Solubility

Solubility Solute: Is any substance that dissolves in a liquid or gas substance (solvent) to create a solution Solvent: A solvent is a liquid or gas that dissolves a solid, liquid, or gaseous solute, to produce a solution.

Solubility:- A maximum amount of a solute (solid, liquid and gas) could be dissolved in a specific amount of a solvent.

Ways of Expressing Concentration Mole Fraction, Molarity, and Molality

Solution types

Solubility rules: That simple rule is “like dissolves like” and it is based on the polarity of the systems i.e. polar molecules dissolve in polar solvents (e.g. water, alcohols) non-polar molecules dissolves in non-polar solvents (e.g. n-hexane). The polarity of organic molecules is determined by polar bonds due to the presence of electronegative atoms (e.g. N, O) in polar functional groups such as amines (-NH2) and alcohols (-OH )

The electrons in a water molecule are not distributed equally about the molecule, but are more concentrated near the oxygen atom and less concentrated near the hydrogen atoms. This phenomenon is due to the difference in the electronegativity values of O (3.5) and H (2.1), and the fact that water is a bent (unsymmetrical) molecule.

Soap Structure Soap molecules have two distinct parts: Hydrophilic polar head. Hydrophobic nonpolar tail.

Organic Functional Groups

Organic Molecules with Several Functional Groups

Solubility process When a solute (solid or liquid) dissolved in a suitable solvent, the structural units of the solute becomes separated from each other, and the spaces between them occupied by the solvent molecules.

Polar solvent: A solvent in which behave an electrostatic force between the different charges (positive & negative ) charges and have the ability to solvate the polar solutes. Non Polar solvent: A non-polar solvent is one with molecules that have roughly the same electrical charge on all sides. Non-polar solvents are typically hydrocarbons, such as pentane and hexane.

2- Polar aprotic solvent: Polar solvent doesn't contain acidic proton. 1-Polar protic solvent:- Types polar solvent in which containing an acidic hydrogen (proton) attached directly to Oxygen, Nitrogen and sometimes Halogens atom. Example:- RCOOH,ROH, NH3 & HX 2- Polar aprotic solvent: Polar solvent doesn't contain acidic proton. Example:- Acetone, DMF & DMSO

Factors Affecting Solubility Intermolecular Forces Surface area. Temperature Pressure

Ionic compounds that have especially strong ionic bonds do not dissolve in water because the water molecules cannot provide sufficient hydration energy to separate the ions. Bond strengths of ionic compounds are related to the charges and sizes of the ions. The lattice energy generally increases with the increasing charge and decreasing size of the ions. This trend is reflected in the solubility rules for ionic compounds in water.

For example, Fe(OH)3 (containing a trivalent Fe3+ ion and a monovalent OH- ion) is insoluble in water, whereas NaOH (containing monovalent Na+ and OH- ions) is soluble in water. The hydration energy supplied by water is insufficient to disrupt the large Fe(OH)3 crystal lattice energy and dissolve the compound.

Many polar covalent compounds are also soluble in water Many polar covalent compounds are also soluble in water. For a molecular (covalent) compound to be polar, two criteria must be met. The molecule must contain at least one polar covalent bond, and its molecular geometry must be unsymmetrical. For example, methane, CH4, is a nonpolar compound, whereas methyl chloride, CH3Cl, and ammonia, NH3, are both polar covalent compounds.

Methane Methyl chloride Ammonia For a covalent bond to be appreciably polar, the difference in the electronegativity values of the two atoms in the bond must be 0.4 or higher. In a methane molecule, the electronegativity difference for the C—H bond is 0.4 (see Table 1 ), slightly polar at best.

The N—H bond in ammonia is polar (0 The N—H bond in ammonia is polar (0.9 difference in electronegativity values). Due to the presence of the lone pair of electrons on nitrogen, the molecule is asymmetrical, the shifts of electron density within each N—H bond do not cancel. Therefore, the ammonia molecule is polar. The solubility of a polar compound in water is enhanced if the compound can form hydrogen bond with water. Hydrogen bonding occurs between molecules that contain an O—H, N—H, or F—H bond. Oxygen, nitrogen and fluorine are small, highly electronegative elements containing lone pairs of electrons. They can form hydrogen bond with hydrogen atoms in adjacent molecules.

For example, methanol, CH3OH, can form hydrogen bond with water For example, methanol, CH3OH, can form hydrogen bond with water. The OH group is highly polar; therefore, methanol dissolves in water. When one liquid dissolves in another, the liquids are said to be miscible. On the other hand, 1-hexanol, CH3CH2CH2CH2CH2CH2OH, is insoluble in water. As the number of carbon atoms increase in an alcohol molecule, the polarity of the OH group is less significant, and the molecule becomes less polar. If the number of hydroxyl groups (OH) in a molecule increases, such as in (1,6-hexanediol), HOCH2CH2CH2CH2CH2CH2OH, the more hydrogen bonds it can form with water molecules and the more soluble it is in water.

H-bond H-bond The dashed lines represent hydrogen bonds formed between methanol and water. The over the H atom indicates a partial positive charge and the over the O atom indicates a partial negative charge.