Lecture 6 Flashcards
What is a tie line? How do we use it to interpret a phase diagram? What is the lever rule?
found in notes on pages 1-2 (in the actual notebook).
What are partially miscible liquids?
they are liquids that do not mix in all proportions at all temperatures.
What is the phase separation of two immiscible liquids? (describe what a graph would look like)
An illustration of the system is shown in notes on page 2.
Such phase diagrams can be depicted by looking at what happens when you change the composition at constant temp and when you change the temp at constant composition.
- change of composition at constant temp:
iMPORTANT: The main point is to determine the abundances of the relative liquids to the composition, which can be done using the lever rule.
GENERAL UNDERSTANDING: Within the system illustrated on page two of the notes, where given at a constant temp T, when a small amount of b is added there is still one phase in the system where B is then dissolved in A, as the composition of B increases eventually B would stop dissolving in A and the system will have two phases at equilibrium.
Any point that has these two phases in the temperature-composition diagram can be represented by the tie line to show the relative amounts of each phase (which is calculated using the lever rule).
As we increase the composition of B, at the same temp, eventually we will go to one phase where liquid A is fully dissolved in liquid B. Note that the abundance of Iiquid B dissolved in liquid A is greater than the abundance of liquid A dissolved in liquid B.
Keep in mind that the phases of both A and B for constant Temp are the same it is just that the relative amount of A and B change.
The change of temp at a constant composition:
IMPORTANT: The point of looking at this is to view the miscibility changes as we add more and more of a liquid into another liquid.
GENERAL UNDERSTANDING: In the case of the system illustrated, as the temperature increases the graph gets narrower and the miscibility increases. This is because as we increase the temp then the rich part of A becomes richer in B and the rich part of B becomes richer in A.
What is the upper and lower critical solution Temp?
The upper critical solution temp is the highest temperature at which phase separation occurs as illustrated in notes on page 2. Past this point, the liquids are fully miscible.
The lower critical solution temp is the lowest temperature where phase separation occurs past this temp the liquids split into two phases. The reason for this is that the liquids in the mixture form weak complexes which at high temp get broken causing phase separation.
Some situations have both lower and higher critical solution temps which is caused by the mixture forming a weak complex that breaks when the temperature increases but then later on the system homogenizes due to larger temps. This is illustrated on notes on page 3.
What does the distillation of partly miscible liquids with low boiling azotropes look like? (explain what happens and if there are any special features).
There are two different situations when it comes to the distillation of partially miscible liquids that have low-boiling azeotropes:
- The liquid is fully miscible before the boiling point of the solution Illustrated on notes on page 3:
GENERAL INFO: The distillation of a mixture of composition a1 would lead to a vapour composition of b1 which when cooled will result in a single-phase composition of b2, further cooling will lead to phase separation, which in the case of distillation only occurs at the first drop of the distillate. When distillation increases the liquid composition changes. In the end, when the whole sample has evaporated and condensed, the composition is back to a1.
- There is no upper critical solution illustrated in notes on page 3:
GENERAL INFO: In this phase situation when we take the mixture from the composition of a1 and distal it we will have a composition of b3 which is a two-phase mixture.
If we take an isopleth line to cross through the azeotropes we notice some interesting behaviors. At the beginning of the line at e1 we will notice it is a two-phase mixture, after increasing the temp at e2 all of the liquid vaporizes to gas at a single characteristic temperature like how a single component system would do. E2 represents the lowest temperature at which the solid can be present.
If we take a horizontal line at the point of E2 we see that there is an equilibrium between the two liquid phases and the vapour where anything below the line represents the two phases of the liquid and anything above the line represents a liquid phase (either rich of A or rich of B) and the vapour.
Notes: It is common to see this type of diagram as both the partially miscible and low boiling point azeotrope properties work to cause phase separation.
Explain a temperature–composition phase diagram for two almost immiscible solids and they’re completely miscible
liquid, as well as explain what are Eutectics.
Found in notes on page 3.
What is an Eutectic halt?
As we go from a liquid composition of a1 and start cooling we will first reach the composition of a2 at which solid B starts to solidify which is an exothermic process that slows down the cooling process of the liquids.
Once the liquid reaches the eutectic composition, the temperature remains constant as the whole sample solidifies, this halt is known as the eutectic halt.
notes that there are some halts due to the solid-liquid boundaries. The longest Eutectic halts give the location of the eutectic composition and its melting temp.
This information is illustrated by a COOLING CURVE on Atkins’ page 219
Incongruent and congruent melting?
Inongurat melting is when a component melts into its component and does not itself form a liquid phase since its liquid phase is highly unstable
Congruent melting is when it can form a liquid phase
Examples page 220 - 221 ‘Atkins’
What is the Gibbs phase rule?
found in notes on pages 4-5
What is a ternary system and how is it represented in terms of the phase rule?
A ternary system is a system that has 3 components, which can be represented with a special phase diagram (illustrated in notes on page 5).
Implementing the Phase rule to a ternary system gives the equation F= 5 - P, if both temperature and pressure were set to be constant the equation is altered to F=3-P.
If the system has two phases then there is only one degree of freedom, which comes from the fact that if you change the composition of one component the other two components are also changed. This is represented as an area on the diagram.
If the system has 3 phases then there is zero degrees of freedom, and that can be represented as a single dot on the diagram.
How are Tie-lines constructed in ternary systems?
In such a system the tie lines in the two-phase region are constructed experimentally by determining the compositions of the two phases that are in equilibrium, marking them on the diagram, and then joining them with a straight line
What is the general interpretation of the phase diagram of 3 components with 2 being partially immiscible with each other
found in notes on page 5.
