The solution process

1
Properties of Solutions

• The solution process
• To form a solution requires overcoming
  intermolecular interactions between solute
  molecules and solvent molecules so solute
  molecules and solvent molecules can be
  homogeneously distributed throughout the bulk of
  the solution.
• The kinds of interactions to be overcome depend
  on the nature of the solutes and solvents.
• Dissolving NaCl in H2O involves breaking ion-ion
  interactions in NaCl and the hydrogen bonds in
  H2O.
• Interactions between Na and H2O and Cl- and
  water will be established to stablize the
  solution solvation occurs - hydration if H2O is
  solvent.

2
Properties of Solutions
Enthalpy changes accompanying solution
formation Exothermic heat of solution
Endothermic heat of solution

• DHsolnDH1DH2DH3
• DHsoln(NaOH)-44.48 kJ/mol
• DHsoln(NH4NO3)26.4 kJ/mol

3
Properties of Solutions

• Enthalpy changes accompanying solution formation
• DH3 represents the enthalpy of interaction
  between solute particles and solvent particles.
• Should DH3 be small in magnitude, the formation
  of the solution will be too endothermic to
  form.
• This occurs when non-polar solvents interact with
  ionic or polar solutes.
• Solution formation and increase in disorder
• Generally, processes that are exothermic are
  spontaneous, but not all spontaneous processes
  are exothermic.
• The formation of an aqueous solution of NH4NO3 is
  endothermic.
• One factor that influences the spontaneity of
  processes besides energy change is the degree of
  randomness created as the result of the process.
• Forming an aqueous solution of NH4NO3 requires
  breaking up the order associated with the
  hydrogen bonded water molecules and the highly
  ordered lattice of NH4NO3.
• The solution is more disordered than the unmixed
  components of the solution.
• If the ordered pure solution components have
  strong interparticle attractive interactions it
  is possible that the disorder created by forming
  a solution will not compensate for these
  energetic interactions.

4
Properties of Solutions

• Factors affecting solubility
• Solute-solvent interactions
• The solubility of gases in liquid solvents
  depends on the strength of London dispersion
  forces between gaseous solute and liquid solvent.
• Solubilities of gases in H2O at 20 oC and 1 atm
  gas pressure

• Polar liquids dissolve readily in polar solvents
  because of dipole-dipole attractive interactions
  between solute and solvent.
• Many pairs of liquids are mutually soluble in all
  ratios they are miscible.
• Acetone is miscible with
  water, whereas 2-pentanone
• dissolves to
  the extent of 4.7 g/100 g water at 20 oC and is
  therefore not miscible with water.

5
Properties of Solutions

• Factors affecting solubility
• Solute-solvent interactions
• Hydrogen bonding is important in determining the
  solubility of solutes in water.
• The short chain alcohols are miscible with water
  but longer chain alcohols are not. For short
  chain alcohols, the hydrogen bonding between
  alcohol molecules is similar in strength to
  that in water
• Nonpolar liquids dissolve readily in nonpolar
  solvents but not polar solvents.
• The dipole-dipole attractive forces in the polar
  substance cannot be overcome by interactions
  between the nonpolar solvent and the polar
  solute.

6
Properties of Solutions

• Factors affecting solubility
• Pressure Effects and the solubility of gases
• Henrys law the solubility of a gas in a liquid
  is proportional to the pressure of the gas above
  the liquid.
• SgkPg where Sg is the solubility of
  the gas in the liquid
• Pg is the pressure of the gas above
  the liquid
• k is the Henrys law constant that
  depends on the liquid-gas pair and
  temperature.
• Example if the pressure of CO2 above the water
  in carbonated water is 4.0 atm and the Henrys
  law constant for CO2 in water at 25 oC is 3.1 x
  10-3 mol/L-atm, what is the concentration of
  CO2 in water under these conditions?

7
Properties of Solutions

• Factors affecting solubility
• Temperature Effects
• Gases decrease their solubilities with increasing
  temperature
• The solution process for gases is exothermic
• Gas liquid solvent saturated
  solution heat energy
• DHsolutionlt0
• If the temperature is increased, heat energy is
  added to the solution
• The result is to shift the equilibrium to the
  left removing gas from the solution and reducing
  the gas concentration in the solution
• LeChâteliers principle states if a stress is
  applied to a system at equilibrium, the system
  will respond to relieve the stress.
• Most solids increase their solubilities in
  liquids as temperature increases because the
  solution phenomenon is endothermic. See Fig.
  14.9, p. 655.
• solid solvent heat solution

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9
Properties of Solutions

• Colligative Properties are properties of
  solutions that depend on the number of
  particles of solvent or solute present in a
  solution. Colligative properties thus depend on
  the concentration of solvent or solute present
  in a solution.
• The vapor pressure of a solution containing a
  nonvolatile solute is less than the vapor
  pressure of the pure solvent,
• The vapor pressure is a measure of the escaping
  tendency of liquid molecules.
• The fraction of solvent molecules on the surface
  of a solution is lowered by the dissolved solute
  molecules. The escaping tendency is proportional
  to the mole fraction of volatile molecules at
  the surface of the liquid.
• Raoults law gives the relationship between the
  vapor pressure of a volatile liquid containing
  a nonvolatile solute and the mole fraction of
  volatile liquid.
• PAXAPAo where PA is the vapor pressure of the
  solution
• XA is the mole fraction of solvent
• Pao is the vapor pressure of pure
  solvent

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Properties of Solutions

• Colligative Properties
• Vapor Pressure
• Example The vapor pressure of pure water at 110
  oC is 1070 torr. A solution of ethylene glycol
  and water has a vapor pressure of 1.00 atm at 110
  oC. What is the mole fraction of ethylene glycol
  in the solution?

• Vapor pressure of a mixture of volatile liquids
• Raoults law and Daltons law of partial
  pressures can be combined. For 2 volatile
  liquids A and B
• PAXAPAo and PBXBPBo (Note XA XB 1)
• PtotalPAPB XAPAo XBPBo (conditions for an
  ideal solution)
• Example Pbenzeneo75 torr and Ptolueneo22 torr
  at 20 oC
• if X0.500, Pbenzene(0.500)(75 torr)37.5
  torr and Ptol(0.500)(22)11.0 torr
• Ptotal(37.511.0)torr48.5 torr
  Xbenz(vapor)37.5/48.5 0.773

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Properties of Solutions

• Colligative Properties
• Boiling point elevation occurs as a result of
  lowering the vapor pressure of a liquid
  containing a nonvolatile solute.
• In order to reach any vapor pressure the
  temperature of the solution must be higher than
  the temperature of the pure liquid. See Fig.
  14.13, p. 661.
• In this Figure, the red line corresponds to the
  vapor pressure of pure benzene vs. temperature
  whereas the blue line is the vapor pressure of a
  2.0 m solution of a nonvolatile solute in
  benzene.
• The increase in boiling temperature is
  proportional to the concentration of the solute
• DtbpKbpm where Dtbp is the increase in the
  boiling temperature of the
  solvent
• Kbp is the boiling point elevation
  constant for the solvent.
• m is the molality of solute in the
  solution.

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14
Properties of Solutions

• Freezing point depression also results from a
  lowering of the vapor pressure of a liquid
  containing a nonvolatile solute.
• The decrease in freezing temperature is
  proportional to the concentration of solute
• DtfpKfpm where Dtfp is the decrease in the
  boiling temperature of solvent
• Kfp is the freezing point depression constant
  for the solvent.
• m is the molality of the solute in the
  solution.
• Calculating molar masses from colligative
  properties

Change in vapor pressure, boiling point
elevation, or freezing point depression or
osmotic pressure
Solution concentration
Use mass of solvent
Molar mass
Moles of solute
15
Properties of Solutions

• Calculating molar masses from colligative
  properties
• Example 0.640 g azulene (empirical formula C5H4)
  is dissolved in 99.0 g benzene. The boiling
  point of the solution is 80.23 oC. What is the
  molecular formula of azulene?

16
Properties of Solutions

• Colligative Properties
• Freezing point depression
• For solutions of ionic compounds, the molality
  will be an integer multiple of the compounds
  molality.
• For NaCl, there are 2 moles of particles per mole
  NaCl, so a 1 molal solution of NaCl is 2 molal
  in particles
• For CaCl2, there are 3 moles of particles per
  mole CaCl2, so a 1 molal solution of CaCl2 is 3
  molal in particles
• The expected effect on freezing temperature and
  boiling temperature for a 1 molal solution of
  NaCl or CaCl2will be 2 or 3 times that of a 1
  molal solution of a nondissociating solid.
• A 1.00 m solution of NaCl should freeze at
  2x(-1.86) oC.
• Experimentally it is found that the freezing
  point of solutions of ionic solids is not quite
  as low as predicted.
• This results because ion pairs are formed in
  solution because of ion-ion interactions,
  reducing the expected molality of the solute.

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Properties of Solutions

• Colligative Properties
• Freezing point depression
• The vant Hoff factor, i, is a measure of the
  dissociation of an ionic compound

Vant Hoff factors for several substances at 25
oC
18
Properties of Solutions

• Colligative Properties
• Osmosis is the transport of solvent molecules
  through a semipermiable membrane from a
  solution of high solvent concentration to a
  solution of lower solvent concentration.
• Solvent molecules move from the solution of lower
  solute concentration to the solution of higher
  solute concentration in an attempt to dilute the
  solution with higher solute concentration.
• Osmosis will cease when the pressure resulting
  from transport of solvent to the solution of
  higher solute concentration prevents further
  transport of solvent through the membrane.
• The osmotic pressure, ?, is related to the
  concentration of the solution, and the absolute
  temperature
• This is analogous to the ideal gas law, where P?
  and n/Vc

where c is the molarity of the solution

• Isotonic solutions have the same osmotic
  pressure.
• A hypotonic solution has lower osmotic pressure
  than another solution.
• A hypertonic solution has higher osmotic pressure
  than another solution.

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20
Properties of Solutions

• Colligative Properties
• Osmosis
• Example Lysozyme is an enyme tha breaks
  bacterial cell walls. A solution containing
  0.150 g in 210 mL of solution has an osmotic
  pressure of 0.953 torr at 25 oC. What is the
  molecular weight of of lysozyme?

21
Properties of Solutions
Colloids are suspensions of particles that have a
size between approximately 10 Å and 2,000Å
dispersed in another medium.

• Colloidal particles can be made up of an
  agglomeration of many small particles or
  molecules or could be a single, large molecule.

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Properties of Solutions

• Colloids
• Hydrophilic colloids are stabilized as a result
  of having polar or ionic groups on their
  surfaces which are water loving.
• The strong attractive interactions with water
  keeps the large particles in suspension.
• Hydrophobic colloids are stabilized by adsorption
  of ions on the particle surfaces.
• Adsorption is the binding of a substance to the
  surface of bulk matter.
• The adsorbed ions interact with water to keep the
  particles suspended.
• The charges on the surfaces of different
  particles are of the same sign which keeps them
  apart and prevents agglomerating and
  precipitating.
• Soaps and detergents consist of a long
  hydrophobic tail and a hydrophilic end.
• The hydrophobic tail inserts into oil or other
  hydrophobic materials and causes the particles
  to be suspended in water as the hydrophilic end
  will be on the outside of the particle

-

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Properties of Solutions

• Colloids
• Soaps and detergents behave as emulsifying agents
  causing oils to be suspended in aqueous
  dispersing medium.
• Removal of colloidal particles from suspension
• Colloidal particles in dispersed in liquids
  stabilized by adsorption of ions can be
  coagulated by adding an inert electrolyte - HNO3
  - and heating the suspension.
• Solid particles suspended in air can be
  precipitated by producing a surface charge on
  the particles and attracting them to an
  oppositely charged electrode.

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