Solutions and Mixtures

SolutionsSolutions are stable, homogeneous mixtures

Particles are evenly distributed through the mixtureWill not spontaneously separate

There are two parts: solute and solvent

Parts of a SolutionSolute: the substance being dissolvedSolvent: the substance doing the dissolving (think water: universal solvent)

Classes of SolutionsClasses are based on the final state of matter of the sol’nThree classes:

Gaseous sol’ns: mixtures of two gasesLiquid sol’ns: gas, liquid, or solid is dissolved in a liquidSolid 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:

MolarityMolalityMole fraction

MolarityMolarity (M): # of moles of solute dissolved in each liter of sol’n M = moles of solute

liters of sol’nFour different labels: molarity, molar, M, or mol/LDependent on tempVolumetric 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 16.0 g of barium chloride in enough water to give 450 mL of sol’n?

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

MolalityMolality (m): # of moles dissolved in each kilogram of solvent

m = moles of solutekg of solvent

Independent of temp

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 0.125g of chromium and 81.3g of iron?

Mole FractionMole fraction (X): # of moles of one component divided by the total # of moles in the sol’n

X = moles of componenttotal 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 TermsMost sol’ns are combinations of liquids and solidsMiscible: liquids or gases that will dissolve in each otherImmiscible: liquids or gases that will not dissolve in each otherSoluble: able to be dissolved in a particular solventInsoluble: not able to be dissolved in a particular solvent

ConcentrationsConcentrated solution: a large amount of solute is dissolved in the solventDilute solution: a small amount of solute is dissolved in the solvent

SaturationSaturated sol’n: a sol’n that has dissolved all the solute it can hold at a given temp

NOT the same as concentratedSol’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 tempSupersaturated sol’n: sol’n that contains more solute than a saturated one has at that temp

It is unstableCrystallization from Supersaturated Solutions of Sodium Acetate

Determining SaturationAdd a solute crystal to a sol’n

If dissolves: unsaturatedIf does not dissolve: saturatedIf excess solute comes out: supersaturated

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

Formation of Soln (cont)Solvation: process of surrounding solute particles with solvent particlesHydration: solvation involving waterFormation of a sol’n involves:

Breaking the attractions among solute particlesBreaking the attractions among solvent particlesFormation of attractions between solute and solvent particles

Heat of SolutionDefined as overall energy change that occurs during solution formationEnergy is required for formation of sol’ns

Breaking attractions – endothermicForming attractions – exothermic

Overall process (net change) depends on the balance between breaking and forming attractions

If breaking requires more energy – endothermicIf forming releases more energy - exothermic

SolubilityDefined as the maximum amount of a solute that will dissolve in a given amount of solvent at a specific tempDetermined experimentallyUsually expressed in grams of solute per 100 grams of solvent at a specific temp and pressure

SolubilityBased on the idea of attractions between particlesOverall polarity determines solubility:

“Like dissolves like”Polar dissolves polarNonpolar dissolves nonpolar

Picture of a soap micelle.

Solvation

The extent to which one substance dissolves in another depends on:

State of solute and solventTemp and pressure

Solubilities change with temp (the key to preparing a supersaturated sol’n)

Solubility of solids and liquids increasesSolubility of gases in liquids decreases

Solubilities of Substances in Water

Solubilities of Gases in Water

Henry’s LawSolubilities of solids and liquids are not appreciably affected by pressure, but solubility of a gas in a liquid is strongly influenced by pressureHenry’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

Because dissolving occurs at the surface, stirring/shaking allows more solvent to come in contact with soluteIncreasing surface area does the same thingAs 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 SolutionsIonic compounds dissociate in water

Dissociate: a process using energy to separate a compound into ions in water

Molecular compounds may also dissociate in waterSome aqueous solutions conduct electricity

Aqueous Soln’sConductance: the measurement of a sol’n’s ability to conduct electrical energyElectrolyte: 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 PropertiesA physical property that is dependent on the # of particles present rather than on the size, mass, or characteristics of those particlesFour main properties:

Vapor pressure reductionBoiling point elevationFreezing point depressionOsmotic pressure

Vapor Pressure ReductionVapor 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 ElevationBecause vapor pressure is lower, a higher temp will be needed to make the sol’n boilThe 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 DepressionThe presence of solute molecules will lower the freezing point of a substanceThe more solute particles in sol’n, the greater the lowering of the freezing ptFreezing pt depression: the ability of a dissolved solute to lower the freezing point of its sol’nWhy does salt melt ice?

Osmotic PressureWhen 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 osmosisIf 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 MixturesSuspension: mixture that appears to be uniform while being stirred, but separates into different phases when agitation ceasesColloid: heterogeneous mixture that contains intermediate size particles evenly distributed through a dispersion medium

Heterogeneous MixturesBrownian Motion: jerky, erratic movement of dispersed particles

Prevent colloids from settling outWhat 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.