12.2 – NOTES Solutions Intro

Endo Exo Solvation Demo

I. What are solutions? A. Characteristics of solutions Soluble – a substance the dissolves in a solvent Insoluble – A substance that does not dissolve in a solvent Liquids Immiscible – when a solvent will not dissolve a solute Liquids Miscible – when a solvent will dissolve a solute

Solvation in aqueous solutions – - process called solvation - attractive forces exist between solute and solvent particles as well as solute-solvent; - if attractive forces b/t solute and solvent is stronger  solvent surrounds solute and dissolves the solute - hydration – solvation in water; like dissolves like;

Aqueous solutions of ionic compounds - water is polar b/c + H and –O make a dipole If you place a crystal (NaCl) in water, water molecules collide with the surface of the crystal b/c water molecules are in constant random motion (Kinetic Theory) Attraction between negatives (O & Cl) to positives (Na & H) is greater than the ions (in the crystal) to each other  the ions break away at the surface exposing more of the crystal until the entire crystal is dissolved Not all ionic compounds are dissolved Gypsum (CaSO4) mixed with water makes plaster – a mixture CaSO4 attractive forces are strong enough that water cannot overcome them;

Aqueous solutions of molecular compounds Many molecular compounds are also soluble in water; sucrose for example Compounds that have –OH sites that can H-bond are often water soluble alcohols – vary in solubility oil – does not have –OH sites (composed of only carbons and hydrogen) will not dissolve in water If a molecular compound is NOT soluble in water b/c lacks H- bonding sites, it will be soluble in another non-polar solvent like benzene;

Factors that affect rate of solvation Agitating the mixture Increasing the surface area and increasing the temperature will increase the rate of solvation b/c increases the number of collisions;

Heat of solution Solute must separate – endothermic energy is initially required to separate the crystal into ions Solvent must separate to make room for solute – endothermic solvent particles must also separate to allow the ions to come between them Solute and solvent attract each other – exothermic when the solute and solvent particles mix, energy is released (exothermic) The overall energy change that occurs during the solution formation is known as the heat of solution When the sum is negative: exothermic – the solute-solvent attraction > solute-solute attraction When the sum is positive: endothermic – the solute-solvent attraction < solute-solute attraction  

C. Solubility – refers to the maximum amount of solute that will dissolve in a given amount of solvent at a specific temperature and pressure - expressed in grams of solute per 100 g of solvent;

Saturated, unsaturated - as the number of solvated ions increases, the same random mixing results in increasingly frequent collisions b/t solvated crystals and the remaining crystals Eventually some colliding particles will collide with the crystal and crystallize or rejoin the crystal When rate of crystallization = rate of solvation then a dynamic equilibrium exists  saturation; If the rate of crystallization < rate of solvation  unsaturated

Supersaturated - contains more dissolved solute than a saturated solution at the same temperature Made when a saturated solution is formed at a higher temperature and then cooled slowly the slow cooling allows the excess solute to remain dissolved at a lower temp very unstable, a seed crystal can cause the excess solute to ppt quickly;

. Factors that influence solubility temperature, pressure (gases), nature of solute/solvent Temperature and solubility; solubility curves most ionic compounds are more soluble at higher temperatures than at lower temperatures; gases are the opposite think KMT, gases have enough energy to readily escape solution, but if we slow the molecules down by lowering the temperature, the molecules do not escape as easily, why pop is better cold,

Pressure and solubility Only affects gases - solubility of a gas in any solvent increases as its external pressure (pressure above the solution) increases how pop works  CO2 is forced into solution by increasing pressure above normal atmospheric pressure the pressure above the liquid decreases when the can/bottle opens and allows the CO2 to escape Henry’s Law Solubility of a gas in a liquid in proportional to the pressure above the liquid; S1/P1 = S2P2

Polarity: “like dissolves like” In order for a solution to form, the solute-solvent attractive forces must be greater than the solute-solute and solvent-solvent attractive forces The solute and solvent will attract if they have similar polarities: polar attracts polar and non-polar attracts non-polar Solvation is dependent upon the strength of the intermolecular forces Two substances with similar strength forces will dissolve while those with different strengths do not;