Phase Rule
Dr. Mohammad Shariare
Phase Equilibria
1. Phase rule
The phase rule is an important generalization dealing with the behaviour of heterogeneous systems. In general with the application of phase rule it is possible to predict qualitatively by means of a diagram the effect of changing pressure, temperature and concentration on a heterogeneous system in equilibrium.
Gibbs' phase rule
Phase rule was proposed by an American Physicist Josiah Willard Gibbs in the 1870, which may be stated mathematically as follows:
F = C – P + 2
where C is the number of components and P is the number of phases in thermodynamic equilibrium with each other.
Typical phases are solids, liquids and gases.
A system involving one pure chemical is an example of a one-component system. Two-component systems, such as mixtures of water and ethanol, have two chemically independent components. F is the number of degrees of freedom, which means the number of intensive properties such as temperature or pressure, which are independent of other intensive variables.
Phase
-A phase (P) is any homogeneous part of a system having all physical and chemical properties same throughout. A system may consist of one phase or more than one phases.
A system containing one phase only is called a homogeneous system. E.g. a system containing only liquid water
A system containing two or more phases is called a heterogeneous system. E.g. a system containing liquid water and water vapour
Components
-Component (C) is the least number of chemical constituents in terms of which the composition of every phase can be expressed by means of a chemical equation
Degrees of freedom
-The least number of variable factors (concentration, temperature, pressure) which must be specified so that the remaining variables are fixed automatically and the system is completely defined.
Phase Diagram
A phase diagram is a plot showing the conditions of pressure and temperature under which two or more physical states can exist together in a state of dynamic equilibrium.
1) Areas – the diagram is divided into three areas which are mentioned as solid (ice), liquid (water) and water vapour. Each of three areas shows the conditions of temperature and pressure under which the respective phase can exist.
2) Line – the lines show the conditions of equilibrium between any two of the three phases 3) Triple point – the three boundary lines enclosing
the three areas on the phase diagram intersect at a common point called the triple point. A triple point shows the conditions under which all the three phases can coexist in equilibrium.
Certain substances such as menthol, thymol, camphor, phenol, salol
etc. when mixed in a particular proportion tend to liquify due to
reduction in their respective melting points. Mixture of such
substances are known as eutectic mixtures. (Greek meaning: Eutectic-
Easy melting)
Phase Equilibria:
Eutectic mixture
Te m p er at u re Liquid
Solid A +
Solution C Solid B + Solution
Eutectic Point
Solid A + Solid B
An
azeotrope
is a mixture of two or more liquids in such a way that its
components cannot be altered by simple distillation. This happens
because, when an azeotrope is boiled, the vapor it produces has
proportionate constituents as the original mixture.
Because their composition is unchanged by distillation,
azeotropes
are
also called
constant boiling mixtures
. The word
azeotrope
is derived
from the Greek words to give the overall meaning, "no change on
boiling".
Each azeotrope has a characteristic boiling point. The boiling point of an azeotrope is either less than the boiling point temperatures of any of its constituents (a positive azeotrope), or greater than the boiling point of any of its constituents (a negative azeotrope).
A well-known example of a positive azeotrope is 95.63% ethanol and 4.37% water (by weight). Ethanol boils at 78.4 °C, water boils at 100 °C, but the azeotrope boils at 78.2 °C, which is lower than either of its constituents. Indeed 78.2 °C is the minimum temperature at which any ethanol/water solution can boil at atmospheric pressure. In general, a positive azeotrope boils at a lower temperature than any other ratio of its constituents. Positive azeotropes are also called minimum boiling mixtures or pressure maximum azeotropes.
Polymorphism is the ability of a material to exist in more than one
crystalline or amorphous form.
Phase Equilibria:
Polymorphism
This phenomenon where compounds exist in more than one crystalline
and/or amorphous forms is termed as polymorphism and the different
crystalline/amorphous forms are known as polymorphs or polymorphic
forms.
Thermodynamically two types – monotropic and enantioropic
There are some other types of polymorphism such as packing
