Experiment #1: Determination of the Solid-Liquid Phase Diagram for Napthalene-Biphenyl Using Thermal Analysis
To apply thermal analysis to the two-component system, naphthalene-biphenyl at atmospheric temperature. The analysis will be represented by a solid-liquid phase diagram (freezing point diagram).
Phase Equilibria and the Gibbs Phase Rule
This experiment is conducted in order to study a condensed system (solid-liquid) at constant temperature (atmospheric temperature). It should be noted that the atmospheric pressure is unlikely to be the equilibrium pressure for the system. However, equilibria in condensed systems are not very sensitive to pressure.
The freezing point is determined at the specific temperature at which a liquid solution of two component, A and B, begin to separate off to a solid form. Between a range of pure A and pure B, the freezing point will vary (as shown by figure A in the appendix).
The figure contained in the appendix has been developed from the ChE 101 Lab Manual . Based upon this figure the following can be observed.
Freezing point curve spans from B to E and then to A based upon the concentration of A and B within the solution.
The eutectic point is labeled at point E. The eutectic point represents when the solid precipitates of A and B as well as the liquid solution are all present in the mixture. Thus this is the lowest freezing point for the two solids. Note that A and B in liquid form are miscible.
Solutions of A and B can exist in equilibrium with solid B within the area of BEC. Solutions of A and B can exist in equilibrium with solid A within the area of AED.
The area above the freezing point curve of BEA represents the liquid mixture of A and B.
The area below CED represents where the 2 solid phases stand with no equilibrium present.
Based on the purpose of keeping the system pressure constant, the Gibbs Phase rule now accommodates this system in the form
DF = C – P + 1
DF – degree of freedom
C – number of components
P – number of phases
The basis of the phase diagram begins with recording a series of T vs. t plots at different A-B compositions. The thermal analysis of the T vs. t plot can be done in three sections.
Change in slope – as the solution is cooled, precipitates start to form. The more precipitates forming causes a diminishing rate of cooling.
A plateau or horizontal line can represent the precipitate of a eutectic solution or a pure solution. System continues to be cooled below the freezing point of both components at a diminished rate up until the eutectic point. From there the temperature is kept constant in order to solidify all the liquid portion remaining.
Supercooling is a phenomena that can be represented by a temperature dip and then a sharp rise. Supercooling occurs when a liquid (still in its liquid state) is taken past its freezing point. Solidification occurs very suddenly.
Since the melting points of naphthalene and biphenyl are dependent on their respective weight compositions, the lever rule can be used to find correlate specific points along the diagram. Though there will be a small degree of error due to human observation, the lever rule is a much faster alternative to methods involving mass balances.
Procedure is given by the ChE 101 Lab Manual .
1. Experiments are divided into two groups. Group #1 will do test tube #1 (mixtures 1-4). Group #2 will do test tube #2 (mixtures 5-8). Tube #1 should contain 17g of naphthalene and tube #2 should contain 18g of biphenyl. Fit tube with thermocouple and stirrer and play in the melting bath until solids have melted. Allow tube and bath to reach equilibrium and record the melting temperature. 2. Set the controlled temperature bath to approximately 3oC below the melting temperature of the contents. Start stopwatch and start...
References:  Enns, K., ChE101 Laboratory Manual, p. 17, 19, (University of Waterloo, Waterloo, 2006)
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