Transcript Chapter 14

Chapter 15
Water and Aqueous
Systems
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Water: The Molecular View
• Water is most often thought of as a (l).(liquid at
room temp)
However, solid water (ice), and gaseous water
(water vapor), also exist in large qty on Earth.
Water is the only sub on Earth that exists in large
qty in all 3 states.
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Modeling Water: H-Bonding
H-bonding:
 a connection between the H atoms on one
molecule and a highly electro -ve atom (O, N,
F) on another.
 one kind of intermolecular attraction
 e.g. O in water molecule and H
in another water molecule.
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H Bonding Versus Covalent Bonding in H2O
• H2O molecules have
both intermolecular
forces (H-bonds) and
• intramolecular
forces (covalent
bonds).
• to vaporize (l) water
… need heat to break
H-bonds
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Water in the Liquid State
The intermolecular attraction among water
molecules results in the formn of H-bonds.
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States of Water
H2O occurs primarily in the (l) and (s) states on Earth, rather
than as a (g).
H-Bonds hold the H2O
molecules together
strongly enough that
they cannot readily
escape into the gaseous
state at ordinary
temperatures.
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States of Water
• That is why H2O has
such a high b.p.
(100° C) for such a
small molecules
• ethanol (C2H5OH)
has a b.p. of 70°C
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Ice Floats
• Ice floats in H2O.
• i.e. the density of the
H2O(s) < that of (l) H2O.
• As H cools from 60°C,
its volume decreases and its
density increases.
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Ice Floats
H2O molecules move < rapidly
• volume of the water
decreases because the
molecules pull together.
• Meanwhile, the mass of H2O
stays the same, so …density
increases.
• Below 4°C, the H2O molecules are beginning to
approach the (s) state → highly organized and
expand →density decreases→ float.
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Water
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Water Evaporation/Condensation
Vaporization
• change in state of (l) to (g) (vapor)
• at or above b.p. (100°C for liquid water) or
• at any temperatures → evaporation
Evaporation
• at surface of a (l). (molecules with enough k.e.
jump …)
• at any temperature (below b.p.)
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Evaporation/Condensation
Condensation
• (g) → (l)
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Water: The Super Solvent
 Most of the H2O on Earth is not pure (in solns).
 because it is an
excellent solvent
for a variety of
solutes.
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Water: The Super Solvent
• H2O is such a versatile solvent that it is
sometimes called the universal solvent.
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Solvents and Solutes
aqueous solution
• water that contains dissolved sub.
solvent
• the dissolving medium in a soln.
• exist in larger amt than solute
solute
• the dissolved particles in a soln
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Solvents and Solutes
both the solute and the solvent pass thru filter papers.
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Solvents and Solutes
• A solvent dissolves the solute.
• solute disperses in the solvent.
• Solvents and solutes may be (g), (l), or (s).
• Solute particles can be atoms, ions, or
molecules.
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The Solution Process
Polar solvents
• e.g. water;
• dissolve polar sub: most ionic cpds (e.g. NaCl),
acids, bases, salts and some covalent cpds (polar
covalent).
Nonpolar solvents
• e.g. gasoline, alcohol, kerosene, thinner,
oil, acetone (organic solvents)
• dissolve nonpolar sub (e.g. grease)
Like dissolves like.
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Electrolytes and Nonelectrolytes
Electrolyte
• a cpd that conducts an electric current when it is
in an aq soln or in the molten state. (ionic cpds)
• All ionic cpds (salts) are electrolytes →
dissociate into ions.
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The Solution Process
Polar solvents
• such as water
• dissolve ionic cpds and polar covalent cpds
such as HCl, NH3
nonpolar solvents
• such as gasoline
• dissolve nonsolve nonpolar cpds (covalent)
such as wax, oil,
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Concentrated vs Dilute
• Don’t use strong and weak to soln concns.
Use ‘concentrated’
and ‘dilute’ to
describe concn.
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Unsaturated vs Saturated
show soln concentration
• how much solute is present relative to the
max amt the soln could hold.
Unsaturated soln:
• If the amt of solute dissolved is < the max
that could be dissolved.
Saturated soln:
• holds the max amt of solute per amt of
the soln under the given conditions.
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Solubility
Solubility of a substance
• The amt of a substance that can be dissolved in
a given amt of solvent.
• a measure of this ability for a particular
substance in a particular solvent, = the qty of
substance dissolving in a fixed qty of solvent
to form a saturated soln under specified
temperature and pressure.
• in g per 100 g of solvent, moles per mole, etc
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Unsaturated vs Saturated
Supersaturated solution
• contain more solute than the usual max amt and
are unstable.
• cannot permanently hold the excess solute in
soln and may release it suddenly.
• have to be prepared carefully.
• made by dissolving a solute in the soln at an elevated
temp
• solubility is higher than at room temp, and then
• slowly cooling the soln.
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Factors Affecting Solubiity
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Suspensions
A suspension
• a mixture from which particles settle out upon
standing. e.g. sand mixes with water
• particles are much larger and denser--- do not
stay suspended indefinitely.
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Suspensions
Suspensions are heterogenous as at least 2 sub can be
clearly identified.
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Colloidal Systems
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Colloids
colloid
• a heterogeneous mixture
• spread throughout the dispersion medium.
• Particles smaller than those in suspensions
and larger than those in solns.
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Colloids
The Tyndall Effect
The scattering of visible light by colloidal and
suspension particles.
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Colloids
• Particles in colloids and suspensions reflect or
scatter light in all directions. Solns do not
scatter light → path of light is visible
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Colloids
Emulsions
 a colloidal dispersion of a (l) in a (l).
An emulsifying agent is essential for the
formn of an emulsion and for maintaining
the emulsion’s stability.
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Colloids
• Mayonnaise is heterogeneous mixture of oil and
vinegar.
• quickly separate w/o
the presence of egg
yolk, (an emulsifying
agent).
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Property
System
Solution
Colloid
Suspension
Particle
type
Particle size
Ions, atoms, small
molecules
Large molecules or Largest particles
particles
or aggregates
0.1-1nm
1-1000nm
1000nm and >
Effect of
light
Effect of
gravity
No scattering
Exhibits Tyndall
effect
Exhibits Tyndall
effect
Stable, does not
separate
Stable, does not
separate
Unstable,
sediment forms
Filtration
Particles not
retained on filter
Particles not
Particles retained
retained on filter
on filter
Uniformity Homogeneous
Heterogeneous
Heterogeneous
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CST problem 2
Water is a polar solvent, while hexane is a nonpolar solvent.
Solute
Water
Hexane
NH4Cl, ammonium chloride
Soluble
insoluble
C10H8, Napththalene
Insoluble
Soluble
C2H5OH, ethanol
Soluble
Soluble
CO(NH2)2, urea
Soluble
Insoluble
Which of the examples above illustrates a nonpolar solute in a polar
solvent?
A
NH4Cl in water
B
C10H8 in water
C
C2H5OH in hexane
D
CO(NH2)2 in hexane
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The End
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