Determination of freezing points of pure substances with Cobra4 TEC

Transcription

1 Determination of freezing points of pure substances TEC Related concept Crystallization point, Gibbs free energy, enthalpy, entropy, heat of fusion, freezing point depression. Principle When a pure substance is heated or cooled, the temperature of it does not continually increase or decrease when it undergoes a change in the state of aggregation. Instead of this, and despite the continuing external supply or removal of heat respectively, the temperature of it remains constant until the change in phase has been completed. This can be used for the determination of the melting point of the substance. Equipment 1 Cobra4 USB-Link Cobra4 Sensor-Unit Chemistry Software measure Cobra Hotplate Magnetic Stirrer, 5 l, 230 V Support rod, stainless steel, 50cm Right angle clamp Universal clamp Magnetic stirring bar, 8 mm, cylindrical Separator for magnetic bars Thermocouple, NiCr-Ni, C Beaker, 600 ml, high Melting point determination tubes Test tubes 160/16 mm Teclu burner, universal, air reg Safety gas tubing with couplings Lighter for natural / liquified gases Spoon, nickel plated steel Mortar with pestle, 150 ml, porcelane Silicone oil, 500 ml Naphthalene, white, 250 g Palmitic acid for synthesis, 250 g Additional equipment 1 PC, Windows XP or higher Fig. 1: Experimental set-up. P PHYWE Systeme GmbH & Co. KG All rights reserved 1

2 TEC Determination of freezing points of pure substances Safety instructions When handling chemicals, you should wear suitable protective gloves, safety goggles, and suitable clothing. Please refer to the appendix for detailed safety instructions. Tasks 1. Measure the change in temperature during the change in the state of aggregation of naphthalene and palmitic acid. 2. Determine the melting points of the three substances from your measurements. Set-up and preparation - Prepare the used furnace crucibles as follows: Take an ordinary test tube (16/160 mm) and heat it 60 to 70 mm under the edge in a teclu burner flame. Pull the heated test tube out, so that a thin walled tube is developed. Close this tube by melting in a distance of about 60 mm from the taper region (see fig. 2). You have to prepare one of these vessels for every analysed sample. - Set up the experiment as shown in Fig 1. - Connect the thermocouples to the T1 and T2 inlets of the Cobra4 sensor unit Chemistry. - Connect the Cobra4 Sensor-Unit Chemistry to the Cobra4 USB Link. - Attach it to the retort stand with the holder for Cobra4 and a right angle clamp. - Place the beaker on the magnetic heater. - Pour some silicone oil into the beaker and put a stirring bar into it. - Attach the thermocouples to the retort stand using two right angle clamps and universal clamps. - One of the thermocouples has to immerse in the silicone oil. - Fill a sample of the pulverised substance into the lower area of a furnace crucible. - Put the second thermocouple in a melting point tube and place it into the sample. Fig. 2: Used furnace crucible. Procedure - Start the PC and connect the Cobra4 USB Link via a cable with the computer. - Boot the experiment Determination of freezing points of pure substances (experiment > open experiment). The measurement parameters for this experiment are loaded now. - Adjust the stirrer to a medium stirring speed (Note: Do not allow the magnetic stirring bar to hit against the measuring electrode.) - Switch on the heater and adjust it to a temperature of approximately 120 C. - Start the measurement with. - When the change in the state of aggregation has occurred and the temperature remains constant terminate the measurement with. 2 PHYWE Systeme GmbH & Co. KG All rights reserved P

3 Determination of freezing points of pure substances TEC - Send all data to measure (see Fig. 3). - Save the measurement (File > Save measurement as ). - Fig. 4 shows the graph for the measurement of the freezing curve of naphthalene. Fig. 3: Window which appears after measurement. Fig. 4:Crystallization curve of naphthalene Theory and evaluation The melting point of a substance is the temperature at which it changes state from solid to liquid. At this point both phases are in equilibrium. When considered as the temperature of the reverse change from liquid to solid it is referred to as the freezing point. For most substances, melting and freezing points are approximately equal. The melting point depends on pressure and is usually specified at standard pressure. For the determination of melting points there exist many laboratory techniques. A basic melting point apparatus consists of an oil bath with a transparent window and a simple magnifier. A sample of the analysed substance is placed in a thin glass tube which is partially immersed in the oil bath. The oil bath is heated and with the aid of the magnifier the disappearance of the individual crystals at a certain temperature can be observed. The melting point of a pure substance is always higher and has a smaller range than the melting point of a mixture of the substance. That s why melting points can be used to characterise and identify organic and inorganic substances. For this one can find the melting points of many compounds as open data. The change from solid to liquid requires the heat of fusion. At the freezing point, the enthalpy and the entropy of the material are decreasing. Freezing happens when the Gibbs free energy of the solid becomes lower than the Gibbs free energy of the liquid. When a melt cools, the oscillation amplitude of the individual particles decreases with the decreasing temperature. It is hereby possible for the melting temperature of the substance to be gone below without freezing taking place, giving what is called a supercooled melt. Spontaneous solidification then raises the temperature back to the melting temperature of the substance, whereby the particles again cluster to a crystal structure. The heat energy that was required to melt the substance is hereby released as latent P PHYWE Systeme GmbH & Co. KG All rights reserved 3

4 TEC Determination of freezing points of pure substances heat of fusion. This counteracts a further cooling effect and leads to a constant temperature during the following course of the process. The amount of heat that is released during freezing is hereby exactly the same as the amount that was required for the sample to melt. When no further heat of fusion is released because the freezing process has been completed, then cooling again occurs due to a thermal interaction To determine the freezing point of the analysed substance one can use the crystallization curve. The curve for naphthalene is shown in figure 4. With you can open the data table and read the temperature value at the maximum of the curve at about 155 sec. In the example the determined value is 80.0 C. Data and results Naphthalene: determined value: 80.0 C literature value: 80.2 C Palmitic acid: determined value: 62.3 C literature value: 62.6 C Notes In general, cooling curves are easier to record than heating curves. In the latter, inhomogeneous temperature distribution in the liquid phase always leads to greater fluctuations in the temperature values than those of cooling curves Deviations of the temperatures obtained from the caused by the NiCr-Ni thermocouples used. These can show higher or lower temperatures than the actual ones because of their production against a different type of temperature measurement device known to show temperatures accurately. This experiment can also be carried out using Cobra4 sensor unit 2 x Temperature, NiCr-Ni (article number ) instead of the Cobra4 sensor unit Chemistry. Disposal Do not dispose napththalene via household waste. To avoid environmental contamination collect the waste in suitable and closable labelled container for recovery or disposal. Appendix Hazard symbol, signal word Naphthalene - Hazard statements H351: Suspected of causing cancer H302: Harmful if swallowed H410: Very toxic to aquat- Precautionary statements P273: Avoid release to the environment P281: Use personal protective equipment as required 4 PHYWE Systeme GmbH & Co. KG All rights reserved P

5 Determination of freezing points of pure substances TEC Attention Palmitic acid ic life with long lasting effects - - P : IF exposed or concerned: Get medical advice/attention P PHYWE Systeme GmbH & Co. KG All rights reserved 5

6 TEC Determination of freezing points of pure substances Room for notes 6 PHYWE Systeme GmbH & Co. KG All rights reserved P