Melting Point Measurement: Determination of Identity and Purity of Organic Compounds PDF

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Lab Report on Melting Point Measurement. It is to determine the identity and purity of an Organic Compound....

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Experiment #1 Melting Point Measurement: Determination of Identity and Purity of Organic Compounds Dawson College, Department of Chemistry and Chemical Technology Organic Chemistry, 202-BZF-05, section 00010, fall semester, 2021

Amine Ouachani, 2034815

Introduction When molecules enter a change of state, the strength and tightness of the intermolecular forces also get altered. In solid compounds, the molecules are tightly held close to each other through Van der Waals forces. Therefore, the movement within a solid compound is quite restricted. In liquid compounds, the molecules are held less tightly to each other, thus giving more free movement to the molecules, all the while staying fairly close to each other. Whenever a compound goes from solid to liquid, the process is referred to as "melting". In order to do so, heat must be applied to the solid, which will loosen the intermolecular forces by increasing the energy of the vibrations of the solid molecules. Hence, the molecules will be less stranded, consequently becoming more free to roam around, just like in a liquid compound. The stronger the intermolecular forces, the more heat must be applied in order to move the molecules out of their fixed positions. Since temperature measures the amount of heat, it will also increase accordingly. The temperature that measures the amount of heat that must be put in order for a solid to melt is referred to as "melting point". Molecular symmetry also plays a role in determining the amount of heat that must be put in order to break out from fixed positions. The melting point of pure solids have been measured experimentally throughout the years, as it is impossible to measure the melting point strictly through theoretical analysis. Pure organic solids usually melt between 25°C to 350°C. However, the melting point is determined by a range between two temperatures, as it is common for solids to have multiple temperatures within a certain range at which the solid is still melting. This occurs because of the fact that heat takes a certain amount of time before fully penetrating into the solid, on top of the fact that pure substances also contain small impurities. Nevertheless, mixing two different pure substances together will cause a melting point depression, meaning that the start of the melting point of the mixed solution will be lower than both of the pure substances’ melting point starts. This is because of the fact that since they are composed of different molecules, they fit less well when held together because of the different molecules’ shapes and sizes. This causes the bonds to be loosened, therefore decreasing the amount of heat it takes in order to displace the molecules from their fixed positions. Moreover, when mixing two different solids, the melting point is widened. When mixing the same pure solids together, no change occurs to the melting point range. The objective of this experiment is to find the identity of an unknown chemical by determining its melting point. In order to do so, we will be first determining the melting point of the unknown and matching it to a known substance that has a similar melting point. Then, we will be mixing those substances together. If the melting point stays the same, the unknown chemical’s identity is the same as the known chemical. If the melting point is altered, then the chemicals are different.

Procedure Refer to laboratory manual.

Data and Results Table 1 : Melting Point Measurement Of Different Compounds Name Of Compound Adipic Acid Benzoin Unknown compound

Experimental Melting Point Range (°C) 152-153 135-137 122-123

Theoretical Melting Point Range (°C) 152-153 136-137 -

Table 2 : Melting Point Measurement Of Different Solutions Solution Adipic Acid + Benzoin Unknown compound + Benzoic Acid Unknown compound + Benzamide

Melting Point Measurement (°C) 125-151 122-123 104-105

Identity of the unknown compound : Benzoic Acid Error Analysis : Careless preparation of a solution can lead to either a solution that is wrongly mixed or a solution that is poorly mixed. Both of them can lead to the wrong identification of the unknown compound, as the first scenario would cause another substance than the one desired to be mixed with the unknown compound, therefore interfering in the accurate representation of the melting point of the desired known compound with the unknown compound. The second scenario would lead to the conclusion that the melting point of the unknown substance is the same as the mixed solution while in reality, the compound was never mixed with the known substance in the first place. The human eye is also unfortunately not an accurate tool in order to determine the exact melting point range, as it is sometimes difficult to determine precisely when a substance is starting to melt as well as finishing to melt. This is why running multiple experiments, as we did, is of extreme importance, as it reduced the margin of error. Moreover, running the melting-point apparatus at 1°C/min rather than 10°C/min for each trial should’ve been more emphasized, as the reading of the exact melting point is more precise when the temperature is going up more slowly. Additionally, the amount of substance put inside each tube affects the temperature at which the substance melts. The more substance there is, the more heat it takes in order to melt the solid. Therefore, another source of error is the fact that the tubes can either carry too much solid (which would higher the melting point range) or not enough solid (which would lower the melting point range).

Answers to Questions Referring to the data obtained from the experiment, 1. Provide theoretical justification for the identity of your unknown. The measurement of the melting point range of the unknown compound was found to be between 122°C-123°C. Therefore, comparing the melting point range to the closest known substances’ melting point ranges shown in the table of page 3 of the lab manual, the unknown compound could either be Benzoic acid (literature melting point of 121°C -122°C) or Benzamide (literature melting point of 128°C -129°C). Mixing the unknown compound with Benzamide drastically reduces the melting point range to 104°C-105°C, thus indicating that the compound is not Benzamide. However, mixing the unknown compound with Benzoic acid leads to the melting point range being approximately the same, at 122°C-123°C. Hence, we can conclude that the molecules of the unknown compound fit well with the molecules of Benzoic acid, consequently not affecting the tightness of the intermolecular forces nor reducing the melting point range, on top of widening it. Thereupon, the unknown compound’s identity is Benzoic acid. 2. Is your unknown sample pure or not? Provide a brief explanation. Yes, the unknown sample is pure. The melting point range of the unknown sample was found to be between 122°C-123°C. If the unknown sample was impure, then the melting point range would be much wider. Additionally, the solution between the unknown and Benzoic acid would have given a drastically different melting point range than the literature melting point range of Benzoic acid, which is between 121°C-122°C.

Conclusion The identity of unknown chemical no. 14 was found to be Benzoic acid since its melting point range (122°C-123°C) matches that of Benzoic acid (121°C-122°C), on top of the fact that when mixing the two compounds together, the melting point range stays similar (122°C-123°C). This indicates that the molecules fit well together, therefore not affecting the intermolecular forces by reducing their stiffness, making the melting point range start at a lower point as well as wider. Systematic sources of error include the fact that the melting point apparatus was set at 10°C/min for the majority of the experiment, which reduces the preciseness at which the human eye can read the exact temperature at which the solid melts. The results of the experiment agree with the theory that if two different compounds are mixed, the melting point range will be lower than both of the pure substances as well as wider, as shown by the mixing of Adipic acid with Benzoin as well as the unknown chemical (Benzoic Acid) with Benzamide.

References Lab manual of experiment #1 (Melting Point Measurement) (Pavan M. V. Raja & Andrew R. Barron), (March 21st 2021, 11). 2.1: Melting Point Analysis. [HTTP]. Available : https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Physical_Methods_in_Chemistry_and_Na no_Science_(Barron)/02%3A_Physical_and_Thermal_Analysis/2.01%3A_Melting_Point_Analysis [September 26th 2021]....