Solutions and Mixtures

Solutions Solutions are stable, homogeneous mixtures Particles are evenly distributed through the mixture Will not spontaneously separate There are two parts: solute and solvent

Parts of a Solution Solute: the substance being dissolved Solvent: the substance doing the dissolving (think water: universal solvent)

Classes of Solutions Classes are based on the final state of matter of the sol’n Three classes: Gaseous sol’ns: mixtures of two gases Liquid sol’ns: gas, liquid, or solid is dissolved in a liquid Solid soln’s: solid and a liquid or two solids are melted, mixed, or cooled Alloys: solid solutions containing metals

Solution of a gas (hydrogen) in a solid (palladium)

Examples of solid solutions

Ways to Describe Solutions There are three ways to describe concentrations: Molarity Molality Mole fraction

Molarity Molarity (M): # of moles of solute dissolved in each liter of sol’n M = moles of solute liters of sol’n Four different labels: molarity, molar, M, or mol/L Dependent on temp Volumetric flasks are the best containers for making a sol’n of a precise molarity

What is the molarity of a sol’n prepared by dissolving 16 What is the molarity of a sol’n prepared by dissolving 16.0 g of barium chloride in enough water to give 450 mL of sol’n?

How many grams of potassium chloride are needed to prepare 0 How many grams of potassium chloride are needed to prepare 0.750L of a 1.50M sol’n of potassium chloride?

Molality Molality (m): # of moles dissolved in each kilogram of solvent m = moles of solute kg of solvent Independent of temp

Calculate the molality of a sol’n prepared by dissolving 20 Calculate the molality of a sol’n prepared by dissolving 20.4 g of sodium chloride in 192g of water.

What is the molality of a solid sol’n containing 0 What is the molality of a solid sol’n containing 0.125g of chromium and 81.3g of iron?

Mole Fraction Mole fraction (X): # of moles of one component divided by the total # of moles in the sol’n X = moles of component total moles of sol’n Used to compare solute and solvent

What are the mole fractions of ethyl alcohol (C2H5OH) and water in a sol’n prepared by adding 50.0g of ethyl alcohol to 50.0g of water?

A gas mixture contains the following gases with the mole fractions indicated: CH4 (0.510), C2H6 (0.431), C3H8 (0.011), and C4H10 (0.013). The mixture also contains the gas acetylene (C2H2). What is the mole fraction of acetylene?

Solution Terms Most sol’ns are combinations of liquids and solids Miscible: liquids or gases that will dissolve in each other Immiscible: liquids or gases that will not dissolve in each other Soluble: able to be dissolved in a particular solvent Insoluble: not able to be dissolved in a particular solvent

Concentrations Concentrated solution: a large amount of solute is dissolved in the solvent Dilute solution: a small amount of solute is dissolved in the solvent

Saturation Saturated sol’n: a sol’n that has dissolved all the solute it can hold at a given temp NOT the same as concentrated Sol’n can be saturated but not concentrated and vise versa

Saturation (cont) Unsaturated sol’n: any sol’n that can dissolve more solute at a given temp Supersaturated sol’n: sol’n that contains more solute than a saturated one has at that temp It is unstable Crystallization from Supersaturated Solutions of Sodium Acetate

Determining Saturation Add a solute crystal to a sol’n If dissolves: unsaturated If does not dissolve: saturated If excess solute comes out: supersaturated

The Formation of Sol’ns The process of dissolving takes place at the surface of the solid solute Water molecules orient themselves on the surface of the crystal so that they can separate the pieces and pull them into sol’n Once separated, the pieces are surrounded by water molecules

Formation of Soln (cont) Solvation: process of surrounding solute particles with solvent particles Hydration: solvation involving water Formation of a sol’n involves: Breaking the attractions among solute particles Breaking the attractions among solvent particles Formation of attractions between solute and solvent particles

Heat of Solution Defined as overall energy change that occurs during solution formation Energy is required for formation of sol’ns Breaking attractions – endothermic Forming attractions – exothermic Overall process (net change) depends on the balance between breaking and forming attractions If breaking requires more energy – endothermic If forming releases more energy - exothermic

Solubility Defined as the maximum amount of a solute that will dissolve in a given amount of solvent at a specific temp Determined experimentally Usually expressed in grams of solute per 100 grams of solvent at a specific temp and pressure

Solubility Based on the idea of attractions between particles Overall polarity determines solubility: “Like dissolves like” Polar dissolves polar Nonpolar dissolves nonpolar

Picture of a soap micelle.

Solvation The extent to which one substance dissolves in another depends on: State of solute and solvent Temp and pressure Solubilities change with temp (the key to preparing a supersaturated sol’n) Solubility of solids and liquids increases Solubility of gases in liquids decreases

Solubilities of Substances in Water

Solubilities of Gases in Water

Henry’s Law Solubilities of solids and liquids are not appreciably affected by pressure, but solubility of a gas in a liquid is strongly influenced by pressure Henry’s Law: the amount of gas dissolved in a solvent is proportional to the partial pressure of that gas over the solvent

The solubility of a gas depends upon its partial pressure above the solution.

Factors that Affect Rate of Solvation Must increase the collisions between solute and solvent particles to increase rate at which solute dissolves. Three ways: Agitate mixture (stir, shake) Increase surface area (breaking solute into small pieces) Increase temp of solvent

Increasing surface area does the same thing Because dissolving occurs at the surface, stirring/shaking allows more solvent to come in contact with solute Increasing surface area does the same thing As temp increases, solvent particles move faster, which causes more particles to come in contact with the solute. Particles of solvent have more energy to remove particles from the solid solute

Aqueous Solutions Ionic compounds dissociate in water Dissociate: a process using energy to separate a compound into ions in water Molecular compounds may also dissociate in water Some aqueous solutions conduct electricity

Aqueous Soln’s Conductance: the measurement of a sol’n’s ability to conduct electrical energy Electrolyte: any substance that will conduct an electric current (by means of movement of ions), when dissolved in sol’n Nonelectrolyte: any substance that will not conduct an electric current when dissolved in water

Colligative Properties A physical property that is dependent on the # of particles present rather than on the size, mass, or characteristics of those particles Four main properties: Vapor pressure reduction Boiling point elevation Freezing point depression Osmotic pressure

Vapor Pressure Reduction Vapor pressure of a solvent containing a nonvolatile solute is lower than the vapor pressure of the pure solvent Volatile means tending to evaporate readily Vapor pressure reduction is proportional to concentration

When a solute is added to the solvent, some of the solute molecules occupy the space near the surface of the liquid, as shown in the figure to the left. When a solute is dissolved in a solvent, the number of solvent molecules near the surface decreases, and the vapor pressure of the solvent decreases.

Boiling Point Elevation Because vapor pressure is lower, a higher temp will be needed to make the sol’n boil The amount by which the boiling point is raised is the boiling point elevation (ΔTb) It is directly proportional to the sol’n’s molarity

A close-up plot of vapor pressure versus temperature for pure water (red curve) and for a 1.0 M NaCl solution (green curve). Pure water boils at 100.0°C, but the solution does not boil until 101.0°C.

Freezing Point Depression The presence of solute molecules will lower the freezing point of a substance The more solute particles in sol’n, the greater the lowering of the freezing pt Freezing pt depression: the ability of a dissolved solute to lower the freezing point of its sol’n Why does salt melt ice?

Osmotic Pressure When a semipermeable membrane is placed between 2 sol’ns so that only solvent particles can move through it, a net flow of solvent molecules will occur from the less concentrated sol’n to the more concentrated sol’n (osmosis) Osmotic pressure: the pressure required to prevent osmosis If 2 different sol’ns have identical osmotic pressure, no osmosis will occur (isotonic)

A solution inside the bulb is separated from pure solvent in the container by a semipermeable membrane. Net passage of solvent from the container through the membrane occurs, and the liquid in the tube rises until an equilibrium is reached. At equilibrium, the osmotic pressure exerted by the column of liquid in the tube is sufficient to prevent further net passage of solvent.

Heterogeneous Mixtures Suspension: mixture that appears to be uniform while being stirred, but separates into different phases when agitation ceases Colloid: heterogeneous mixture that contains intermediate size particles evenly distributed through a dispersion medium

Heterogeneous Mixtures Brownian Motion: jerky, erratic movement of dispersed particles Prevent colloids from settling out What is Brownian Motion? Tyndall effect: scattering of light by dispersed particles

The light beam is not visible as it passes through a true solution (right), but it is readily visible as it passes through colloidal silver in water.

Tyndall effect in clouds.