Two component system

1. Two component system

2. Lead Silver(Pb-Ag) System.
Potassium IodideWater(KI-H2O) System.
Ferric Chloride Water(FeCl3-H2O) System.

3. Some important terms …
 Phase.
 Components.
 Degree of freedom.
 Homogeneous system.
 Heterogeneous system.
 Phase rule.
 Significance of phase rule.
 Limitations of phase rule.
 Phase diagram.
 Eutectic system.
 Eutectic point.

4. Phase (P):
• “A phase is defined as any homogeneous physically
distinct and mechanically separable portion of
heterogeneous system in equilibrium. “
• Examples:
• A gas mixture constituents a single phase (p = 1).
Since gases are completely miscible.
• Immiscible liquids constitute different phases. Thus
carbon tetra chloride (CCl4) is immiscible in water
(H2O).
• For this system p =2.
• Miscible liquids constitute a single phase. Thus
alcohol is immiscible in water (H2O).
• For this system p =1.

5. Components (C):
“The number of components of a system at equilibrium
is defined as the smallest number of independent
constituents by means of which composition of each
phase can be expressed directly (or) in terms of chemical
equation.”
For a system,
Number of components = (number of chemical species)
– (number of independent equations).
C = N –E

6. Examples:
KCl-NaBr-H2O system ; C=4
KCl + NaBr ⇌ NaCl + KBr
Number of chemical species , N =5(KCl,NaBr,H2O,NaCl,KBr)
Number of independent equations is E=1.
C = 5-1 = 4.
KCl-NaCl-H2O system ; C=3
Since there is no equation (E=0), N=3.
Ice ⇌ water ⇌ water vapor
H2O H2O H2O C=1.

7. Degrees of freedom (F):
The number of independent variables such as temperature,
pressure, concentration which must be specified in order to
define the system completely.”
If F = 0, then system is known as invariant system.
If F = 1, then system is known as uni variant (or) mono
variant system.
If F = 2, then system is known as bi variant (or) di variant
system.
If F = 3, then system is known as tri variant system.

8. Examples :
PV = nRT (for ideal gas)
P = RT/V (or) p = CRT
Here P, C, T‘s are intensive variables. If ‘P’and ‘T’are
fixed, then the volume (V) will have a definite value.
For a pure gas, F = 2
F = C – P + 2 = 1 – 1 + 2 = 2 (C = 1, p = 1),
Bi variant.
For a gaseous system consisting of two gases, F = 3.
F = 2 -1 + 2 = 3,
Tri variant.

9. Homogeneous system:
It is a system, which is uniform through out in physical
and chemical properties.
Ex: the solution of NaCl in water is homogeneous
system.
Heterogeneous system:
It is a system which consists of parts with different
physical & chemical properties.
Ex: the ice - water - water vapor system is a
heterogeneous system.
Because there are portions which physically distinct and
mechanically separable from one another.

10. Phase rule:A rule relating the possible no of phases,
constituents & degrees of freedom in a chemical
system.(F=C-P+2) by Gibbs.
• The phase rule at equilibrium is depend only on
• temperature,
• pressure and concentration variations
• and is not influenced by
• gravity,
• surface forces,
• electrical and magnetic forces,
• then the number of degrees of freedom (F) exceeds the
difference of number of components (C) and the
number of phases(P) by 2.

11. Significance of phase rule:
 Phase rule is applicable both physical & chemical
equilibria.
 Phase rule is applicable to microscopic systems.
 We can conveniently classify equilibrium states in
terms of phases, components and degrees of freedom.
 The behavior of system can be predicted under
different conditions.
 According to phase rule, different systems behave
similarly if they have same degrees of freedom.

12. Limitations of phase rule:
• Phase rule is applicable only for those systems which are in
equilibrium.
• Only three degrees of freedom namely temperature, pressure
and composition are allowed to influence the equilibrium
systems.
• Under the same conditions of temperature and pressure all the
phases of the system must be present.
• It considers only the number of phases rather than their
amounts.

13. Phase diagrams:
When a system goes from one phase to another phase,
without change of chemical composition is known as
“phase Transition.”
Examples:
 Melting (solid to liquid).
 Boiling (liquid to gas).
 Condensation (gas to liquid).
A diagram which illustrates the conditions of
equilibrium between various phases of a substance is
called a “phase diagram.”

14. Two component system:
In a system of two components, when p = 1
F = C – p + 2
= 2 -1 +2 =3
• This means that three variables must be specified in
order to describe the condition of the phase.
Ex: Ag – Pb system.

15. Eutectic system:
A binary system in which two components are miscible
in all proportions in the liquid state, but do not react
chemically and each component has the property of
lowering each others melting point is known as eutectic
(easy to melt)system.
A solid solution of a two component system which has
the lowest freezing point of all the possible mixture of the
component is known as “Eutectic mixture”.
And the minimum freezing point of the eutectic mixture is
known as “Eutectic point”.

16. Lead Silver (Pb-Ag) System:

17. Lead Silver (Pb-Ag) System:
 L(327°C) is the melting point of Pb.
 Addition of Ag lowers the melting point of Pb and LM
is the melting point curve of Pb in the presence silver.
 Along LM, solid Pb and the melt are in equilibrium.
 Point N(961°C) represents the melting point of Ag.
 Its melting point is lowered by the addition of Pb along
the curve NM.
 Hence NM is the melting point curve of Ag in the
presence of Pb.
 Along NM solid silver is in equilibrium with the melt.
 The system along LM or NM is monovariant.
 The curves LM & NM intersects at M, where three
phases solid Pb, solid Ag & melt are in equilibrium.

18. Lead Silver (Pb-Ag) System:
 The system is invariant and M represents the eutectic
point.(303°C, 2.6% Ag by mass)

19. Potassium Iodide Water(KI-H2O) System:

20. KI-H20 System:
 L(0°C) is the freezing point of water and is lowered by the
addition of salt(KI).
 Hence after addition of salt(KI), LM is the freezing point of
water in the presence of KI.
 Along LM ice separates out from the solution and system is
monovarient.
 NM is the solubility curve of KI.
 M Pt of KI cannot be realized in this phase diagram because it
has high M Pt as compared to critical temperature of H20.
 From slope of the curve it reveals that the solubility of KI
increases with the rise in temperature.
 At M where LM & NM meet, three phases(ice, solid KI &
solution) are in equilibrium, the system is invariant.
 M is the eutectic point(-23°C, 52%KI by mass).

21. KI-H20 System:
 In case of system involving salt & water, the eutectic
point is generally known as cryohydric point.
 The temperature at M is the cryohydric temperature.
 Solution of composition M is the cryohydric solution.
 For such systems the lowest temperature that can be
attained is the cryohydric temperature and it is the
characteristic of each system.

22. Ferric Chloride Water(FeCl3-H2O) System:

23. Ferric Chloride Water(FeCl3-H2O) System:
 In this system four congruently melting compounds are
formed which are,
• dodecahydrate(Fe2Cl6.12H2O)
• heptahydrate(Fe2Cl6.7H2O)
• pentahydrate (Fe2Cl6.5H2O)
• tetrahydrate(Fe2Cl6.4H2O).
 The phase diagram consists of four maxima
corresponding to the formation of these hydrates.
 Points N,P,R,T represents the congruent melting
points of dodeca,hepta,penta and tetra hydrates
respectively.
 The congruent melting point of a salt hydrate is also
known as the dystectic point.

24. Ferric Chloride Water(FeCl3-H2O) System:
 There are five cryohydric points at M,O,Q,S and U.
 L is the melting point of ice.
 Addition of Fe2Cl6 lowers the melting point along LM.
 At the cryohydric point M the solution becomes
saturated w.r.t dodecahydrate and represents the
lowest temperature that can be attained with this
system.
 Curves MNO,OPQ,QRS and STU represents the
solubilities of dodeca,hepta,penta and tetra
respectively while UV represents the solubility
characteristic of the anhydrous salt.
 In the diagram the solubility of each hydrate increases
with the rise of temperature.

25. Ferric Chloride Water(FeCl3-H2O) System:
 Now consider the phase changes that result when an
unsaturated solution represented by point k is
concentrated isothermally by adding anhydrous ferric
chloride along ku.
 Firstly a saturated solution of dodecahydrate results at
l.
 At m the whole mass solidifies to form dodecahydrate
which melts when more of ferric chloride is added.
 Dodecahydrate disappears beyond n and between n
and o an unsaturated solution exists.
 The solution becomes saturated with respect to
heptahydrate at o.

26. Ferric Chloride Water(FeCl3-H2O) System:
 Further addition of ferric chloride increases the
amount of solid heptahydrate in the solution and at p
the whole solutions solidifies yielding heptahydrate.
 The heptahydrate persists upto q.
 Between q and r the solution remains unsaturated, at r
the pentahydrate begins to crystallize out.
 At s solidification of whole mass into pentahydrate
occurs.
 Between s and t a mixture of penta and tetrahydrate
exists which is completely converted into tetrahydrate
at t, beyond which tetrahydrate decomposes into
anhydrous ferric chloride and at u only the anhydrous
salt remains.