(a) Temperature Temperature Figure 4.1 The phase diagrams referred to in (a) Exe
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Question
(a) Temperature Temperature Figure 4.1 The phase diagrams referred to in (a) Exercise 4A.1(a) and (b) Exercise 4A.1 (b). How many phases a f the points in Figs 4. la and 167 What is the maximum number of phases that can be in mutual equilibrium in a (a) two component (b) three component and (c) four component system? Explain why four phases cannot be in equilibrium in a one-component system. What is the physical reason for the fact that the chemical potential of a pure substance decreases as the temperature is raised? Difference between normal and standard boiling points Difference between normal and standard freezing point. How does the melting point of a substance responses under pressure? How does the vapor of a substance responses under pressure? Discuss the effect of temperature and pressure on the chemical potential of a pure substance from the following two equations:Explanation / Answer
1. The number of phases at each point are as follows:
In figure (a) In figure (b)
At point a=1 ,b=3,c=3,d=2
At point a=1,b=2 c=3 d=2
2.The maximum number of phases that can be in mutual equilibrium follows the Gibbs phase rule.
i.e F = 2-P +N ,where F=degree of freedom ,P=number of phase, N=number of components.
So for the maximum number of phases degree of freedom should become zero.
Therefore for a) two-component system
if F=0 and C=2 then this will result in P=4.
b)three component system
when F=0 and C=3 then P won't be equal to 5. As it's not necessary that to attain equilibrium F has to be zero. It might happen that the 3 component system will be having two degrees of freedom.
i.e P=3 phases.
c)For four component system, too phases will be 3 only.At this point, you should know that C=s-r where s=number of species and r=number of independent relations among intensive variables(such as temperature,
Pressure and concentration) of individual phases.
3.Gibbs phase rule doesn't permit 4 phases in one component system.
i.e F=0 ,N=1 then P=3 only.
4.To obey the second law of thermodynamics the potential has to decrease else the process will become spontaneous and hence the equilibrium condition will be lost.
5.The boiling point of a substance is the amount of temperature at which the surrounding pressure of the liquid equals the vapor pressure of the liquid.
Standard boiling point is when the surrounding pressure is set to 1 bar.
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