Melting Point

Abstract

Finding the melting point of an organic substance is a practical and efficient way for scientists to identify an unknown substance or determine a known substance's level of purity. When organic substances are mixed together in varying degrees they take on a melting characteristic that is lower and broader than in its pure form. This property was manipulated in the lab to observe the various melting points of Naphthalene and Biphenyl when the percentage of composition was altered. A eutectic point of 45 oC at 52% Naphthalene was determined from the class data. An unknown crystalline substance was identified to be Benzophenone by mixing and finding its melting point with known substances.

Introduction

Organic compounds are categorized by their physical properties and/or chemical reactions. Naphthalene is classified as an aromatic hydrocarbon that consists of two benzene rings that are fused together. Its physical appearance is that of white crystal flakes. Solid crystalline materials consist of a three dimensional structure that is called the crystal lattice. Organic crystalline solids are held in fixed positions by cohesive forces that are fairly weak which gives them a low to moderate melting point (mp). When an organic crystalline solid is heated it absorbs the energy and the molecules begin to vibrate, eventually overcoming the cohesive forces. Once the molecules begin to move past one another, the structure starts to collapse and there is an appearance of fluid (lower end of melting range) which continues until the substance is entirely liquid (upper end of melting range) â„-.

Melting points are used in identifying unknown substances and assessing the purity of a compound. Pure substances like 100% Biphenyl will have a sharp melting temperature (68.93 oC). A compound that is mixed or has even a small amount of impurity will have a lower melting point and broader melting range. This characteristic is beneficial in determining a substances' purity. The closer mixture is to the actual melting point and exhibits a narrow melting point range, the better its purity. There is one general exception, a eutectic mixture. This mixture is proportioned so that the melting point is at the lowest possible level and exhibits a sharp melting point.

Melting point depression (lower mp) aids in identifying an unknown compound. By first determining an unknown's melting point it can then be mixed with known compounds whose melting points are close to that of the unknown. If the two compounds are the same then melting point depression will not be exhibited.

Experimental

Materials

The experimental procedures for this lab are found in the PSU Lab manual І and Organic Laboratory Techniques 3rd edition Ñ-. Known melting point values were obtained from the CRC Handbook of Chemistry and Physics 88th edition 4. All chemicals were obtained from the PSU stockroom. Electronically melted samples were performed using a "Mel-Temp" electronic device. Manual melting temp procedures were performed using a Thiele Tube apparatus set-up.

Part A

Two samples of pure naphthalene and biphenyl were finely ground in a spot plate and then placed in a standard closed end capillary tube Ñ-. The samples were then attached to a thermometer and placed in a Thiele tube filled with oil. The tube was heated by a Bunsen burner at approximately 1 oC per minute. Melting point temperatures were recorded at the first indication of fluid and then when the sample had turned completely liquid. Two trials were completed on each sample І.

The melting point of 10% naphthalene / 90% Biphenyl mixture was determined by electronic instrumentation. Two samples were prepared and placed in closed end capillary tubes and then inserted into the "Mel-Temp" device beginning at 30oC and set at 22 volts to ensure gradual heating; approximately 1oC per minute. Two trials were completed on each sample. The results were then categorized and combined with other data sets collected by the laboratory students. This data was then used to find the eutectic point of the two substances І.

Part B

Unknown crystalline substance "A" was assigned for identification. Two samples were prepared using the same technique in part A. The melting point was then measured on a Mel-Temp device beginning at 30 oC and set at 22 volts. Two known substances with melting points close to the unknown sample were obtained and mixed 50% unknown/50% known. The melting points were then measured using the Mel-Temp device and then compared to see if any results exhibited a melting point depression; which can be used to rule out one from the other І.

Results and Discussion

Part A

The melting point of Naphthalene was determined to be 78.8-80.5 oC (table1). This range is within 1.7 degrees of the CRC value of 80.26 oC. This discrepancy can be accounted for by error in the thermometer reading or by heating the material too quickly. No discoloration was observed when heating any of the samples which rules out decomposition of product. The standard laboratory thermometer was not calibrated prior to testing which can account for lack of precision. It was also observed that using the Thiele tube provided difficulty in controlling the rate of temperature; several times the flame had to be removed to slow the heating process down.

Table 1: Melting Point of Naphthalene Samples

Composition

(% Weight) Trial No. Low(oC) High(oC)

100 N 1 79.0 80.0

100 N 2 78.5 81.0

Mean 78.8 80.5

N = naphthalene

CRC: 80.26 oC

The melting point of Biphenyl was determined to be 65.8 - 68.0 oC (table 2). This range is within 3.13 degrees of the CRC value of 68.93 oC. No discoloration of product was observed and samples were approximately the same size. Procedures followed were identical to that of the pure Naphthalene. It was determined that the lack of precision in the data was due a combination of error in technique and the thermometer not being calibrated; post calibration showed the lab thermometer off by approximately 2 degrees.

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