Determination of the Solid-Liquid Phase Diagram for ...

University of Waterloo

Chemical Engineering Department

ChE101: Chemical Engineering Concepts 2: Laboratory

Experiment #1:

Determination of the Solid-Liquid Phase Diagram for Naphthalene-Biphenyl Using Thermal Analysis

Hilary Lockie 20168263

Performed: May 13th, 2005 Submitted: May 24th, 2005

Group #1 Hilary Lockie

Alvin Wan Adil Habib

1.0 INTRODUCTION

1.1 Objective: The purpose of this experiment is to use thermal analysis to study naphthalene-biphenyl (two combined components) to determine the freezing point of the system at atmospheric pressure. From the experiment, it will be possible to develop the resulting solid-liquid phase diagram.

1.2 Theoretical Principles:

? Gibbs Phase Rule

The Gibbs phase rule is a method of calculating degrees of freedom of a system.

Degrees of freedom are the intensive properties (independent of system size) for the

system. The phase rule is given by the following equation:

DF = 2 + c -

(1)

where: DF = degrees of freedom (intensive properties)

c = number of chemical species

= number of phases in the system

In a system with two components and two phases, like the naphthalene-biphenyl mixture used in this lab, the degree of freedom is 2 (2 + 2 components ? 2 phases).1

? Freezing Point

The freezing point of a solution indicates the point at which a solid begins to form as

the solution cools. This is a determined point that depends on the composition of the

solution.

? Lever Rule

The lever rule provides a method for finding the fraction of liquid and vapor (or solid,

in the case of this lab) in a two-phase, two-component system when the temperature

and pressure are known, as well as the overall composition.

The fraction of the masses of the phases is found through the following equation:

L = xS - xF

(2)

S xF - xL

where: L and S are the mass of the liquid and solid phases, respectively

xS, xL and xF are the mass fractions of the solid phase, liquid phase and total mixture

These are all calculated with respect to one component of the solution.1

? Phase Diagrams

Phase diagrams are graphs that give information on the equilibrium temperature and

pressure for a particular compound. The equilibria occur for the solid-liquid plateau,

liquid-vapor plateau and solid-vapor plateau. In this experiment, the phase diagram is

shown for the solid-liquid equilibrium point, and varies from 100% composition of

naphthalene to 100% composition of biphenyl. Further examples of this particular type of

phase diagram are in the ChE101 lab manual and in the preliminary report questions.

Theoretical phase diagrams can be developed from the following equation:

T f = K f m

(3)

where: Tf is the freezing point depression Kf is a proportionality constant m is the molality (mol solute/kg solvent)1

? Cooling Curves (from the ChE101 lab manual)

Cooling curves have characteristics that are distinct:

(a) The rate of cooling lessens with time, since the cooling process releases heat from

the system. The curve will become less steep.

(b) A plateau occurs at the eutectic temperature (lowest possible for solidification). The

temperature stays constant until all of the liquid has solidified, at which time the solid

finishes cooling.

(c) A sudden dip and rise in temperature can occur from supercooling the liquid. The

solution cools below its normal freezing point and becomes unstable, so solidification is

sudden. This is avoidable by continuously stirring the liquid.

2.0 EXPERIMENTAL 2.1 Apparatus: Apparatus 1: Hot Water Bath

Test tube

Beaker Naphthalene-biphenyl mixture H2O (boiling)

Hot plate

Figure 2.1: Hot Water Bath

Apparatus 2: Controlled Temperature Water Bath

Temperature controlled water bath

Stirrer

Test tube equipped with thermocouple

Temperature regulator Naphthalene-biphenyl mixture

H2O

Figure 2.2: Controlled Temperature Water Bath

Chemical materials include reagent grade naphthalene and biphenyl (See next section for safety data for these chemicals).

2.2 Procedure: Procedure is as outlined in the ChE101 lab manual.

3.0 SAFETY 3.1 Biphenyl

Major handling and hazard issues with biphenyl stem from its organic nature, and its effect on the environment. Biphenyl is toxic to aquatic organisms and can cause long term damage. This means that the chemical should be disposed of in an organic waste container. It is also an irritant for the eyes, respiratory system and skin, so lab workers should be cautious to avoid inhaling the fumes and should wear protective eyewear at all times, and gloves when handling the chemical in potentially hazardous situations (i.e. if the biphenyl is in an open container).

First aid procedures for contact with skin or eyes are to wash the area extensively with large volumes of water. In the case of skin contact, soap is also recommended. In a case of inhalation, remove the person to an area of fresh air. In case of ingestion, wash mouth out with copious amounts of water and contact poison control.

3.2 Naphthalene Naphthalene, like biphenyl, is very hazardous to the environment and should be

disposed of in an organic waste container. It is an irritant like biphenyl, to the eyes, skin and respiratory system. Lab workers should be cautious to avoid inhaling fumes, and should wear protective eyewear and gloves when handling the chemical.

First aid procedures are the same as with biphenyl. Poison control should be contacted in cases of dermal contact, eye contact or ingestion. Area of contact should be washed extensively with water and clothing should be disposed of in a safe manner.

Naphthalene has a flash point of 80?C, so it should be kept away from hot plates and other areas of high heat. In case of a flare-up, the fire can be extinguished with water or a fire extinguisher. The fumes from ignited naphthalene are toxic, so extreme care should be taken to avoid fire.

3.3 Other Hazards Other aspects of the experiment that could be potentially dangerous include the hot

water being used to heat the chemicals and the hot plate. Workers should be cautious to keep hands well away from hot surfaces and mixtures, and should use appropriate handling devices (gloves, test tube tongs, etc.) when necessary.

4.0 RESULTS AND DISCUSSION

4.1 Results:

Table 4.1: Composition of Naphthalene-Biphenyl for Each Test

Mixture

1 2 3 4 5 6 7 8

Mass (g) Naphthalene

17 17 17 17 0 2 6 12

Mass (g) Biphenyl

0 3 7 14 18 18 18 18

Total Mass (g)

17 20 24 31 18 20 24 30

Start Temperature (?C) 85 80 70 60 75 70 60 45

By plotting the freezing curve for each of the eight mixtures (graphs and tables in Appendix C), the freezing point for each mixture can be found. In the graphs in Appendix C, the freezing point is plotted by a horizontal dotted line. In turn, these temperatures are put together to create Figure 4.1.

Table 4.2: Freezing Point of Naphthalene-Biphenyl Mixtures

Mixture

1

2

3

4

5

6

Freezing Point Tf (?C) 80.8 72.5 63.3 51.8 69.3 62.3

%wt Biphenyl

0

15 30 45 100 90

Naphthalene-Biphenyl Solid-Liquid Phase Diagram

7 49.6 75

8 38.0 60

90

Melting Point of Naphthalene

80 70

Liquid Mixture

Melting Point of Biphenyl

Temperature Tf (?C)

60

Solid Naphthalene

50

= Liquid Mixture

Solid Biphenyl = Liquid Mixture

40

TE

Solid Naphthalene

E

and Solid Biphenyl

30 0

10

20

30

40

50

60

70

80

90

100

%wt Biphenyl

Figure 4.1: Naphthalene-Biphenyl Liquid-Solid Phase Diagram

4.2 Discussion: From this diagram, it can be shown that the melting point of naphthalene is

approximately 81?C and that of biphenyl is approximately 69.5?C. These values are quite accurate compared to the sample data given, giving error of only about 1% from the data table. From physical property tables, the actual melting point of naphthalene is 80.0?C1 and of biphenyl is 70?C2. This means the data gathered in the experiment are also precise. The eutectic point was found to be 38?C. This was not as accurate as the melting point temperatures (it was off by 2.3?C from the sample data). This difference can be accounted for by experimental errors.

The difference between the freezing points plotted for 15%, 30% and 45% biphenyl and the solid-liquid equilibrium line is larger than expected. This could be because the experiment was halted prematurely for these mixtures, so the temperature was not as low as it should have been. It is also possible that the compounds did not contain the right percentage of biphenyl (this is discussed in Sources of Error), so the temperature would be accurate for the amount of biphenyl the mixture contained. Mixture 4, in particular, was stopped when the temperature began to drop at 0.1?C per second as opposed to 0.2 - 0.3?C each second (Figure C1.4).

Overall, the results appear fairly precise. None of the values stray too far from the fitted line of equilibrium. This can be used to conclude that there were no extreme errors in calculations or measurements.

4.3 Sources of Error: ? There were a few sources of error in this experiment, but the largest source involved the mixing of chemicals. Each time more biphenyl was added to the compound, some residue was left in the plastic tray. Another aspect of this occurred because, as the experiment progressed, the temperature was lowered. This caused the solution, while in liquid form, to solidify on the upper portion of the test tube. This inhibited the stirring and lessened the volume of the solution that was being cooled to the freezing point. ? To accommodate for solution solidifying at the top of the test tube, it would be useful to use either bigger test tubes or less substance so the test tube does not become overfilled ? and can therefore be completely submerged into the water

bath to above the level of the solution. This way, all of the solution melts each time more solid chemical is added. ? Test tube tongs and a scale closer to the laboratory site would be beneficial for safety concerns. Workers used bare hands (or latex gloved hands) to carry hot test tube, increasing the risk of burns. Carrying chemicals across the room is also potentially hazardous should one trip. A shorter distance minimizes that risk.

5.0 CONCLUSIONS ? The solid-liquid phase diagram for naphthalene-biphenyl is shown in figure 4.1. It reflects closely the sample data given, and the freezing points of both components are precise to reference numbers. ? Error from loss of substance accounted partially for variances in points from the melting (freezing) point line. ? The eutectic point calculated from this lab is approximately 38.0?C. Melting point of biphenyl was calculated to be about 69.5?C and that of naphthalene was about 81?C.

6.0 REFERENCES 1. Felder and Rousseau, Elementary Principles of Chemical Processes (Third Edition),

Wiley, USA: 2000. 2. "Biphenyl," , 27 Apr 2005, accessed 23 May 2005.

 ................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download