Water and Aqueous Systems
Chapter 17
Objectives
1. Describe the hydrogen bonding that
occurs in water
2. Explain the high surface tension and low
vapor pressure of water in terms of
hydrogen bonding
3. Account for the high heat of vaporization
and the high boiling point of water in
terms of hydrogen bonding
4. Explain why ice floats on water
The Water Molecule
• O-H bonds are highly polar covalent
bonds.
• the O atom has a slightly negative
charge. H has a slightly positive charge.
• Hydrogen bonding- occurs when
hydrogen is attracted to a very
electronegative element (N, O, and F).
Hydrogen bonding in water
The Unique Properties of Water
1.
2.
3.
4.
5.
6.
High surface tension
Low vapor pressure
High specific heat capacity
High heat of vaporization
High boiling point
Low density of ice
Surface properties
• Surface tension: the inward pull that
tends to minimize the
surface area of a liquid.
• Surfactant: surface active agent- surface
tension is decreased by detergents
(interferes with hydrogen bonding)
Why are water droplets
spherical as they fall?
Surface tension
Surface tension
Water on your car….
Vaporization and Vapor Pressure
• Hydrogen bonding allows few water
particles to vaporize, resulting in low vapor
pressure above the surface of water
Water’s low vapor pressure
Specific Heat Capacity
• Water’s high specific heat capacity helps
to moderate daily air temperatures around
large bodies of water.
• (Water’s specific heat
is more than 4 times
that of most metals.)
Water’s high heat of vaporization
• Because of hydrogen bonding,
water absorbs large amounts
of heat as it vaporizes.
• Water releases large
amounts of heat as it
condenses.
Water’s high boiling point
• Hydrogen bonding causes water’s high
boiling point
Ice
• Ice has an open framework structure, the
density of ice is lower than that of water.
Objectives
1. Explain the significance of the statement
“like dissolves like”
2. Distinguish between strong electrolytes,
weak electrolytes, and nonelectrolytes,
giving examples of each
What is a solution?
• Solution – homogeneous mixture
– Solvent – substance present in largest amount
(the dissolving medium)
– Solutes – dissolved particles in the lesser
amount
– Aqueous solution – water is the solvent
Solvation…
• the process that occurs as a solute
dissolves.
• There are two types of solvation:
– Ionic compounds dissolve by dissociation.
– Covalent compounds dissolve by molecular
solvation.
Solubility of Ionic Substances:
Dissociation
• The positive and negative ions of a salt come
apart (dissociate) as a salt dissolves.
Dissociation vs.
Molecular Solvation
• Dissociation:
NaCl(s)  Na +(aq) + Cl -(aq)
Note: the ions come apart from each other
(electrolytes)
• Molecular solvation:
C6H12O6(s)  C6H12O6(aq)
Note: no dissociation occurs
(nonelectrolytes)
“Like dissolves like”
• Polar solutes dissolve in polar solvents
(water is polar, so it dissolves polar
substances, either ionic or molecular).
• Nonpolar solutes dissolve in nonpolar
solvents (oil dissolves in kerosene).
Solubility of Polar Substances
• Ethanol is soluble in water because of the
polar OH bond.
• Why is solid sucrose (C12H22O11), table
sugar, soluble in water?
Substances Insoluble in Water
• Nonpolar oil does not interact with polar
water.
Electrolytes
• Electrolytes- conduct an electric current
in solution or in molten state.
(many ionic compounds)
• Nonelectrolytes- do not conduct an
electric current in solution or in molten
state.
(molecular compounds)
Electrolytes and nonelectrolytes
The dissolving process
Electrolytes
• Strong electrolytes: solute completely
breaks apart in solution
[includes soluble salts (such as KCl),
inorganic (strong) acids (such as HNO3),
inorganic (strong) bases (such as NaOH)]
Electrolytes
• Weak electrolytes: only a fraction of the
solute breaks apart into solution
[includes poorly soluble salts (such as PbCl2),
organic (weak) acids (such as HC2H3O2), and
organic (weak) bases (such as NH3)]
Electrolytes
• Nonelectrolyte – does not conduct when
in solution- does not break apart into ions.
[includes most organic or molecular
compounds (such as glucose)]
Electrolytes
Water of hydration
• The water in a crystal is called water of
hydration.
• A hydrate is a compound that includes
water of hydration.
Water of hydration
• A hygroscopic substance removes water
from the air. These substances are called
dessicants.
Heterogeneous Aqueous Systems
• Suspensions are mixtures from which
particles settle out upon standing. (The
particles are much larger than those in a
solution.)
• Colloids are heterogeneous mixtures that
contain particles that are intermediate in
size between suspensions and true
solutions
Colloidal Systems
• Colloids exhibit the Tyndall effect, which is
the scattering of light in all directions.
• Colloids include milk, mayonnaise,
marshmallows, egg white, blood, and
paint.
Solutions and their Behavior
Chapter 18
Objectives
1. Identify factors that determine the
rate at which a solute dissolves
2. Identify factors that affect the
solubility of a solute in solution
3. Calculate the solubility of a gas in a
liquid under various pressure
conditions
Factors affecting the rate of
dissolving
• How could you speed up the
dissolving of sugar in a glass of iced
tea?
• Which dissolves faster, table salt or
rock salt?
Factors affecting the rate of
dissolving
• Temperature – increasing the
temperature speeds up the rate of
dissolving
• Agitation – stirring speeds up the
rate of dissolving
• Particle size – smaller particles
dissolve faster than large particles
(surface area)
Solubility
• The solubility- the amount that
dissolves in a given quantity of
solvent at a given temperature.
Solubility
• A saturated solution contains the
maximum amount of solute at a
constant temperature.
Solubility
• An unsaturated solution contains less
solute than a saturated solution.
• A supersaturated solution contains
more solute than a saturated solution.
(This occurs when a solution is saturated
and then allowed to cool but all of the
solid remains dissolved. It is an unstable
solution, adding a crystal causes
precipitation.)
What would happen…
• …if you added more sugar to a
saturated sugar solution and
stirred?
• …if you added more sugar to an
unsaturated sugar solution and
stirred?
Factors affecting the
solubility of a substance
• How could you increase the amount
of sugar that would eventually
dissolve in a glass of tea?
Factors affecting the
solubility of a substance
•
Only two factors affect the amount of
solute that can dissolve.
1. Temperature affects solubility of both
solids and gases in liquid solvents.
2. Pressure affects solubility of gases in
liquid solvents.
Factors affecting the
solubility of a substance
• The solubility of most solid substances
increases as the temperature of the
solvent increases.
• For a few substances, the reverse occurs.
The Effect of Temperature on the
Solubility of Solids
The Effect of Temperature on Gas
Solubility
• Increasing the temperature of a dissolved
gas solution decreases the concentration
of the gas.
Have you ever tried a hot Dr. Pepper? Heat it in
a pan on the stove, pour a cup, and it has no
bubbles!
Thermal pollution occurs when hot water is
added to a lake, the dissolved oxygen
levels fall in the water and it kills the fish.
The Effect of Temperature on Gas
Solubility
The Effect of Pressure on Gas
Solubility
• Increasing the pressure of a gas over the
surface of a solvent increases the
solubility of the gas in the solvent.
• In a bottled soda, the pressure of CO2
over the liquid is high and when the cap is
opened, the pressure is reduced and
bubbles begin to come out of the solution.
The Effect of Pressure on Gas
Solubility
• Henry’s Law: at a given temperature the
solubility (S) of a gas in a liquid is directly
proportional to the pressure (P) of the gas
above the liquid.
S1/P1 = S2/P2
Question:
• If the solubility of a gas in water is 0.77 g/L
at 3.5 atm pressure, what is the solubility
(in g/L) at 1.0 atm?
(The temperature is held constant at
25oC.)
• Answer: S1/P1 = S2/P2
0.77 g/L / 3.5 atm = S2 / 1.0 atm
S2 = 0.22 g/L