Chapter 16: Mixtures & Solutions

Heterogeneous Mixtures Mixture: a combination of two or more pure substances in which each pure substance retains its individual chemical properties Can be homogeneous and heterogeneous Heterogeneous mixture: a mixture that does not have a uniform composition and in which the individual substances remain distinct. Suspensions: a mixture containing particles that settle out if left undisturbed Particles are large enough for gravity to cause the particles to settle Ex. Muddy water (particles can be separated when poured through a filter) Ex. Latex paint Colloid: heterogeneous mixtures of intermediate sized particles (between 1 nm and 1000 nm) and do not settle out. Ex. Milk

The most abundant substance in a mixture is the dispersion medium. Colloids are categorized according to the phases of their dispersion particles and dispersing mediums.

Tyndall effect: when dispersed colloid particles scatter light. Brownian motion: the jerky, random movements of particles in a liquid colloid, from the results of particle collisions. Robert Brown (1773-1858) Noticed random movements of pollen grains dispersed in water Results from a collision of particles of the dispersion medium with the dispersed particles  prevents settling Tyndall effect: when dispersed colloid particles scatter light. Example: headlights in fog

Homogeneous Mixtures Solution: a homogeneous mixture of two or more substances 2 Parts of a simple solution: Solute: the component that is dissolved (least abundant part) Solvent: the dissolving agent (most abundant part) Aqueous Solution: a substance (solute) is dissolved in water (solvent) Complex Solutions: More than one solute

Common Solution Formation Terms A substance that dissolves in a solvent is soluble. Substances that form a solution Ex. Salt and water A substance that does not dissolve in a solvent is insoluble. Ex. Sand and gasoline Two liquids that are soluble in each other in any proportion are miscible. Ex. Water and ethanol  alcoholic beverages Two liquids that can be mixed but separate shortly after are immiscible. Ex. Oil and Vinegar

Solvation Solvation: the process of surrounding solute particles with solvent particles to form a solution. Solvent particles break their interactions to form interactions with the solute particles Oil does not form a solution with water because there is little attraction between polar water molecules and nonpolar oil molecules. Hydration: solvation with water as the solvent During solvation, the solute must separate into particles and move apart, which requires energy. The overall energy change that occurs during solution formation is called the heat of solution.

Factors That Affect Solvation Agitation: brings fresh solvent into contact with the solute Temperature: the solvent particles have more kinetic energy, increasing the force and frequency of solvent-solute collisions Particle Size: smaller solute particles have greater surface area and therefore more solvent-solute collisions occur

Solubility Solubility: maximum amount of substance that dissolves in a given quantity of a solvent at a certain temperature Depends on the nature of the solute and solvent Saturated solutions: contains the maximum amount of solute that can be dissolved at a given temperature Unsaturated solution: a solution that contains less solute than a saturated solution Supersaturated solution: under suitable conditions, the solution contains a greater amount of solute than a saturated solutions To form a supersaturated solution, a saturated solution is formed at high temperature and then slowly cooled Supersaturated solutions are unstable

Factors that Affect Solubility Nature of the solute/solvent “Like dissolves like” Nonpolar solvents (carbon tetrachloride) dissolve nonpolar solutes (grease) Polar solvents (water) dissolve ionic compounds (salt) and polar solute Temperature Solid dissolving in a liquid  solubility tends to increase as temperature increases Gas dissolving in a liquid  solubility tends to decrease as temperature decreases Pressure Gas solubility increases as the pressure above the solution increases Ex. Carbonated beverages: bottled under high pressure to increase the CO2 solubility in the water

Henry’s Law: at a given temperature, the solubility of a gas in a liquid is directly proportional to the pressure of the gas above the liquid Example: The solubility of a gas in water is 0.22g/L at 20.0kPa of pressure. What is the solubility at 115 kPa?

Solution Concentration Concentration: a measure of the amount of solute that is dissolved in a specific quantity of solvent Qualitative Descriptions: Dilute solutions: contains a small amount of solute Concentrated solution: contains a lot of solute

Percent by mass Usually describes a solid dissolved in a liquid Expressed as a percentage Ratio of the solute quantity to the total solution quantity Example: What is the mass percent of a solution with 13.25g of CaCl2 dissolved in 450g of water?

Percent by Volume Usually describes solutions with both parts as liquids Expressed as a percentage Ratio of the solute quantity to the total solution quantity Example: What is the percent by volume of a solution with 45.1 mL of ethanol mixed with 375.0 mL of water?

Molarity Number of moles of solute dissolved in 1L of solution Example 1: What is the concentration when 35.4 g of LiOH is added to enough water to form a solution with a volume of 450mL? Example 2: How would you prepare 375.0 mL of 1.2 M Na2CO3?

Molality The ratio of moles of solute dissolved in 1 kg of solvent Useful to eliminate volume changes due to temperature 1m is a molal solution Example: What is the molal concentration of a solution made by dissolving 67.2 g Mg(NO3)2 in 1250.0 mL of water

Mole Fraction Ratio of the number of moles of solute to the total number of moles of solute and solvent Example: Calculate the mole fraction when 34.5 g of NaCl is dissolved in 250 g of water. XA = mole fraction n = number of moles A/B = substances

Making Dilutions Chemicals are shipped and stored in high concentrations We dilute these concentrated solutions for use in experiments To dilute an acid: Add acid to the required amount of water This generates the least amount of heat Example: How would you prepare 250.0 mL of 3.0 M H2SO4 from a stock solution that is 18.4 M? M1V1 = M2V2

Colligative Properties Colligative Properties: physical properties of solutions that are affected by the number of particles but not by the identity of dissolved solute particles. Electrolytes: an ionic compound whose aqueous solution conducts electricity Ions separate (dissociate) during the hydration process Increases the number of dissolved particles in solution Ex. 1 mole of NaCl breaks into 1 mol Na+ and 1 mol Cl- 3 mole CaBr2  3 mol Ca2+ and 6 mol Br- Nonelectrolytes: many molecular compounds dissolve in water but do not dissociate Ex. Sugar C12H22O11 These solutions do not conduct electricity No separation = less of an impact on colligative properties

Vapor Pressure Lowering Vapor Pressure: pressure exerted in a close container by liquid particles that have escaped the liquid’s surface and entered the gas state Adding a nonvolatile solute to a solvent lowers the solvent’s vapor pressure Solute particles occupy some of the surface area (fewer particles enter the gaseous state) Solute particles interact with the solvent particles therefore requiring more energy for the solvent to escape The greater the number of solute particles, the lower the vapor pressure

Boiling Point Elevation Boiling occurs when: vapor pressure = atmospheric pressure More heat is needed to supply additional kinetic energy to raise the vapor pressure to atmospheric pressure. The amount of increase is dependent upon the amount of solute The boiling point of a solution is higher than the boiling point of a pure solvent ΔTb = Kbm ΔTb is the boiling point elevation Kb is the molal boiling point elevation constant m represents molality

Freezing Point Depression Solute particles interfere with the solvent particle attractive forces This prevents the solvent from freezing at its normal temperature The freezing point of a solution is always lower than that of the pure solvent Amount of decrease is dependent on amount of solute ΔTf = Kfm ΔTf is the freezing point depression Kf is the freezing point depression constant m is molality

What is the boiling and freezing temperature of the solution of 23 What is the boiling and freezing temperature of the solution of 23.45 g NaI in 345 g of water.

Osmotic Pressure Osmosis: the diffusion of a solvent through a semipermeable membrane Water molecules diffuse across the membrane from the dilute solution to the concentrated solution Osmotic pressure: the amount of additional pressure caused by water molecules that moved into the concentrated solution