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EXPERIMENT 3 FREEZING POINT DEPRESSION FPD_pdb.wpd CHM 1041 Molar Mass Determination in PDB January 9, 2002 I. THEORETICAL DISCUSSION Background: The following describes the changes in phase of a substance as heat is lost at a steady rate at constant pressure, and attempts to explain these changes by understanding conditions at the molecular level. The illustration of a hypothetical cooling curve of a pure substance aids this discussion (see figure 1 ). Figure 1 Hypothetical Cooling Curve Note that the gas, liquid, and solid have different heat capacities and cool at different rates. This is responsible for the different negative slopes for cooling of each phase in the above curve. Kinetic energy is the energy of motion and is directly proportional to the absolute temperature. At high temperatures, a stable compound will have enough kinetic energy to exist as independent molecules in the gaseous state. As heat is lost, the temperature and average kinetic energy of the gas decline. A point is reached at which intermolecular attractive forces become competitive with the kinetic energy and the molecules begin to associate in the liquid phase. Here the molecules maintain continuous contact but still retain enough energy to migrate randomly throughout the system. As gas molecules continue to condense to liquid, potential energy is lost as heat and the temperature of the system remains the same until all of the gas has liquefied (this is the boiling temperature of the liquid). Now with only liquid present, the liquid cools and the temperature drops until the average kinetic energy becomes low enough for the intermolecular forces to lock the molecules in place within a solid crystal lattice; the molecules are no longer free to wander. As solidification (freezing) continues, the temperature of the system remains constant as potential energy converts to heat and is released (this the freezing or melting temperature of the liquid). When conversion to solid is complete, the temperature of the system then declines as the solid itself cools. The molecules within the lattice vibrate in place, and as the temperature falls a minimum in vibrational energy is approached (absolute temperature, 0 K , or -273°C). Figure 2 Cooling Curve of Liquid Solvent The Freezing Point of Pure Liquid: This experiment will focus on the region in which the cooling liquid approaches and achieves the freezing point (The boxed area on cooling curve in figure 1 is seen close up in figure 2 ). At the freezing point both liquid and solid are present. If the system becomes thermally insulated from its
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2 surroundings, that is, heat can neither enter or leave the system, a state of equilibrium will be established. Here the number of molecules moving from solid to liquid is the same as the number of molecules moving from liquid to solid. This means that the relative quantities of solid and liquid present will remain constant. Liquid
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