Môn Hóa Lý: Chapter 10 liquid vapour

• The boiling point of a solution is the point at which enough energy has been added to overcome the intermolecular forces that hold the solute in the solution.• At this point, the molec

Dr Ngo Thanh An

Chapter 10 – Solution and Liquid-vapor equilibrium

Solutions

• are homogeneous mixtures of two or more

substances

• consist of a solvent and one or more solutes

Solutions: solute and solvent

Solutes

• spread evenly throughout the solution

• cannot be separated by filtration

• can be separated by evaporation

• are not visible but can give a color to the

solution

Nature of solute in solution

• The solute and solvent in a solution can be a solid, liquid, and/or a gas.

Examples of solutions

• is the most common solvent

• is a polar molecule

• forms hydrogen bonds between the hydrogen atom in one molecule and the oxygen atom

in a different water molecule

Water

Possible combinations of solutes and solvents

Combination of solutes and solvents in solutions

Na+ and Cl– ions

• on the surface of a NaCl crystal are attracted to polar

water molecules

• are hydrated in solution by many H2O molecules

surrounding each ion

Formation of a solution

Two substances form a solution

• when there is an attraction between the particles of the solute and solvent

• when a polar solvent (such as water) dissolves polar solutes (such as sugar) and/or ionic solutes (such as NaCl)

• when a nonpolar solvent such as hexane (C6H14) dissolves nonpolar solutes such as oil or grease

Like dissolves like

Water and a polar solute

• The boiling point of a solution is the point at which enough energy has been added to overcome the intermolecular forces that hold the solute in the solution.

• At this point, the molecules gain enough kinetic energy to produce a pressure that is greater than the atmospheric pressure keeping them in solution

• Once this point is reached, the solution vaporizes (becomes a gas)

Boling point

-• Saturated vapor: A vapor that is about to condense.

• Saturated liquid–vapor mixture: The state at which the liquid and vapor phases coexist in equilibrium.

• Superheated vapor: A vapor that is not about to condense (i.e., not a saturated vapor).

• The freezing point of a solution is the point where enough energy has been removed from the solution to slow the molecules down and increase intermolecular forces so the solution becomes a solid

Freezing point

Vapor pressure or equilibrium vapor pressure is defined as the pressure exerted by a vapour in

thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system The equilibrium vapor pressure is an indication of a liquid's evaporation rate It relates to the tendency

of particles to escape from the liquid (or a solid) A substance with a high vapor pressure at normal temperatures is often referred to as volatile.

Vapor pressure

Mass percentage (weight percentage):

Mole fraction: The amount of a given component (in moles) divided by the total amount (in

of mass total

component of

mass

component the

of percentage

×

=

mass

msolute = moles solute per kilogram solvent

= moles per kg or (mol kg-1)

Molarity

csolute = moles solute per volume solution

= moles per liter of solution (mol L-1)

Solution composition

 Having same intermolecular force

Ideal solution

• Different intermolecular force: fA-A ≠ fB-B ≠ fA-B

• when mixed, ∆U ≠ 0; ∆H ≠ 0; ∆V ≠ 0

• Not satisfying the ideal equations

Non-ideal solution

Factors Affecting Solubility

1. Structure Effects

2. Pressure Effects

3. Temperature Effects for Aqueous Solutions

Gas solubility in liquid

Molecular Interactions

–Polar molecules , water soluble, hydrophilic (water loving)

• E.g., Vitamins B and C; water-soluble

–Non-polar molecules , soluble in non-polar molecules, hydrophobic (water fearing)

• E.g., Vitamins A, D, K and E; fat-soluble

1 Structure effects

Gas solubility in liquid

2 Pressure effects

Gas solubility in liquid

Gas i (Pi) = Solution (Concentration xi)

i

x K

P

=

Henry’s Law (for dilute solutions): The mole fraction of volatile solute is

proportional to the vapor pressure of the solute.

X = kH P

kH = Henry’s Law constant, X = mole fraction.

Increasing the partial pressure of a gas over a liquid increases the amount of

gas dissolved in the liquid.

kH depends on temperature.

Gas solubility in liquid

When the partial pressure of nitrogen over a sample of water at 19.4°C is 9.20

atm, the concentration of nitrogen in the water is 5.76 x 10-3 mol L-1

Compute Henry’s law constant for nitrogen in water at this temperature.

Gas solubility in liquid

The solubility of some

solids as a function of temperature.

The aqueous solubility of most solids increase with increasing

temperature, some decrease with temp.

3 Temperature effects

Gas solubility in liquid

The solubility of some gases in water as a function of

temperature at a constant pressure of 1 atm.

3 Temperature effects

Gas solubility in liquid

Consider equilibrium:

Gas i ↔ Solution (Conc xi) + ∆ H1

Solid i ↔ Solution (Conc xi)+ ∆ H2

/ (

)

(

solution

x solid

gas x

solution

x

i i

Applying Van’t Hoff equation:

Solute is dissolved following steps: i(gas, solid) → iliq → isol , therefore:

∆ H1 = λ cond.+ ∆ Hdilute + ∆ Hsolvate ≈ λ cond = λ i

∆ H2 = λ melt.+ ∆ Hdilute + ∆ Hsolvate ≈ λ melt = λ i

Integrating both sides:

λi = const and P = const.

To: condensation or melting temperature of pure substance i

1x

i

T

dT

R

x ln

When two liquids are mixed, it could be:

Two substances A, B (in solution) are in equilibrium with A, B (in gas phase)

Necessary parameters to determine system’s state: x, y, T, P (x: mole fraction in liquid phase, y: mole fraction in gas phase)

1 Raoult’s Law, non-volatile solute

• Consider a non-volatile solute (component 2) dissolved in a volatile solvent (component 1).

• X1 = the mole fraction of solvent

o A A

x P

P

x P

o B

o

A

B

o B B

o

A

B

o B A

o A B

A

x P P

P

P

x P x

P

P

x P x

P P

P

P

− +

=

1

x y

1

1 + −

=

α α

A

B A

B

x

x y

y

α

=

B B

B P x

A A

A P x

A

B vapor

A

B A

B

P

P n

n y

x y

1

1 + −

=

α α

4 T-x diagram:

I Two-miscible component solution (ideal)

) RT /

exp(

K P

) RT /

exp(

K P

BB

0B

AA

0A

o B

o

A P P x P

B

A A

B B

A

RT

K RT

K RT

Binary-System Phase Diagram with Three Variables: P, T and x

 The solid part of the surface represents the

“liquid + vapor” region.

 Above it we have the liquid phase and

below it we have the vapor phase.

 On the T-P planes we have pure liquid

curves where the boiling point curves can be

seen

I Two-miscible component solution (ideal)

 On the P-x plane we have the normal

pressure-composition phase diagram It can be viewed at

different temperatures.

 On the T-x plane we have the

temperature-composition phase diagram, which is more

commonly used in experimental work since it is more

convenient to fix P rather than T.

I Two-miscible component solution (ideal)

If P= const 

 The T-x (isobaric) phase diagram is represented by a

double curve or a lens, above which the vapor phase exists

and below which the liquid phase exists, unlike the case in the

P-x phase diagram.

 The lowest part of the lens corresponds to the pure liquid

with the highest vapor pressure “the one that vaporizes more

easily”.

 The opposite is true for the highest part end of the diagram.

I Two-miscible component solution (ideal)

) ( x B g

vapor of

(mass) mole

phase liquid

of

(mass)

Qa

Qa mole

=

a Vapor pressure

If Preal-vapor > Pideal-vapor : positive deviation system

If Preal-vapor < Pideal-vapor : negative deviation system

Cause: fA-A ≠ fA-B ≠ fB-B

II Two-miscible component solution (real)

Liquid-Vapor Equilibrium Deviating from Raoult’s

Law

When we have positive deviation from Raoult’s law,

Ptot could be in some parts greater than both P*A

When we have negative deviation from Raoult’s law,

Ptot could be in some parts less than P*A and P*B

As a result, a minimum in the pressure-composition

curve is observed.

II Two-miscible component solution (real)

Liquid-Vapor Equilibrium Deviating from Raoult’s

Law

If the P-x plot shows maximum (positive deviation

from Raoult’s law), the T-x plot will show a minimum.

Liquid-Vapor Equilibrium Deviating from Raoult’s

Law

II Two-miscible component solution (real)

If the P-x plot shows minimum (negative deviation from

Raoult’s law), the T-x plot will show a maximum.

II Two-miscible component solution (real)

Liquid-Vapor Equilibrium Deviating from Raoult’s

Law

Azeotropes or azeotropic mixtures

An azeotropic mixture “boiling without changing” is the mixture whose liquid and vapor phases have identical compositions of the two

components.

II Two-miscible component solution (real)

• Some mixtures of liquids, because of attractions or repulsions between the molecules, do not behave ideally

• These mixtures do not obey Raoult’s Law

• An azeotrope is a mixture with a fixed composition that cannot be altered by either simple or fractional

Azeotropes with minimum boiling points at 1 atm pressure

Azeotropes with maximum boiling points at 1 atm pressure

II Two-miscible component solution (real)

Distillation is based on the fact that the vapour of a boiling mixture will be richer

in the components that have lower boiling points.

Therefore, when this vapour is cooled and condensed, the condensate will contain more volatile components At the same time, the original mixture will contain more of the less volatile material

Distillation is a process wherein a liquid or vapour mixture of two or more substances is separated into its component fractions of desired purity, by the

application and removal of heat.

Application: - Separation of mixtures of liquids into their components

- most important processes of the chemical industry

- a common method for this separation is distillation

III Distillation

• Vaporization is the conversion of a liquid to a gas.

• The enthalpy of vaporization ( ∆Hvapn) is the quantity of heat that must be absorbed to vaporize a given amount of liquid at a constant temperature.

• Condensation is the reverse of vaporization The enthalpy of condensation ( ∆Hcondn) accompanies this change of a gas to a liquid.

• Enthalpy is a function of state: therefore, if a liquid is vaporized and the vapor condensed at constant temperature, the total ∆H must be zero:

∆Hvapn + ∆Hcondn = 0

∆Hcondn = – ∆Hvapn

III Distillation

Some enthalpies (Heats) of Vaporization at 298 K

III Distillation

• The vapor pressure of a liquid is the partial pressure exerted by the vapor when it is in dynamic equilibrium with the liquid at a constant temperature.

vaporization

condensation

III Distillation

Liquid–Vapor Equilibrium

More vapor forms; rate of condensation

of that vapor increases …

… until equilibrium is attained.

III Distillation

III Distillation

• Boiling point : the temperature at which the vapor pressure of the liquid equals the external pressure.

• Normal boiling point : boiling point at 1 atm.

• Critical temperature (Tc): the highest temperature at which a liquid can exist.

• The critical pressure , Pc, is the vapor pressure at the critical temperature.

• The condition corresponding to a temperature of Tc and a pressure of Pc is called the critical point.

III Distillation

(i) Simple distillation- difference in boiling points of compounds is more than 40ºC e.g.– chloroform (b p 334K) and

aniline (b p 457K)

III Distillation

(ii) Fractional distillation- difference in boiling points of compounds is less than 40ºC e.g – acetone (b p 329K) and methyl alcohol (b p 338K).

III Distillation

(iii) Vacuum distillation- used for organic compounds which decompose at or below their boiling points e.g Glycerol.

III Distillation

(iv) Steam distillation- used for organic compounds which are immiscible with water and are steam volatile e.g Aniline.

III Distillation

How to remove the aezotrope point?

T

P1

P2 P3

1. Changing pressure.

2. Adding the third component

III Distillation

1 The total vapor pressure above an immiscible system is equal to the sum of the vapor pressures independent of their relative amounts

PT = P˚A + P˚B + P˚C + …

2 The mixture will boil at a temperature typically lower than either liquid

3 The mole fraction of each component (nA and nB) in the vapor phase is given by the ratio of its partial pressure over the total pressure:

nA = P˚A/PT and nB = P˚B/PT

If the vapor is condensed, the resulting distillate has the same composition The ratio of the mole fractions for A and B in the distillate is then

given by equation: nA/nB = P˚A/P˚B

IV Two-immiscible component solution

IV Two-immiscible component solution

0

Pout 0

Application: Steam distillation

IV Two-immiscible component solution

IV Two-immiscible component solution

V Partially miscible liquid