Solutions, Concentrations, Colligative Properties and Colloids

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• Solutions, Concentrations, Colligative Properties
  and Colloids
• Unit IX
• Ch. 15

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• Solution - homogeneous mixture of several
  substances a solid dissolved in a liquid cannot
  be separated into its components by means of
  filtration
• Solvent - the predominant substance in a solution
  (water is the most common solvent used)
• Solute - the less abundant substance in the
  solution

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• Dissociation - the separation of ions from each
  other
• Solvation - the surrounding of solute particles
  by solvent particles
• Hydration - the solution process (with water
  molecules surrounding particles of the solute)
  with water as the solvent
• Miscibility - property of mutual solubility two
  substances are said to be completely miscible if
  they are mutually soluble in all proportions
  (water and alcohol). Two liquids that do not mix
  are said to be immiscible (oil and vinegar)
• Suspension - a mixture in which the particles are
  large enough that they can settle out of solution
  due to the force of gravity

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Solvent-Solute Interaction

• Solvent/Solute Combination
• Likes dissolve likes
• polar solvents (water) dissolve polar substances
  (NaCl, CaSO4)
• nonpolar solvents (hexane) dissolve nonpolar
  substances (oils, grease)
• Hydration - the solution process (with water
  molecules surrounding particles of the solute)
  with water as the solvent

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Solubility

• Solution Equilibrium
• exists when the number of particles leaving the
  surface of the solid equals the number of
  particles returning to the solid surface
• Saturated solution
• exists when the solution is at equilibrium since
  the liquid cannot hold any more of the solute
  molecules it has reached a maximum

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• Unsaturated solution
• exists prior to the time that solution
  equilibrium is established
• Supersaturated Solution
• the solution holds more solute than will normally
  dissolve in the solvent at that temperature
• such solutions are metastable and usually revert
  to saturated solutions when slightly disturbed

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Solubility Values

• Solubility - the maximum amount of a solute that
  will dissolve in a given quantity of solvent at
  saturation (usually 100 mL) at a given
  temperature and pressure
• Solubility Curves - are used to describe the
  behavior of solutes in solvents
• Precipitation Reactions - used to separate
  substances based on the difference in their
  solubilities in the solvent of interest

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Solutes Electrolytes and Nonelectrolytes

• An electrolyte is a substance that when dissolved
  in water yields a solution that conducts electric
  current (NaCl, ...)
• A nonelectrolyte is a substance that when
  dissolved in water does NOT conduct electricity
  (sugar)

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Factors Affecting the Rate of Dissolution

• Dissolving Effects
• since the dissolution process is a surface
  effect, the rate of solution is affected by the
  amount of surface area exposed to fresh solvent
• Three factors which can affect the rate of
  dissolution
• Increase the surface area of the solute
• Agitate the solution
• Heat the solvent

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Effect of Pressure on Solubility

• Pressure
• Pressure has very little effect on the solubility
  of solids or liquids in liquid solvents. However,
  pressure has a significant effect on the
  solubility of gases in liquids.
• Henry's Law The solubility of a gas in a liquid
  is directly proportional to the partial pressure
  of that gas on the surface of the liquid

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Effect of Temperature on Solubility

• Temperature
• The solubility of a gas in a liquid is inversely
  proportional to the temperature.
• The solubility of a solid or liquid in a liquid
  is directly proportional to the temperature.

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Heats of Solutions

• Solvation A solute particle that is surrounded
  by solvent particles is said to be solvated.
• The heat of solution is the amount of heat energy
  that is absorbed or released when a specific
  amount of a solute is dissolved in a solvent.

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• Concentration

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Concentration

• Molarity - moles solute/Liter solution
• Molality moles solute/kg solvent
• Mole Fraction - moles solute/moles solution
• Mass Percentage - (grams solute / grams solution)
  x 100
• The numerator contains the quantity of the solute
• The denominator contains the quantity of solution
  or solvent

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Problems

• Molarity
• 1) 193 g MgBr2 is dissolved in 500 mL water.
  Calculate the Molarity of bromide in the
  resulting solution.
• 2) 8.28 g Ca(C5H9O2)2 is dissolved in 250 mL H2O.
  Calculate the Molarity of calcium in the
  solution.
• Molality
• 3) 52.0 g K2CO3 dissolved in 518 g H2O. Calculate
  the molality of K2CO3 in the solution.
• 4) 75 g NaCl is dissolved in 430 g H2O. Calculate
  the molality of NaCl in the solution.

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• Colligative Properties

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Compounds in Aqueous Solution

• Dissociation - The separation of ions that occurs
  when an ionic compound dissolves
• Precipitation Reactions - A chemical reaction in
  which one or more of the products formed by the
  combination of ionic substances is only slightly
  soluble in the solvent (usually water).
• Ag Cl- ----gt AgCl(s)
• Net Ionic Equations
• Write only those ions/substances that take part
  in the reaction
• Spectator ions - they are present, but not
  involved in the reaction

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Ionization

• Ionization is the process in which ions are
  formed from solute molecules by the action of the
  solvent
• Hydronium Ion - the H3O ion. It is formed by the
  addition of a free proton to a water molecule
• Strong and Weak Electrolytes
• A strong electrolyte is a compound of which all
  or almost all of the dissolved material exists as
  ions in an aqueous solution. An example of this
  would be hydrogen chloride, sodium chloride.
• A weak electrolyte is a compound of which a
  relatively small amount of the dissolved compound
  exists as ions in an aqueous solution. Acetic
  acid is a weak electrolyte. It is also referred
  to as a weak acid for the same reason.

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Colligative Properties

• Colligative Properties are properties of
  solutions that are determined by the number of
  particles in solution rather than the type of
  particles.
• vapor pressure lowering
• freezing point depression
• boiling point elevation
• rate of diffusion through a membrane (osmosis)
• Vapor Pressure Lowering
• Raoult's Law
• A nonvolatile solute added to a pure solvent will
  lower the vapor pressure of the solvent. This is
  because fewer solvent particles are at the
  surface
• VPsolution mfsolvent x VPpure solvent
• VP is proportional to the mole fraction of the
  solvent

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Vapor Pressure Lowering
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Problem 1

• Sugar in solution. What is the vapor pressure of
  water at 70oC if 1.00 x 102 g H2O dissolved 2 x
  102 of sucrose (C12H22O11, MW 342 g/mole).
• First calculate the mole fraction of the water
  moles H2O / total number of moles of water and
  sugar
• 100 g H2O/18 g/mole 5.56 moles H2O
• 200 g sucrose/342 g/mole 0.585 moles
• total number of moles 6.14 moles
• mole fraction m.f. 5.56 moles H2O/6.14 moles
  solution 0.905
• VPH2O, 70oC 31.2 kPa
• Using Raoult's Law
• VPsolution m.f.solvent x VPsolvent
• 0.905 x 31.2 kPa
• 28.3 kPa

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Problem 2

• You have a solution of Mercury (I) chloride,
  HgCl2, m.f. 0.163, temp 25oC VPH2O, 25oC
  3.2 kPa.
• Calculate the vapor pressure of the solution
• VPsolution m.f.solvent x VPsolvent
• 0.837 x 3.2 kPa
• 2.68 kPa

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Boiling Point Elevation (DTbp) and Freezing Point
Depression (DTfp)

• Due to the reduced vapor pressure of the liquids,
  the freezing point is depressed and the boiling
  is increased.
• Calculation of DTbp and DTfp
• 1m 1 molal solution of sugar 1 mole sugar / 1
  kg H2O (i1)
• 1m 1 molal solution NaCl 2 mole solute / 1 kg
  H2O Na and Cl- ( i 2 )
• 1m 1 molal solution CaCl2 3 mole solute / 1
  kg H2O Ca2, and 2 Cl- ( i 3 )
• Kbp molal boiling point constant 0.515oC
• Kfp molal freezing point constant 1.853oC
• DTbp i m Kbp
• DTfp i m Kfp

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Problem

• Sugar in water. Calculate the freezing point and
  boiling point for the solution described.
• Calculate the molality of a sugar solution
  consisting of 85.0 g sugar (MW 342 g/mole)
  dissolved in 392 g water (0.392 kg H2O).
• molality moles sugar / kg water (85 g / 342
  g/mole) / 0.392 kg water 0.249 mol/0.392 kg
  0.635 m
• DTbp i m Kbp 0.635 x 0.515 0.327 oC
• boiling point 100 DTbp 100 0.327 oC
  100.327 oC
• freezing point 0 - DTfp 0 - 1.18 oC
• -1.18 oC

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Osmotic Pressure

• Semipermeable membrane
• only allows certain types of particles to pass
  through it.
• Osmosis
• the movement of solvent through a membrane from
  an area of higher solvent concentration to an
  area of lower solvent concentration.
• Dynamic Equilibrium
• is finally reached when the rate of flow from the
  pure solvent side is balanced by the rate of flow
  from the solute side due to increased back
  pressure (osmotic pressure)
• Osmotic pressure results from two things
• concentration difference of solvent on two sides
  of a membrane
• solute particles cannot pass through the membrane
  whereas solvent particles can.

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Electrolytes and Colligative Properties

• Due to the reduced vapor pressure of the liquids,
  the freezing point is depressed and the boiling
  is increased. This can be seen clearly from the
  diagram in p. 436. The amount of the vapor
  pressure reduction is based on the molal
  concentration of the solute. Pay close attention
  to the number of particles that are in solution
  for every particle that dissolves.

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• Colloids

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Solutions and Colloids

• Solution - homogeneous mixture of several
  substances
• a solid dissolved in a liquid
• cannot be separated into its components by means
  of filtration
• Solvent - the predominant substance in a solution
  (water is the most common solvent used)
• Solute - the less abundant substance in the
  solution

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• Dissociation - the separation of ions from each
  other
• Solvation - the surrounding of solute particles
  by solvent particles
• Suspension - a mixture in which the particles are
  large enough that they can settle out of solution
  due to the force of gravity

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Colloids

• Colloids are mixtures composed of two phases
• the dispersed phase
• the continuous phase

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• Colloidal particles
• larger than atoms or molecules
• smaller than the particles in suspensions
• can be seen under a microscope
• settle out upon standing

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Classification of Colloids

• Dispersed Continuous
  Phase Phase
• Aerosols Solid/Liquid Gas
• Foams Gas Liquid/Solid
• Emulsions Liquid Liquid/Solid
• Sols Solid Liquid/Solid

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Colloidal Size

• Diameter
• Solutions lt 1 nm permanent
• Colloids 1 - 100 nm permanent
• Suspensions gt 100 nm will settle out
  (gravity)

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• Tyndall Effect
• the scattering of light by colloidal particles
• Brownian Motion
• continuous random motion of particles (due to
  collisions)
• Adsorption
• solid and liquid surfaces attract and hold
  substances with which them come into contact

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Rules for Net Ionic Equations

• Rule
• 1 HCl, HBr, and HI are strong all other binary
  acids are weak
• 2 Ternary acids If oxygens gt hydrogens by 2
  or more, they are strong H2SO4, HClO4
• 3 Polyprotic acids 2nd, 3rd, ... ionizations are
  weak
• 4 Bases Group IA and IIA hydroxides are strong
  (except Be) all others are weak
• 5 Salts if soluble write in ionic form
• 6 Oxides undissociated form
• 7 Gases molecular form

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Salts and Solutions

• Salt - a crystalline compound composed of the
  negative ion of an acid and the positive ion of a
  base.
• Acid Base ----gt Salt H2O
• HCl NaOH ----gt H2O NaCl
• This is a "neutralization reaction"
• Salts are also formed by the reaction of acidic
  or basic anhydrides with a corresponding base,
  acid or anhydride
• Basic Anhydride Acid
• Na2O H2SO4 ----gt Na2SO4 H2O
• Acidic Anhydride Base
• 2 NaOH SO3 ----gt Na2SO4 H2O
• Basic Anhydride Acidic Anhydride
• Na2O SO3 ----gt Na2SO4

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Salts and Solutions (contd)

• Nomenclature
• Binary acids produce salts ending in "-ide"
• Ternary acids produce salts ending in "-ate",
  "-ous", or "-ite
• Acidic and Basic Salts, each ion is named
  separately
• NaHC2O4 Sodium Hydrogen Oxalate
• Pb2(OH)2CO3 Lead (II) Hydroxide Carbonate