Experiment (1) Melting Point Determination PDF

Experiment (1) Determination of Melting Points Purpose: The purpose of this experiment is to determine the melting points of organic compounds to identify compounds and check the purity of compound. Theory: - The melting point (m.p)°C is the...

Experiment (1) Determination of Melting Points Purpose: The purpose of this experiment is to determine the melting points of organic compounds to identify compounds and check the purity of compound. Theory: - The melting point (m.p)°C is the temperature at which the binding forces between molecules are overcome to change the compound from a solid to a liquid. - The nature and strength of these intermolecular forces are responsible for the observed differences in melting point. In general, if the forces are strong, the melting point will be high, and if they are relatively weak, the melting point will be low - The melting point is the temperature at which the first crystal begins to melt to the temperature at which melting is complete. - Melting point range: The interval between the temperature at which a solid sample just begins to turn to liquid and the temperature at which the entire sample becomes liquid. or range of temperatures in which the first crystal starts to melt until the temperature at which the last crystal just disappears. Melting begins when the crystal structure of the sample begins to “slump,” or form liquid drops. The factors that affect the melting point 1- Molecular weight Melting points are higher for higher molecular weight compounds. The reason the melting point increases with the weight is that it takes more energy  to separate larger molecules from a crystalline structure than it takes to separate smaller ones. 2- -Impurities Impurities decrease the melting point. contaminants normally lower the melting point and broaden the melting point range Pure samples usually have sharp melting points, for example (149.5-150) °C, impure samples of the same compounds melt at lower temperatures and over a wider range, for example (145-148)°C. 3- The nature of the organic compounds Ionic compounds have high melting points, While the covalent compounds have a lower melting point than ionic compounds, because of the strength of this electrostatic attraction. Breaking up an ionic crystal lattice requires considerable energy as a result, ionic compounds have high melting points. Moreover, some covalent compounds have polar molecules in which one end is more electronegative than the other, such polar compounds have a higher melting point than nonpolar molecules. Procedure: 1. Take a capillary tube of length 5-6cm. seal its one end by inserting the end of the capillary tube horizontally into a Bunsen flame for a few seconds. 2. Take a small quantity of the compound whose melting point is to be determined on a porous plate and bash it with a spatula. 3. Introduce the powdered compound in the capillary tube by introducing the open end of the capillary tube into the powdered compound and gently rotating it. Gently tap the capillary tube against the porous plate so that the compound sinks into the closed end. Repeat the procedure of introducing and tapping three to four times to fill the capillary tube to a level of 2-3 mm. Remove any trace of the powder stuck on the outer surface. 4. Carefully place the capillary tube of solid into one of the slots in the heating block of the Melt Station. 5. Start data collection. On the Melt Station, turn the control knob to the Rapid Heat area. The red LED will come on, indicating the Melt Station is heating. Rapid Heat will warm your solid sample at a rate of >10°C/min. 6. When the temperature is within about 10°C of the expected melting temperature of a sample, turn the control dial to that temperature, which will slow the heating rate to ~1.5°C/min. Carefully observe your sample. 7. *It is essential to heat slowly near the melting temperature in order to evenly distribute heat. If the sample is heated too rapidly, the resulting melting will occur over a wide range of temperatures. 8. Record the melting range, which begins when the sample first starts to melt and ends when the sample is completely melted. 9. Prepare additional samples to test by repeating the steps. Observe the temperature of the heating block in the meter screen. After the heating block cools to a suitably low temperature, place new samples in the Melt Station and repeat the melting point testing procedure as needed. 10. Enter your data in the experimental data table. Refer to the reference table to identify your unknown(s). Questions: 1. Why is this method not used for finding the melting points of inorganic compounds? 2. Why could the rate of heating influence the melting point? Lab Report Form and Reference Tables Experimental Data: sample Melting point range Compound Identity 1 2 Reference Table: compound Melting Point Range (C) Benzoic acid 121-123 Acetylsalicylic acid 134-136 Salicylic acid 158-160