Download Chapter 13 - "Water and Solutions"

•Water and Solutions
• Water is the most abundant liquid on the earth and is
necessary for all life. Because of water's great dissolving
properties, any sample is a solution containing solids, other
liquids, and gases from the environment. This stream also
carries suspended, ground-up rocks, called rock flour, from
a nearby glacier.
• Solutions
• Introduction
– A solution is a homogeneous mixture of ions or
molecules of two or more substances.
– Two parts
• Solvent is the component that is in the largest quantity
• Solute is the component that is dissolved in the solvent.
– If one of the components of a solution is a liquid it is
usually the solvent.
– If the solvent is water then the solution is identified as an
aqueous solution.
• Above this city there are at least three kinds of solutions.
These are (1) gas in gas-oxygen dissolved in nitrogen, (2)
liquid in gas-water vapor dissolved in air, and (3) solid in
gas-tiny particles of smoke dissolved in the air.
• Concentration of Solutions
– The concentration of a solution describes the relative
amounts of the solute and solvent.
• Concentrated solution contain large amounts of solute.
• Dilute solutions contain little solute.
– Different ways to express concentration
• Parts per million (ppm)
• Parts per billion (ppb)
• Percent by volume – the volume of solute per 100 volumes of
solution
• Molarity – number of moles of solute per liter of solution
• There are different ways to express the concentration of a
solution. How many different ways can you identify in this
photograph?
• Three ways to express
the amount of solute in a
solution: (A) as parts
(e.g., parts per million),
this is 2 parts per 100;
(B) as a percent by
volume, this is 2 percent
by volume; (C) as
percent by weight, this
is 2 percent by weight.
• Salinity is a measure of the amount of salts
dissolved in 1kg of solution. If 1,000 g of seawater
were evaporated, 35.0 g of salts would remain as
965.0 g of water leave.
• Solubility
– A saturated solution is an equilibrium mixture that
exists between solute and solvent where the rate of
dissolving is equal to the rate at which the solute is
coming out of solution.
– Solubility is the concentration of solute that can be
dissolved to make a saturated solution at a given
temperature.
• It is important to note that solubility varies with temperature
• Solubility of most ionic solids increases with increasing
temperature
• Solubility of most gases decreases with increasing temperature.
• Approximate
solubility
curves for
sodium nitrate,
potassium
nitrate,
potassium
chloride, and
sodium
chloride.
• Water Solutions
• (A) A tetrahedron is a four-sided pyramid. (B) Molecules
with four bonding electron pairs have a tetrahedral shape
like that of the CCI4 molecule illustrated here. Note that the
carbon atom is in the center of the pyramid, with chlorine
atoms at each of the four corners. (C) A water molecule has
two bonding pairs and two lone pairs in a tetrahedral
arrangement. (D) The water molecule has an angular
arrangement called bent. If something were attached to the
two lone pairs, it would be a tetrahedral arrangement.
• Properties of Water Molecules
– Water is a polar molecule since the oxygen has a greater
electronegativity than the hydrogen.
• The electrons spend more time around the oxygen,
giving it a partial negative charge and giving the
hydrogen a partial positive charge.
• It is this charge differential that allows most water
soluble compounds to dissolve in water.
– The polarity of the water molecule allows it to set up
intermolecular forces with other molecules.
• The positive end of the water molecule can attract the negative
part of another molecule
• The negative end of water molecule can attract the positive end
of another molecule.
• These types of attractions are called van der Waals forces.
– The intermolecular forces between a hydrogen atom and
an electronegative atom such as oxygen is called a
hydrogen bond.
• A hydrogen bond is a weak bond between a partial positive
hydrogen and a partial negative atom on another molecule.
• (A) The water
molecule is polar,
with centers of
positive and
negative charges.
(B) Attractions
between these
positive and
negative centers
establish hydrogen
bonds between
adjacent molecules.
• The hexagonal structure of ice. Hydrogen bonding between
the oxygen atom and two hydrogen atoms of other water
molecules results in a tetrahedral arrangement, which forms
the open, hexagonal structure of ice.
• The density of water from OOC to 10O C. The density of
water is at a maximum at 4O C, becoming less dense as it is
cooled or warmed from this temperature. Hydrogen bonding
explains this unusual behavior.
• The Dissolving Process
– There is a limit to the solubility of any molecule in a
solution
– Some substance do not dissolve to any appreciable
amount in a solution.
– Fluids that can mix freely with one another are called
miscible
– Fluids that cannot mix freely are called immiscible
• Water is
polar, and
carbon
tetrachloride
is nonpolar.
Since like
dissolves
like, water
and carbon
tetrachloride
are
immiscible.
• Soap cleans oil and grease because one end of the soap
molecule is soluble in water, and the other end is soluble in
oil and grease. Thus, the soap molecule provides a link
between two substances that would otherwise be
immiscible.
• An ionic solid dissolves in water because the number of
water molecules around the surface is greater than the
number of other ions of the solid. The attraction between
polar water molecules and a charged ion enables the water
molecules to pull ions away from the crystal, a process
called dissolving.
• Properties of Solutions
– Electrolytes
• Ionic substances dissolved in an aqueous solution
create a charge differential in the solution
• Since there is a charge differential there is a potential
difference and electrical charge can flow between
unlike charged areas.
• A solution which can conduct an electrical charge is
called an electrolyte
• A solution which cannot conduct an electrical charge
is called a nonelectrolyte.
• Ionic compounds ionize in water, disassociate to form
their independent, charged particles.
• Acids ionize in water to protonate the water molecule
and form a hydronium ion H3O+
• (A) Water solutions
that conduct an
electric current are
called electrolytes.
(B) Water solutions
that do not conduct
electricity are
called
nonelectrolytes.
• Three representations of water and hydrogen chloride in an
ionizing reaction. (A) Sketches of molecules involved in the
reaction. (B) Electron dot equation of the reaction. (C) The
chemical equation for the reaction. Each of these
representations shows the hydrogen being pulled away from
the chlorine atom to form H3O+, the hydronium ion.
– Boiling Point
• The boiling point of a solution is the point at which
enough energy has been added to overcome the
intermolecular forces that hold the solute in the
solution.
• At this point, the molecules gain enough kinetic
energy to produce a pressure that is greater than the
atmospheric pressure keeping them in solution
• Once this point is reached, the solution vaporizes
(becomes a gas)
• The rate of evaporation, and thus the vapor pressure, is less
for a solution than for a solvent in the pure state. The greater
the solute concentration, the less the vapor pressure.
– Freezing Point
• The freezing point of a solution is the point where
enough energy has been removed from the solution to
slow the molecules down and increase intermolecular
forces so the solution becomes a solid
• Since ionic compounds
dissolve by separation of
ions, they provide more
particles in solution than
molecular compounds. (A)
A mole of sugar provides
Avogadro's number of
particles. (B) A mole of
NaCl provides two times
Avogadro's number of
particles. (C) A mole of
CaCl2 provides three times
Avogadro's number of
particles.
• Acids, Bases, and Salts
• Properties of Acids and Bases
– The properties of acids are:
•
•
•
•
Sour taste
Change blue litmus paper to red.
React with metals to release hydrogen gas.
Neutralize bases, forming water and salts
– The properties of bases are:
•
•
•
•
Bitter taste
Bases turn red litmus paper blue.
Feel slippery to the skin.
Neutralize acids, forming water and salts.
• (A)Acid solutions will change the color of blue litmus to
red. (B) Solutions of bases will change the color of red
litmus to blue.
– Explaining Acid-Base Properties
• Acid base properties are explained by looking at how
they ionize in water.
• Equilibrium occurs when two opposing reactions
happen at the same time and rate.
– H2O + H2O  H3O+ + OH• This reaction can occur in both directions and
chemical equilibrium is reached when the rate of the
reaction to the right (the forward reaction) is equal to
the rate of the reaction to the left (the reverse
reaction)
• An acid is defined as a proton donor
• A base is defined as any substance that is a proton
acceptor
– Strong and Weak Acids and Bases
• Strong acids ionize completely in water
• Weak acids ionize only partially in water
• Strong bases ionize completely in water.
• Weak bases ionize only partially in water.
• Hydrochloric acid (HCl) has the common name of muriatic
acid. Hydrochloric acid is a strong acid used in swimming
pools, soil acidifiers, and stain removers.
– The pH Scale
• Acid base strength is a measure of relative H3O+ and OHconcentrations.
• Acidic solutions have concentrations of H3O+ above 1 X 10-7
• Neutral solutions have H3O+ concentrations equal to 1 X 10-7
• Basic solutions have H3O+ concentrations less than 1 X 10-7
• A neutral solution has a pH of 7, acidic solutions have pH
below 7, and basic solutions have pH above 7.
• Each division of the pH scale is an increase or decrease in
concentration to a power of 10, which makes the scale a
logarithmic scale
• The pH scale.
• The pH increases as the acidic strength of these substances
decreases from left to right. Did you know that lemon juice
is more acidic than vinegar? That a soft drink is more acidic
than orange juice or grapefruit juice?
• Properties of Salts
– A salt is produced by a neutralization reaction between an
acid and a base.
• A salt is any ionic compound, except for hydroxide or oxides
– Hard and Soft Water
• A solution of Ca2+ and Mg2+ ions is called hard water because
soap does not lather up very well in it.
– The Ca2+ and Mg2+ replace the sodium or potassium ions
in the soap and become insoluble
• A water softener exchanges sodium ions for the calcium and
magnesium ions of hard water. Thus, the water is now soft,
but it contains the same number of ions as before.
– Buffers
• A buffer is a mixture of a weak acid with the salt of
the acid.
• A buffer has the ability to resist changes in pH when
small amounts of acids or bases are added to the
solution
• If small amounts of acid is added, hydronium ions are
neutralized by reacting with the salt in solution
• If small amounts of base is added, hydroxide ions are
neutralized by reacting with the acid.