Solutions Chapter 15 Chapter 16

Chapter 15 Many unique and important properties of water (high surface tension and low vapor pressure) result from hydrogen bonding

Surface tension Skin-like property of water Molecules of water within body of water form hydrogen bonds with other water molecules around it, on all sides and the attraction is balanced. However, at the surface there is an unbalance attraction and the molecules at the surface tend to be drawn inward. Inward force, or pull, tends to minimize the surface area of the liquid = Surface tension.

Vapor pressure Measure of the force exerted by a gas above a liquid in a sealed container Result of molecules escaping from the surface of liquid and entering vapor phase. A dynamic equilibrium exists between a vapor and the liquid. Hydrogen bonding between water molecules explains waters low vapor pressure. Hydrogen bonds hold water molecules together and keeps them in the liquid phase (evaporation is slow)

Solvents and solutes Solvent: dissolving medium Solute: dissolved particles Solvent dissolves the solute. Solute becomes dispersed in solvent. Solution is formed. “Likes dissolve likes”

Solvation As individual solute ions break away from the crystal, the negatively and positively charged ions become surrounded by solvent molecules and the ionic crystal dissolves. Solvation is the process by which the positive and negative ions become surrounded by solvent molecules.

Chapter 16 solutions

Solution formation The compositions of the solvent and the solute determine whether a substance will dissolve. Stirring (agitation), temperature, and the surface area of the dissolving particles determine how fast the substance will dissolve. Contact of solute with the solvent.

Solubility The amount of solute that dissolves in a given quantity of a solvent at a specific temperature and pressure to produce a saturated solution. Saturated solution: contains the max. amount of solute for a given quantity of solvent at a constant temperature and pressure. Solubility is expressed in grams of solute per 100 grams of solvent or in grams per liter. Unsaturated solution: solution that contains less solute than a saturated solution at given temp and pressure

Solubility Miscible: when two liquids dissolve into each other completely Immiscible: liquids that are insoluble in on another (oil and water)

Factors affecting solubility Temperature affects solubility of solid, liquid, and gas solutes in a solvent Temperature and pressure affect solubility of gas solutes in a solvent Supersaturated solution: contains more solute that it can theoretically hold at a given temperature.

Pressure Gas solubility increases as the partial pressure of the gas above the solution increases. Henry’s law: at a given temperature, the solubility (S) of a gas in a liquid is directly proportional to the pressure (P) of the gas above the liquid.

Molarity Concentration: measure of the amount of solute that is dissolved in a given quantity of solvent. Dilute solution: small amount of solute Concentrated solution: large amount of solute Molarity (M) is the number of moles of solute dissolved in one liter of solution. Also known as molar concentration.

Dilutions Diluting a solution reduces the number of moles of solute per unit volume, but the total number of moles of solute in solution does not change.

Percent solutions The concentration of a solution in percent can be expressed in two ways: as the ratio of the volume of the solute to the volume of the solution or as the ratio of the mass of the solute to the mass of the solution.

Colligative Properties A property that depends only upon the number of solute particles, and not upon their identity. Vapor-pressure lowering Boiling-point elevation Freezing-point depression

Vapor-Pressure Lowering The decrease in a solution’s vapor pressure is proportional to the number of particles the solute makes in solution. Ionic solutes that dissociate have greater affects on the vapor pressure than does a non-dissociated solute such as glucose. NaCl produces 2 particles in solution, sodium ion and chloride ion. CaCl2 produces three particles, a calcium ion and 2 chloride ions. Glucose produces only 1 particle because it does not dissociate.

Freezing-Point Depression When a substance freezes, the particles of solid take on an orderly pattern. The presence of solute disrupts the formation of this pattern. More kinetic energy must be removed from a solution than from the pure solvent to cause the solution to solidify. Freezing point of a solution is lower than the freezing point of the pure solvent. The magnitude of the freezing-point depression is proportional to the number of solute particles dissolved in the solvent and does not depend upon their identity. Ice-bite and antifreeze

Boiling-Point Elevation Boiling point is the temp at which the vapor pressure of the liquid equals atmospheric pressure. Because of the decrease in vapor pressure due to the added solute, additional kinetic energy must be added to raise the vapor pressure of the liquid phase of the solution to atmospheric pressure and initiate boiling. Boiling point of solution is higher than the boiling point of the pure solvent. The magnitude of the boiling-point elevation is proportional to the number of solute particles dissolved in the solvent. Antifreeze

Calculations involving colligative properties Molality Mole fraction

Molality Molality (m) is the number of moles of solute dissolved in 1 kilogram of solvent. Also known as molal concentration.

Mole Fraction Ratio of moles of a solute to the total number of moles of solvent and solute.