Solutions College Chemistry

Types of Solutions Remember, a solution is a homogenous mixture of two or more substances Saturated solution – contains the maximum number of solute that will dissolve in a solvent at a specific temperature Supersaturate solution – contains more solute than is present in a saturated solution Not stable Eventually the extra solute will come out through crystallization as crystals Unsaturated solution – contains less solute that is present in a saturated solution

Types of Solutions

Molecular View of the Solution Process the intermolecular attractions that hold molecules together in liquids in solids is also responsible for the formation of solutions The ease of solution formation depends on these three types of interactions: 1. solvent-solvent interaction 2. solute-solute interaction 3. solvent-solute interaction

Molecular View of the Solution Process For simplicity, we can think of the solution process as happening in three distinct steps 1. separation of solvent molecules 2. separation of solute molecules 3. solvent and solute molecules mix Process can be endothermic or exothermic DHsoln = DH1 + DH2 + DH3

Molecular View of the Solution Process

Molecular View of the Solution Process Solute-solvent interaction > solute-solute AND solvent-solvent Reaction is favorable and exothermic DHsoln < 0 Solute-solvent interaction < solute-solute AND solvent-solvent Reaction isn’t favorable and is endothermic DHsoln > 0

Molecular View of the Solution Process So if particles are more attractive to each other than the solute-solvent, why does dissociation take place? Two things govern dissociation 1. energy (exothermic is more favorable) 2. tendency towards disorder (entropy) Molecules want to be in a state of disorder (solution), do not enjoy being in order (just solute or just solvent)

Predicting Solubility BP Oil Spill Predicting Solubility “like dissolves like” help us determine what is soluble (or dissolves) in another molecule non-polar solute will dissolve in non-polar solvents and polar solutes will dissolve in polar solvents This is why oil (non-polar) and water (polar) will not mix Also, compounds with similar intermolecular forces will be solubility in each other Ex: CCl4 (carbon tetrachloride) and C6H6 (benzene) are both non-polar and have dispersion forces only  fairly soluble Ex: alcohols (with –OH) dissolve in water

Predicting Solubility Miscibility – ability of two liquids to be completely soluble in each other in all proportions Immiscible – not soluble Solubility rules allows us to determine what ionic compounds will dissolve in water Generally, ionic compounds will dissolve in polar solvents and covalent compounds will dissolve only in nonpolar solvents Covalent compounds lack a dipole moment and cannot effectively solvate an ionic compound (such as NaCl) Solvation – process in which an ion or molecule is surrounded by solvent molecules in a specific manner

Solvation

Example 12.1 Predict the relative solubilities of the following cases: (a) Bromine (Br2) in benzene (C6H6, m = 0 D) and in water (H2O, m = 1.87 D) (b) KCl in carbon tetrachloride (CCl4, m = 0 D) and in liquid ammonia (NH3, m = 1.46 D) (c) formaldehyde (CH2O) in carbon disulfide (CS2, m = 0 D) and in water

Example 12.1 Predict the relative solubilities of the following cases: (a) Bromine (Br2) in benzene (C6H6, m = 0 D) and in water (H2O, m = 1.87 D) Br2 is nonpolar as in benzene (dispersion forces), water is polar Br2 should only dissolve in benzene (b) KCl in carbon tetrachloride (CCl4, m = 0 D) and in liquid ammonia (NH3, m = 1.46 D) KCl is ionic, more soluble in ammonia since it has a larger dipole (c) formaldehyde (CH2O) in carbon disulfide (CS2, m = 0 D) and in water Formaldehyde is polar, carbon disulfide is nonpolar, and water is polar Formaldehyde can also hydrogen bond with water and it is more soluble in water