Transcript 15.2 Homogeneous Aqueous Systems

15.2 Homogeneous Aqueous Systems >
Chapter 15
Water and Aqueous Systems
15.1 Water and Its Properties
15.2 Homogeneous Aqueous
Systems
15.3 Heterogeneous Aqueous
Systems
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15.2 Homogeneous Aqueous Systems >
CHEMISTRY
& YOU
How can you make a pickle glow?
Although it sounds
absurd, an
ordinary dill pickle
from the deli can
be a source of
light when
connected to an
electric current!
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15.2 Homogeneous Aqueous Systems > Solutions
Solutions
What types of substances
dissolve most readily in water?
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15.2 Homogeneous Aqueous Systems > Solutions
Solutions
What types of substances
dissolve most readily in water?
• An aqueous solution is water that
contains dissolved substances.
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15.2 Homogeneous Aqueous Systems > Solutions
Solvents and Solutes
• In a solution, the dissolving medium
is the solvent.
• The dissolved particles in a solution
are the solute.
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15.2 Homogeneous Aqueous Systems > Solutions
Solvents and Solutes
• A solvent dissolves the solute.
• The solute becomes dispersed in the
solvent.
• Solvents and solutes may be gases,
liquids, or solids.
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15.2 Homogeneous Aqueous Systems > Solutions
Solvents and Solutes
• Solutions are homogeneous mixtures.
• Solute particles can be atoms, ions, or
molecules.
• If you filter a solution through filter
paper, both the solute and solvent pass
through the filter.
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15.2 Homogeneous Aqueous Systems > Solutions
Solvents and Solutes
Substances that dissolve most
readily in water include ionic
compounds and polar covalent
compounds.
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15.2 Homogeneous Aqueous Systems > Solutions
Solvents and Solutes
Substances that dissolve most
readily in water include ionic
compounds and polar covalent
compounds.
• Nonpolar covalent compounds, such as
methane, and compounds found in oil, grease,
and gasoline, do not dissolve in water.
• However, oil and grease will dissolve in
gasoline.
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15.2 Homogeneous Aqueous Systems > Solutions
The Solution Process
• A water molecule is polar, with a partial
negative charge on the oxygen atom and
partial positive charges on the hydrogen
atoms.
• As individual solute ions break away from
the crystal, the negatively and positively
charged ions become surrounded by
solvent molecules and the ionic crystal
dissolves.
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15.2 Homogeneous Aqueous Systems > Solutions
The Solution Process
Solvated ions
The process by which
the positive and
negative ions of an
ionic solid become
surrounded by
solvent molecules is
called solvation.
Surface of ionic solid
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15.2 Homogeneous Aqueous Systems > Solutions
The Solution Process
• Polar solvents such as
water dissolve ionic
compounds and polar
compounds.
• Nonpolar solvents such
as gasoline dissolve
nonpolar compounds.
• This relationship can be
summed up in the
expression “like dissolves
like.”
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15.2 Homogeneous Aqueous Systems >
Which of these compounds should not
dissolve in water?
A. HCl
B. C4H10
C. KI
D. NH3
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15.2 Homogeneous Aqueous Systems >
Which of these compounds should not
dissolve in water?
A. HCl
B. C4H10
C. KI
D. NH3
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15.2 Homogeneous Aqueous Systems >
Electrolytes and
Nonelectrolytes
Electrolytes and Nonelectrolytes
Why are all ionic compounds
electrolytes?
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15.2 Homogeneous Aqueous Systems >
Electrolytes and
Nonelectrolytes
Electrolytes and Nonelectrolytes
Why are all ionic compounds
electrolytes?
• An electrolyte is a compound that
conducts an electric current when it
is in an aqueous solution or in the
molten state.
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15.2 Homogeneous Aqueous Systems >
Electrolytes and
Nonelectrolytes
All ionic compounds are
electrolytes because they
dissociate into ions.
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15.2 Homogeneous Aqueous Systems >
Electrolytes and
Nonelectrolytes
In order for the bulb to light, an
electric current must flow
between the two electrodes that
are immersed in the solution.
• Sodium chloride, a
strong electrolyte, is
nearly 100%
dissociated into ions
in water.
To (+)
electrode
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To (–)
electrode
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15.2 Homogeneous Aqueous Systems >
Electrolytes and
Nonelectrolytes
In order for the bulb to light, an
electric current must flow
between the two electrodes that
are immersed in the solution.
• Mercury(II) chloride, a
weak electrolyte, is
only partially
dissociated in water.
To (+)
electrode
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To (–)
electrode
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15.2 Homogeneous Aqueous Systems >
Electrolytes and
Nonelectrolytes
In order for the bulb to light, an
electric current must flow
between the two electrodes that
are immersed in the solution.
• Glucose, a nonelectrolyte,
does not dissociate in
water.
• A nonelectrolyte is a
compound that does not
conduct an electric current
in either an aqueous
solution or the molten state.
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To (+)
electrode
To (–)
electrode
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15.2 Homogeneous Aqueous Systems >
Electrolytes and
Nonelectrolytes
Some polar molecular compounds are
nonelectrolytes in the pure state but
become electrolytes when they dissolve
in water.
• This change occurs because such
compounds ionize in solution.
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15.2 Homogeneous Aqueous Systems >
Electrolytes and
Nonelectrolytes
Some polar molecular compounds are
nonelectrolytes in the pure state but
become electrolytes when they dissolve
in water.
• For example, ammonia (NH3(g)) is not an
electrolyte in the pure state.
• Yet an aqueous solution of ammonia conducts an
electric current because ammonium ions (NH4+)
and hydroxide ions (OH–) form when ammonia
dissolves in water.
NH3(g) + H2O(l)  NH4+(aq) + OH–(aq)
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15.2 Homogeneous Aqueous Systems >
Electrolytes and
Nonelectrolytes
Not all electrolytes conduct electric
current to the same degree.
• In a solution that contains a strong
electrolyte, all or nearly all of the solute
exists as ions.
• A weak electrolyte conducts an electric
current poorly because only a fraction of the
solute in the solution exists as ions.
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15.2 Homogeneous Aqueous Systems >
Electrolytes and
Nonelectrolytes
Your cells use electrolytes, such as sodium
and potassium ions, to carry electrical
impulses across themselves and to other cells.
• An electrolyte imbalance
can occur if you become
dehydrated.
• When you exercise,
you can lose water
and electrolytes from
your body through
perspiration.
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15.2 Homogeneous Aqueous Systems >
CHEMISTRY
& YOU
Pickles contain table salt. Why can
electric current flow through a pickle,
causing it to glow?
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15.2 Homogeneous Aqueous Systems >
CHEMISTRY
& YOU
Pickles contain table salt. Why can
electric current flow through a pickle,
causing it to glow?
Electrolytes conduct an electric current
when they are in an aqueous solution.
Table salt, or NaCl, is a strong
electrolyte. The water and salt in the
pickle form a solution that conducts an
electric current. The electric current
causes the pickle to glow.
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15.2 Homogeneous Aqueous Systems >
Explain why you must be extremely
careful when using electricity near a
swimming pool.
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15.2 Homogeneous Aqueous Systems >
Explain why you must be extremely
careful when using electricity near a
swimming pool.
The chlorinated water in a swimming
pool is a solution that can conduct an
electric current. If a current is
introduced into the water, any
swimmers could be shocked.
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15.2 Homogeneous Aqueous Systems > Hydrates
Hydrates
Why do hydrates easily lose and
regain water?
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15.2 Homogeneous Aqueous Systems > Hydrates
The water contained in a crystal is called
the water of hydration or water of
crystallization.
• A compound that contains water of
hydration is called a hydrate.
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15.2 Homogeneous Aqueous Systems > Hydrates
The forces holding the water
molecules in hydrates are not very
strong, so the water is easily lost
and regained.
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15.2 Homogeneous Aqueous Systems > Hydrates
The forces holding the water
molecules in hydrates are not very
strong, so the water is easily lost
and regained.
• A substance that is anhydrous does not
contain water.
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15.2 Homogeneous Aqueous Systems > Hydrates
CuSO45H2O(s)
Heating of a
sample of
blue
CuSO45H2O
begins.
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+ heat
– heat
CuSO4(s) + 5H2O(g)
After a time,
much of the
blue hydrate
has been
converted to
white
anhydrous
CuSO4.
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15.2 Homogeneous Aqueous Systems > Hydrates
• A piece of filter paper that
has been dipped in an
aqueous solution of cobalt(II)
chloride and then dried is
blue in color (anhydrous
CoCl2).
• When the paper is exposed
to moist air, it turns pink
because of the formation of
the hydrate cobalt(II)
chloride hexahydrate
(CoCl26H2O).
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15.2 Homogeneous Aqueous Systems > Hydrates
Each hydrate contains a fixed quantity of
water and has a definite composition.
Some Common Hydrates
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Formula
Chemical name
Common name
MgSO47H2O
Magnesium sulfate heptahydrate
Epsom salt
Ba(OH)28H2O
Barium hydroxide octahydrate
CaCl22H2O
Calcium chloride dihydrate
CuSO45H2O
Copper(II) sulfate pentahydrate
Blue vitriol
Na2SO410H2O
Sodium sulfate decahydrate
Glauber’s salt
KAl(SO4)212H2O
Potassium aluminum sulfate
dodecahydrate
Alum
Na2B4O710H2O
Sodium tetraborate decahydrate
Borax
FeSO47H2O
Iron(II) sulfate heptahydrate
Green vitriol
H2SO4H2O
Sulfuric acid hydrate (mp 8.6oC)
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15.2 Homogeneous Aqueous Systems > Hydrates
To determine what percent by mass of a
hydrate is water:
• First determine the mass of water in one mole of
hydrate.
• Then determine the molar mass of the hydrate.
• The percent by mass of water can be calculated
using the following equation:
mass of water
Percent by mass H2O = mass of hydrate  100%
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15.2 Homogeneous Aqueous Systems > Hydrates
Efflorescent Hydrates
The water molecules in hydrates are held
by weak forces, so hydrates often have
an appreciable vapor pressure.
• If a hydrate has a vapor pressure higher than
the pressure of water vapor in the air, the
hydrate will lose its water of hydration, or
effloresce.
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15.2 Homogeneous Aqueous Systems > Hydrates
Hygroscopic Hydrates
Hydrated ionic compounds that have low
vapor pressure remove water from moist
air to form higher hydrates.
• These hydrates and other compounds that
remove moisture from air are called
hygroscopic.
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15.2 Homogeneous Aqueous Systems > Hydrates
Hygroscopic Hydrates
Calcium chloride monohydrate
spontaneously absorbs a second molecule
of water when exposed to moist air.
• Calcium chloride is used as
a desiccant in the laboratory.
• A desiccant is a substance
used to absorb moisture
from the air and create a dry
atmosphere.
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15.2 Homogeneous Aqueous Systems >
Sample Problem 15.1
Finding the Percent by Mass of Water in
a Hydrate
Calculate the percent by mass
of water in washing soda,
sodium carbonate
decahydrate (Na2CO310H2O).
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15.2 Homogeneous Aqueous Systems >
Sample Problem 15.1
1 Analyze List the known and the unknown.
To determine the percent by mass, divide the
mass of water in one mole of the hydrate by the
molar mass of the hydrate and multiply by 100%.
KNOWN
formula of hydrate = Na2CO310H2O
UNKNOWN
percent H2O = ? %
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15.2 Homogeneous Aqueous Systems >
Sample Problem 15.1
2 Calculate Solve for the unknown.
Determine the mass of 10 mol of water.
mass of 10 mol H2O = 10[(2  1.0 g) + 16.0 g] = 180.0 g
For every 1 mol of
Na2CO310H2O, there
are 10 mol of H2O.
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15.2 Homogeneous Aqueous Systems >
Sample Problem 15.1
2 Calculate Solve for the unknown.
Determine the mass of 1 mol of the
hydrated compound.
molar mass of
= (2  23.0 g) + 12.0 g + (3  16.0 g) + 180.0 g
Na2CO310H2O
= 286.0 g
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15.2 Homogeneous Aqueous Systems >
Sample Problem 15.1
2 Calculate Solve for the unknown.
Calculate the percent by mass of water in
the hydrate.
mass of water
percent by mass H2O = mass of hydrate  100%
=
180.0 g
286.0 g  100%
= 62.94%
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15.2 Homogeneous Aqueous Systems >
Sample Problem 15.1
3 Evaluate Does the result make sense?
• The mass of the water accounts for
more than half the molar mass of the
compound.
• So, a percentage greater than 50%
is expected.
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15.2 Homogeneous Aqueous Systems > Hydrates
Deliquescent Compounds
Some compounds are so hygroscopic that
they become wet when exposed to
normally moist air.
• These compounds are deliquescent, which
means that they remove sufficient water from
the air to dissolve completely and form
solutions.
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15.2 Homogeneous Aqueous Systems > Hydrates
Pellets of sodium hydroxide are deliquescent.
For this reason,
containers of NaOH
should always be
tightly stoppered.
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The solution formed by a
deliquescent substance has
a lower vapor pressure than
that of the water in the air.
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15.2 Homogeneous Aqueous Systems >
Calculate the percent by mass of water
in epsom salt, magnesium sulfate
heptahydrate (MgSO47H2O).
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15.2 Homogeneous Aqueous Systems >
Calculate the percent by mass of water
in epsom salt, magnesium sulfate
heptahydrate (MgSO47H2O).
mass of H2O = 7  ( 2  1.0 g + 16.0 g) = 126.0 g
molar mass of = 24.3 g + 32.1 g + (4  16.0 g) + 126.0 g
MgSO47H2O
= 246.4 g
126.0 g
percent by mass H2O = 246.4 g  100% = 51.14%
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15.2 Homogeneous Aqueous Systems > Key Concepts and
Key Equation
Substances that dissolve most readily in
water include ionic compounds and polar
covalent compounds.
All ionic compounds are electrolytes
because they dissociate into ions.
The forces holding the water molecules in
hydrates are not very strong, so the water
is easily lost and regained.
mass of water
percent by mass H2O = mass of hydrate  100%
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15.2 Homogeneous Aqueous Systems > Glossary Terms
• aqueous solution: water that contains
dissolved substances
• solvent: the dissolving medium in a solution
• solute: dissolved particles in a solution
• solvation: a process that occurs when an
ionic solute dissolves; in solution, solvent
molecules surround the positive and negative
ions
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15.2 Homogeneous Aqueous Systems > Glossary Terms
• electrolyte: a compound that conducts an
electric current when it is in an aqueous
solution or in the molten state; all ionic
compounds are electrolytes, but most
covalent compounds are not
• nonelectrolyte: a compound that does not
conduct an electric current in aqueous
solution or in the molten state
• strong electrolyte: a solution in which a
large portion of the solute exists as ions
• weak electrolyte: a solution that conducts
electricity poorly because only a fraction of
the solute exists as ions
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15.2 Homogeneous Aqueous Systems > Glossary Terms
• water of hydration: water molecules that are
an integral part of a crystal structure
• hydrate: a compound that has a specific
number of water molecules bound to each
formula unit
• anhydrous: a substance that does not
contain water
• effloresce: to lose water of hydration; the
process occurs when the hydrate has a vapor
pressure higher than that of water vapor in
the air
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15.2 Homogeneous Aqueous Systems > Glossary Terms
• hygroscopic: a term describing salts and
other compounds that remove moisture from
the air
• desiccant: a hygroscopic substance used as
a drying agent
• deliquescent: describes a substance that
removes sufficient water from the air to form a
solution; the solution formed has a lower
vapor pressure than that of the water in the
air
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15.2 Homogeneous Aqueous Systems >
BIG IDEA
Bonding and Interactions
• Ionic compounds and polar covalent
compounds dissolve most readily in
water to form aqueous solutions.
• Ionic compounds dissolve in water
when the polar water molecules attract
the ions of the solute, causing the
individual solute ions to break away
from the ionic crystal.
55
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15.2 Homogeneous Aqueous Systems >
END OF 15.2
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