Solute-Solvent Interactions Ch. 13/14: Solutions Solute-Solvent Interactions

“like dissolves like” Rule of thumb for determining whether two substances will dissolve one another Polar substances usually dissolve only polar Nonpolar usually dissolve only nonpolar Also includes type of bonding and types of intermolecular forces Would ionic compounds be considered polar or nonpolar?

Hydration Charged ends of the water molecules attract ions in an ionic compound They surround the ions and keep them away from other ions As ions are pulled away from a crystal, more ions beneath are exposed Eventually, all the ions are evenly dispersed throughout the solution

Hydrates CoCl2.6H2O (cobalt chloride hexahydrate) Ionic compounds that incorporate water molecules into their structure when they crystallize Each have a specific number of molecules attached What would happen if we crushed a hydrate and then heated it up? CoCl2 (cobalt chloride, anhydrous)

Nonpolar Solvents Molecular compounds Ex: carbon tetrachloride, CCl4 Toluene, C6H5CH3 Do not usually dissolve ionic compounds because they do not have a strong enough attraction to the ions to pull them apart

Miscibility Miscible Liquids that mix well in any proportion Ex: benzene and carbon tetrachloride because they are both nonpolar and don’t have strong IMFs Immiscible Two liquids that are not soluble in each other Ex: fats and oils in nonpolar liquids like toluene

Miscibility Some substances have parts that are polar and nonpolar so are miscible in both types of liquids

Pressure and Solubility Little effect on solubility of liquids and solids Increase in P increases solubility of gases in liquids Gas + solvent ↔ solution When P is increased, it puts more gas molecules in contact with the surface of the liquid so more will be dissolved into the solution Using Le Châtelier’s principle, as P is increased the reaction shifts towards the side of the reaction with less gas molecules

Pressure and Solubility

Henry’s Law The solubility of a gas in a liquid is directly proportional to the partial pressure of that gas on the surface of that liquid at a constant T Named after William Henry, English chemist Effervescence Quick escape of bubbles of gas from a liquid in which it was dissolved In bottling plants, CO2 gas is forced into the soda with 5-10 atm of pressure

Gas Pressure and Solubility

Temperature and Solubility Gases: Increasing T usually decreases gas solubility As the molecules get more KE, more can escape into gas phase Solids increases solid solubility Some have larger changes in solubility than others Some solids even decrease in solubility

Heats of Solution Dissolution of some ionic solids release energy and others absorb energy How would each feel on the outside of beaker? Which is endothermic/exothermic? Solute particles must be separated Solvent particles must be separated Solvent and solute particles mix

Heats of Solution Usually energy is required for the first two steps to occur Amount depends on the strength of IMFs of each Usually, energy is released in step three Depends on how strong the IMFs are between the solute and solvent Heat of solution Net amount of heat absorbed or released when solute is dissolved in a solvent

Heats of Solution Heat of Solution = Step 1 + Step 2 + Step 3 Is negative (exothermic) when sum of the positive values from Steps 1&2 is less than the negative value in Step 3