Solutions

1
Solutions

• Chapter 14

2
Key concepts

• Understand the solvation process at the molecular
  level.
• Be able to qualitatively describe energy changes
  during solution formation.
• Understand how entropy affects the solution
  process.
• Know the terms solute, solvent, saturated,
  unsaturated, and supersaturated.
• Know factors contributing to miscibility and
  immiscibility.
• Know how pressure affects solubility of gases
  Henrys law
• Understand the effects of colligative properties
  Raoults Law, freezing/boiling point changes,
  and osmosis.
• Know what a colloidal suspension is and how it
  may differ from a solution.

3
Solution process

• When intermolecular attractions between solute
  molecules are overcome by intermolecular
  attractions with solvent.
• solute breaks up to become dissolved within
  solvent.
• process of solvation

4
Solids and liquids
Overcoming intermolecular forces between solvent
molecules (h-bonds in water)
5
Solids and liquids
Overcoming intermolecular forces between solute
molecules/ions
6
Solids and liquids
Solvent molecules surround and interact with
solute (solvation) Solvation energy (hydration
energy) step b step c
7
Solvation and heat

• The heat flow associated with formation of a
  solution is called the heat of solution
  (??Hsolution)
• If solvation process is exothermic, then
  ??Hsolution is ___________.
• If solvation process is endothermic, then
  ??Hsolution is ___________.
• DEMO Ammonium nitrate vs calcium chloride

8
Two thermodynamic principles of solution
formation

• spontaneous processes tend to be exothermic
  (??Hsolution is ?)
• processes where entropy increases tend to occur
  spontaneously
• solution formation favored by an increase in
  entropy
• Solution formation may involve an endothermic
  process IF the entropy change outweighs the need
  to absorb heat.

9
Liquid-liquid interactions

• solute-solvent interactions like dissolves like
• miscible
• immiscible
• Molecular red rover

10
Dissolving gases in liquids

• Miscibility rules for liquids also apply to
  gas/liquid solutions.
• Usually, aqueous solutions of gases occur when
• Hydrogen bonding is possible with the gas
• The gas ionizes extensively in water
• Rxn between gas and water takes place

11
dynamic equilibrium of solutions

• solvent solute ? solution
• saturated solution in equilibrium with
  undissolved solute
• unsaturated solution not enough solute
  available to reach equilibrium.

12
crystallization

• for crystallization to take place, two conditions
  must be met
• solution must reach saturation point
• crystal order must be established
• Supersaturated solution an unstable solution
  with a concentration higher than the equilibrium
  concentration.
• Solution is beyond normal saturation point, but a
  crystal order has not yet established.

13
Temperature effects on solubility

• Increasing temperature may increase OR decrease
  solubility. Generally
• Solubility of substances that undergo endothermic
  dissolution ________ with increasing temperature
• Solubility of substances that undergo exothermic
  dissolution ________ with increasing temperature

14
Dissolving gases in solution why your pop
bottle fizzes over.

• The solubility of a gas is directly proportional
  to partial pressure of the gas.
• if pressure on gas increases, the solubility of
  the gas also increases.
• Henrys law
• Pg k Cg
• solubility and temperature

15
Colligative properties

• Physical properties of solutions that depend on
  the total number of solutes (not type of solute)
  are colligative properties.
• We will look at several types of colligative
  properties.

16
Raoults Law solutes and vapor pressure

• Partial pressure of a liquid solvent is dependent
  upon the amount of solute it contains.
• Psolvent Xsolvent P0solvent
• This form of the law applies only to
  non-volatile, non-ionizing solutes in ideal
  solutions.

17
Raoults law for two volatile liquids

• Two volatile liquids mutually effect each others
  partial pressures.
• PA XAP0A
• PB XBP0B

18
Deviations from ideality

• Positive and negative
• Depend on interactions between the two
  substances.
• Mixtures of polar/non-polar substances usually
  lead to _______ deviations of the ideal pressure.
• Mixtures of polar substances usually lead to
  ________ deviations of the ideal pressure.

19
Fractional distillation

• A process by which two volatile liquids are
  separated.
• Each time boiling and condensation takes place,
  the resulting liquid contains an increasing
  amount of the more volatile component.
• Fractional distillation columns are designed to
  facilitate this process.

20
Boiling point elevation/freezing point depression

• ?T kf m or
• ?T kb m
• ?T is change in temperature
• Must include molality of all solutes in solution.
• Kf/b constants have been determined for several
  substances
• (p 556)
• What is the change in freezing point temperature
  for a 0.100 molal solution of sucrose in water?
• What is the change in freezing point temperature
  for a 0.100 molal solution of K2CO3 in water?

21
Ion association

• Ions in solution occasionally stick to each
  other. When this happens, the combination
  behaves as one particle in solution.
• The effective concentration of ions in solution
  depends on the amount of association that occurs.

22
Physical behavior of ionic solutions
Non-ideal ionic solutions
Near ideal ionic solutions
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
23
vant Hoff factor

• Table 14-3 (p. 561). What happens as the number
  of ions per formula unit increases?
• What will the actual ??Tf be for a 0.100 m
  solution of K2CO3?

24
Osmotic pressure making pickles
h
Height of column ? pressure head from osmosis
25
Calculating osmotic pressure

• For dilute solutions, the osmotic pressure is
  given by
• MRT
• Example 0.100 molar solution of sucrose in
  water at 20 ?C.
• Osmotic pressure plays a key role in biological
  processes.

26
Molecular mass of polymers

• A 2.30 g sample of a polymer is dissolved in 250
  mL of water, resulting in an osmotic pressure of
  5.40 torr.
• What is the molecular mass of the polymer?

27
Reverse osmosis

• If a pressure is applied to the solution side of
  the system, greater than the osmotic pressure
  (2.40 atm in our sugar example), pure liquid can
  be forced out of the membrane.
• Everything moves in the opposite direction
• Desalination of seawater26 atm pressure required.

28
Colloids

• Colloids, or colloidal suspensions, are in an
  in-between area between homogeneous and
  heterogenous mixtures.
• Solute-like particles are dispersed in a
  solvent-like medium.
• Colloidal suspensions disperse and scatter light
  (Tyndall effect)
• Example milk.

29
Hydrophilic colloids

• Hydrophilic colloids form between water and
  dispersed phases containing polar groups about
  their surfaces.
• Example hemoglobin

30
Hydrophobic colloids

• Hydrophobic dispersed particles are not attracted
  to water. In order to produce a colloidal
  suspension, we must introduce an emulsifying
  agent.
• How to make your own mayo.
• Soaps are excellent examples of emulsifying
  agents.

31
Micelles

• Contain a polar head and a non-polar tail.
• Non-polar side attracts to non-polar substances
  (dirt, grease, etc).
• Polar side attracts to water.
• Micelles surround non-polar particle, suspending
  it in aqueous solution.