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7 Phase Equilibria - 1

77 PPhhaassee EEqquuiilliibbrriiaa ((II)) OOnnee--CCoommppoonneenntt SSyysstteemmss ((IIII)) TTwwoo--CCoommppoonneenntt SSyysstteemmss ((IIIIII)) TThhrreeee--CCoommppoonneenntt SSyysstteemmss

OOnnee--CCoommppoonneenntt SSyysstteemmss (I) Definition of a phase

A phase is any part of a system which is physically separated from other parts of the system by a distinct boundary. A phase can be a solid, liquid, vapor (gas) or aqueous solution which is uniform in both chemical composition and physical state i.e. homogenous. • For example, a mixture of gases consists of one phase only. This is because in the

mixture, the gases are uniformly distributed and there is no distinct boundary between them.

• However, a mixture of oil and water consists of two different liquid phases. This is

because even though both of them are liquids, there is a distinct boundary between the two liquids.

• For a mixture of solids, each solid is regarded as having one phase. • A system with one phase only is described as a homogeneous system while a

system consisting of more than one phase is described as a heterogeneous system. For example, a mixture of ethanol and water is a homogeneous system, whereas a mixture of ice and water is a heterogeneous system.

(II) Phase equilibrium

(i) Changing of physical states involves changes of phase, but not changes of

chemical composition. For example, whenever a solid changes to a liquid (e.g. melting of ice) or a liquid changes to a gas (e.g. vaporization of water), a phase transition occurs.

A phase equilibrium is a balance between phases, that is, the coexistence of two or more phases in a state of dynamic equilibrium. The forward process is taking place at the same rate as the backward process and therefore the relative quantity of each phase remains unchanged unless the external condition is altered.


(ii) A component is a chemical species which may be used to specify the composition of a system. Phase equilibria are classified according to the number of components present in a system. For example, a three phase system of water (i.e. water, ice and vapor) is a one-component system. This is the constituent substance of the three phases is water only.

System Phase Component Number of

components An ice block 1 solid phase Water One A bottle of perfume 1 liquid phase and 1 gaseous

phase (A small amount of perfume vapor is found over liquid perfume.)

Perfume One

A small amount of sodium chloride added to a saturated sodium chloride solution

1 solid phase and 1 liquid phase (A small amount of undissolved salt is found in solution.)

Sodium chloride and water

Two

Iodine in a separating funnel containing water and 1, 1, 1- trichloroethane

2 liquid phases Iodine, water and 1, 1, 1- trichloroethane

Three

(iii) Phase diagrams

The equilibrium between a liquid and its vapor is not the only dynamic equilibrium that can exist between states of matter. Under appropriate conditions of temperature and pressure a solid can be in equilibrium with its liquid state or even with its state. A phase diagram is a graph summarizing the conditions under which equilibria exist between the different states of matter. It also allows us to predict the phase of a substance that is stable at any given temperature and pressure. The general form of a phase diagram for a substance that exhibits three phases is shown in the following diagram. It contains three important curves, each of which represents the conditions of temperature and pressure at which the various phases can coexist at equilibrium.

1. The line from A to B is the vapor-pressure curve of the liquid. It represents

the equilibrium between the liquid and gas phases. The point on this curve where the vapor pressure is 1 atm is the normal boiling point of the substance. The vapor-pressure curve ends at the critical point (B), which is at the critical temperature and critical pressure of the substance. Beyond the critical point the liquid and gas phases become indistinguishable.

Note: For every substance, there exists a temperature, the critical

temperature above which the gas cannot be liquefied, regardless of the applied pressure.

Critical temperature is the highest temperature at which a substance can exist as a liquid.

Critical pressure is the pressure required to bring about liquefaction at the critical temperature.


2. The line AC is the sublimation curve which represents the variation in the

vapor pressure of the solid as it sublimes at different temperatures.

3. The line from AD is the melting point curve (fusion curve) which represents the change in melting point of the solid with increasing pressure. This line usually slopes slightly to the right as pressure sure increases. For most substances the solid is denser than the liquid; therefore, an increase in pressure usually favors the more compact solid phase. Thus, higher temperatures are required to melt the solid at higher pressures. The melting point of a substance is identical to its freezing point. The two differs only in the temperature direction from which the phase change is approached. The melting point at 1 atm is the normal melting point.

• Point A, where the three curves intersect, is known as the triple point. All

three phases are at equilibrium at this temperature and pressure. • Any other point on the three curves represents an equilibrium between two

phases. • Any point on the diagram that does not fall on a line corresponds to

conditions under which one phase is present. • The gas phase is the stable phase at low and high temperatures. The

conditions under which the solid phase is stable extend to low temperatures and high pressures. The stability range for liquids between the other two regions.

(III) Phase diagram of water

The phase diagram is divided by three solid lines into three regions in which only one particular phase exists.

Figure. General shape for a phase diagram for a one component system.


1. Curve TB (melting point curve or fusion curve) represents the variation of melting point with pressure.

2. Curve TC (vapor-pressure curve or vaporization curve) represents the variation

of boiling point with pressure. 3. Curve TA (sublimation curve) represents the variation of sublimation temperature

with pressure. For example, at 100oC and 1.0 atm, water and steam are in equilibrium.

4. The dotted curve TD represents the supercooling curve. Supercooling occurs

when water is rapidly cooled below 0oC at 1 atm and remains in the liquid state. This occurs because water molecules are unable to orientate themselves into the crystalline structure and it remains as a liquid at a temperature below the freezing point. The supercooled condition is a unstable (metastable) state.

When some substance such as dust particles acting as external nuclei is added or

when the temperature is further lowered, ice would form rapidly. Energy is released and brings the temperature back to the melting point, where water continues to freeze.

5. Point T is the triple point of water (0.01oC and 0.006 atm). At the triple point, all

the three phases, ice, water and water vapor (steam) coexist and are in equilibrium. The temperature and pressure must be kept constant in order to maintain all three phases at equilibrium.

6. Point C is the critical point of water (374oC, 218 atm). Beyond the critical point,

the physical nature of water and steam cannot be distinguished. Above the temperature at the critical point (374oC for water), water vapor cannot be liquefied no matter how much pressure is applied .

Water vapor

(Liquid water)

Phase diagram of water


NOTE: The phase diagram of water is not a typical example of a one-component system because line TB (melting point curve) slopes to the left. The melting point decreases as the pressure increases. This occurs only for substances which expand on freezing. This unusual behavior occurs because of the open structure of the regular packing of water molecules in ice. This is exactly what happens in ice skating. The narrow blade of the skate exerts a high pressure on the ice and lowers the melting point. This causes the ice to melt, and also provides a good lubrication for the skate.

The above effect can also be explained by Le Chatelier's principle. When the pressure of the system is increased, the system will tend to reduce its volume. The equilibrium will shift to the right because water molecules are packed more closely in liquid water and thereby reduces its volume.

(IV) Phase diagram of carbon dioxide

There are two main differences between the phase diagram of carbon dioxide and that of water.

(i) The melting point (fusion) curve (line TB) slopes upwards from left to right, i.e.

the melting point increases as the pressure increases, a characteristic behavior for substances which contract on freezing. This is due to the closed packing of carbon dioxide molecules in the solid crystal, and hence the density of the solid phase is higher than that of the liquid phase. According to Le Chatelier's principle, when the pressure of the system increases, the system will tend to reduce its volume. The following equilibrium will shift to the left. Therefore, carbon dioxide tends to exist in solid phase when the pressure increases.

H2O(s) H2O(l)lower higher density density

Figure. Phase diagram of CO2

CO2 (s) CO2 (l)higher lower density density


(ii) The triple point T (-57oC, 5.1 atm) of carbon dioxide is above one atmospheric

pressure. This means that liquid carbon dioxide does not exist at ordinary atmospheric pressure, and solid carbon dioxide (also known as dry ice) sublimes when left exposed to the atmosphere. The sublimation of carbon dioxide results in a low temperature which causes water vapor in the air to form a mist.

Note: Iodine and naphthalene also have a similar behavior as that of carbon

dioxide. Their vapor-pressure curve are far beyond the ordinary atmospheric pressure. Their solids sublime, but do not melt, when they are exposed to the atmosphere.

Example 1 Using the figure on the right, determine the state that water exists under the following conditions. (a) 50oC and 0.1 kPa (b) –30oC and 50.0 kPa (c) 105oC and 1000 kPa (d) 30oC and 100 kPa Solution:


TTwwoo--CCoommppoonneenntt SSyysstteemmss (V) Two-component system - mixture of two miscible liquids

A mixture of two miscible liquids forms a two component system with a single phase. To be miscible these two liquids must be able to mix in any proportions. At a molecular level these liquids must have basically similar chemical structures and hence similar intermolecular interactions. The individual components are relatively free to diffuse throughout the solution and therefore there cannot be any boundary separating one component from another. When two miscible liquids of similar chemical structures are mixed in a ratio of 1 : 1, the boiling point of the mixture is approximately the average of the boiling points of its component liquids. For example, when one mole of hexane, C6H14 (b.p. = 69oC) and one mole of heptane, C7H16 (b.p. = 98oC) are mixed, the boiling point of the mixture is 83oC. Also, the boiling points of different mixtures of hexane and heptane vary almost linearly with the composition or mole fractions of the constituents. Such a solution is known as an ideal solution and obeys the Raoult’s law.

However, mixtures of liquids having dissimilar structures have marked deviations from

the linear boiling point-composition relationship. Mixtures such as trichloromethane and propanone, ethanol and cyclohexane are typical examples of non-ideal solutions.

(VI) Raoult's law for ideal solutions

Raoult's law states that the vapor pressure of a component in a mixture at a given temperature is directly proportional to its mole fraction, and is equal to the product of its mole fraction and the vapor pressure of the pure component at that temperature.

For a mixture of two liquids A and B forming an ideal solution,

partial pressure (partial vapor pressure) of A: PA = χA PoA

partial pressure (partial vapor pressure) of B: PB = χB PoB

where χA = mole fractions of A =

χB = mole fractions of B =

98

69

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t(o C)

0% 100% hexane 100% 0% heptane

Plot of the boiling point against concentration ofa mixture of hexane and heptane

nA nA + nB

nB nA + nB

nA = no. of moles of A nB = no. of moles of B


and PoA and Po

B are the vapour pressures of pure A and pure B respectively. According to Dalton's law, the total pressure of the mixture is equal to the sum of the partial pressures of the two components A and B, i.e.,

Ptotal = PA + PB = χA Po

A + χB PoB

Note: Liquid mixtures that obey the Raoult’s law are said to be ideal solutions.

Conditions for forming ideal solutions:

In order to form an ideal solution consisting of two components A and B, the two components should have similar chemical structures. Consequently, the intermolecular attraction between the molecules in the mixture is approximately equal to that in pure A and that in pure B. Hence, when components A and B are mixed to form an ideal solution, there is very little enthalpy change or volume change. Also, because of the similarities of intermolecular attraction, the tendency of the molecules of A and B in the mixture to change from the liquid phase to the vapor phase is nearly the same as that in pure A and pure B.

Example 2 A mixture of 0.2 mole of alcohol A and 0.5 mole of alcohol B has a total vapor pressure of 40 mmHg at 298 K. If the mixture obeys Raoult's law, find the vapor pressure of pure B at 298 K provided that the vapor pressure of pure A is 20 mmHg at 298 K. Solution: Example 3 Determine the total pressure of a mixture of methanol and ethanol at 298 K based on information given below.

Methanol Ethanol Mass in mixture (g) 64 92 Relative molecular mass 32 46 Vapor pressure at 298 K (mmHg) 90 45


Solution:

Vapor pressure-composition curve

(i) Vapor pressure ∼ mole fraction at constant temperature As the partial pressures of components A and B are directly proportional to the mole fractions of A and B in the mixture respectively, the graph gives two straight lines when plotting PA against χA and PB against χB respectively. The plot of Ptotal (i.e. PA + PB) against the mole fraction of the mixture also gives a straight line.

Graph of vapor pressure against composition of an idealsolution at a constant temperature.

χA χB


(ii) Boiling point ∼ mole fraction at constant pressure

The boiling point of a liquid mixture is the temperature at which its total vapor pressure (i.e. Ptotal ) is equal to 1 atm. As the total vapor pressure of a mixture is related to its composition, the boiling point of the mixture is also related to its composition.

Note: 1. It should be noted that very few liquid mixtures have the kinds of vapor pressure-composition curve or the boiling point-composition curve as shown above because no mixture is perfectly ideal. As the intermolecular attraction between molecules in the mixture cannot be exactly the same as that in the pure liquids, all mixtures show some deviations from Raoult's law.

2. However, the following mixtures are some examples of nearly ideal

solutions because the pairs of molecules in each mixture have very similar structures. They therefore have similar curves as shown above. 1. benzene and methylbenzene 2. bromomethane and iodomethane 3. propan-l-ol and propan-2-ol 4. hexane and heptane

Example 4 The vapor pressures of pure octane and pure 2-methylheptane at 303 K are 19.0 mmHg and 27.0 mmHg respectively. (a) Draw the structural formulae of octane and 2-methylheptane. (b) Calculate the vapor pressure of a mixture of the two liquids at 303 K, given that the

mole fraction of octane is 0.3. (c) What assumption is made before calculating the vapor pressure of the mixture ? (d) The boiling points of octane and 2-methylheptane are 126oC and 112oC respectively.

Estimate the boiling point of the mixture in (b).

χA χB

Graph of boiling point against composition of an ideal solution at a constant pressure.


Solution: (VII) Deviations from Raoult's law - Non-ideal solutions

Most liquid mixtures are not ideal solutions and they show deviations from Raoult's law. They are known as non-ideal solutions. Even for solutions that are considered ideal, their characteristics as ideal solutions only hold for very dilute solutions. (i) Positive deviation from Raoult.'s law

A positive deviation from Raoult's law means that the vapor pressure of a liquid mixture is greater than that predicted according to the law, i.e.

PA > χA Po

A and PB > χB PoB

As shown in the curves, there are maximum vapor pressure and a minimum boiling point

χA χB

χAχB


In a mixture of A and B that shows positive deviation from Raoult's law, the average intermolecular attraction between a molecule of A and a molecule of B is weaker than the average of that between two A molecules in pure A and that between two B molecules in pure B. Therefore, the tendency of the molecules in the mixture to escape from the liquid surface is higher than those expected for an ideal mixture. This results in a higher vapor pressure and a lower boiling point for the mixture than those expected for an ideal mixture. Besides, the process of mixing is always associated with an expansion in volume and absorption of heat.

For example, the mixture of cyclohexane and ethanol is a non-ideal mixture that shows positive deviation from Raoult's law.

In pure cyclohexane, the cyclohexane molecules are held together by weak intermolecular forces. However, in pure ethanol, the ethanol molecules are held together by strong intermolecular hydrogen bonds.

When cyclohexane is added to ethanol, cyclohexane molecules get in between ethanol molecules, and this results in the breaking up of a certain portion of the hydrogen bonds. Therefore, a weaker overall intermolecular attraction is found after mixing, and the molecules have a higher tendency to escape. This makes the total vapor pressure higher and the boiling point of the mixture lower than those predicted according to Raoult's law. Moreover, as a certain portion of the hydrogen bonds is broken in the process of mixing, heat is absorbed from the surroundings and the temperature of the solution is decreased.

The following solutions exhibit positive deviations from Raoult’s law:

carbon disulphide and propane trichloromethane and ethanol (ii) Negative deviation from Raoult's law

A negative deviation from Raoult's law means that the vapour pressure of a liquid mixture is smaller than those predicted according to Raoult's law, i.e.

PA < χA Po

A and PB < χB PoB

In a mixture of A and B that shows negative deviation from Raoult's law, the average intermolecular attraction between a molecule of A and a molecule of B is stronger than the average of that between two A molecules in pure A


and that between two B molecules in pure B. Therefore, the tendency of the molecules to escape from the liquid surface is lower than that expected for an ideal mixture. The mixture thus has a lower vapor pressure and a higher boiling point than those expected for an ideal mixture. Besides, the process is always associated with a reduction in volume and evolution of heat.

As shown in the graphs, there are a minimum vapor pressure and a maximum boiling point. Example of solution that shows negative deviation from Raoult's law: Mixture of propanone and trichloromethane In pure propanone and trichloromethane, their respective molecules are held together by weak van der Waal's forces. However, after mixing the two liquids, strong intermolecular hydrogen bonds are formed between propanone and trichloromethane molecules. As a result, there is an stronger overall intermolecular attraction after mixing. Thus, the molecules have a lower tendency to escape. This in turn makes the total vapor pressure lower and the boiling point of the mixture higher than those predicted according to Raoult's law. Besides, as a certain amount of hydrogen bonds is formed in the process of mixing, heat is released to the surroundings and the temperature of the solution is increased.

χAχB

χA

χB

The vapor pressure-composition curvefor a non-ideal solution that showsnegative deviation from Raoult's law ata constant temperature.

The boiling point-composition curvefor a non-ideal solution that showsnegative deviation from Raoult'slaw at a constant pressure.


(VIII) Fractional Distillation

An ideal or nearly ideal mixture can be separated into their components by fractional distillation. The separation is based upon the difference in boiling point of the components. In the process, the vapor formed above the liquid mixture is always richer in the more volatile component than the less volatile one.

1. Referring to the above diagram, when a liquid mixture of A and B with

composition x is heated, it will boil at temperature T1. The vapor in equilibrium with the solution has a composition x', which is richer in A than x, as shown by the line (1) → (2).

2. On cooling, this vapor of composition x' condenses. If the condensate is heated

again, it will boil at temperature T2,. The vapor formed has a composition x", which is richer in A than x'. This is shown by the line (3) → (4).

3. Again, on cooling, this vapor of composition x" condenses. And if the condensate

is heated, it will boil at temperature T3. The vapor formed has a composition x"', which is richer in A than x". This is represented by the line (5) → (6).

4. By continuing this process of successive condensation and boiling, the vapor

becomes richer and richer in A, and the reside liquid also becomes gradually richer in B. Thus, theoretically, an ideal (nearly ideal solution) of two liquids can be separated into pure components by fractional distillation.

Note: It would be very tedious to separate mixtures by repeated vaporization and

condensation as described above. Instead, the separation is performed as a continuous operation by using a fractionating column. It consists of a long glass tube packed with a large number of glass beads which provide a large surface area for condensing and allows the vapor and liquid to come to equilibrium along the column.

The boiling point-composition curve for the vapor and liquid phases of an ideal solution formed by mixing components A and B. 0 Mole fraction χA 1.0

1.0 Mole fraction χB 0


Action of the fractionating column: When the mixture of A and B boils in the flask, the vapors rise up the fractionating column. On the surface of each bead, vapors condense to give a liquid which is richer in the less volatile component B. The vapors that continue to rise becomes richer in the more volatile component A. As the condensed liquid flows down the column, it meets hot upgoing vapors, and an equilibrium is established between them. The process of vaporization and condensation repeats all along the column. A sufficiently long fractionating column will result in distilling off the vapor of pure A at the top of the column while the residue of pure B will be left eventually in the pear-shaped flask.

In petroleum refining, a fractionating tower is used to separate different components of petroleum.

In the fractionating tower, vapors condense on each plate to give a liquid which is richer in less volatile components. The vapors that continue to rise will be richer in more volatile components. An equilibrium is established when the upward flowing vapors meet the downward flowing liquid. The processes of vaporization and condensation repeat all along the series of trays. The vapors thus cool progressively up the tower, and different fractions of petroleum can be drawn off from different levels of the tower.

The laboratory set-up of fractional distillation.

Figure. Refining petroleum by fractional distillation.

The compartments are separated by trays .

Holes in the trays allowvapors to go up and overflow pipes allowcondensed liquids to godown.


(IX) Azeotropic mixtures

(i) Mixtures that show Negative deviation from Raouit's law

Liquid mixtures which deviate negatively from Raoult's law show a maximum in the boiling point-composition curve.

As shown in above diagram, when a liquid mixture of A and B with composition x is boiled, the vapor in equilibrium with the solution has composition x' which is richer in B, as shown by the line (1) → (2). On cooling, this vapor of composition x' condenses to form a liquid of the same composition x', and this is represented by the line (2) → (3). Vaporization and condensation continue until only the pure B is obtained in the distillate. The residue liquid obtained ultimately will have the composition M. It has a constant boiling point which is the maximum boiling point shown in the curves. This mixture is known as an azeotropic mixture (also known as a constant boiling mixture). In an azeotropic mixture, the composition of the vapor and that of the liquid are the same. Therefore, it is not possible to obtain pure A in this manner.

Similarly, as shown on the other side of the diagram, when the mixture is boiled starting from composition y, repeated distillation occurs until the pure component A is obtained in the distillate. The residue liquid will ultimately be the azeotropic mixture with maximum boiling point and of composition M. It is therefore not possible to obtain pure B in this manner. Therefore, the two components in such a mixture cannot be completely separated by fractional distillation.

0 Mole fraction χA 1.0 1.0 Mole fraction χB 0


(i) Mixtures that show positive deviation from Raouit's law

Liquid mixtures which deviate positively from Raoult's law show a minimum in the boiling point-composition curve.

As shown in the following diagram, when a liquid mixture of A and B with composition x is boiled, the vapor in equilibrium with the solution will have a composition x' which is richer in A. This is represented by the line (1) → (2). On cooling, this vapor of composition x' condenses to form a liquid of the same composition x', and this is shown by the line (2) → (3). Vaporization and condensation continue until only the pure B is left in the residue liquid. The distillate will ultimately be the azeotropic mixture with the minimum boiling point and the composition M. The similar but reversed case is shown on the other side of the diagram. Starting from a composition y, the vapor will have a composition y' which is richer in B. Repeated distillation will again yield the azeotropic mixture in the distillate and pure A in the liquid phase. Note: 1. An azeotropic mixture is formed once the vapour curve and liquid

curve overlap. 2. An azeotropic mixture has a constant boiling point, and it cannot be

separated by fractional distillation.

0 Mole fraction χA 1.0 1.0 Mole fraction χB 0


Example 5 At one atmospheric pressure, nitric acid (b.p. = 87oC) and water (b.p = 100oC) form a constant boiling mixture. It has a boiling point of 122oC and contains 65% by mass of nitric acid. (a) Define the term constant boiling mixture. (b) Sketch and label fully the boiling point-composition diagram for the mixture of nitric

acid and water. (c) What changes would take place when nitric acid is added to water? Explain briefly. (d) State Raoult's law. Explain what is meant by the statement that 'a nitric acid-water

mixture shows a negative deviation from Raoult's law'. (e) Explain the changes in temperature and composition of the liquid mixture during the

distillation of a nitric acid-water mixture containing initially 20% by mass of nitric acid?

Solution:


Example 6 At 50oC the vapor pressure of benzene is 273 mmHg and that of methylbenzene is 95.0 mmHg. (i) Assuming ideal behavior, determine the total vapor pressure at 50oC of a mixture of

15.62 g of benzene and 73.70 g of methylbenzene, and the composition of the vapor formed from this mixture. (Relative molecular masses: benzene = 78.11; methylbenzene = 92.13)

(ii) In an actual experiment the vapor pressure observed in the above system is 148 mmHg. Briefly describe the origin of any difference.

(iii) If a small amount of this distillate is collected and distilled for a second time at 50oC, calculate the composition of the first droplet of the second distillate. The mixture of methylbenzene and benzene is assumed to behave ideally over the entire range of composition. (p.488 and 493, Fillan II)

Solution:


TThhrreeee--CCoommppoonneenntt SSyysstteemmss If a solute is added to two layers of immiscible solvents in both of which it is soluble, it will dissolve in the two solvents to different extent. This gives a system of two phases (two immiscible solvents) and three components (two solvents and one solute). (X) Partition of a solute between two immiscible solvents

(i) Partition law

The partition law (also called distribution law) states that at a given temperature, the ratio of the concentrations of a solute in two immiscible solvents is constant when dynamic equilibrium has been reached. There is no unit for the partition coefficient.

For example, when iodine is added to a beaker containing water and 1,1,1-trichloroethane which are immiscible, iodine will dissolve in both layers to different extent. The dissolved iodine molecules will be moving across the interface between the two solvents in opposite directions. When the rates of movement of iodine molecules in both directions are equal, a dynamic equilibrium is established. When this occurs, the concentrations of iodine in water and 1,1,1-trichloroethane reach a particular ratio, the partition (distribution) coefficient (constant). Note that the partition law does not apply to systems where the solute does not have the same molecular form in both solvents. For example, when ethanoic acid is dissolved in water, the intermolecular hydrogen bonds between ethanoic acid molecules are broken as stronger hydrogen bonds between water molecules and ethanoic acid molecules are formed. However, when ethanoic acid is dissolved in benzene, the hydrogen bonds between ethanoic acid molecules remain intact. and exists as dimmers. For this reason, the partition law cannot be applied to the distribution of ethanoic acid between water and benzene.

(ii) Solvent extraction

Solvent extraction is a method of separating a substance from a mixture, using the principle of partition equilibrium of a solute between two immiscible solvents. In organic preparations, the organic products can be separated by shaking the product mixture with a suitable organic solvent such as hexane which is immiscible with water in a separating funnel. The organic product tends to be more soluble in the organic solvent and the inorganic impurities, being soluble in water, will remain in the aqueous layer. After shaking the mixture in a separating funnel and allowing it to settle, an organic and an

Partition coefficient, KD = Concentration of solute in solvent 1 Concentration of solute in solvent 2

Ethanoic acid dimmer

A separating funnel


aqueous layer can be obtained. The two layers can then be separated and the organic layer with most of the organic compound can be obtained. The organic solvent can further be removed by distillation.

Taking separating iodine as another example. Iodine can be extracted from water by adding hexane. Hexane is less dense than water and forms the upper layer whereas water forms the lower layer. Iodine is much soluble in hexane than in water and can therefore be extracted by collecting the upper layer and distilling off the organic solvent.

Example 7 An organic compound X has a partition coefficient of 30 in ethoxyethane and water. Suppose we have 3.1 g of X in 50 cm3 of water and 50 cm3 of ethoxyethane is then added to extract X from water. How much X is extracted by ethoxyethane ? Solution: Example 8 The partition coefficient KD of compound A between two immiscible liquids B and C is 20. If 5 g of A was dissolved in 50 cm3 of liquid C and extracted with 100 cm3 of liquid B, determine the mass of A in liquid B. Solution:


Example 9 Three different 1,1,1-trichloroethane solutions containing different concentrations of iodine were used in an experiment. At 298 K, a 100 cm3 portion of each solution was shaken with 100 cm3 portions of water respectively until equilibria was reached. The volume of 0.10 M sodium thiosulphate required for titration of the iodine in (A) 2.0 cm3 of the 1,1,1-trichloroethanc layer, and (B) 50.0 cm3 of the aqueous layer are as follows:

Volume of thiosulphate solution used (cm3) Experiment number A B

1 20.50 6.00 2 35.30 10.40 3 53.20 15.60

Determine the average partition coefficient of iodine between 1,1,1-trichloroethane and water. Solution:


Example 10 At 298 K, 50 cm3 of an aqueous solution containing 6 g of solute Y is in equilibrium with 100 cm3 of an ether solution containing 108 g of Y. Calculate the mass of Y that could be extracted from 100 cm3 of an aqueous solution containing 10 g of Y by shaking it with (a) 100 cm3 of fresh ether, and (b) 50 cm3 of fresh ether twice at 298 K. Solution:


(XI) Paper chromatography

Paper chromatography is a method used to separate and analyze mixtures. For example, it can be used to separate a mixture of dyes.

The filter paper, which contains a thin film of water trapped on it, forms the stationary phase. The solvent is called the mobile phase or eluent. The solvent moves up the filter paper by capillary action. As the solvent moves across the sample spot of a mixture of dyes, partition of the dyes between the stationary phase and the mobile phase occurs. The states of partition of the dyes depend on:

1. the tendency of the dyes to attach to the adsorbed water, and 2. the tendency of the dyes to dissolve in the solvent.

As different dyes have different states of partition between the mobile and stationary phases, they would be moved upwards to different extent. Dyes which are more soluble in the solvent than in water are moved up by the mobile phase at a faster rate. Conversely, dyes which are more soluble in water than in the solvent are moved up by the mobile phase at a slower rate. As fresh solvent is continuously moving up, solvent extraction occurs continuously along the filter paper.

When the solvent front (i.e. distance travelled by the solvent) almost reaches the top of the paper, the paper is removed and dried. The filter paper so obtained is known as the chromatogram. As the dyes are colored, they are easily observed after separation. But for a mixture of substances that are colorless (e.g. in the separation of amino acids), the chromatogram is sprayed with ninhydrin to color the amino acids. The solvent front and the positions of the spot are marked and the distances traveled by spot and solvent are measured.

The ratio of the distance travelled by a spot to that travelled by the solvent is known as the Rf value, i.e.,

Note: Rf value is always less than one.

Cover

Solvent front

Chromatography paper

Two components in a sample Glass container

Solvent

Original samples

Set-up of paper chromatography

Rf =Distance travelled by spot Distance travelled by solvent


Rf values of components A and B can be determined as follows:

The Rf value of any particular substance is the same for a particular solvent at a given temperature, therefore different components in the mixture can be identified. The following table lists the Rf values of some amino acids in two different solvents at a given temperature.

Rf value of component A = d2d1

Rf value of component B = d3d1

bcbf

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