Phase equilibria, Chemistry

Assignment Help:

Melting and freezing points:

When a pure solid substance is heated through its melting point (melting temperature), the solid changes to a liquid. At the melting point equilibrium is established between the solid and liquid phases. Conversely, when a liquid is cooled through its freezing point or freezing temperature, the liquid is transformed to a solid, and at the freezing point, equilibrium is established so the melting and freezing temperatures of a pure substance are identical and in this case the terms can be used interchangeably.

Boiling point:

At the boiling point of a liquid A, equilibrium is established for the transition of A between liquid and vapor: 

1892_boiling point.png

Again, the equation 2046_boiling point1.pngcan be applied to this equilibrium, but in this case with the changes in volume and entropy being ΔV=V(g)-V(1) and ΔS=S(g)-S(1), for the transformation between liquid and vapor. ΔS have a positive value, as substances have more freedom of movement in the vapor than in the liquid. ΔV is always positive and is much larger than that observed in a melting transition.

Phase diagrams of a single species:

The boiling equilibrium condition is represented as a line on a plot of the pressure, p against the temperature, T (Fig. 1). In this plot, this condition can be represented as a line of positive gradient, so that any points on this line correspond to the situation where vapor and liquid are at equilibrium.

627_equilibria.png

Fig. 1. Pressure-temperature plot (phase diagrams) resulting from plotting the boiling point and melting point equilibrium condition lines. (a) Normal plot; (b) plot for water.

Especially when applied to multi-component (multisubstance) systems (see Topic D5), is the number of degree of freedom, F, of the system. This is the minimum number of variables that can be varied without changing the number of phases in the system. F=2 for the regions where liquid, gas or solid only are present, as changes in two variables (for example p and T) are possible without a phase change occurring. This corresponds to being able to move in two directions on the phase diagram (Fig. 2a). On the freezing point or the melting point lines, F=1, as movement in one direction, down and up the line only, is possible in order to maintain the two phases at equilibrium. That means there is only one independent variable,

348_boiling point2.png

Fig. 2. The concept of the number of degrees of freedom, F, as applied to movement on a phase diagram. (a) F=2; (b) F=1; (c) F=0. as the two variables are related by the equation of the line (Fig. 2b). At the triple point, which occurs at one fixed point on the diagram, no movement is possible and F=0 (Fig.2c).


Related Discussions:- Phase equilibria

Construct a thermodynamical born-haber-cycle, Use the given data to constru...

Use the given data to construct a thermodynamical Born-Haber-cycle of the formation of potassium chloride from its elements and calculate the lattice energy of potassium chloride.

Radis of curvater, what is the radis of carvater eassy words

what is the radis of carvater eassy words

What are molecular orbitals, What are molecular orbitals? How are they form...

What are molecular orbitals? How are they formed? What does molecular orbital theory states? what is the significance of molecular orbital theory? Ans) Molecular orbitals are for

Explain cycloaddition reaction, This cycloaddition reaction involves sixtee...

This cycloaddition reaction involves sixteen electrons; hence, it must be suprafacial on one component and antarafacial on the other.  Thus, it must be a [14s + 2a] or a [14a + 2s]

Iupac nomenclature, what are the most important rules of iupac nomenclature...

what are the most important rules of iupac nomenclature that a student should know

Find the pressure in the droplet, Q. The pressure in a liquid droplet of ra...

Q. The pressure in a liquid droplet of radius r is greater than the pressure of the surrounding equilibrated gas phase by a quantity 2 =r, where is the surface tension. (a) Cons

The formula of its phosphate, Chloride of metal is MCl 2  . The formula of...

Chloride of metal is MCl 2  . The formula of its phosphate will be: (1) M 2 PO 4                                                (2)  M 3 (Po 4 ) 2 (3)  M 2 (

Write Your Message!

Captcha
Free Assignment Quote

Assured A++ Grade

Get guaranteed satisfaction & time on delivery in every assignment order you paid with us! We ensure premium quality solution document along with free turntin report!

All rights reserved! Copyrights ©2019-2020 ExpertsMind IT Educational Pvt Ltd