Transcript Chapter 11 Water and Solutions Water • The universal solvent. It has the ability to dissolve most molecules. • In living systems these molecules.

Chapter 11
Water and Solutions
Water
• The universal solvent. It has the ability to dissolve most
molecules.
• In living systems these molecules can then be transported
from one place to another by diffusion or by some kind of a
circulatory system.
• Liquid water has a higher density than ice (solid water). Ice
thus floats on the surface of liquid water. Fish and other
organisms can then live below the ice in natural bodies of
water.
• Water has a very high specific heat. Therefore, large bodies
of water can moderate temperature by absorbing great
amounts of heat.
• Water has a very high latent heat. This means that a great
amount of heat is needed to evaporate water. This is what
occurs when people perspire, we get rid of a lot of heat
from our bodies, thus enabling us to withstand high
temperatures.
Bonds in the Water Molecule
• The H-O bonds in the water molecule are polar covalent.
The oxygen atom attracts the electrons more than the
hydrogen atoms because it is more electronegative.
• Since the molecule has a “V” shape:
O
Negative end
H
H Positive end
• The oxygen end of the molecule is therefore more
negative and the hydrogen end of the molecule is more
positive.
• The molecule is said to be polar and thus possesses a
dipole (a negative and a positive end).
• As a result it can dissolve many ionic compounds, which
have a positive ion and a negative ion which are
attracted to the opposite charge in the two sides of the
water molecule. The negative ion will be attracted to the
positive side (the hydrogens), and the positive ion will be
attracted to the negative side (the oxygen).
Acids, Bases, and Salts
• Salts are ionic compounds. The hardness
of water is related to ionic compounds or
salts dissolved in water.
• The acidity of soils determines how well
plants grow.
• Acid rain is a by product of industry and
automobiles. This is harmful for living
organisms.
Acids, bases and salts
• Acids have sour
tastes
• Changes litmus
paper from blue to
red
• Acids react with
metals releasing
hydrogen gas
• Acids neutralize
bases forming water
and salt
Acids, bases and salts
• Bases have bitter taste
(caffeine)
• Bases turn litmus
paper blue
• Basic solutions feel
slippery on skin.
• Bases neutralize acids
forming water and salts
Acids, bases and salts
Acid - any substance that is a
proton donor when dissolved
in water.
A proton is a hydrogen ion,
H+
Hydronium ion
This is the same as:
H2O
HCl
H+(aq) + Cl-(aq)
Bases
• Many bases are ionic compounds which
contain hydroxide ion (OH-) as the
negative ion.
• They produce OH- when dissolved in
water:
HO
NaOH
Na+(aq) + OH-(aq)
2
Proton acceptor
A proton is a hydrogen ion (H+)
Acids, bases and salts
• Other bases, like ammonia, NH3, will produce the OHonce it is dissolved in water, since a hydrogen ion gets
transferred from a water molecule to the ammonia
molecule.
H+OHHydroxide ion
Base - Any substance that is a proton
acceptor when dissolved in water.
Proton acceptor
Acids, bases and salts
• What happens when acids and bases
mix?
The H+ from the acid and the OH- from the base react to form water:
H+ (aq) + OH-(aq)
H2O(l)
This is called neutralization
Strong Acids
• A strong acid is one that ionizes (dissociates) 100% in aqueous
solution.
• HNO3
H2O
H+(aq) + NO3-(aq)
100%
• Acids can be recognized because the formula begins with “H”.
• Common strong acids include:
HCl hydrochloric acid
HBr hydrobromic acid
HI
hydroiodic acid
H2SO4 sulfuric acid
HNO3 nitric acid
Weak Acids
• Weak acids only partially ionize (dissociate) in aqueous
solution. They only produce a small percentage of
hydrogen ions (H+).
H2O
• HF
not 100%
H+(aq) + F-(aq)
• A double arrow indicates an equilibrium, so the reaction
occurs both ways, to the right and to the left.
• The arrow to the left is longer because there is more of
the non ionized acid present than of the ions at any
given time.
Weak Acids
• Common weak acids include:
HC2H3O2 acetic acid
H2CO3 carbonic acid
• HC2H3O2
H 2O
H+(aq) + C2H3O2-(aq)
In the case of acetic acid, only about 1%
of the acid molecules ionize in aqueous
solution.
Strong Bases
• Strong bases are those that completely (100%)
ionize (dissociate) in aqueous solution.
NaOH H2O
Na+(aq) + OH-(aq)
100%
• Common strong bases include:
NaOH sodium hydroxide, common name lye, which
is used to make soap.
KOH potassium hydroxide
Also the other Group IA and Group IIA hydroxides
except for Be(OH)2 and Mg(OH)2.
Weak Bases
• Weak bases only partially ionize
(dissociate) in aqueous solution.
Mg(OH)2
Mg2+(aq) + 2OH-(aq)
not 100%
NH4+(aq) + OH-(aq)
NH3(aq) + H2O
not 100%
The pH Scale
• The strength of acids and bases is
measured using the pH scale.
• A neutral substance, like water, which is
neither an acid nor a base, has a pH of 7.
• Acidic solutions (acids dissolved in water)
have a pH lower than 7.
• Basic solutions (bases dissolved in water)
have a pH higher than 7.
Salts
• When an acid reacts with a base a salt (ionic
compound, except bases) is formed. Water is
also formed:
HCl + NaOH
acid base
2HNO3 + Ca(OH)2
acid base
NaCl + H2O
salt
water
Ca(NO3)2 + 2H2O
salt
water
• Neutralization always occurs 100% between an
acid and a base even if they are weak.
Importance of Salts
• Salts are important in the diet, since we need
them as electrolytes and as a source of certain
elements. These are minerals.
• Plants also require the elements that can be
provided by certain salts and are added to plants
as fertilizers. For example, K, N (supplied as
nitrates, NO3-), and P (supplied as phosphates,
PO43-).
• Not all salts are soluble in water. Insoluble salts
is what gives rise to hard water that ends up
forming rings in your bathtub because when
mixed with soap insoluble salts are formed.
Electrolytes
• Electrolytes are substances or solutions which
conduct electricity. Often these will be aqueous
solutions.
• In order to conduct electricity there has to be a
flow of electrons through the solution.
• In order for there to be a flow of electrons
through a solution there have to be ions in the
solution.
• The more ions there are the stronger the
electrolyte will be.
Only ionic compounds
Or acids dissolved in
water will produce ions in
solution and electricity can
be then conducted through the
Solution.
Ionic Compounds dissolved in
Water
• NaCl(s)
CaCO3(s)
H2O
H2O
Na+(aq) + Cl-(aq)
CaCO3(s)
Non Electrolytes
• All covalent compounds except for acids and
ammonia are non electrolytes, since no ions are
present in solution.
• Water is a covalent compound, so it is a non
electrolyte, so are all alcohols and most other
substances which are liquids or gases at room
temperature.
• Acids are unique, since they are covalent
compounds but they ionize when dissolved in
water. All acids are soluble in water.
Practice Exercises
• p. 294-296 Applying the Concepts:
# 20, 21, 22, 23, 24, 25, 26, 27
• New Book: p. 319-322 # 3, 9, 12, 13, 14,
15, 16, 18, 32, 33, 37, 38, 39, 40, 41, 42,
43, 44, 46, 49.
Review for Chapter 11 (cont.)
• Water is polar and is able to
dissolve many ionic compounds. It
also dissolves most acids and
many bases.
• Ionization of strong and weak acids
in aqueous solution (100% for
strong acids, equilibrium for weak
acids). (Strong acids are sulfuric,
nitric, hydrochloric, hydrobromic,
and hydroiodic acids. The rest are
weak acids.)
• Ionization of strong and weak
bases in aqueous solution.(100%
for strong bases, equilibrium for
weak bases). (Strong bases are
the group IA and IIA (alkali metals
and alkaline earth metals, except
Mg(OH)2 and Be(OH)2) combined
with hydroxide ion.
• Properties of acids and bases.
• How ammonia, NH3 is a base
because it reacts with water to
form NH4+ and OH- in aqueous
solution.
• Neutralization reactions: Acid +
Base react to produce salt
(ionic compound) plus water.
• The pH scale: 0-14. 7 is
neutral, like water. Less than7
is acidic, higher than 7 is
basic.
• The importance of salts: They
provide electrolytes to living
organisms, including plants,
animals and humans and they
are a source of some essential
elements.