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SPECEFGOC II 04 Solutions 4

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This lecture explores non-ideal liquid-vapor systems, including those exhibiting positive and negative deviations from Raoult's Law leading to azeotropism, partially miscible liquid systems with critical solution temperatures, and immiscible liquid systems, culminating in the explanation of steam distillation.

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In today's

lecture, I'll touch upon non ideal

liquid vapor systems. The ones that do

not obey Roll's law showing both

positive and negative

deviations resulting in an interesting

feature called aotropism.

Then we'll take partially missible liquid

liquid

systems. Three good examples are there.

Phenol water system, triathile ammine

water system, nicotine water system. And

lastly, we look at immissible liquid

systems to explain one interesting

distillation process called steam

distillation. Non ideal systems. As I told

told

you this is the system which doesn't

obey R's

law. There are two types. There can be

positive deviation from R's law where

the vapor pressure will be higher than

predicted from R's law. And we can also

have systems showing negative deviation

from R's law where vapor pressure will

be lower than predicted from R's law.

see the phase diagram or rather a

pressure composition diagram. You can

immediately notice the

difference in the diagram. If you look

at the dotted line shows the expected

variation in vapor pressure if the

liquids were to behave

ideally. But it is clear that the

experimental values of vapor pressure

show positive deviation. The curve is

much above that is expected from R's

components. Now here it needs an

explanation why liquids show positive

deviation. It depends on the interaction

between the

molecules. If the interaction between

the molecules of the same type that is A

and A, if it is stronger than between A

and B, it is likely that B may not find

place in the liquid state. A tries to

push it out.

Similarly, B molecules will be liking B

molecules. Therefore, A molecules are

shunted out. Molecules are unassociable

that way. If you don't like me, I don't

like you. And that's exactly what is

observed in the case of systems which

show positive deviation. Now look at the

total pressure. The total pressure which

is the sum of the partial vapor

pressures of the two components shows a

maximum. And the example that we have

here is the mixture of carbon dulfide

and acetone which shows a vapor pressure

maximum. But the diagram in which we

plot temperature versus composition is

more useful. Here is the diagram. You

can clearly see that the two liquids

have boiling points that are much

different. A is lower boiling. That

means it has higher vapor pressure. B is

high boiling which has lower vapor

pressure and you see there is a minimum

here which is called the boiling point

minimum. So these systems are the ones

which show boiling point minima. Let us

try to explain the usefulness of these

type of diagrams. You start with a

liquid mixture with composition

corresponding to A1 and heat

it. When we reach the point A2 on the curve

curve

here which can be considered as the

boiling point curve of various mixtures.

At this point A2 the liquid mixture

starts boiling. The vapors that emerge

out has the composition given by A2

prime. And if you condense this

vapor, we get a liquid having a

composition given by A3. If we heat it,

it liberates vapors having a composition A3

prime. And successively if you do this

condense the vapor to get the liquid and

again evaporate that liquid collected

ultimately we reach the composition

corresponding to the point C at which

the liquid that we have shows a sharp

boiling point. It forms a vapor having

the same composition as the liquid that

we started with. And this liquid is set

to have eiotropic

composition. What do you mean by aotropic

aotropic

mixture? Eiotropic mixture is the one

which boils without changing

composition. An example of the system

that we have here is offered by a

mixture of ethanol and water. Water has

a boiling point 100 and ethanol

78.5. You have a system a mixture of

ethanol and water which gives us a

boiling point minimum which has 95%

ethanol. Let us start with non ideal

systems. The systems which show positive

deviation from R's law. A pressure

versus composition diagram is here. You

can see both the liquids show vapor

pressures that are much more than their

ideal vapor pressures as expected from

rolls law which are shown by the dotted

line. Why both the liquids show higher

vapor pressure in the mixture than their

ideal pressures? The reason is not

difficult to

find. It has something to do with the

molecular interactions.

interaction between molecules of A and

interaction between molecules of B and

between A and B. If molecules of A like

molecules of A much more than B, then

there is a chance that B molecules try

to escape and it's vice versa.

Similarly, A molecules are not liked by

B. Therefore, they also tend to escape.

Molecules are also unassociable in a

sense. As a

consequence, if you look at the total

pressure which is shown here, which is

the sum of the partial vapor pressures

of the two

components, the curve shows a maximum.

So these are the systems where the vapor

pressure maximum can be seen. An example

that we have here is a mixture of carbon

dulfide and acetone. There are several

examples of this type.

But what really matters to us a phase

diagram which we get if you plot

temperature versus composition. You can

see the liquid with higher vapor

pressure shows lower boiling point and a

liquid with lower vapor pressure shows

higher boiling point.

So we have A and B with the differing

boiling points and the boiling point

curve of the mixture which is shown here

shows a minimum. So these are the

systems which show boiling point

minima. The upper curve

shows the composition of the vapor that

is in phase equilibrium with the liquid.

Let us try to use this

diagram. Let us start with a liquid

mixture having

composition a prime and heat it. Heating

is no problem because we have two

degrees of freedom. And when we reach

point A2 on the curve, it starts

boiling. The vapors emerge and the

vapors have a composition given by point A2

prime. And if we condense this vapor, we

get a liquid having a composition given

by the point A3 which is different from

that of the original composition which

has rich become richer in the liquid A

in this case which is having lower

boiling point.

And if you take this liquid and heat it,

you get a vapor having composition A3

prime. And successively if you condense

it and then vaporize it, condense it,

vaporize it, we end up with the liquid

having a composition corresponding to

point C. It's interesting here. If you

heat this liquid mixture, it will boil

at a constant temperature without change

in composition. All other compositions

will lead to a difference in the

composition of the vapor except this

point. Therefore, this point C is

interesting. It is called the aotropic

composition. What is meant by eiotropic

composition? We look at slightly later.

An example of this type of system is

offered by ethanol water. It gives us an

esotropic mixture having the composition

95% of ethanol which is called the

rectified spirit. There are systems

which show negative deviation from

Raul's law and a consequence is very

clear here. You can see the vapor

pressure is lower than expected from

Raul's law and the total vapor pressure

shows a minimum which is much less than

the ideal vapor pressure expected for

the liquid system. An example that we

have here is mixture of tricloromthane

and acetone.

Let us look at the diagram in which we

have plotted temperature versus

composition as in the earlier case. It

shows a boiling point

maximum and similar point C which is the

esotropic composition where the liquid

shows a constant boiling point which is

therefore called constant boiling

mixture and there is no change in the

composition. The rest of the discussions

are very similar. If you heat a liquid

of composition

A1, it starts boiling at A2 to give a

vapor having a composition given by A2

prime. If you remove this vapor, the

liquid composition starts drifting

towards A3 and ultimately reaches the

point C and further separation of the

two liquids is not possible. So it is

possible for us to separate the two

liquids till we reach point C the

esotropic composition. So one of the

usefulness of the diagrams is to find

out conditions where the two liquids can

be separated in what is called

distillation. But here we end up always

with anotropic mixture whether the

system shows a maximum in the boiling

point or a minimum in the boiling point.

So that needs explanation. These

mixtures as I told you are called

esotropic mixtures. Let us look at an

example of a system which shows boiling

point maximum. That means shows an esotropic

esotropic

mixture. This is offered

by nitric acid. Nitric acid forms an

isotropic mixture with water having 68%

nitric acid. You can see there is a

boiling point maximum. The temperature is

120.5° which is more than the boiling

point of nitric acid and

water. Let us now try to explain what is

meant by

esotropic. As we have

seen a mixture of this composition boils

without change in

composition. Our attempt to separate the

mixture by distillation fails.

entire phase diagram has this point

where the distillation is not possible.

All other mixtures can be distilled in

order to improve the purity

levels. We have an example here which is

like nitric acid. A mixture of HCl water

also gives anotropic mixture having 80%

water with boiling point of

108.6° centigrade. There are systems

showing boiling point maximum. One is

trricchloromthane acetone mixture. The

other is nitric acid water mixture and

as I told you there are systems showing

boiling point maximum. Ethanol water is

one example that we have seen. Another

is offered by dioxane water

system. Why isotropic mixtures boil at constant

constant

temperature? We can look for an answer

in the phase rule. We use the equation F

is equal to C minus P + 2. In this case,

it's a two component system. That's why

C is equal to 2. Number of phases is

two. And hence the number of degrees of

freedom available to us is

two. But during the experiment, we have

kept pressure constant at one

atmosphere. We lose one degree of

freedom. So we have only one degree of

freedom available to us. But there is a

restriction imposed on the

system. What is that restriction? At point

point

C, you can see here where the two curves

meet, the vaporous curve and the

liquidous curve meet. What does that

mean? The composition of the

vapor and the composition of the liquid

are the same. And this

restriction takes away the last degree

of freedom that we have. That means the

system becomes

invariant. The mixture therefore has to

boil at constant temperature without

change in composition. Phase rule has

answer to this problem. But then the

question arises how to remove esotropism

because such a mixture cannot be

subjected to distillation to separate

the two

components. We can try various methods.

One of them is freezing the mixture so

that one of the components present in

the mixture may freeze and hence can be

separated. We can resort to selective

absorption. Put a good absorbent which

selectively takes away by absorption one

of the components especially we try for

minor component that is present in the

mixture or by chemical

reaction add some reagent which can

react with it so that it can be

eliminated or we resort to extraction of

the minor component if you have a

suitable solvent extraction

procedure but we have heard of absolute alcohol

alcohol

In the lamps you might have seen there

are two bottles on one which is written

alcohol and on the other absolute

alcohol. What is the

difference? Difference is here alcohol

and water form an esotropic mixture containing

containing 95.6%

95.6%

alcohol which we generally call

rectified spirit. Some water can be

removed from this by chemical reaction

with the quick lime and magnesium ribbon

but about 1% water still remains in it.

It's very difficult to remove water from

alcohol because of the presence of

aotrophism in the system. Then how do

you prepare absolute alcohol which is

nothing but 100% alcohol? Let us try a

method. We use esotropism to kill

esotropism. To the esotropic mixture containing

containing

95.6% alcohol. You add a calculated

amount of benzene and distill the

mixture. What

happens? Benzene has a tendency to form

an esotropic mixture with alcohol and

water. That means a turnary esotropic

mixture is formed which distills off very

very

easily. Entire water is taken

out and what is

left alcohol and a small amount of

benzene which is deliberately added in

excess in order to ensure that all the

water is removed. On further heating

this binary esotropic mixture starts

boiling and a binary esotropic mixture

of alcohol and benzin distills off

taking away all the benzene leaving

behind 100% alcohol that is our absolute

alcohol. one of the best examples where

we can use eotropism to remove eotropism

and prepare absolute alcohol. Let us

move on to partially missible

liquids. We have several examples of

partially missible

liquids. We have dealt with completely

missible liquids in our la last episode

where we discuss solid vapor equilibria

the systems that obey Roll's law. In

this part, we'll take up partially

missible liquids like phenol water

system. Let us look at phenol water

system. The phase diagram is

here. A very simple. You have a

dome-shaped region in which the system

breaks into two phases and outside this

there is complete missibility. Phenol

and water are partially missible.

Therefore, if you take

water at any temperature, say

T1, and add phenol slowly with

stirring, phenol dissolves initially,

but when the solubility limit is reached

here on the dome-shaped curve here, the

system splits into two

phases. One with this composition, the

other with this composition. As you

continue to add additional phenol into

it, the relative amounts of the two

phases will change

change

and finally we reach a system in which

there is large excess of phenol and

again the missisibility is attained.

Therefore, on either side of the

dome-shaped area, we have complete

missibility. But within that, the system

is always split into two

phases. You can see here if we increase the

the

temperature, the compositions of the two

phases that are in equilibrium start

coming closer together. In other words,

the length of the tie line joining the

compositions of the two phases decreases

and ultimately the length becomes zero

that is a point which is called critical

solution temperature at which the two

become missible. Above this temperature

they always form one phase. So point C

is important. It is called the critical

solution temperature. Let us apply phase

rule equation to this. The equation is

F= C minus P +

2. In the one-phase region where phenol

and water are completely

missible, C is equal to 2. Two component

system phase is 1. Therefore, we can

find out the number of degrees of

freedom which is

three. As pressure is kept constant, we

are left with two degrees of freedom.

The system is

bariant. In the two-phase region where

there is partial

missisibility, number of degrees of

freedom works out to be one. The system

is univariant system. But at critical

solution temperature, that is at point

C, the two phases become identical. The

composition of the two phases become

identical. Hence the number of degrees

of freedom F becomes zero. the system

becomes invariant

system. Point C as I told you is called

critical solution temperature

abbreviated as CST. Since it is the

maximum in the curve in this particular

case it is called upper CST. It is a

characteristic of the system. It needs

some explanation here. Where do we use this?

this?

The domestic disinfectant that we get

what we call generally

phenile is a system like this. It's a

dark liquid but

clear. If you add water to it, it becomes

becomes

milky. So why it turns milky? We take

into account the effect of chemicals or

impurities on CST. CST may be made to

increase or decrease with the addition

of certain impurities deliberately. In

this particular case, you add a

substance which suppresses the CST of

phenol water system so that when you add

water, it goes into the two-phase region

from the one-phase region when it was

supplied to you.

Let us see the changes that are taking

place when we deal with such phase

diagrams. You start with the phenol

water mixture with this

composition. As you heat it and say take

it to temperature

T1 with this original composition, the

system is in two phases. One with the

composition A and other the phase that

is in equilibrium is with composition B.

And as you heat it, say to temperature

T2, the composition now of the two

phases is A prime and B prime. And the

relative amounts of the two phases are

given by the T line. The two arms of the

T line. And you can see one of the arms

start decreasing in length. And as you

increase the temperature, ultimately we

reach the point AP prime here. And you

can see that the amount of one of the

phases in this particular case BP prime

decreases almost to zero and further

increase in temperature makes the two

liquids completely missible. So we

attain missibility from a two-phase

system by heating and the missisibility

is attained by the disappearance of one

of the phases. If you start with phenol rich

rich

phase, you will have systems giving rise

to missibility

where the water-rich phase

disappears. If you start with a

particular composition in this case

given by X prime and heat it you can

clearly see that the two phases remain

in equilibrium till we reach the point C

at which both the phases are present and

missibility is attained till the

last point the two phases will be

present which means misibility is

attained when the compositions of the

two phases that are present in

equilibrium become

identical and therefore this point is

very characteristic of the

system and as I told you it is called

upper critical solution temperature.

There are other systems which

show lower CST like the one that we have

here. A

system given by water and trimethile

amine exactly like phenol water

system but the CST the critical solution

temperature is at the lowest point here.

This is called lower CST.

Why missibility is attained at lower

temperature which is unusual. Generally

we know that the solubility goes on

increasing as we raise the temperature

and ultimately missibility is attained

at a particular temperature. In this

particular case the missisibility is

there at lower temperature with the

increase in temperature the

missisibility decreases.

Probably in such systems there are some

molecular interactions which are

operative at lower temperature which

become weaker as the temperature is

raised. When you have systems like these

two upper and lower CST and interesting

system we have here is the nicotine

water system which shows both lower CST

as well as upper CST. The explanation is

similar. We have complete missibility

outside this circle and within this you

have the system in which there are two

phases in

equilibrium. Let us now look

at systems involving emissible liquids.

If we take two emissible

liquids one and two depending on density

they form separate layers. Liquid one

which is denser is cut off from the

vapor phase. Therefore only the

molecules of liquid two which is lighter

will have chance to escape to the vapor

phase. This is very clear and

therefore in such systems if we bring

about agitation very strongly both the

liquids will have chance to enter the

vapor phase so that the vapor will be in

equilibrium with both the liquids and

total vapor pressure in such cases will

be the sum of the partial vapor

pressures but in the pure state that

means P is equal to PA KN plus PB KN. It

implies that such liquid mixtures will

boil at a temperature which is lower

than the boiling point of either of the

liquids. So each liquid behaves

independently and therefore contributes

to the vapor pressure as if it is a pure liquid.

liquid.

This is true irrespective of the

relative amount of each of the liquids

present in the mixture provided their

amounts are sufficient

enough so that they exist in equilibrium

with their vapor. Now boiling point of the

the

mixture. If you take water as one of the

liquids which is immissible with the

other liquid which we takes like

nitroenzine, hydroenzine etc. even

enolene for that matter. The liquid

mixture will boil at a temperature below

100° centigrade. Take for example

boiling point of pure water as we know

is 100° centiggrade and that of analine

is 184° centigrade. the mixture boils

below 100° say around 98°

centigrade. Exactly similar arguments

holds good for any other liquid mixture

whether water is present or not. So

instead of boiling with water, it is

advantageous to use

steam because it heats up the liquid

mixture as well as it brings about

agitation which is a requirement for

both the liquids to be in equilibrium

with their vapors. This is the assembly

that we use in the distillation

process. So that the liquid mixture that

is present is made to boil over by

passing steam where the steam passed at

vigorously agitates the mixture. The

vapor gets carried water as well as that

of the liquid that is immissible with

water and collected in the distillate.

The process is called the steam

distillation. Obviously because we are

using steam. Why steam distillation?

Normal distillation of such

compounds is taking place at high

temperature because these are high

boiling liquids. They may be unstable at

that temperature. Therefore, if we use

steam distillation, they can be made to

boil at much lower temperature even

below 100° centigrade in this case

because water is being used. What is the

application? Definitely it is to

separate and purify compounds of this

type which we have mentioned earlier

which are set to be steam volatile

compounds. These are generally the

natural products that are very sensitive

to heat. Some of them are steam volatile

and steam distillation can be

effectively used to extract such plant

materials like eucalyptus, citrus oils

and natural oils which are used in

perfumery from the plants. It is found

that in steam distillation the

composition of the distillate is fixed

irrespective of the relative amounts of

the liquids present in the mixture. And

to explain this we can use phase rule

equation f is equal to c minus p + 2.

And if you substitute the value of c as

2 p as 3 we get f is equal to 1. But we

already have kept pressure at constant

at 1 atmosphere which means that the

system becomes invariant. The degrees of

freedom f becomes zero.

Therefore the mixture of the two

imissible liquids will boil at constant

temperature with fixed composition of

the vapor. That means the distillate

will have a fixed composition. We can

calculate this by using the familiar

equations that we have and you are

already familiar with it. We have come

across these equations which relate the

pressure, the partial pressure, the mole

fraction in the vapor phase, the number

of moles that are present in the

system and ultimately we can

calculate and show that the ratio of the

number of moles of each of the solutes

or each of the components present in the

distillate is constant. And in fact we

can calculate the weight ratio by

substituting the values of molecular

weight in the equation. And so that w a

by wb is equal to pa 0 into m a divided

by pb0 into ma where ma and mb stands

for the molecular weight a constant

ratio as expected. This equation in fact

has been used earlier to find out the

molecular weight of compounds that are

freshly isolated. Let us summarize what

we have seen. We have seen today non

ideal systems which show positive and

negative deviation from R's

law. We saw what areottopic mixtures.

We looked at partially missible liquid

systems which show upper CST, lower CST

as well as upper and lower CST. The

examples are here. Finally, we looked at

imissible liquid systems which are

interesting because we find a process

very important in organic chemistry that

is steam

distillation. So this brings us to the

end of our discussions on phase equilibrium.

equilibrium.

What we looked at? We started with the

definitions of terms involved in phase

equilibrium. We derived the phase rule

equation. Then saw the application of

phase rule to one component

system like water, carbon

dioxide, sulfur etc.

then used our

uh knowledge for applying it to two

component systems where we saw solid liquid

liquid

equilibria, liquid vapor

equilibria, liquid gas equilibria

equilibria

involving missible

liquids, partially missible liquids and

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YouTube Transcript:SPECEFGOC II 04 Solutions 4