SOLUTIONS AND SOLUBILITY

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: F= C – P + 2 F= Degree of freedom (T, P, C) C= Number of components P= number of phases At constant T and P (F= C- P)

Phase solubility curves: Phase solubility curves: Phase solubility diagram for a pure substance F=1 F=0  A- B: Conc is below saturation  B- C: Conc is above saturation  B – C: has no slope, indicating purity  Point D: Solubility of pure substance. A)For pure substance: Saturation

B) For non- pure substance: (one impurity) Phase solubility curve for substance contain one impurity  A- B: Conc is below saturation for both (1 phase)  At B: Saturation with major component  From B to C: Conc is above saturation with major component and below saturation for minor one (2 phases)  Section C – D: Saturation with both components (3 phases)  Value of AE: Solubility of major component.  Value of EF: Solubility of minor component  At BC: Pure solid major F=2 F= 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 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 aqueous medium () Prediction of solubility in aqueous medium (Dilute solution) Predicting the solubility of solutes in aqueous media depends on: Molecular surface area of solute (substituents) 2. Nature of the key chemical groups in the solute.

Prediction of solubility in aqueous medium (Dilute solution) The solubility with of molecular surface area. A. The the solute molecule, the the cavity required. A. The larger the solute molecule, the larger the cavity required. B. The the number of contacts created. B. The greater the number of contacts created. The term w12 is a measure of solute-solvent interactions (SOLVATION). A. Interactions involving the non-polar part of the solute. B. Interactions with the polar portion of the solute.

Solvation 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-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-electrolytes

of the equatorial –OH with the structure of water in bulk. Compatibility of the equatorial –OH with the structure of water in bulk. Axial hydroxyl -OH groups cannot bond with water without distorting it. Axial hydroxyl -OH groups cannot bond with water without distorting it. Differences in the Lattice Energies of the crystals may also contribute. Differences in the Lattice Energies of the crystals may also contribute. Equatorial -OH Axial -OH

A cyclohexane molecule AXIAL -H Equatorial -H

Hydration of electrolyes and ionic 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 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 around 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 the bound layer which is less ordered than bulk water.  Bulk water layer

Ionic species Hydration

Water Structure Breakers and Structure Makers Ions, which include all the alkali and halide ions except Li+ and F-, are called structure breakers. Ions, which include all the alkali and halide ions except Li+ and F-, are called structure breakers. Li < Na < K < Rb < Cs in size (Monovalent alkali metals) Li < Na < K < Rb < Cs in size (Monovalent alkali metals) Li+ and F-, and many polyvalent ions, for example Al3+, increase the structured nature of water beyond the immediate hydration layer, and are therefore structure makers. Li+ and F-, and many polyvalent ions, for example Al3+, increase the structured nature of water beyond the immediate hydration layer, and are therefore structure makers. Na+ is considered as weak structure maker. Na+ is considered as weak structure maker.

Hydration numbers Hydration number: the number of water molecules in he primary hydration layer. (tightly bound layer) Hydration number: the number of water molecules in he primary hydration layer. (tightly bound layer) Solvation number: the number of solvent molecules in the primary layer Solvation number: the number of solvent molecules 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 size the The solvation numbers decrease with increase of ion size the ? Ionic force field diminishes with increasing radius. Ionic force field diminishes with increasing radius.

Polar Solvent (H2O) I) Electrolytes II) Non- electrolytes I) Electrolytes Strong (NaCl) and weak (AgCl) electrolytes interact with water (polar), through dipole-dipole interactions, where Ionization occurs first Ionization occurs first Followed by hydration Followed by hydration II) Non-electrolytes through dipole-dipole interactions e.g. phenols, alcohols, aldehydes, ketones, amines. The hydration occurs through the formation of solute/solvent unit through dipole-dipole interactions.

Non-polar Solvent I) Induced dipole Induced 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-polar Solvent I) Permanent dipole -induced dipole II) Intermediate solvent I) Permanent dipole-induced dipole: e.g. alcohol (semi-polar) dissolves in benzene (non-polar) through induction of temporary dipole in benzene molecule. Examples of semi-polar solvents are: alcohols and ketones. II) Intermediate solvent (Cosolvency): e.g. acetone increases the miscibility of ether (non-polar) in water (polar).