Phase equilibrium
Phase:-phase may be defined as any part of the system which is homogenous, physically distinct and
mechanically separable part of the system e g ; ice and water in equilibrium constitute a two phase
system .Likewise two immiscible liquids (benzene and ethanol) constitute a two phase system as there
exists an interface separating the two phases. However; two liquids that are completely miscible with
each other ( water and ethanol) form a single phase system since there is no physical boundary
separating the two phases .Here one must keep in mind that definition of phase is not simply restricted
to the states of matter as illustrated by the heterogeneous equilibrium below
CaCO3(s) ↔ CaO(s) + CO2(g) ,the mixture contains two states of matter(solid and gas) but the number
of phases are three. A gaseous mixture of nitrogen and hydrogen constitute a single phase system since
it is homogenous as a whole and there is no interface separating the two phases.
Components:-components refers to the minimum number of independently variable chemical
entities by means of which the composition of each phase/system can be described completely.
Consider the Autopyrolysis of water
H2O(I) ↔H(aq)
+
+ OH(aq)
-
The number of chemical entities present in the system here are three(H2O,H+
,OH-
) but it is a one
component system. It can be seen that if the concentration of any two species is arbitrarily set, the
concentration of third is fixed and can be calculated from equilibrium constant. The equilibrium
expression for this process is Kw =[ H
+
][OH-
].Therefore, if the concentration of H+
is known the
concentration of OHis set .Analogously if the concentration of OH- is known ,the concentration of H
+
is
set .Hence, it is a one component system.
It can be computed using the formula
C=S-R where S refers to the number of chemical constituents present in the system and R refers to the
number of restrictions imposed on the independent variation of these constituents.
Consider the decomposition of MgCO3 .
MgCO3(s) ↔ MgO(s) +CO2(g), the number of chemical entities present, S is three I.e. MgCO3,MgO,
CO2 and the number of equilibrium restrictions ,R is one that is the eqm. restricts the independent
variation of these constituents .So the number of components is 2.
Let us take the example of acetic acid and ethanol. At the first sight we might predict two components
because there are two constituents HoAc and EtOH, however these constituents react to give ethyl
acetate and water. Therefore ethyl acetate and water are also present at equilibrium along with the
reactants ,this raises the number of components to four .But because of equilibrium condition and
since at equilibrium [ EtOH] =[HoH] ,the number of components is reduced back to two .If we consider
the decomposition of PCl5
