Lesson 14.2 Acid and Base Strengths
Suggested Reading
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Zumdahl Chapter 14 Section 14.1, 14.2, 14.6, & 14.7
Essential Question

What is the relationship between the strength of an acid (base) the the
extent of dissociation?
Learning Objectives:.




Distinguish between strong and weak acids and bases in terms of the
extent of dissociation, reaction with water and electrical conductivity.
State whether a given acid or base is strong or weak.
Distinguish between strong and weak acids and bases, and determine
the relative strengths of acids and bases using experimental data.
Write the equilibrium expression for acid dissociation in water.
Introduction
The Bronsted-Lowry concept considers an acid-base reaction as a protontransfer reaction. It is useful to consider acid-base reactions as competitions
between species for protons. From this point of view, you can order acids by
their relative strengths as proton donors. The strong acids are those that lose
their protons more easily than other acids. Similarly, the stronger bases are
the ones that hold on to protons more strongly than other bases. Lets look at
objective one from above.
Distinguish between strong and weak acids and bases
in terms of the extent of dissociation, reaction with
water and electrical conductivity:
Strong Acid
Completely dissociates
Weak Acid
Strong Base
Weak Base
Partly dissociates
Completely dissociates
Partly dissociates
Reactive with water
Slightly reactive with water Reactive with water
(equilibrium position is far (equilibrium position is far (equilibrium position is
right)
left)
far right)
High conductivity
Weak conductivity
High conductivity
Slightly reactive with
water (equilibrium
position is far left)
Weak conductivity
Explaining the Term Strong Acid
When an acid dissolves in water, a proton (hydrogen ion) is transferred to a
water molecule to produce a hydronium ion and a negative ion (anion). In
general,
where HA is a strong acid, H3O+ is the hydronium ion, and A- is an anion.
These reactions are all reversible, but in some cases, the acid is so good at
giving away hydrogen ions that we can think of the reaction as being one-way.
The acid is virtually 100% ionized (completely dissociated).
For example, when hydrogen chloride (HCl in gaseous form is called
hydrogen chloride. Watch out for this!) dissolves in water to make hydrochloric
acid, so little of the reverse reaction happens that we can write:
At any one time, virtually 100% of the hydrogen chloride will have reacted to
produce hydronium ions and chloride ions. Hydrogen chloride is described as
a strong acid. A strong acid is one which is virtually 100% ionized in
solution. Other common strong acids include sulphuric acid and nitric acid.
Refer to your list of the 7 strong acids that you had to memorize for chapter
4.
You may find the equation for the ionisation written in a simplified form:
This shows the hydrogen chloride dissolved in the water splitting to give
hydrogen ions and chloride ions in solution. This version is often used just to
make things look easier. If you use it, remember that the water is actually
involved, and that when you write H+(aq) what you really mean is a
hydronium ion, H3O+.
Explaining the Term Weak Acid
A weak acid is one which doesn't ionize fully when it is dissolved in water.
Ethanoic acid is a typical weak acid. It reacts with water to produce
hydroxonium ions and ethanoate ions, but the reverse reaction is more
successful than the forward one. The ions react very easily to reform the acid
and the water.
At any one time, only about 1% of the ethanoic acid molecules have
converted into ions. The rest remain as ethanoic acid molecules.
Most organic acids are weak. Hydrogen fluoride (dissolving in water to
produce hydrofluoric acid) is a weak inorganic acid that you should be familiar
with.
Explaining the Term Strong Base
A strong base is something like sodium hydroxide or potassium hydroxide
which, like a strong acid, fully ionizes. You can think of the compound as
being 100% split up into metal ions and hydroxide ions in solution.
Each mole of sodium hydroxide dissolves to give a mole of hydroxide ions in
solution.
Some strong bases like calcium hydroxide aren't very soluble in water. That
doesn't matter - what does dissolve is still 100% ionized into calcium ions and
hydroxide ions. Calcium hydroxide still counts as a strong base because of
that 100% ionization.
Explaining the Term Weak Base
Ammonia is a typical weak base. Ammonia itself obviously doesn't contain
hydroxide ions, but it reacts with water to produce ammonium ions and
hydroxide ions.
However, the reaction is reversible, and at any one time about 99% of the
ammonia is still present as ammonia molecules. Only about 1% has actually
produced hydroxide ions.
A weak base is one which doesn't convert fully into hydroxide ions in solution.
Properties of Acidic and Basic Solution
Run the animation on the pHet web site to explore properties of acidic and
basic solutions (you will need to click on the test apparatus and drag it into the
solution to see the result. To find the simulation go to the following web site
and click on Acid-Base solutions.
http://phet.colorado.edu/en/simulations/category/chemistry
Relative Strengths of Acids and Bases
By comparing various acid-base reactions you can construct a table of relative
strengths of acids and bases. In this context the terms stronger and weaker
are used in a comparative sense.
An acid-base reaction normally goes in the direction of the weaker acid. You
can use this fact to compare the relative strengths of any two acids, write acidbase equations, and to predict the direction of an acid-base reaction. Consider
the following reaction.
In the reaction above, hydrogen chloride is the stronger acid, so the
reaction tends to go from left to right (equilibrium position is on the right).
HCl sits above the hydronium ion on the table to the right as a result.
Now consider the ionization (dissociation) of acetic acid, a
weak acid.
Practice: What acid-base definitions can you apply to this reaction?
Experiment shows that in a 0.1 M solution of acetic acid, only about 1% of
the acetic acid is ionized. This implies that acetic acid is a weaker acid than
hydronium ion, so the reaction tends to go from right to left (equilibrium
position is on the left). Furthermore, we see that hydronium ion sits above
acetic acid in the table.
A definite relationship exists between acid and base strengths. The
strongest acids have the weakest conjugate bases, and the strongest
bases have the weakest conjugate acids. Notice how the chart shows that
the strengths are inversely related.
Molecular Structure and Acid Strength
The strength of an acid depends on how easily the proton, H+, is lost from an
H-A species. By understanding the factors that determine the ease of proton
loss, you will be able to predict the relative strengths of similar acids.
Two factors are important in determining relative acid strengths. One is the
polarity of the bond with the H atom. In a polar bond, the H atom has a
partial positive charge, as shown in the bond between H and F. The more
polarized the bond is, the more easily the proton is removed and the
greater the acid strength. The second factor determining acid strength is
the size of the atom, A. The larger the atom A, the weaker is the bond and
the greater the acid strength.
Consider a series of binary acids, HA, formed from a given column of
elements of the periodic table. As you go down the column of elements,
each time adding a shell of electrons to the atom, the radius increases. The
size of atom A is the dominant factor in determining acid strength. Thus,
acid strength increases in going down a column of elements in the periodic
table.
As you go across a row of elements, the atomic radium decreases and the
effective nuclear charge increases. As the effective nuclear charge
increases the polarity of the H-A bond becomes the dominant factor in
determining acid strength. Thus, acid strength increases across a row.
Now consider the oxoacids. An oxoacid has the structure H-O-X. The acidic
acid is always attached to the O, which is in turn attached to an atom X.
Bond polarity appears to be the dominant factor in determining relative
strengths of the oxoacids. This, in turn, depends on the electronegativity of
X. If the electronegativity of X is large, the H-O bond is relatively polar and
the acid strength is large. Thus, for a series of oxoacids of the same
structure, differing only in the atom X, the acid strength increases with the
electronegativity of X.
Consider, for example, the acids HClO, HBrO, and HIO. The
electronegativity of Group VIIA elements decreases going down the column
of elements, so the order of acid strength is
HIO < HBrO < HClO
For a series of oxoacids, such as the oxoacids for chlorine or the oxacids of
bromine, the acid strength increases with the number of oxygen.
In the series above perchloric acid is the strongest while hypochlorous acid is
the weakest.
The last thing you need to know about acid strength and molecular structure is
this. For polyprotic acids such as phosphoric acid, the acid strength decreases
with each ionization.
The Equilibrium Expression for Acid Dissociation in
Water
The simplest acid-base equilibria are those in which a single acid or base
solute reacts with water. An acid reacts with water to produce hydronium ion
and the conjugate base ion. The process is called acid ionization or acid
dissociation. Consider the acid dissociation of acetic acid
The reaction involves the transfer of a proton from acetic acid to water. Often
this reaction is just written as HC2H3O2(aq) ⇌ H+(aq) + C2H3O2-(aq). In this
case the transfer of the proton is not as explicit, however, the production of H +
is expressed. Because acetic acid is a weak acid, it dissociates to a small
extent (about 1% or less). For a strong acid (or base) such as HCl which
dissociates completely, the concentrations of ions are determined by the
stoichiometry of the reaction from the initial concentration. However, for a
weak acid (or base), the concentration of the ions in solution are determined
from the acid-ionization constant (or acid dissociation constant), which is
the equilibrium constant for the ionization of a weak acid (or base) (Ka). Thus,
Ka tells us something about the extent of dissociation. For the general reaction
or HA ⇌ H+ + A-
The equilibrium constant expression is
I can use either [H3O+] or [H+] in the expression above, they are equivalent.
For weak bases, the concentration of the ions are
determined from the base-ionization constant (Kb). For
base dissociation with the general form B(aq) + H2O(l) ⇌ HB+(aq) + OH-(aq),
the equilibirum constant is equal to
The electrons, illustrating the Lewis base concept, in the expression above
are not necessary.
HOMEWORK: Practice exercises 16.4-16.5