pure Urea have higher melting points than mixtures of Cinnamic Acid and Urea. This shows that mixtures of Cinnamic Acid and Urea have weak intermolecular forces holding together it’s solid state so it takes a small amount of energy to allow it to change its state and so it has a low melting point range. References: “Cinnamic Acid.” National Center for Biotechnology Information. PubChem Compound Database, U.S. National Library of Medicine, pubchem.ncbi.nlm.nih.gov/compound/Cinnamic- acid. “Urea.” National Center for Biotechnology Information. PubChem Compound Database , U.S. National Library of Medicine, pubchem.ncbi.nlm.nih.gov/compound/Urea. Williamson, Kenneth L., and Katherine M. Masters. Macroscale and Microscale Organic Experiments. 7th ed., Cengage Learning PTR, 2017.
