CHAPTER 14 Solutions

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CHAPTER 14 Solutions
Solution homogeneous (uniform) mixture
Solution vs Colloidal Suspension
Clear vs Colorless
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Components of Solutions
Component a substance that is part of the
solution
Solvent present in the greatest amount Solute
other than solvent (lower amount)
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Ways of Expressing Concentration
Concentration intensive property that conveys
the amount of solute relative to the amount of
either solvent or solution
Qualitative ways Dilute Concentrated
Contrast dilute vs weak concentrated vs strong
Quantitative ways Numeric representation Seve
ral ways
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Molarity
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Mole Fraction
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Molality
Value is independent of temperature
Good for freezing point depression and boiling
point elevation experiments (see Section 14.5)
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Mass percentage
e.g. Solution of HCl that is 36 by mass contains
36 g of HCl for every 100 g solution
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Parts per Million (ppm) and Parts per Billion
(ppb)
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Example
How many grams of sulfur (S8) must be dissolved
in 100.0 g naphthalene (C10H8) to make a 0.12 m
solution?
Suggestion start with m and express in explicit
terms so that the units are clearly visible
Apply the necessary factors so that the final
answer (g of sulfur) is obtained.
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Example for Molarity
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Percent H2SO4
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Percent H2SO4 (2)
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Percent H2SO4 (3)
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Percent H2SO4 (4)
Or can solve in one step
Now calculate by mass if given molarity (same
problem but reversed)
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The Solution Process
Solution process one substance is dispersed
uniformly throughout another
Page 562 Dissolution of an ionic substance
Solvation (general) vs Hydration (water only)
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Saturated Solutions and Solubility
Crystallization opposite of the solution
process (solute becomes reattached to surface
of crystal)
Saturated solution no more solid will dissolve
and a dynamic equilibrium exists between the
solution and the undissolved solid
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More Saturated Solutions and Solubility
Solubility amount of solute needed to form a
saturated solution in a given quantity of
solvent
Solubility can be expressed different
ways (e.g. g/mL or molarity) and is
temperature dependent
Solubility of NaCl in water at 0 ?C 35.7 g
per 100 mL of water
Unsaturated solution less solute than maximum
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Supersaturation
Supersaturated Solution a solution that contains
a greater amount of solute than that needed to
form a saturated solution.
Page 563 supersaturated solution of sodium
acetate
Increase the solubility by increasing the
temperature and then slowly cool when cooled,
the ions have trouble arranging to form a crystal
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Factors That Influence Solution Formation
Whether or not two substances will mix is
influenced by two factors, a change (lowering) in
energy and an increase in the disorder of the
system.
If the energy is lower as a result of mixing the
two materials, the process is exothermic. This
favors the solution process. A solution will not
form if ?Hsoln is too endothermic.
Processes in which the disorder of the system
increases tend to occur spontaneously
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Factors That Influence Solution Formation (2)
Occasionally, it is observed that a solution will
form from two substances even though the enthalpy
of solution is positive (endothermic) or close to
zero.
The reason for the mixing is related to an
increase in disorder associated with the mixing
process (Figure 14.3).
If two nonpolar substances like C6H14 and CCl4
are mixed, the change in energy is minimal due to
the weak intermolecular forces encountered.
However, when they are mixed, the system is in a
more random or disordered state. This is a
statistically more likely arrangement compared to
when the two materials remain separated in the
same container.
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Factors Affecting Solubility
Solute-Solvent Interactions
Attractive forces between solute and solvent will
increase the ability to mix. In some cases, you
can predict solubility trends by looking at the
chemical structure and recognizing the types of
intermolecular forces encountered (Ch 13).
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More Factors Affecting Solubility
Polar liquids tend to dissolve in polar solvents
due to dipole-dipole and H-bonding interactions
Miscible soluble in all proportions
(liquids) Immiscible do not dissolve in one
another
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Factors Affecting Solubility (cont.)
Nonpolar liquids tend to be insoluble in polar
liquids e.g. hexane (C6H14) does not dissolve in
water. Increasing the nonpolar character of a
substance will decrease its solubility in water.
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Summary of Factors Affecting Solubility
Can increase the solubility by increasing the
number of polar groups (e.g. sucrose - page 565)
Ions are highly attracted to water molecules. In
some cases increasing the ionic character of a
compound that is not very polar and can increase
the solubility of water. e.g. I2 can be converted
to I3- (triiodide ion) to help dissolve it in
water Organic compounds are sometimes extracted
after reacting them with acids and bases.
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Predicting Solubility
Benzoic Acid
Sodium Benzoate
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Using Reactions To Affect Solubility
An acid (-CO2H) can be made ionic by reacting
with NaOH A base (-NH2) can be made ionic by
reacting with HCl
Other compounds will not be affected and will
remain soluble in organic solvents
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Extraction
http//chemistry.mtu.edu/kmsmith/Organic/Manuals/
2411/Extractions/Extractions.pdf ACID-BASE
EXTRACTION SEPARATION OF AN ORGANIC ACID, AN
ORGANIC BASE, AND A NEUTRAL COMPOUND (10/8/02)
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Heat of Solution (?Hsoln)
MgSO4 dissolved in water ?Hsoln -91 kJ/mol
WARM
Solution (or beaker) gets warm or cold?
NH4NO3 dissolved in water ?Hsoln 26 kJ/mol
COLD
Solution (or beaker) gets warm or cold?
Figure 14.7 Enthalpy changes that accompany
the solution process
Table 14.1 Enthalpy of Solution and Solubility
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Pressure Effects (Factors Affecting Solubility)
Figure 14.8 Effect of pressure on solubility of
gas
Solubility of a gas increases in direct
proportion to its partial pressure above the
solution
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Sample Exercise 14.5
Calculate the concentration of CO2 in a soft
drink that is bottled with a partial pressure of
CO2 of 0.33 atm over a liquid at 25 ?C. The
Henry's law constant for CO2 in water at this
temperature is 4.48 x 10-5 M/mm Hg.
Pg (0.33 atm)(760 mm Hg/1 atm) 2.5 x 102
mm Hg
Sg kPg (4.48 x 10-5 M/mm Hg)(2.5 x 102 mm Hg)
0.011 M
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Temperature Effects (Factors Affecting
Solubility)
The solubility of gases in water decreases with
increasing temperature. Le Chatelier's Principle
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14.4 Colligative Properites
Colligative properties properties of solutions
that depend upon the quantity (concentration) but
not the kind or identity of the solute particles
Colligative means "depending on the collection"
Four colligative properties will be reviewed
e.g. Lowering of Vapor Pressure
0.1 m sucrose vs 0.2 m sucrose
0.1 m sucrose vs 0.1 m glucose
0.1 m sucrose vs 0.1 m NaCl
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Lowering of Vapor Pressure
Figure Vapor pressure of solvent with and
without solute
Raoult's law Psolv XsolvP?solv
Psolv is partial pressure above soln Xsolv is
mole fraction P?solv is vapor pressure of pure
solvent
e.g. A solution is formed in which Xwater 0.800
and Xglucose 0.200. The vapor pressure of water
at 20 ?C is 17.5 mm Hg. Pwater (0.800)(17.5 mm
Hg) 14.0 mm Hg
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Boiling-Point Elevation and Freezing-Point
Depression
Figure 14.12 Phase diagrams for a pure solvent
and a solution
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Solutions Containing Ions
Salt or Sodium Chloride is used on roads because
it dissolves in water and it has a low molar mass
(i.e. can dissolve a lot of particles with a
small mass).
Consider the dissociation of ions when predicting
how the colligative property is
influenced. NaCl(s) ?Na(aq) Cl-(aq)
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Predict Which Solution Has the Lowest Freezing
Point Based on Number or Dissolved Particles
3) 0.15 m in particles
1) 0.30 m in particles
2) 0.20 m in particles
5) 0.050 to 0.10 m in particles
4) 0.10 m in particles
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Ion Pairing and van't Hoff Factors
Because cations are attracted to anions,
solutions containing ionic compounds sometimes
behave differently than what we would predict
based on the dissociation of ions.
Cations can sometimes associate with anions in
solution and form an ion pair. This makes the
two ions behave as a single particle and, from
the standpoint of colligative properties, reduces
the effect (e.g. freezing point depression)
compared to what one my expect.
Table 14.5 van't Hoff factors and freezing point
depression
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Osmosis
Figure 14.14/14.15 Two solutions separated by
a semipermeable membrane
Osmosis the net movement of solvent is always
toward the solution with the higher solute
concentration.