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Chapter 11- Properties of
Solutions
Sections 13.1 - 13.3
Dissolving Solubility
Read pg 529 543
pg 564 #1, 2, 3, 4, 6, 12, 13, 17, 19, 22, 25, 27, 29, 31,
84
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Solutions
Solutions are homogeneous mixtures of two
or more pure substances.
In a solution, the solute is dispersed uniformlythroughout the solvent.
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How Does a Solution Form?
As a solution forms, the solvent pulls solute
particles apart and surrounds, orsolvates,
them.
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QuickTime and a
Sorenson Video decompressorare needed to see this picture.
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How Does a Solution Form
If an ionic salt is
soluble in water, it is
because the ion-dipole interactions
are strong enough
to overcome the
lattice energy of thesalt crystal.
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From weakest to strongest, rank the
following solutions in terms of solvent
solute interactions: NaCl in water, butane
(C4H10) in benzene (C6H6), water in ethanol.
1. NaCl in water < C4H10 in C6H6 < water in ethanol2. Water in ethanol < NaCl in water < C4H10 in C6H6
3. C4H10 in C6H6 < water in ethanol < NaCl in water
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Correct Answer:
Butane in benzene will have
only weak dispersion forceinteractions. Water in
ethanol will exhibit much
stronger hydrogen-bondinginteractions. However, NaCl
in water will show iondipole
interactions because NaCl
will dissolve into ions.
1. NaCl in water < C4H10 in C6H6 < water in ethanol
2. Water in ethanol < NaCl in water < C4H
10in C
6H
6
3. C4H10 in C6H6 < water in ethanol < NaCl in water
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Energy Changes in Solution
Simply put, three
processes affect the
energetics of the
process:Separation of solute
particles
Separation of solventparticles
New interactions
between solute and
solvent
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Energy Changes in Solution
The enthalpy
change of the
overallprocess
depends on
Hfor each ofthese steps.
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Why Do Endothermic
Processes Occur?
Things do not tend
to occur
spontaneously (i.e.,
without outside
intervention) unless
the energy of the
system is lowered.
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Why Do Endothermic
Processes Occur?
Yet we know that
in some
processes, like
the dissolution of
NH4
NO3
in water,
heat is absorbed,
not released.
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Enthalpy Is Only Part of the Picture
The reason is that
increasing the
disorder orrandomness
(known as
entropy) of asystem tends to
lower the energy
of the system.
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Enthalpy Is Only Part of the Picture
So even though
enthalpy may
increase, theoverall energy of
the system can
still decrease ifthe system
becomes more
disordered.
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Water vapor reacts with excess solid
sodium sulfate to form the hydrated formof the salt. The chemical reaction is
Does the entropy increase or decrease?
SAMPLE EXERCISE 13.1 Assessing Entropy Change
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Student, Beware!
Just because a substance disappears when it
comes in contact with a solvent, it doesntmean the substance dissolved.
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Student, Beware!
Dissolution is a physical changeyou can
get back the original solute by evaporatingthe solvent.
If you cant, the substance didnt dissolve,
it reacted.
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Types of Solutions
Saturated
Solvent holds as
much solute as is
possible at thattemperature.
Dissolved solute is in
dynamic equilibriumwith solid solute
particles.
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Types of Solutions
Unsaturated
Less than the
maximum amount
of solute for that
temperature is
dissolved in thesolvent.
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Types of Solutions
Supersaturated
Solvent holds more solute than is normally
possible at that temperature.
These solutions are unstable; crystallization
can usually be stimulated by adding a seed
crystal or scratching the side of the flask.
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Factors Affecting Solubility
Chemists use the
axiom like dissolves
like:Polar substances
tend to dissolve in
polar solvents.
Nonpolar
substances tend to
dissolve in nonpolar
solvents.
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Factors Affecting Solubility
The more similar
the intermolecular
attractions, themore likely one
substance is to be
soluble in
another.
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Factors Affecting Solubility
Glucose (which has
hydrogen bonding)
is very soluble in
water, while
cyclohexane (which
only has dispersion
forces) is not.
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Factors Affecting Solubility
Vitamin A is soluble in nonpolar
compounds (like fats).
Vitamin C is soluble in water.
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SAMPLE EXERCISE 13.2 Predicting Solubility Patterns
Predict whether each of the following
substances is more likely to dissolve in carbontetrachloride (CCl4) or in water: C7H16, Na2SO4,
HCl, and I2.SolutionC7H16 is a hydrocarbon, so it is molecular
and nonpolar. Na2SO4, a compound containing ametal and nonmetals, is ionic; HCl, a diatomic
molecule containing two nonmetals that differ in
electronegativity, is polar; and I2, a diatomic
molecule with atoms of equal electronegativity, is
nonpolar. We would therefore predict that C7H16
and I2 would be more soluble in the nonpolar CCl4
than in polar H2O, whereas water would be thebetter solvent for Na SO and HCl.
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Answer :C5H12 < C5H11 Cl < C5H11 OH