Solutions Chapter 15 Chapter 15 Solutions

Characteristics of Solutions Solute – substance that dissolves Solvent – dissolving medium Soluble – substance dissolves in a solvent Immiscible – two liquids can be mixed but separate shortly after they cease to be mixed.

Solvation in Aqueous Solutions Solvation – the process of surrounding solute particles with solvent particles to form a solution General Rule: “like dissolves like” –Examine the bonding, polarity and intermolecular forces of the particles. Solvation occurs when the attraction between the dipoles and ions of the solute and solvent is greater than the attraction of the ions in the solute.

Rate of Solvation Three common ways to increase rate: 1.Agitating the mixture. 2.Increasing the surface area of the solute. 3.Increasing the temp of the solvent. Heat of Solution – the overall energy change that occurs during the solution formation process

Solubility Solubility – the maximum amount of solute that will dissolve in a given amount of solvent at a specified temp and pressure. Saturated Solution – max amount of dissolved solute for a given amount of solvent (at specific temp and pressure) Unsaturated Solution – dissolved solute for a given temp and pressure is less than a saturated solution When solvation rate equals the crystallization rate, a dynamic equilibrium has been reached.

Factors That Affect Solubility Generally, solubility increases with increased temperature Allows formation of supersaturated solutions. Supersaturated Solutions – contains more dissolved solute than a saturated solution at the same temp. video Solubility of gas increases as its external pressure increases.

Henry’s Law S 1 S 2 P 1 P 2 Most commonly solved for S 2 : S1P2S1P2 P 1 Do page = S 2 =

Solution Concentration Concentration – a measure of how much solute is dissolved in a specific amount of solvent or solution. Percent by mass is usually used to describe solution of a solid dissolved in a liquid. mass of solute mass of solution Percent by mass = X 100 P

Concentration Percent by volume is usually used to describe solution of a liquid dissolved in a liquid. volume of solute volume of solution Do page Molarity (M) – the number of moles of solute dissolved per liter of solution. moles of solute liters of solution X 100 Percent by volume = Molarity (M) = Percent by volume is usually used to describe solution of a liquid dissolved in a liquid. volume of solute volume of solution Do page Molarity (M) – the number of moles of solute dissolved per liter of solution. moles of solute liters of solution X 100 Percent by volume =

Find M What is the molarity of a solution prepared by adding 6.3 grams of KCl to enough water to make 3.4 liters of solution?

Preparing Molar Solutions Example: How many grams of MgCl 2 would be dissolved in 1.0 liter of a 0.50 molar solution of MgCl 2 ? 1.0 L 0.50 moles grams MgCl 2 1 L 1 mole MgCl 2 = 48 grams MgCl 2 Do page

Diluting Solutions Dilution lowers the concentration because more solvent particles are added as the number of solute particles remains the same. This relationship as an expression: M 1 V 1 = M 2 V 2 Example: What volume of a 3.50M CaI 2 stock solution would you use to make a.600 L of a 1.75M CaI 2 solution? Do page

Molality Molality (m) – the ratio of the number of moles of solute dissolved in 1 kg of solvent. moles of solute moles of solute kilogram of solvent 1000 g of solvent Example: What is the molality of 25.0 g of CaCO 3 dissolved in g of water? Do page =Molality (m) =

Mole Fraction Mole Fraction (X) – the ratio of the number of moles of solute in solution to the total number of moles of solute and solvent. n solute n solute + n solvent Example: What is the mole fraction of HNO 3 in an aqueous solution that contains 36.87% HNO 3 by mass? What is the mole fraction of water? Do page X solute =

Colligative Properties Colligative Properties – a physical property of a solution that depends on the number, but not the identity, of the dissolved solute particles. Depends on amount (how much/how many). Electrolytes –Strong Electrolytes: produce many ions in solution –Weak Electrolytes: produce only a few ions in solution –Nonelectrolytes: doesn’t produce ions or conduct electricity.

Vapor Pressure Lowering The greater the number of solute particles in solvent, the lower the resulting vapor pressure. (Colligative Property) Relative effect can be predicted based on whether the solute is an electrolyte or a nonelectrolyte.

Increasing Pressure Increasing Temp Pure Solvent solution SOLID GAS LIQUID P TfTf TbTb

Boiling Point Elevation Boiling Point Elevation – the temp difference between a solution’s boiling point and a pure solvent’s boiling point. For nonelectrolytes: Δ T b = K b m For electrolytes: Δ T b = i K b m K b for water is ◦ C/m (see table 15-4)

Freezing Point Depression Freezing Point Depression – the difference in temp between its freezing point and the freezing point of its pure solvent For nonelectrolytes: ΔT f = K f m For electrolytes: ΔT f = iK f m K f for water is 1.86 ◦ C/m (see table 15-5) Do page

Osmosis – the diffusion of solvent particles across a semipermeable membrane from an area of higher solvent concentration to an area of lower solvent concentration. Semipermeable membranes allow some but not all kinds of particles to cross it. –Ex: membranes surrounding all living cells Osmotic Pressure (¶) – the amount of additional pressure caused by the water molecules that moved into the solution.

Heterogeneous Mixtures Mixture – combination of two or more substances that keep their basic identity Contain substances that exist in distinct phases. Two types: –Suspensions –Colloids

Suspensions Suspension – mixture containing particles that settle out if left undisturbed. –Ex: cornstarch in water, muddy water Suspended particles (1000nm) are much larger than solvated particles (1nm) so gravity quickly pulls them out of solution. Thixotropic – substances that are in a solid-like state but quickly begin to flow like a liquid when stirred. –Ex: house paint that quickly thickens to a solid

Colloids Colloid – heterogeneous mixture of intermediate size particles (between 1nm and 1000nm). Colloids are categorized by the phase of their dispersed particles and dispersing mediums. CategoryDispersed Particles Dispersing Particles Example Solid SolSolid Colored gems SolSolidLiquidBlood, gelatin Solid EmulsionLiquidSolidButter, cheese EmulsionLiquid Milk, mayonnaise Solid FoamGasSolidMarshmallow, soaps that float FoamGasLiquidWhipped cream, beaten egg white AerosolSolidGasSmoke, dust in air AerosolLiquidGasSpray deodorant, clouds Types of Colloids

Brownian Motion Brownian Motion - the erratic movement of colloid particles. Occurs when particles of the dispersion medium collide with the dispersed particles Electrostatic forces keep the particles in the colloid from settling to the bottom. Heat, addition of an electrolyte, or any other interference with the electrostatic layering will destroy the colloid.

The Tyndall Effect Dispersed colloid particles are large enough to scatter light. –known as the Tyndall Effect Solutions never create the Tyndall Effect. Suspensions do create this effect. Examples: rays of sunlight passing through smoke-filled air, lights through fog at night