dissolution and solubilityxx_ppt [compatibility mode]

8/19/2019 Dissolution and Solubilityxx_ppt [Compatibility Mode]

1/45

DISSOLUTION AND SOLUBILITY


2/45

Solutions

Solutions are homogeneous mixtures of two

or more pure substances.

In a solution, the solute is dispersed uniformly

throughout the solvent.


3/45

Ways of ExpressingWays of Expressing

Concentrations ofConcentrations ofSolutionsSolutions


4/45

Mass Percentage

Mass % of A =mass of A in solution

total mass of solution × 100


5/45

Molarity (M)

1-L flask (1000.0 mL)

Convenient way to prepare solution in lab,

add 0.5 mol

solute

dilute to mark

get 0.5 M

solution

solutionof liters

soluteof moles (M)Molarity =


6/45


7/45

Parts per Million (ppm) and Parts per

Billion (ppb)

1 part

4 parts

= = 0.25 part per part

10xsolutionof masstotal

solutionincomponentof mass

componentof ppm

6

=


8/45

moles of A

total moles in solution X A =

Mole Fraction ( X )


9/45

THE PROCESS OF

DISSOLUTION


10/45

State of Matter

Kinetic theory of matter Gas; molecules move independently

Solid; thermal motion of molecules is

restricted to vibration about their mean

position

Liquid; thermal motion of molecules is

intermediate between gas and solid


11/45

Solutions

How does a solid dissolveinto a liquid?

What !drives" thedissolution process?

What are the energetic of

dissolution?


12/45

How Does a Solution Form?


13/45

How Does a Solution Form?

1. Solvent molecules attracted to surface ions.2. Each ion is surrounded by solvent molecules.

3. Energy change

Ionic solid dissolving in water


14/45

How Does a Solution Form


15/45

Degree of saturation

Saturated solution

Ø Solvent holds as muchsolute as is possible at

that temperature.

Ø Undissolved solidremains in flask.

Ø Dissolved solute is indynamic equilibrium

with solid solute

particles.


16/45


Unsaturated Solution

Ø Less than themaximum amount of

solute for thattemperature is

dissolved in the

solvent.

Ø No solid remains in

flask.


17/45


Supersaturated

Ø Solvent holds more solute than is normallypossible at that temperature.

Ø These solutions are unstable; crystallization canoften be stimulated by adding a #seed crystal$ or

scratching the side of the flask.


18/45

Crystallization: opposite of the

solution process (solute becomes

reattached to surface of crystal)

Solute + Solvent Solution ←

→

ecrystalliz

dissolve

Saturated solution: no more solid will dissolve and

a dynamic equilibrium exists between the solution

and the undissolved solid


19/45


Unsaturated, Saturated or Supersaturated?

⇒ Depends on How much solute can bedissolved in a solution


20/45


21/45

Energy Changes in Solution

To determine the enthalpy change, we divide theprocess into 3 steps.1. Separation of solute particles ( %H 1).

2. Separation of solvent particles to make !holes" ( %H 2).

3. Formation of new interactions between solute andsolvent ( %H 3).

& We define the enthalpy change in the solutionprocess as

%H soln = %H 1 + %H 2 + %H 3

& %H soln can either be positive or negative dependingon the intermolecular forces


22/45


23/45

Energy Changes in Solution (cont.) The reaction may be exothermic when heat is

released as a result of solubility process, and

the flask gets warm (less common).e.gcalcium hydroxide , calcium acetate , in this

case solubility decreases as temperature

rises.

The reaction may be endothermic when thesystem consumed heat to do the solubility

process, and the flask gets cold (more

common) e.g KNO3 , KBr , in this case

solubility increases as temperature increases.

The reaction is neither exothermic nor

endothermic( %H = 0) e.g NaCl , in this case

the solubility is not affected by temperature


24/45

SOLUBILITY OF LIQUID IN LIQUID


25/45

More Factors Affecting Solubility

Polar liquids tend to dissolve in polar solvents

due to dipole-dipole and H-bonding interactions

CH3CCH3

acetone

||O

CH3CH2OH

ethanol

Miscible: soluble in all proportions (liquids)

Immiscible: do not dissolve in one another


26/45

Factors Affecting Solubility (cont.)

Nonpolar liquids tend to be insoluble in polar

liquids; e.g. hexane (C6H14) does not dissolve inwater. Increasing the nonpolar character of a

substance will decrease its solubility in water.

Solubility and Polarity Solubility in Solubility inAlcohol H2O C6H14

CH3CH2OH ∞ 0.12

CH3CH2CH2OH ∞ ∞

CH3CH2CH2CH2OH 0.11 ∞

CH3CH2CH2CH2CH2OH 0.030 ∞ CH3CH2CH2CH2CH2CH2OH 0.0058 ∞

mol alcohol/100 g solvent


27/45

SOLUBILITY OF SOLIDS IN LIQUID(SOLUTE SOLVENT

INTERACTION)


28/45

DISSOLUTION MECHANISM

1. Interfacial reaction that results in liberationof solute molecule from the solid phase, Thesolution in contact with solid will becomesaturated (Cs).

2. The solute molecules migrate through theboundary layers surrounding the crystal to thebulk of solution, at which concentration is (C).

Boundary layer is nearly static and surroundall wetted surface of the solid.

Mass transfer is slowly through the boundarylayer to the bulk of solution.


29/45


H= unstirred layer

Small film of liquidsurrounding drug particle

Bulk solution


30/45



31/45

Plot of concentration gradient

Concentration

h thickness of the

stagnant layer

Distance from surface

h

Cs

Cb

Csat

C = C b

stagnant

water layer


32/45


33/45

DISSOLUTION RATE

• NOYES-WHITNEY EQUATION

dm/dt= DACs /h

'where dm/dt is the rate of mass transfer of

solute molecule through the diffusion layer Cs is the concentration of the solute at

surface available for migration

D is the diffusion coefficient (m2/s)

A is the area available for molecularmigration

H is the thickness of the boundary layer


34/45

SOLUBILITY OF GASES IN LIQUID


35/45

SOLUBILITY OF GASES IN LIQUID

In general, the solubility of

gases in water increases

with increasing mass.

Why?

Larger molecules have

stronger dispersion

forces.

Usually, increase

pressure will increasegas solubility


36/45


37/45

1.Effect of Structure

Chemists use the axiom

#like dissolves like$:

Ø Polar substances tend

to dissolve in polarsolvents.

ØNonpolar substancestend to dissolve in

nonpolar solvents.


38/45

1. Effect of Structure (cont.)

The stronger the

intermolecular

attractions between

solute and solvent,

the more likely thesolute will dissolve.

Example: ethanol in water

Ethanol = CH3CH2OH

Intermolecular forces = H-bonds; dipole-dipole; dispersion

(Van der Waal bond)

Ions in water also have ion-dipole forces.


39/45

1. Effect of Structure (cont.)

Glucose (which has

hydrogen bonding)

is very soluble in

water.Cyclohexane (which

only has dispersion

forces) is not water-

soluble.


40/45

1. Effect of structure (cont.)

Vitamins A, D, K and E is soluble in nonpolarcompounds (like fats).

Vitamins B and C is soluble in water.


41/45

Which

vitamin is

water-solubleand which is

fat-soluble?


42/45

2. Effect of Pressure on solubility

Increasing pressure above solution forces more gas todissolve.


43/45

2. Effect of Pressure (cont.)

The solubility of liquids and solids

does not change appreciably with

partial pressure.

But, the solubility of a gas in a liquid isdirectly proportional to the partial

pressure above that gas. Effect of

pressure is significant.


44/45


Solubility of a gas in a liquid is a function of thepressure of the gas.

The higher the pressure, the more molecules ofgas are close to the solvent and the greater thechance of a gas molecule striking the surface and

entering the solution. & Therefore, the higher the pressure, the greaterthe solubility.

& The lower the pressure, the fewer molecules ofgas are close to the solvent and the lower the


45/45


Henry"s LawThe amount of a gas diss olved in a

so lut ion is di rect ly proport ion to the

pressu re of the gas above the solut ion

W = kP where

W is the mass of gas dissolved in thesolvent (Mol fraction);

k is the Henry"s law constant for that gas

in that solvent;

P is the partial pressure of the gas abovethe liquid.

dissolution and solubilityxx_ppt [compatibility mode]

Documents