Chapter 12.2 The Solution Process

POINT > Describe the dissolving process POINT > Identify factors that affect dissolving POINT > Define solubility and saturation POINT > Describe effects of temperature and pressure on solubility POINT > Examine the energetics of the solution process

POINT > Describe the dissolving process As a solution forms, the solvent particles pull solute particles apart and surround, or solvate, them.

POINT > Describe the dissolving process The intermolecular forces between solute and solvent particles must be strong enough to compete with those between solute particles and those between solvent particles

POINT > Describe the dissolving process For example, if an ionic salt is soluble in water, it is because the ion-dipole interactions are strong enough to overcome the lattice energy of the salt crystal

AgCl is insoluble in water. Why is this? WB CHECK: AgCl is insoluble in water. Why is this? a) Ionic bonding is stronger than covalent bonding b) Water is not attracted to the AgCl crystal surface c) The intermolecular forces between water molecules and AgCl particles are balanced d) The AgCl lattice energy is stronger than the attractions between water and AgCl

POINT > Identify 3 factors that affect dissolving rate ? 1. Increasing the surface area for interactions to occur (i.e. crushing up the solute crystal) 2. Mixing/stirring the solution: increases solvent/solute contact 3. Heating the solution: increases solvent/solute contact

POINT > Define solubility and saturation Solubility is a description of how well a solute dissolves in a solvent. For example: “NaCl is highly soluble in water” “sucrose is highly soluble in water” “lead sulfate is insoluble in water” “water is poorly soluble in gasoline” What governs this? Intermolecular forces (of course!)

WB CHECK: What are three factors that affect the rate of dissolving a solute in a solvent?

POINT > Define solubility and saturation Chemists use the axiom “like dissolves like”: Polar substances tend to dissolve in polar solvents Nonpolar substances tend to dissolve in nonpolar solvents So, fats, oils, waxes, hydrocarbons, etc. (all nonpolar), are poorly soluble in polar compounds like water

POINT > Define solubility and saturation The more similar the intermolecular attractions, the more likely one substance is to be soluble in another (ex. ethanol in water)

POINT > Define solubility and saturation Solubility is quantified as the mass of solute that will dissolve in a specified amount of solvent at a given temperature For example, solubility of potassium nitrate in water: 31.6g per 100.0g H2O at 20.0C At this point the solution is “saturated”

POINT > Define solubility and saturation Saturated: The solvent holds as much solute as is possible at that temperature Dissolved solute is in dynamic equilibrium with solid solute particles

POINT > Define solubility and saturation Unsaturated: Less than the maximum amount of solute is dissolved in the solvent

POINT > Define solubility and saturation Supersaturated: Solvent holds more solute than is normally possible at that temperature These solutions are unstable; crystallization can occurs by adding a “seed crystal” or even scratching the side of the flask

POINT > Describe effects of temperature and pressure on solubility The solubility of liquids and solids does not change appreciably with pressure The solubility of a gas in a liquid is directly proportional to its pressure (Read Henry’s Law)

POINT > Describe effects of temperature and pressure on solubility Generally, the solubility of solid solutes in liquid solvents increases with increased temperature The effect varies widely depending on the solute

POINT > Describe effects of temperature and pressure on solubility The opposite is true of gases: Carbonated soft drinks are more “bubbly” if stored in the refrigerator Warm lakes have less O2 dissolved in them than cool lakes

WB CHECK: Which is true? a) Solubility of gases is unaffected by temperature b) Solubility of a solid in a liquid increases with increasing pressure c) Solubility of a gas in a liquid decreases with higher pressure d) Solubility of a gas in a liquid decreases with higher temperature

POINT > Examine the energetics of the solution process Three processes affect the energetics of the solution process: 1. Separation of solute particles (requires energy)

POINT > Examine the energetics of the solution process Three processes affect the energetics of the solution process: 1. Separation of solute particles (requires energy) 2. Separation of solvent particles (requires energy)

POINT > Examine the energetics of the solution process Three processes affect the energetics of the solution process: 1. Separation of solute particles (requires energy) 2. Separation of solvent particles (requires energy) 3. New interactions between solute and solvent (releases energy)

POINT > Examine the energetics of the solution process The enthalpy change, H, of the overall process depends on H for each of these 3 steps

POINT > Examine the energetics of the solution process The enthalpy change of the overall process depends on H for each of these steps

Homework: READ THE BOOK! Complete Section 12.2 F.A. #1-7