NAME________________________________ PER ________ DATE DUE ___________________
ACTIVE LEARNING I N C HEMISTRY E DUCATION
"ALICE"
CHAPTER 21
ACIDS
AND
BASES
Behavior In Water
21-1
©1997, A.J. Girondi
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© 1997 A.J. Girondi, Ph.D.
505 Latshmere Drive
Harrisburg, PA 17109
[email protected]
Website: www.geocities.com/Athens/Oracle/2041
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©1997, A.J. Girondi
SECTION 21.1
Acids and Bases - The Arrhenius Definitions
In preceding chapters you have studied substances such as gases, salts, metals, and so forth.
These labels describe classes of chemical substances which have certain properties in common. This
chapter will introduce you to two additional classes of substances known as acids and bases.
Many years ago it was found that when certain substances were placed into water, the resulting
solutions had a sour taste. When the early alchemists discovered this sour taste, they called these
substances acids. The alchemists were fond of using Latin names and phrases when describing their
work. The Latin word meaning "sour" is acidus. The alchemists discovered that these acid solutions were
sometimes capable of dissolving compounds that would not dissolve in plain water. You are probably
familiar with the sour taste of vinegar. Vinegar contains acetic acid. The Latin word for vinegar is acetum,
reflecting its sour taste.
Alchemists also worked with another kind of substance prepared from the ashes of dried plants.
When these ashes were placed in water, some of the components in the ashes dissolved in the water. It
was found that when these "ash solutions" were mixed with acid solutions, the sour taste of the acid
disappeared. These substances derived from ashes were named bases. Early chemists found that a
base could neutralize, or cancel out, the properties of an acid.
Chemical descriptions of acids and bases have developed and improved during the past 300
years. The actual ions and molecules present in acids and bases have been identified. It was found that
the common ion present in acid solutions was the positively charged hydrogen ion, H1+, while the most
common ion found in solutions of bases was the negatively-charged OH1- ion. What is the name of this
ion? {1}_________________________
In modern chemistry there are three ways to define acids and bases. In this chapter, we will
examine two of these ways. Please keep in mind that in this chapter we will be discussing how acids
behave when you put them into pure water, and how bases behave when you put them into pure water.
In the next chapter, you will study how acids and bases react with each other.
The first and simplest definition of acids was provided by the Swedish chemist, Arrhenius, in
1887. He defined an acid as a compound that contains hydrogen and which would produce hydrogen
ions (H 1+) when you dissolve it in water. For example, when hydrogen chloride gas (HCl) is dissolved in
water it breaks up or "dissociates" into hydrogen ions and chloride ions:
HCl(g) -----> H1+(aq) + Cl1-(aq)
This solution which contains hydrogen and chloride ions is called hydrochloric acid. It is the acid which aids
digestion in your stomach. Its acidic properties are due to the presence of the hydrogen ions, H1+.
We will generally show hydrogen
as the first element in the formula
of an acid.
In some acids, hydrogens which
are part of a polyatomic ion do not
dissociate into hydrogen ions.
HCl
HC2 H3 O2
acetate ion
In the case of acetic acid (the acid in vinegar) the first H in the formula represents what we call an "acidic"
hydrogen, which is an H that can form an H1+ ion when the molecule is put into water. The three H's which
are part of the acetate ion (C2H3O21-) are not "acidic" H's:
HC2H3O2(aq) <===> H1+(aq) + C2H3O21-(aq)
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©1997, A.J. Girondi
The Arrhenius definition of bases is the simplest one. According to Arrhenius, a base is a
compound that contains the hydroxide ion, and produces hydroxide ions (OH1-) when it is dissolved in
water. When solid sodium hydroxide is dissolved in water, it breaks up or "dissociates" into sodium ions
and hydroxide ions:
NaOH(s) -----> Na1+(aq) + OH1-(aq)
A water solution of NaOH has the properties of a base because of the presence of the hydroxide ions.
Sometimes basic solutions are described as being "alkaline."
The Arrhenius Definitions:
An acid produces H1+ in water solution
A base produces OH1- in water solution
General Properties of Acidic Solutions
they taste sour
they neutralize bases
they affect chemical indicators
they are electrolytes, meaning they conduct electricity
Problem 1. Complete the following equations, showing how the acids listed dissociate in water to form
hydrogen ions and a negatively-charged ion (anion).
a. HBr(aq)
__________________________________________________
b. HI (aq)
__________________________________________________
c.
__________________________________________________
HClO4(aq)
General Properties of Basic Solutions
they taste bitter
they neutralize acids
they feel slippery
they affect chemical indicators
they are electrolytes, meaning they conduct electricity
Problem 2. Complete the following equations, showing how the bases listed dissociate in water to form
OH1- ions.
a. KOH (s)
__________________________________________________
b. LiOH(s)
__________________________________________________
c. CsOH(s)
__________________________________________________
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©1997, A.J. Girondi
SECTION 21.2
The Bronsted–Lowry Definitions
Arrhenius's definitions were generally accepted by chemists of his time. They explained many
unanswered questions about acids and bases. His definitions of acids and bases are still widely used by
chemists who work with aqueous (water) solutions.
However, eventually compounds were discovered that had the properties of bases, but which did
not contain the OH1- ion. This meant that a better definition of bases was needed. The Arrhenius
definition was no longer adequate. A definition was needed which could explain why some compounds,
other than those containing hydroxide, had "basic" properties. The term "alkaline" is sometimes used to
describe substances which have "basic" properties. To solve this problem, a second definition of acids
and bases was suggested by Thomas M. Lowry and Johannes N. Bronsted. It will be helpful for you to
realize that a hydrogen ion (H1+) can also be called a proton, since all that is left of a hydrogen atom which
has lost its electron is a proton.
Hydrogen Ion = H1+ = a proton
H
H1+
If a hydrogen atom loses its
electron, it becomes a hydrogen
ion - just a proton.
A common hydrogen atom
consists of one proton in the
nucleus and one electron.
Bronsted and Lowry proposed that an acid be defined as a molecule or ion that can give away or donate a
proton (H 1+) to some other particle. A base was defined as a substance that can combine with or accept a
proton (H 1+) from some other particle. According to the Bronsted–Lowry concept, an acid became known
as a proton donor, and a base as a proton acceptor.
The Bronsted–Lowry Definitions
An acid donates a proton (H1+)
A base accepts a proton (H1+)
Note that the Arrhenius definition of an acid and the Bronsted–Lowry definition of an acid are very similar.
Both definitions refer to the formation of H 1+:
HCl ----> H1+ + Cl1-.
However, the Arrhenius definition of a base refers only to substances which can provide OH1- ions in
solution, whereas, the Bronsted–Lowry definition of a base refers to any particle which can accept a
hydrogen ion (proton), H1+. Perhaps the following example will help. Arrhenius would consider NaOH to
be a base because it forms OH1- when you put it into water solution:
NaOH(s) ----> Na1+(aq) + OH1-(aq)
Bronsted and Lowry would consider NaOH to be a base because a solution of it contains the OH1- ion
which is a proton acceptor:
H1+(aq) + OH1-(aq) ----> HOH(l)
When OH1- accepts a proton its forms a molecule of water, HOH.
Now remember, when Arrhenius defined a base he was thinking only of OH 1-. However, other
particles such as the fluoride ion, F1-, can also act a proton acceptors. For example:
H1+(aq) + F1-(aq) ----> HF(aq)
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©1997, A.J. Girondi
Solutions which contain the fluoride ion have "basic" properties similar to solutions which contain the
hydroxide ion, OH 1-. Other particles which can function as bases (proton-acceptors) of this kind include
both molecules and ions like C 2H3O21-, NH3, CO32-, and many others. Therefore, many more particles can
be classified as bases according to Bronsted–Lowry than according to Arrhenius. Here are a few more
equations which illustrate how these bases function as proton-acceptors:
H1+ (aq) + C2H3O21-(aq) ----> HC2H3O2(aq)
H1+(aq) + NH3(aq) ----> NH41+(aq)
H1+(aq) + CO32-(aq) ----> HCO31-(aq)
What is it about particles like OH 1-, F 1-, C2H3O21-, NH3 , and CO32- that allows them to function as proton
acceptors? If you look at the electron dot structure of these ions, you will see that they have one or more
unshared pairs of electrons:
1-
1-
The OH1- ion has 3 unshared pairs of
electrons, while the F 1- ion has 4 unshared
pairs.
OH
F
hydroxide ion (a base)
fluoride ion (a base)
The H1+ ion is seeking the stable helium configuration (1s2). It can achieve that configuration by sharing a
pair of electrons with another particle. So, protons (H1+) tend to bond to particles which have an unshared
pair of electrons in their valence shells. Many such particles can act as proton–acceptors which are bases
according to the {2} ______________________definition.
Check out the electron-dot structures of
OH1- and the F1- ions below:
1H1+ +
OH
H OH
+
H1+
F
1-
H F
If you examine the electronic structures of other bases like NH 3 molecules or acetate ions, C2H3O21-, you
will see that they also have unshared pairs of electrons:
1+
H1+
+
H
N
H
H
H1+ +
H
C
H
H
H
H
1–
H
H
N
H
O
C
H
O
C
H
O
C
OH
You might ask, "Why do acids lose H1+ when they are put into water?" Well, the
reason is that water molecules take them! You see, since water has two
unshared pairs of electrons on the oxygen atom, it too can accept protons and
function as a base. Note the electron-dot structure of water shown at right.
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O
H
H
©1997, A.J. Girondi
To illustrate how water can act as a proton-acceptor, consider the equation below showing the reaction or
"dissociation" of HCl in water:
HCl(g) + H2O(l) ----> H3O1+(aq) + Cl1-(aq)
The shorthand way of writing this equation is:
HCl(g) -----> H1+(aq) + Cl1-(aq)
(Note that HCl is a gas and therefore is accompanied by the (g) subscript before it is put into water. Most of the other acids you will see will
be written using the (aq) subscript.)
When you include water in the equation, you must represent the hydrogen
ion as H3O1+ instead of as H1+. This is actually a more accurate
representation of what happens. The H 3O1+ particle is known as the
hydronium ion. It is a water molecule which is bonded to a proton (H1+). Its
electron-dot structure is shown at right.
H
O
H
1+
H
The hydronium ion forms when water acts as a base and accepts a proton from an acid molecule like HCl.
H1+
H Cl
+
O H
H
H O H
H
1+
+
Cl
1-
Equations representing what happens when you put an acid molecule in water can, therefore, be written
in two ways. You can choose to show the water and the hydronium ion (the more accurate way to do it), or
you can choose not to show them (the shorthand way of writing it). Below are the equations representing
what happens when you put nitric acid in water:
HNO3(aq) + H2O(l) ----> H3O1+(aq) + NO31-(aq)
or
HNO3(aq) ----> H1+(aq) + NO31-(aq)
According to the Bronsted-Lowry definition, water can act as a base since it can accept protons. (Later
you will learn that water can also give away a proton and, therefore, act as an acid.)
Problem 3. For each of the acids below, write two forms of the equation which represents what
happens when you put them into water.
a. HBr
_____________________________________
_____________________________________
b. HClO4
_____________________________________
_____________________________________
Not all particles with unshared pairs of electrons make good bases (proton acceptors) in water. For
example, the chloride ion has the necessary unshared pairs of electrons, but it is not a basic ion. It is
described as being neutral. It is NOT a proton acceptor:
H1+
+
Cl
1-
no reaction
Some other neutral ions include Br 1-, I 1-, NO31-, SO42-, and ClO41-.
21-7
©1997, A.J. Girondi
SECTION 21.3
Strong Acids Versus Weak Acids
A. Strong Acids
The reason that solutions of acids and bases are electrolytes is because they contain ions. The
ions move about and carry the electric charge through the solution. The equation below demonstrates the
reaction of hydrochloric acid in water. Note that the ions produced are "aqueous" meaning in water
solution. Acids are hydrogen compounds that form water solutions which contain ions, one of which is
the hydrogen ion.
For example:
HI (aq) ----> H1+(aq) + I 1-(aq)
HI (aq) + H2O(aq) ----> H3O1+(aq) + I 1-(aq)
or
There are only six strong acids, but there are many, many weak acids. The six strong acids are
HBr, HCl, HI, HNO3, H2SO 4, HClO4. You should memorize the six strong acids. You may be wondering
what makes an acid strong or weak. Acids such as HCl and HNO 3, are strong acids because they
dissociate completely to form ions when they are put into water. In other words, all of the molecules of a
strong acid will dissociate into ions when you put the acid into water solution. We say that they are "100
percent dissociated in water." Solutions of strong acids, therefore, contain a high concentration of
hydrogen ions. Perchloric acid, HClO4, is an example:
HClO4(aq) ----> H1+(aq) + ClO41-(aq)
OR
HClO4(aq) + H2O(l) ----> H3O1+(aq) + ClO41-(aq)
HI
hydroiodic acid
HBr
hydrobromic acid
HCl
hydrochloric acid
THE SIX STRONG ACIDS
HClO4
perchloric acid
H2SO 4
sulfuric acid
HNO3
nitric acid
B. Weak Acids
Acetic acid, HC2H3O2, is classified as being weak. Because of the nature of the bonding between
the acidic hydrogen and the acetate ion, a molecule of acetic acid does not dissociate very much in water.
As a result, most molecules of weak acids remain in the form of molecules when they are put into water.
Many of the ions which form when the weak acid molecules dissociate will recombine to form the original
molecules. Thus, the concentration of hydrogen ions is lower than it would have been if the acid had been
strong. Only a small percentage of molecules of a weak acid will be dissociated at any given point in time.
An equilibrium is established in which the equilibrium is strongly favored toward the reactants (<---):
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©1997, A.J. Girondi
HC2H3O2(aq) + H2O(l) <====> H3O1+(aq) + C2H3O21-(aq)
low concentration
of hydronium ions
double arrow indicates equilibrium
Or, using the shortcut (but less accurate) representation:
HC2H3O2(aq) <====> H1+(aq) + C2H3O21-(aq)
The use of the double-headed arrow indicates that the acid is weak and exists mostly in the form of
molecules (HC 2H3O2) rather than as ions. Equilibrium is characteristic of {3}__________ acids in water.
When equilibrium is established, the system contains mostly reactants and, therefore, not many ions. This
fact provides us with a method for determining the strength of acids. The greater the concentration of
ions in an acid solution, the better the solution will conduct electricity. Since stronger acids dissociate into
ions much, much better than weak ones do, the stronger acids are {4}___________ conductors of
electricity. You will use this property to determine the relative strengths of some acids in the next activity.
There are many weak acids. However, at this point you will be expected to memorize the names
and formulas of only the three listed below.
SOME WEAK ACIDS
HC2H3O2
Acetic Acid
HF
Hydrofluoric Acid
H3PO 4
Phosphoric Acid
Weak acids form an equilibrium system in water in which the reverse reaction is favored. Why?
Each of the two reactions involved in such an equilibrium system is an acid-base reaction. In the system
shown below, HF is a weak acid. In the forward reaction (---->) the acid is HF and the base is
{5}__________. That is, the HF is donating a proton, and the H 2O is accepting it to form the products on
the right. In the reverse reaction (<----) the acid is H3O1+ and the base is {6}_________. That is, the H3O1+
is donating a proton, and the F1- is accepting it to form the products on the left:
HF + H2 O <====> H3 O1+ + F1acid
base
acid
base
It turns out that H3O1+ is a stronger proton donor (acid) than HF and F1- is a stronger proton acceptor
(base) than H2O. Therefore, the reverse reaction (<----) is better than the forward (---->) one. When
equilibrium is established, there will be much more HF and H2O in the solution than H3O1+ and F1-.
Solutions of weak acids, therefore, have a relatively low concentration of H3O1+ ions.
Problem 4. Complete the following equations, showing how the weak acids listed below dissociate in
water to form ions. The anions contained in these acids are ClO31- and OCl 1-. Be sure to use a doubleheaded arrow in the equation. Include H2O in the equations.
a. HClO3(aq)
______________________________________________
b. HOCl(aq)
______________________________________________
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©1997, A.J. Girondi
ACTIVITY 21.4
Comparing the Conductivity of Strong and Weak Acids
There is a fairly simple method that can be used to determine the extent to which an acid or base
dissociates. This method involves measuring the conductivity of a solution. When ions are present in a
solution, it is possible for the solution to conduct electricity. Other things being equal, the greater the
concentration of ions in a solution, the greater the electrical current that will pass through the solution.
The number of ions which form when an acid or base is added to water depends on the degree of
dissociation. Since stronger acids and bases dissociate much more than weaker ones do, they are much
better conductors of electric current. The greater the electrical conductivity of an acid or base solution,
the stronger it is.
Your teacher will give you specific instructions about how to use the conductivity device. Use it to
measure the conductivity of the three acids listed in Table 21.1. Record your observations in the spaces
provided in the table. If the apparatus has a meter, record the meter reading in the table. If it has a light
bulb, record the strengths of the acids as high, medium, or low depending on the brightness of the bulb.
If the bulb does not light, this is probably because the ion concentration is too low to allow enough current
to flow to light the bulb. It does not necessarily mean that there are no ions in the solution. (Hint: one of
the acids should be rated "high;" one should be rated "medium;" and one should be rated "low." You
determine which is which.
Table 21.1
Conductivity of Acid Solutions
Acid Formula
Acid Name
Conductivity
0.01M HCl
hydrochloric
_________
0.01M HC2H3O2
acetic
_________
0.01M H8C6O7
citric
_________
Based on the results, what conclusions can you draw about the relative strengths of these three acids?
{7}____________________________________________________________________________
Even though all three acid solutions have the same concentration, they do not conduct the same amount
of electric current and are not equally strong. Why not?
{8}___________________________________
______________________________________________________________________________
______________________________________________________________________________
SECTION 21.5
The Dissociation Constant of a Weak Acid, K a
As you have already learned, weak acids form an equilibrium system when they are put into water.
There is both a forward and a reverse reaction. We can write equilibrium expressions for weak acid
systems. Let's consider the acetic acid equilibrium system:
HC2H3O2(aq) + H2O(l) <====> H3O1+(aq) + C2H3O21-(aq)
You should recall that an equilibrium expression consists of the product of the molar concentrations of the
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©1997, A.J. Girondi
products divided by the product of the molar concentrations of the reactants. Furthermore, you may recall
that solids and pure liquids are not included in equilibrium expressions. Water is a pure liquid in the
system above. The equilibrium constant for a weak acid is called the dissociation constant and is given
the symbol, K a. The Ka expression for the acetic acid system is given below. Compare the Ka expression
to the equation shown above.
[H 3O1+ ] [C 2H 3 O21− ]
Ka =
[HC 2H 3 O2 ]
Problem
system.
5. The two equilibrium equations below involve weak acids. Write the K a expression for each
a. HNO2(aq) + H2O(l) <===> H3O1+(aq) + NO21-(aq)
Ka =
b. HCN(aq) + H2O(l) <===> H3O1+(aq) + CN1-(aq)
Ka =
Look at the expressions for Ka which you wrote above. Note that as the concentration of hydronium ions,
H3O1+, increases, so does the value of Ka. Weak acids vary considerably in strength. Some are much
stronger than others, although none of them approach the strength of the six "official" strong acids.
Thus, the weak acids which are "strongest" have larger Ka values. Indeed, by comparing K a values, you
can determine which of any given set of acids is strongest. Three weak acids are listed in Table 21.2.
Which is strongest?{9}______________________ Weakest?{10}______________________
Table 21.2
K a Values of Selected Weak Acids
Acid
Dissociation Constant
hypochlorous acid, HClO
formic acid, HCOOH
phosphoric acid, H3PO 4
K a = 3.2 X 10-8
K a = 1.8 X 10-4
K a = 7.1 X 10-3
Reference sources such as textbooks and The Handbook of Chemistry and Physics contain tables of Ka
values of many weak acids.
SECTION 21.6
The Behavior of Strong and Weak Bases In Water
A. Strong Bases
Bases are also divided into groups that are strong and weak. The strong bases are the hydroxide
compounds of most of the Group 1A and 2A metals. Examples include LiOH, NaOH, KOH, Ca(OH) 2, etc.
All other bases are considered to be weak. Strong bases are completely dissociated in water.
Strong base:
NaOH(s) ---------> Na1+(aq) + OH1-(aq)
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©1997, A.J. Girondi
Strong bases form solutions which contain lots of ions. The base (NaOH in this case) simply breaks apart
in water. They are considered to be strong bases because 100% of the compound dissociates into
individual ions when it dissolves. The hydroxides of family 1A metals, like NaOH, are potent bases for two
reasons: (1) they are very soluble in water, and (2) they are 100 percent dissociated in solution. As a
result, strong bases of family 1A can produce lots of OH1- ions in solution. The high concentration of
OH1- ions in strong bases makes them dangerous.
The strong bases are the hydroxide
compounds of the solid metals in
families1A and 2A of the periodic table.
(Except for beryllium and magnesium)
Using a periodic table, you should be able
to write the formulas of the strong bases
from memory.
The hydroxide compounds of the family 2A metals, like Ca(OH)2, are considered to be strong
because, like family 1A hydroxides, they too are 100% dissociated in solution. However, the family 2A
hydroxides are not nearly as soluble as those of family 1A. As a result, while they are strong bases, they
are not as potent because they do not form solutions with high concentrations of OH1- (since they are not
very soluble). Only a small amount of Ca(OH) 2 will dissolve before its solution becomes saturated. In fact,
Ca(OH)2 is called "lime" and is mild enough to be used to neutralize acids in lawn and garden soils.
Mg(OH)2 is used in some stomach antacids like "milk of magnesia." You wouldn't want to use NaOH for
that purpose! NaOH is the active ingredient in many drain cleaners!
Ca(OH)2(s) ----> Ca2+(aq) + 2 OH1-(aq)
Strong base:
[Ca(OH)2 is 100% dissociated, but not much will dissolve.]
Water is not included in equations which show the dissociation of strong bases. Strong and weak acids
actually react with water so H2O can be included in the equation. Strong bases do not react with water;
they just come apart (dissociate).
Problem 6. Write equations showing the complete dissociation of the following strong bases in water.
a.
RbOH(s) ---->
_____________________________
b.
Ba(OH)2(s) ---->
_____________________________
B. Weak Bases
Weak bases are in some ways similar to weak acids. For example, they form ions when you
dissolve them in water. Furthermore, they form equilibrium systems in water, because they dissociate
only slightly (similar to weak acids). Some are polar covalent molecules which react with water to form ions.
The most common example of a weak base is ammonia, NH3.
Ammonia:
NH3(g)
weak
base
+
HOH(l)
<===>
NH41+(aq)
+
OH1-(aq)
weak
acid
Note: Remember that water can also be written as HOH. We will use this form of the formula in equations dealing with
weak bases, because it makes it easier to see what is happening.
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©1997, A.J. Girondi
Some weak bases are ions such as the fluoride ion, F1-:
F1-(aq) + HOH(l) <===> HF(aq) + OH1-(aq)
Weak base:
base
The electron dot notation
of
acid
acid
base
the fluoride ion, F1-,
1-
molecule is shown at right. What is the feature of this
molecule that allows it to function as a base?
{11}______________________________________
F
________________________________________
Notice that water acts as an acid in the last two equations. In previous sections, you saw water acting as a
base. Water can act as an acid because it contains hydrogen which can form ions (H1+), and it contains
oxygen which has unshared pairs of electrons which allow it to act as a Bronsted–Lowry base.
Substances such as water which can act as either acids or bases are said to be amphoteric.
Can act as an acid by
giving away hydrogen
in the form of H1+
HO
H
WATER
Can act as a base by
attracting H1+ to one of its
unshared electron pairs.
There is an important difference between weak bases and strong bases. Both produce hydroxide
ions, OH 1-, when you dissolve them in water; however, with strong bases the OH1- comes directly from
the strong base, like NaOH:
NaOH(s) ----> Na1+(aq) + OH1-(aq)
Note that the formula for a weak base like ammonia, NH3, does not contain hydroxide. The most common
and important weak bases are not hydroxide compounds. Yet, when you dissolve them in water,
hydroxide ions are formed. So, where do these hydroxide ions come from?
In a solution of a weak base, the
OH1- ions come from the water!
The formula of this weak base does
NOT contain hydroxide (OH1-).
NH3(g) + HOH(l)
<===>
NH41+(aq) + OH1-(aq)
Whenever you write an equation showing the reaction of a weak base with water, you should always
include the water in the equation (as is done in the equation above). If you write the equation without
including the water, it will not be balanced:
Wrong ----->
NH3(g) <===> NH41+(aq) + OH1-(aq)
Right ----->
NH3(g) + HOH(l) <===> NH41+(aq) + OH1-(aq)
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©1997, A.J. Girondi
Problem 7. Write balanced equations showing the behavior of the following weak bases in water. Keep
in mind that the particle of weak base will accept a H1+ ion from water.
a. BrO1-(aq)
BrO1-(aq) + HOH(l) <===>
b. S 2-(aq)
S 2-(aq) + HOH(l) <===> __________________________
________________________
Problem 8. Write balanced equations which illustrate the behavior of the substances listed below when
they are added to water. Refer back to the examples in previous sections of this chapter. Be sure to
include charges on ions.
a. HBr (a strong acid)
___________________________________________________
b. LiOH (a strong base)
___________________________________________________
c. HNO2 (a weak acid)
___________________________________________________
d. CN1- (a weak base)
___________________________________________________
(Check to be sure that you showed charges on any ions in the products.)
There are two reactions involved in an equilibrium system. In the acetic acid system, note that in
the forward reaction (--->) we can classify one substance as an acid and one as a base, and we can do the
same thing for the reverse reaction (<---):
HC2H3O2 + H2O(l) <====> C2H3O21-(aq) + H3O1+(aq)
acid
base
base
acid
We are making use of the Bronsted–Lowry definitions of acid and base here.
What molecule is the proton donor in the forward (--->) reaction?
{12}_______________
What ion is the
proton donor in the reverse reaction?{13}_____________________
Problem 9. In the three equations below, label each substance as an acid or a base:
a.
HF
+
H2O
_______
_______
b.
HCO31_______
c.
HCN
+
_______
+
<===>
HBr
<===>
_______
NH3
_______
<===>
H3O1+
_______
+
F1_______
H2CO3 + Br1_______
_______
CN1+
_______
NH41+
_______
At equilibrium, the system contains mostly reactants. This was also the case for the weak acids. In other
words, the weak bases do not form a high concentration of OH1- ions. Even though weak bases do not
contain OH1-, it is still the OH1- ion that makes the solution basic. Notice that the OH1- ions which are
produced in a solution of a weak base are formed indirectly. By that we mean that the OH1- ions do not
come from the base itself, but they come from the water! Examine the following equation.
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©1997, A.J. Girondi
In solutions of weak bases, the
OH1- ions come from the water!
NO21-(aq) + HOH(l) <===> HNO2(aq) + OH1-(aq)
base
acid
acid
base
As you can see, some negative ions (anions) like NO21- can acts as weak bases in water:
In solutions of strong bases like NaOH, the OH1- ion comes from the base itself. Water is not shown in the
equation which represents a strong base in water:
NaOH ----> Na1+ + OH1Explain the difference in the source of the OH1- ions in solutions of strong bases versus solutions of weak
bases:
{14}______________________________________________________________________
______________________________________________________________________________
Since weak bases involve equilibrium systems, an equilibrium expression can be written. For the
system below:
NO21-(aq) + HOH(l) <===> HNO2(aq) + OH1-(aq)
The equilibrium expression =
[HNO2 ] [OH1- ]
[NO 21− ]
Water is omitted since it is a pure liquid. This expression is equal to a constant which is given the symbol
K b. It is known as the dissociation constant of a weak base:
Kb =
[HNO2 ] [OH1- ]
[NO 21 − ]
Notice that Kb gets larger as the
concentration of OH1- increases.
You probably already realize that the larger the Kb value, the stronger the base. A very common and
important weak base is ammonia, NH3. Ammonia is very useful in the production of explosives and
fertilizers. When it is dissolved in water, the solution is usually called "ammonium hydroxide." When you
look at the reaction, you can guess why:
water
ammonium ion
ammonia
hydroxide ion
NH3(g) + HOH(l) <===> NH41+(aq) + OH1-(aq)
base
acid
acid
base
The electron dot structure of ammonia is shown in the space at
the right. What feature makes it a base (even though it's a
weak one)? {15}___________________________________
21-15
H
N
H
H
©1997, A.J. Girondi
The products of the reaction are an ammonium ion and a hydroxide ion. However, since this is a weak
base , a better name for the solution would be "ammonia water." Explain why:
{16}__________________
______________________________________________________________________________
______________________________________________________________________________
When 100 of these are added to water
In fact, if you put 100 ammonia molecules in
water, only one would be dissociated at
equilibrium. Thus, for every 100 ammonia
molecules put into a solution, only one OH1ion would be formed at equilibrium.
NH3(g) + HOH(l) <===> NH41+(aq) + OH1-(aq)
1
99
Number of particles present at equilibrium
Problem
equation.
10. The equations below involve weak bases in water. Write the Kb expression for each
a. CN1-(aq) + H2O(l) <===> HCN(aq) + OH1-(aq)
Kb =
b. NH3(aq) + H2O(l) <===> NH41+(aq) + OH1-(aq)
Kb =
Three weak bases are listed below along with their K b values. Which is strongest?{17}________________
Weakest?{18}___________________
Base
ammonia, NH3
fluoride ion, F 1sulfite ion, SO 32-
Dissociation Constant
K b = 1.8 X 10-5
K b = 1.4 X 10-11
K b = 1.8 X 10-7
It is not easy to identify a weak base just by looking at its formula. You would have to draw its
electron-dot structure to see if it contains an unshared pair of valence electrons, and you would have to
study how it reacts with water. Some particles like the chloride ion, Cl1-, do not act as bases even though
they do have unshared pairs of valence electrons. One simple way of explaining this is to say that if the
Cl1- ion were to accept a proton, it would form a molecule of HCl. However, HCl is a strong acid, and strong
acids are completely dissociated in water solutions. Therefore, HCl cannot form. Particles such as Cl1which would form a molecule of a strong acid by accepting a proton cannot, therefore, serve as bases.
They are neutral particles. The anions (negative ions) which are found in the six strong acids are neutral
anions. As mentioned earlier in this chapter, they include Cl1-, Br1-, I1-, SO42-, NO31-, and ClO41-.
Keep in mind that the reactions we are considering here all occur in water. Therefore,let's say that
the Cl1- ion is trying to accept a proton from water:
Cl1-(aq) + H2O(l) ----> HCl(aq) + OH1-(l)
<----- This reaction WILL NOT happen!
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©1997, A.J. Girondi
Why won't the reaction shown in the equation above happen? Well, let's just say that the reaction would
result in the formation of a strong acid (HCl in this example). Strong acid molecules like HCl do not exist in
water because they are completely dissociated. Therefore, the reaction does not occur.
ACTIVITY 21.7
Comparing the Conductivity of Strong and Weak Bases
This activity is like Activity 21.4 in which you determined the relative strengths of a few acids. The
method involves measuring the conductivity of a solution. When ions are present in a solution, it is
possible for the solution to conduct electricity. Other things being equal, the greater the concentration of
ions in a solution, the greater the electrical current that will pass through the solution.
The number of ions which form when an acid or base is added to water depends on the degree of
dissociation. Since stronger acids and bases dissociate much more than weaker ones do, they are much
better conductors of electric current. The greater the electrical conductivity of an acid or base solution,
the stronger it is.
Because of your experience in Activity 21.4, you should already know how to use the conductivity
device. Use it to measure the conductivity of the two bases listed in Table 21.3. Record your
observations in the spaces provided in the table. If the apparatus has a meter, record the meter reading
in the table. If it has a light bulb, record the strengths of the acids as high, medium, or low depending on
the brightness of the bulb. If the bulb does not light, this is probably because the ion concentration is too
low to allow enough current to flow to light the bulb. It does not necessarily mean that there are no ions in
the solution. (Hint: one of the acids should be rated "high;" one should be rated "low." You determine
which is which.
Table 21.3
Conductivity of Basic Solutions
Base Formula
Base Name
Conductivity
0.1M NaOH
sodium hydroxide
_________
0.1M NH3
ammonia
_________
Based on the results, what conclusions can you draw about the relative strengths of these two bases?
{19}____________________________________________________________________________
Even though both base solutions are 0.1M, they do not conduct the same amount of electric current and
are not equally strong. Why not?
{20}__________________________________________________
______________________________________________________________________________
SECTION 21.8
Polyprotic Acids
There are only six acids which are generally recognized as being strong (100% dissociated). They
include HCl, HNO3, H2SO 4, HBr, HI, and HClO4. (You should have memorized these by now!) The
number of hydrogens in the formula has nothing to do with the acid's strength. Thus, H2SO 4 is not twice
as strong as the other strong acids. Phosphoric acid, H3PO 4, is weak even though it contains three
hydrogens! Remember, it is the percentage of dissociation that determines an acid's strength. (In other
words, how well the acid molecule reacts with water.) That is, the strength is determined by how many H1+
ions actually form when you put the acid particle in water, not by how many H's are in the acid's formula. If
21-17
©1997, A.J. Girondi
an acid can provide only one H1+ ion, it is said to be monoprotic. The general term for acids which can yield
more than one H1+ ion is "polyprotic."
If an acid can provide two H1+ ions, it is "diprotic."
completely as shown in the equation below:
H2SO 4(aq) + H2O(l)
Sulfuric acid is strong because it ionizes
----> H3O1+(aq) + HSO41-(aq)
Notice that only one hydrogen formed, even though H2SO 4 contains two hydrogens. The second
hydrogen is now part of the HSO41- ion, and it is very hard for a second H1+ to break away since the
HSO 41- ion, with its negative charge, strongly "holds on" to it. As shown in the equation below, a few do
manage to break away, and as a result, the second hydrogen makes a small, very insignificant contribution
to the total number of H1+ ions in a solution of sulfuric acid. The HSO41- ion is a relatively weak acid:
HSO41-(aq) + H2O(l) <===> H3O1+(aq) + SO42-(aq)
This equilibrium reaction does
not produce much H3O1+.
Therefore, when describing the behavior of sulfuric acid in water, it is sufficient for our purposes to say
that it undergoes only one significant step:
H2SO 4(aq) + H2O(l)
----> H3O1+(aq) + HSO41-(aq)
Now, what we have just discussed is the behavior of sulfuric acid in pure water. However, sulfuric acid is
diprotic and it can give up two hydrogen ions when it reacts with a strong base instead of with water. You
will learn more about this when you study acid-base reactions in the next chapter.
Not all acids with more than one "H" are polyprotic. As mentioned earlier
in this chapter, acetic acid has the formula HC2H3O2. Only the first
hydrogen in the formula can dissociate - at all - to form H1+. The other
three hydrogens are part of the acetate ion (a polyatomic ion), C2H3O21-,
and they are bonded differently in the molecule. As a result, they cannot
form H1+ ions. Therefore, it is only a monoprotic acid. Sometimes, those
who work primarily with carbon compounds (organic chemists) write the
formulas for acids differently. For example, instead of writing acetic acid
as HC2H3O2, they may write CH3COOH. The hydrogen that forms H1+
now appears at the end of the formula instead of the beginning. This
alternate way of writing formulas gives more of a clue as to the structure of
the molecule. Note the structure of acetic acid shown at right.
H
H
C
O
C
OH
H
Acetic Acid
CH3COOH or HC2H3O2
In the acetic acid molecule, it is the hydrogen which is bonded to oxygen which can dissociate in
water as an H 1+ ion. The H–O bond is polar, and so is the water molecule. Thus there is an attraction
between them, and water can act as a base and "pull" a few of those particular H's off of the acetic acid
molecule. The C–H bonds are not polar, so those H's are not pulled away by water at all. Acetic acid has
only a 1-step dissociation. The following equation showing electron-dot structures may be helpful:
21-18
©1997, A.J. Girondi
polar water
molecule
nonpolar bond
H
H
O
C
+
C
O
H
1–
H
O
H
H
H
H
O
1+
H
+
H
O
C
H
C
O
H
polar bond
Acetic Acid
Water
Hydronium Ion
Acetate ion
One final word about the hydronium ion. As you know, the hydronium ion forms when water
(which has 2 unshared pairs of electrons) acts as a base and accepts a proton. This is happening in the
equation shown above. Since the hydronium ion, H3O1+, ion still contains one unshared pair of
electrons, students often ask why H3O1+ cannot accept a second proton to form an ion with the formula
H4O2+. The proposed equation for this reaction is shown below.
H
H
O
1+
H
+
H1+
H
H
O
2+
H
This reaction does
NOT occur.
H
Suggest a reason why the hydronium ion cannot act as a base by accepting a proton.
{21}
____________
______________________________________________________________________________
______________________________________________________________________________
21-19
©1997, A.J. Girondi
SECTION 21.9 LEARNING OUTCOMES
This is the end of Chapter 21. Check the learning outcomes below and arrange to take the exam
on Chapter 21. Then, go on to Chapter 22 which is a continuation of the subject of acids and bases.
_____1. List the general properties of acids and bases.
_____2. Compare and contrast the definitions of acids and bases according to the Arrhenius and the
Bronsted–Lowry models.
_____3. Write balanced equations showing the dissociation of strong and weak acids and bases in water.
_____4. Given a list of acids, classify them as strong or weak.
_____5. Given a list of bases, classify them as strong or weak.
_____6. Explain why some acids are strong, while others are weak.
_____7. Explain why some bases are strong, while others are weak.
21-20
©1997, A.J. Girondi
SECTION 21.10 Answers to Questions and Problems
Questions:
{1} hydroxide; {2} Bronsted-Lowry; {3} weak; {4} better; {5} H2O; {6} F1-; {7} strongest is HCl and weakest
is acetic; {8} They do not all dissociate into ions to the same extent. HCl dissociates most, while acetic
dissociates least; {9} phosphoric; {10} hypochlorous; {11} An unshared pair of electrons on the fluoride
ion; {12} HC2H3O2; {13} H3O1+; {14} In solutions of strong bases, the OH1- comes from the base itself,
whereas, in solutions of weak bases, the OH1- comes from the water that the weak base reacts with;
{15} An unshared pair of electrons on the nitrogen atom; {16} The reverse reaction is better than the
forward reaction, so there is more ammonia and water present at equilibrium; {17} NH3; {18} F1-; {19}
Sodium hydroxide is strong but ammonia is weak; {20} The 0.1M NaOH dissociates to a high degree to
form a lot of OH1- ions, but 0.1M ammonia does not; {21} The positively-charged H3O1+ ion will repel the
positively-charged H 1+ ion too much;
Problems:
1.
2.
3.
4.
HBr(aq) ----> H1+(aq) + Br1-(aq)
HI (aq) ----> H1+(aq) + I1-(aq)
HClO4(aq) ----> H1+(aq) + ClO41-(aq)
KOH (s) ----> K1+(aq) + OH1-(aq)
LiOH(s) ----> Li1+(aq) + OH1-(aq)
CsOH(s) ----> Cs1+(aq) + OH1-(aq)
HBr(aq) ----> H1+(aq) + Br1-(aq)
HBr(aq) + H2O(l) ----> H3O1+(aq) + Br1-(aq)
b. HClO4(aq) ----> H1+(aq) + ClO41-(aq)
HClO4(aq) + H2O(l) ----> H3O1+(aq) + ClO41-(aq)
a. HClO3(aq) + H2O(l) <===> H3O1+(aq) + ClO31-(aq)
b. HOCl(aq) + H2O(l) <===> H3O1+(aq) + OCl1-(aq)
a.
b.
c.
a.
b.
c.
a.
[H 3O1+ ] [NO 21− ]
[HNO2 ]
5. a.
Ka =
6.
RbOH(s) ----> Rb1+(aq) + OH1-(aq)
Ba(OH)2(s) ----> Ba2+(aq) + 2 OH1-(aq)
BrO1-(aq) + HOH(l) <===> HBrO(aq) + OH1-(aq)
S 2-(aq) + HOH(l) <===> HS1- + OH1-(aq)
HBr(aq) + H2O(l) ----> H3O1+(aq) + Br1-(aq)
LiOH(s) ----> Li1+(aq) + OH1-(aq)
HNO2(aq) + HOH(l) <===> NO21-(aq) + H3O1+(aq)
CN1-(aq) + HOH(l) <===> HCN(aq) + OH1-(aq
acid, base, acid, base
base, acid, acid, base
acid, base, base, acid
7.
8.
9.
a.
b.
a.
b.
a.
b.
c.
d.
a.
b.
c.
10. a. K b =
[HCN] [OH1- ]
[CN1- ]
b.
b.
Kb =
21-21
Ka =
[H 3O1+ ] [CN1− ]
[HCN]
[NH41+ ] [OH1- ]
[NH3 ]
©1997, A.J. Girondi
SECTION 21.11 Student Notes
21-22
©1997, A.J. Girondi