experiment 1 kjgf

7/28/2019 Experiment 1 kjgf

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[1]

[2]

[3]

[4]

EXPERIMENT 1

SOLID LIQUID PHASE DIAGRAM

Important: bring a formatted 3.5 floppy diskette/USB flash drive for this laboratory you will

need it to save your data files!

Introduction

The relation of cooling curves to phase diagrams form the basis of thermal analysis, an

important technique for constructing phase diagrams. In the solid-liquid phase equilibrium chosen

for study here, the two components, although miscible with one another in the liquid phase, are of

limited solubility in one another as solids. Thus, we can consider them as pure solids, plus a two-

component liquid. Such systems exhibit an eutectic temperature at which the three phases can

coexist in equilibrium at a fixed pressure.

Solid-liquid equilibria differ from liquid-vapour equilibria in that they are essentially

independent of pressure changes on the order of a few atmospheres, owing to the small molar

volume change associated with fusion. This is a consequence of the Clapeyron equation:

We shall be concerned here with temperature-composition diagrams atp = 1 atm.

If the liquid solution behaves ideally, the solubility of each component in the liquid

depends on temperature, according to:

where:

XA andXB are the mole fractions of components A and B, respectively,

HA and HB are the heats of fusion of components A and B, respectively, and

TA and TB are the melting points of the pure components.

Equations [1] and [2] can be represented in the following phase diagram:


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Figure 1.1

The eutectic composition (XE) and eutectic temperature (TE) are given by the intersection of

the two liquid curves. With the aid of graph (a) in Figure 1.1, we can predict the general nature of

the cooling curves in a system of this kind. The curves in graph (b) are plots of temperature against

time obtained when liquid solutions of various compositions are allowed to cool.

When a liquid consisting of pure A is cooled, the temperature falls until solid A begins to

form. The temperature then remains constant until solidification is complete, where upon it fallsagain. It is said that the curve shows a thermal arrest. When a liquid having the eutectic

composition is cooled, the behaviour is similar in that a thermal arrest is obtained.

However, when a liquid of some other composition for example, X1 (see graph (a)) is

cooled, solid A begins to form at temperature T1. This tends to deplete the liquid of component A,

so that its composition passes throughX2,X3, ... and the temperature falls as long as solid A alone

continues to come out of solution.

The abrupt change in slope, which occurs when solid A begins to form, is called a break.

When the composition of the solution finally reachesXE, solid B begins to form together with solid

A and the two solids continue to separate from solution at the temperatureTE until no liquid remains,

and thus an arrest occurs.

The binary solid-liquid phase diagram for the naphthalene-diphenylamine system will beconstructed from cooling curves. Several mixtures of different ratios of the two components will

be melted, and temperature versus time curves will be plotted as the mixtures cool. The

temperatures at which these breaks and arrests occur are plotted as a function of composition of the

mixtures to obtain the phase diagram.


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jumbo magnetic stirrer

ice water bath

to Labpro

temperature probe

stir bar

plexiglasscontainerand lid

medium test tube

600 mL beaker

hotplate

Figure 1.2

7. Click on on the Logger Pro software. This will provide a real time plot of thechanging temperature in the sample. The collection will continue for a maximum of 1000

seconds. However, if you observe a distinct arrest or break in the pattern, you may

terminate the acquisition and begin a new run. Note:keep the water bath cold, especially

for the later runs! Remember that the pure compounds and the eutectic mixture should have

only one arrest! Be sure to pull out the stir bar before dumping waste!

8. Save a file for this run by exporting the data as a text file. Excel can later be used to process

this data. Make sure to use file names that will clearly distinguish your data sets.

9. Find the arrests on the curves for the pure compounds, the breaks on all mixtures, and the

eutectic arrest for two of the latter.

Calculations1. Extract the break and arrest temperatures from the cooling curves. Print out plots of your

cooling curves (Excel can be used). Convert all temperatures to the thermodynamic

temperature scale (i.e., Kelvin) and use these units for all of the calculations.

2. Determine the mole fraction,Xnaphthalene, for all of the mixtures. Determine the mole fraction

of diphenylamine,Xdiphenylamine, for runs 6, 7 and 8.


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3. Using the break and arrest temperatures of these mixtures, plot the two limbs of the solid-

liquid phase diagram (i.e., temperature as a function of the mole fraction of naphthalene).

Runs 1-5 will constitute one limb, and runs 6-8 will make up the other.

4. Draw the liquidus curves, identify the eutectic line and identify the phases present in each

area of the diagram.

5. Determine the eutectic composition and eutectic temperature from the phase diagram.6. Using [1] and [2], plot lnXnaphthalene against 1/Tfor runs 1-5 and plot lnXdiphenylamine against 1/T

for runs 6-8. The plots should be linear with:

slope = -Hi/R and

-slope/intercept = Ti (the melting temperature of the pure component.)

7. Calculate the enthalpy of fusion and the melting point for each component, assuming that

an ideal liquid solution is formed. Compare to literature values by calculating the percent

error forfusHand temperature, as well as the absolute error for the temperature.

Lab Questions

1. A series of Ni-Mn mixtures were prepared and allowed to reach equilibrium at various

temperatures. Use the following data to plot a phase diagram (preferably on an Excelspreadsheet) for the Ni-Mn system. Label the different components and number of phases

in each part of the phase diagram.

Mn-Rich mixtures:

T/(oC) 1260 1200 1150 1100 1050 1000

w(Ni)sol 0.00 0.04 0.08 0.13 0.22 0.45

w(Ni)liq 0.00 0.07 0.12 0.18 0.29 0.45

Ni-Rich mixtures:T/(oC) 1050 1100 1150 1200 1250 1300 1350 1400 1450

w(Ni)sol 0.58 0.64 0.70 0.75 0.80 0.85 0.90 0.96 1.00

w(Ni)liq 0.54 0.62 0.68 0.73 0.78 0.83 0.88 0.94 1.00

2. What is the composition of the solid solution in equilibrium with a liquid mixture of Ni and

Mn at 1000 oC? What is this point designated as?

References

1. Atkins, Peter and Julio de Paula. Physical Chemistry. 7th

ed. New York: W. H. Freeman,2002. 144-148.

2. Shoemaker, David P., Garland, Carl W., and Joseph W. Nibler. Experiments in Physical

Chemistry. 6th ed. New York: McGrawHill, 1996. 215-221.

3. Silbey, Robert J., and Robert A. Alberty. Physical Chemistry. 3rd ed. Wiley, 2001.

Chapter 6, Section 6.9.

experiment 1 kjgf

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