Transcript Document

Priya
Roll no.00-121
BSc. Chem (H)
2nd year
To draw a phase diagram for solid solution
consisting of 2-Naphthol and Naphthalene
Capillary tubes
Bunsen burner
Spatula
Heating block
Electronic weighing machine
Watch glass
2-Naphthol
Naphthalene
These compounds were selected as their
size difference was within 15%, no appreciable
electronegative difference was present between the two
compounds and the crystal structure of each compound in
the solid solution was same.
Capillary method.
This method was used due to the fact that very small quantities
of chemicals were consumed. Also to verify whether the trend followed
during heating curves was same as that of the trend during the cooling
curves,i.e., to check if the phase diagram obtained was of the same trend
on reversing the cooling curve method.
The phase rule is a qualitative treatment of systems in equilibrium. It enables us to predict in general terms the
conditions that must be satisfied for a given system to be in equilibrium and the relations that may be expected
among the variables defining its state.
A phase is defined as any homogeneous and physically distinct part of a system, which is bounded by a surface
and is mechanically separable from other parts of the system. It is the part of the system, which is chemically and
physically uniform throughout.
The number of components of a system at equilibrium is defined as the smallest number of independently
variable constituents by means of which the composition of each phase can be expressed either directly or in terms of
chemical equations.
The number of degrees of freedom is defined as the number of independent variables such as temperature,
pressure and concentration (or composition), which must be specified in order to define the system completely. It is
also referred to variance of the system.
Gibbs phase rule equation: It states that if the equilibrium in a heterogeneous system is not affected by
gravity or by electrical and magnetic forces, the number of degrees of freedom, (F), of the system is related to the
number of components, (C), and the number of phases, (P), existing at equilibrium with one another by the equation
F=C-P+2
In the given experiment the number of components is 2, thus, the Gibbs phase rule applied gives F=4-P
Therefore, the maximum numbers of phases that can exist at invariant point (F=0) are 4, and the
maximum number of degree of freedom would be 3. Thus, a three-dimensional phase diagram should
be constructed to study the properties of a two-component system. In practice, one variable, either the
temperature or pressure is kept constant and only two variables, temperature and mole fraction or
pressure or mole fraction is specified. Under this condition, the modified form of the phase rule is
applied, i.e., F=C-P+1.
A solid solution is a solid state solution of one or more solutes in a solvent. Such a mixture is
considered a solution rather than a compound when the crystal structure of the solvent remains
unchanged by addition of the solutes, and when the mixture remains in a single homogeneous phase.
The solute may incorporate into the solvent crystal lattice substitutionally, by replacing a solvent
particle in the lattice, or interstitially, by fitting into the space between solvent particles. Both of these
types of solid solution affect the properties of the material by distorting the crystal lattice and disrupting
the physical and electrical homogeneity of the solvent material.
Binary systems are classified according to their solid solubility. If both the
components are completely soluble in each other, the system is called isomorphous system.
E.g.: Cu-Ni, Ag-Au, Ge-Si, Al2O3-Cr2O3.
Extent solid solubility for a system of two metallic components can be predicted based on
Hume-Ruthery conditions,summarized in the following:
- Crystal structure of each element of solid solution must be the same.
- Size of atoms of each two elements must not differ by more than 15%.
- Elements should not form compounds with each other i.e. there should be no
appreciable difference in the electro-negativities of the two elements.
- Elements should have the same valency.
All the Hume-Rothery rules are not always applicable for all pairs of elements
which
show complete solid solubility.
In systems other than isomorphous systems i.e. in case of limited solid solubility, there
exist solid state miscibility gaps; number of invariant reactions can take place;
intermediate phases may exist over a range of composition (intermediate solid solutions)
or only at relatively fixed composition (compound). These intermediate phases may
undergo polymorphic transformations, and some may melt at a fixed temperature
(congruent transformations, in which one phase changes to another of the same
composition at definite temperature).
There are two types of solid solutions: a)
Substitutional solid solution- when particles of one substance are replaced by their
lattice positions by atoms of another substance. It is possible when the atoms of the
molecules of the two solids have comparable sizes. In a true Substitutional solid solution, the
replacement of atoms of one substance by atoms of another substance is at random. There is
no order such as replacement of every second atom or every third atom.
b)
Interstitial solid solution- when the particle of one of the solid enters the voids or
interstices of the crystals of the other solid. In this case, the size of the atoms or molecules of
one of the solid crystal is much smaller than that of the other solid crystal atom or molecule .
Phase diagrams for solid solution
Since the two components are miscible in solid and liquid phases, the maximum
number of phases, which can exist at equilibrium, would be only two, and phase rule for such a
system will give the minimum number of degrees of freedom at constant pressure as
F=3 – P =3 – 2 =1
Hence an invariant system is impossible and there will be no discontinuity such as eutectic point on
the phase diagram.
The factors influencing substitution in solid solution are: 1. Size of the atom of the metal: To form a Substitutional solid solution, an atom of one
metal must take place of an atom of another metal. Thus the atomic size of the two metals must
be almost the same. Only atoms whose radii fall within a certain range can be expected to be
accomodated in a particular lattice. If the atom of one metal (solute) were too large, the lattice
of the other metal (solvent) would have to expand to accommodate it. If the atoms were too
small, the regularity of the lattice would have to be distorted. In either case, there is a limit to
the amount of expansion or distortion that a lattice can undergo.
2. Chemical nature of the metal: The two metals should not have very much different first
ionization potential. If the I.P. values are widely different than crystals of a compound of the
metals are formed
3. Ratio of number of electrons in the outer orbitals to the number of atoms:If the atoms
of two elements in the solid solutions are approximately of the same size and if the elements
have similar first ionization potentials, the electron/atom ratio determines the nature of the solid
solution. For the formation of Substitutional solid solution, the (e/A) ratio of the two metals
should be of the order of 1.4.
The completely miscible solid solutions are classified as: i) Continuous series of solid solutions: The heating curve trend followed in the experiment
is explained as follows. The labeling of the phase diagram shown in figure 1. can be easily
understood by first considering the expected heating behavior of the solid solution
represented by the point e. On heating, the state of the overall system will move vertically
upward along the isopleth edcba. The system remains in the solid phase till the point d is
reached where the first drop of liquid of solid solution d’ starts appearing.
Since the obtained liquid solution is more rich in A as compared to the solid
phase, the latter becomes less rich in A and thus, its composition point moves towards the .
B-axis. If the melting of the solid solution is to be continued, the temperature of the solid
phase has to be increased so that its state point moves on line dc’’b’B. During the heating of
the system from d to b, the relative length of the segment of tie line lying to the left of dcb
decreases indicating that the relative amounts of the two solutions change in favour of solid
solution, i.e. more and more of the solid solution liquefies during the heating process. When
the system has been heated to point b, the last trace of solid solution b’ remains to be
liquefied and thus represents the point where practically whole of solid phase is liquefied to
produce solid solution b whose composition is the same as that of starting solid mixture (i.e.,
point e). Further heating from b to a, merely increases the temperature of solid solution.
Figure 1.
Region/ lines
A
B
Curve AbB
Curve AdB
Area above AbB
Area below AdB
Area AbBdA
Description
Melting point of A
Melting point of B
Melting point curve
of solid solution
Fusion point curve
of solid solution
Liquid
Solid solution
A point within this
area represents a
Solid solution (whose
composition lies
On the curve AdB)
in equilibrium with
A liquid solution
(whose composition
Lies on the curve AbB).
No. of Phases(P)
2
2
Degrees of freedom(F)
1
1
2
1
2
1
1
1
2
2
2
1
* Fractional crystallization. On cooling a liquid solution with the composition corresponding to point P, the first
crop of solid solution appears at a temperature corresponding to point P’. This solid phase has the composition
corresponding to point Q, much richer in component B (high melting point) than the original liquid solution.
Suppose the solid solution Q is removed from the system and heated to a temperature represented by
point R when it liquefies. When this liquid is allowed to cool upto temperature R’, a solid solution with
composition S (much richer in component B) separates out. On repeating this process, it is possible to separate the
pure component B in the solid form. The remaining liquid left behind every solidification will be more and more
richer in component A (low melting point). Thus the method of the fractional crystallization will enable us to
separate pure A and B. In actual practice, the melt is allowed to cool till half of the amount is crystallized. The
crystal form is separated and then heated and allowed to cool till half of the liquid is solidified. This is repeated
many times to get pure solid of high melting point. The half of the liquid left in the first crystallization is cooled
till half is crystallized; the crystals are removed and the liquid is allowed to cool till half of it is solidified. On
repeating this process the low melting component is obtained in the pure form. By this method the separation of
Co-Ni, Au-Ag, Au-Pt, Naphthalene--2-Naphthol etc. is carried out.
ii) Solid solutions with minimum melting point: The phase diagram for such a system is shown in figure 2.
The upper curve is the liquidus and the lower curve is solidus. The two curves meet at a temperature lower than
the melting point of either of the components. At this point, the melting and freezing points of the solution are
equal, and the liquid and the solid phases have the same compositions. The systems forming solid solutions of this
type are p-iodochlorobenzene & p-dichlorobenzene, sodium carbonate & potassium carbonate, KCl-KBr, etc.
From this phase diagram, it is clear that the complete separation of the two components is not possible by the
method of fractional crystallization. We can obtain only that component in the pure form, which is present in the
larger proportion in the liquid mixture. For example, let us consider a mixture of composition P (containing excess
of A). On cooling this mixture, the solidification starts at a temperature corresponding to point Q. If the mixture is
further cooled upto the temperature indicated by point R, the solid has the composition given by point a, and the
liquid has the composition given by point b. Now, if the solid a is removed and heated to a temperature P’ and
again cooled to R’ , the solid separates with composition c.
Figure 2.
By repeating this process, it is possible to get pure A. If the liquid b after the first fractionation is treated, we will
ultimately get a mixture of constant composition corresponding to the minimum point in the curve. Similarly, if
we start the fractional crystallization of a mixture richer in B, we shall be able to get pure B and a mixture of A
and B of constant composition.
iii) Solid solutions with maximum melting point: The systems with maximum melting point are rare. One
such example is the binary system d- and l-carvoxime. The complete separation of the components is not
possible by fractional crystallization.
i) Different compositions of the solid solutions were made according to table 1.
ii) The first composition of the solid solution was taken in a test tube.
iii) The composition was melted on the heating block such that all the solid particles
had converted to the liquid form.
iv) The liquid form of the solid solution was then poured onto a watch glass. The
liquid mixture was stirred quickly with a capillary tube before its solidification, so as
to ensure the formation of a homogeneous mixture.
v) The watch glass was then kept undisturbed for a few minutes, to let the liquid
mixture solidify.
vi) The solid thus formed was crushed with a clean spatula to fine particles.
vii) A capillary tube was quarterly filled with the fine particles of the solid mixture.
viii) The temperatures were noted down at which melting had started and completed.
ix) The second composition was taken and the above procedure was repeated.
x) Similarly, the two temperatures of all the compositions were determined.
xi) A graph of temperature v/s composition was plotted.
Room temperature =18oC
Compound A= 2-Naphthol
Compound B= Naphthalene
Melting point of pure 2-Naphthol = 121o C
Melting point of pure Naphthalene = 79o C
T1 = Temperature at which melting starts
T2 = Temperature at which melting is complete
Mass percentage of B =
Mass of compound B
Mass of compound A + Mass of compound B
Mass of
Mass of
Mass percentage
Compound A
Compound B
T1 ( o C)
T2 ( o C)
of B
0.50
0
0
117
118
0.40
0.10
20
100
105
0.30
0.20
40
92
97
0.25
0.25
50
89
94
0.20
0.30
60
85
91
0.10
0.40
80
81
86
0
0.50
100
74
75
Table 1.
TA
oC
TB
Mass % of B
In this experiment, a melting point diagram is plotted. As heating continues more
solid will melt but at ever higher 2-Naphthol compositions. This is because as the
temperature rises the higher 2-Naphthol crystals are the ones becoming more
unstable, and thus melting. Actually, entire crystals are not melting so much as the
more Naphthalene-rich fractions of the crystal melt first.
And just as with crystallization, during melting the melt composition
and crystal composition will move in tandem horizontally across from each other,
but this time up the solidus and liquidus lines until all the crystal is melted. At the
point no crystal is left the line moves straight up the graph as temperature
continues to rise.
In this experiment, a melting point diagram is plotted. It was verified
that the trend followed in heating curves was same as that in cooling
curves.
i) The test tubes and watch glass should be properly cleaned.
ii) the completely melted solid solution should be poured quickly onto the watch
glass.
iii) On pouring the melted solid solution on the watch glass, the solution should be
stirred quickly with the capillary tube before the melt solidifies.
iv) While using the melting point apparatus, it should be made sure that the
temperature of the apparatus is at room temperature. If not, then the apparatus should
be switched off till its temperature returns to the room temperature.