SOLUTIONSSOLUTIONSANDAND

SOLUBILITYSOLUBILITY

Phase Solubility Analysis: (cont.)Phase Solubility Analysis: (cont.) Steps of determination:

1gm 2gm 3gm 4gm

shaken at constant (T,P)

1gm 2gm 3gm 4gm

Equilibrium

Separate solid from solution

Determine amount dissolved

Plot Y axis (solution conc)

Plot X axis (system conc)

Gibbs phase Rule:Gibbs phase Rule:F= C – P + 2F= C – P + 2F= Degree of freedom (T, P, F= Degree of freedom (T, P, C)C)C= Number of componentsC= Number of componentsP= number of phasesP= number of phasesAt constant T and P At constant T and P (F= C- P)(F= C- P)

Phase solubility curves:Phase solubility curves:

Phase solubility diagram for a pure substance

F=1

F=0

A- B:A- B: Conc is below saturation Conc is below saturation B- C: B- C: Conc is Conc is aboveabove saturation saturation B – C:B – C: has no has no slopeslope, indicating , indicating puritypurityPoint D:Point D: Solubility of pure substance. Solubility of pure substance.

A)For pure substance:A)For pure substance:

Saturation

B) B) ForFor non- pure substance: (one non- pure substance: (one impurity)impurity)

•Phase solubility curve for substance contain one impurityA- B:A- B: Conc is below saturation Conc is below saturation for bothfor both (1 phase) (1 phase) At B:At B: Saturation with Saturation with majormajor component component From B to C:From B to C: Conc is Conc is aboveabove saturation with saturation with majormajor component component and below saturation for minor oneand below saturation for minor one (2 phases) (2 phases)Section C – D:Section C – D: Saturation with both components (3 phases) Saturation with both components (3 phases) Value of Value of AEAE:: Solubility of major component. Solubility of major component.Value of Value of EFEF:: Solubility of minor component Solubility of minor component AtAt BC:BC: Pure solid majorPure solid major

F=2

F= 1F=1

F=0

F=1

The Process of Dissolution

1. The solute is separated from other similar molecules

Step 1

2. The solvent molecules are separated sufficiently from other molecules to create space to accommodate the solute

molecule.

Step 2

3. The solute molecule becomes surrounded by solvent molecules

Step 3+

+ W22

- W12

W11

The free energy change of solution is (w11 + w22 -w12)

W11+ W22 > W12 Endothermic

W11+W22 < W12 Exothermic

The Process of Dissolution

The free energy change of solution is (w11 + w22 -w12)

1- Separation of solute from similar molecules to become surrounded by solvent molecules.

2- Separation of solvent from similar molecules to create space to accommodate the solute.

3- Placing the solute molecule in the solvent cavity requires a number of solute-solvent contacts

4- Dissolution occurs if solute-solvent attraction overcomes:

Solute-solute interaction Solvent-solvent interaction.

The Process of Dissolution

Prediction of solubility in Prediction of solubility in aqueous mediumaqueous medium ((Dilute solution))

Predicting the solubility of solutes in aqueous media depends on:

1. Molecular surface area of solute (substituents)

2.2. Nature of the key chemical groups in the Nature of the key chemical groups in the solute. solute.

Prediction of solubility in aqueous Prediction of solubility in aqueous mediummedium

(Dilute solution)(Dilute solution) The solubility with of molecular

surface area.

A.A. The The larger the solute molecule, the the solute molecule, the larger the the cavity required.cavity required.

B.B. The The greater the number of contacts created. the number of contacts created.

The term w12 is a measure of solute-solvent interactions (SOLVATION).

A.A. Interactions involving the non-polar part of the Interactions involving the non-polar part of the solute. solute.

B.B. Interactions with the polar portion of the Interactions with the polar portion of the solute. solute.

Solvation and hydrationSolvation and hydration

Solvation is the process of binding of solvent to solute molecules.

If the solvent is water, the process is hydration

Hydration of non-electrolytesHydration of non-electrolytes

A non-electrolyte does not provide ions in a solution

and therefore current does not flow through such

solution

e.g. Carbohydrates

In a solution of Sucrose, six water molecules are bound to each sucrose molecule as a one unit

Mannitol, sorbitol and inositol are sugar alcohols have very different affinities for water.

The solubility of Sorbitol in water is about 3.5 times that of Mannitol.

(sorbitol has an equatorial –OH group on pyranose sugar).

Hydration of non-electrolytesHydration of non-electrolytes

Compatibility of the equatorial –OH with of the equatorial –OH with the structure of water in bulk. the structure of water in bulk.

Axial hydroxyl -OH groups cannot bond Axial hydroxyl -OH groups cannot bond with water without distorting it. with water without distorting it.

Differences in the Differences in the Lattice EnergiesLattice Energies of the of the crystals may also contribute.crystals may also contribute.

Equatorial -OH

Axial -OH

A cyclohexane molecule

AXIAL -H

Equatorial -H

Hydration of electrolyes and ionic Hydration of electrolyes and ionic groups groups

An electrolyte provides ions in a solution and therefore current flow through such

solution

e.g. Salts as NaCl

Hydration of electrolyes and ionic Hydration of electrolyes and ionic groups groups

All ions in water possess a layer of “Tightly Bound Water”

“Four “ water molecules are in the bound layer of most

monovalent, monatomic ion. The firmly held layer can be regarded as being

in a ‘Frozen’ condition around a positive ion.

Orientation of water molecules Orientation of water molecules around the ionaround the ion

Primary solvent sheath in which the water molecules could be oriented with all the hydrogen atoms of the water molecules pointing outwards. (tightly bound layer)

An intermediate layer of water around An intermediate layer of water around the bound layer which is the bound layer which is less orderedless ordered than than bulk water.bulk water.

Bulk water layerBulk water layer

Ionic species HydrationIonic species Hydration

Water Structure Breakers and Water Structure Breakers and Structure MakersStructure Makers

Ions, which include all the alkali and Ions, which include all the alkali and halide ions except Li+ and F-, are halide ions except Li+ and F-, are calledcalled structure breakersstructure breakers..

Li < Na < K < Rb < Cs in size Li < Na < K < Rb < Cs in size (Monovalent alkali metals)(Monovalent alkali metals)

Li+ and F-, and many polyvalent ions, Li+ and F-, and many polyvalent ions, for example Al3+, increase the for example Al3+, increase the structured nature of water beyond the structured nature of water beyond the immediate hydration layer, and are immediate hydration layer, and are therefore therefore structure makersstructure makers..

Na+ is considered as weak Na+ is considered as weak structure maker.structure maker.

Hydration numbersHydration numbers Hydration number:Hydration number: the number of water molecules the number of water molecules

in he primary hydration layer. (in he primary hydration layer. (tightly bound layertightly bound layer))

Solvation number:Solvation number: the number of solvent molecules the number of solvent molecules in the primary layer in the primary layer

((zero in the case of large ions such as:zero in the case of large ions such as: (iodide, caesium, tetraalkylammonium ions(iodide, caesium, tetraalkylammonium ions). ).

The solvation numbers decrease with increase of ion The solvation numbers decrease with increase of ion size the size the

?? Ionic force field Ionic force field diminishesdiminishes with increasing with increasing

radius.radius.

PolarSolvent (H2O)

I) Electrolytes II) Non-electrolytes

I) Electrolytes

Strong (NaCl) and weak (AgCl) electrolytesStrong (NaCl) and weak (AgCl) electrolytes interact with water (polar), through interact with water (polar), through dipole-dipole-dipole interactionsdipole interactions, where, where

Ionization occurs firstIonization occurs first

Followed by hydrationFollowed by hydration

II) Non-electrolytes

e.g. phenols, alcohols, aldehydes, ketones, amines. The hydration occurs through the formation of solute/solvent unit through through dipole-dipole-dipole interactionsdipole interactions.

Non-polarSolvent

I) Induced dipoleInduced dipole

II) Dimerization

I) Induced dipole-induced dipole:They are able to dissolve other non-polar in which

bonds are weak.e.g. hydrocarbon in hydrocarbon- oil, fat in

petroleum ether.

II) Dimerization:e.g. acetic acid in CCl4.(Consumption of dipole)

Semi-polarSolvent

I) Permanent dipole-induced dipole II) Intermediate solventII) Intermediate solvent

I) Permanent dipole-induced dipole:e.g. alcohol (semi-polar) dissolves in benzene (non-e.g. alcohol (semi-polar) dissolves in benzene (non-

polar) throughpolar) throughinduction of temporary dipole in benzene molecule.induction of temporary dipole in benzene molecule.

Examples of semi-polar solvents are: alcohols and ketones.

II) Intermediate solvent (Cosolvency):II) Intermediate solvent (Cosolvency):e.g. acetone increases the miscibility of ether (non-e.g. acetone increases the miscibility of ether (non-

polar) in water (polar).polar) in water (polar).