Experiment 8
Characterization of Acids and Bases
Over the next two weeks we will explore two classes of chemical reactions that form the
basis of much of your laboratory work this semester: acid/base reactions and
oxidation/reduction reactions. We will explore a third class of chemical reactions—
complex formation and complex dissociation reactions—in the semester’s final two labs.
This week you and a partner will work to identify compounds as acids or bases, to
examine the relationship between an acid’s or a base’s structure and its chemical
reactivity, and consider the difference between an acid’s or a base’s capacity to react
and the strength of its reaction. At least one of you should bring your textbook or
computer to lab so that you can look up information.
Pre-lab Assignment
Before coming to lab, skim through the material in Sections 15.1 – 15.3 to familiarize
yourself with the general properties of aqueous solutions and acid–base reactions.
Preliminary Tests
Obtain approximately 5 mL each of the stock solutions identified as A, B, C, D, E, and F,
placing each in a separate clean test tube. Complete the following preliminary tests on
these solutions, recording your observations in your laboratory notebook.
Preliminary Test 1: Transfer approximately 10 drops of solution A into each of two test
tubes. Add one drop of bromothymol blue to one test tube and one drop of
phenolphthalein to the other test tube. Record your observations in your lab notebook.
Repeat for each of the remaining stock solutions.
Preliminary Test 2: Divide the remaining portion of solution A into two test tubes. Add
a small piece of Mg to the first test tube and add five drops of 1 M Mg(NO3)2 to the
second test tube. Record your observations in your lab notebook. Repeat for each of
the remaining stock solutions.
Based on your results for these preliminary tests, divide the stock solutions into two
groups. In the space below, identify the solutions you placed in each group and explain
the reason(s) for your assignments.
Identifying Acids and Bases
After completing the preliminary tests, share your conclusions with your instructor who
will provide you with the identities of the stock solutions, each of which is a 1 M solution
of an acid or a 1 M solution of a base. In the space below, and considering the
reactivities of compounds A–F with bromothymol blue, Mg, and Mg(NO3)2, propose one
or more simple rules for identifying whether a compound is an acid or a base based on
its chemical formula and/or structure.
Examine the formulas and structures for the compounds in the table below, and predict
whether each is an acid or a base. If you have insufficient information on which to base
a prediction then indicate this. It is okay if you are unsure of your predictions.
formula
structure
O
prediction
formula
structure
NaC2H3O
HC2H3O2
H 3C
OH
2
H 3C
O
H3PO4
HO
HO
O Na
O
P
OH
KH2PO4
HO
P
OH
O
K2HPO4
prediction
O
O Na
OH
O
P
O Na
K3PO4
Na O
P
O Na
O Na
O Na
H
NH3
H
N
H
NH4Cl
Cl
H
N
H
H
H
O
NaHCO3
O
Na2CO3
C
HO
O Na
C
Na
O
O Na
Evaluate your predictions using the available 1 M solutions of each compound and the
tests described above. Based on the results of your tests, list below any changes to
your rules for predicting whether a compound is an acid or a base.
Acid–Base Strength
For a generic acid with the formula HA, the reaction responsible for its acidity is
HA(aq) + H2O(l) D H3O+(aq) + A–(aq)
and for a generic base with the formula B, the reaction responsible for its basicity is
B(aq) + H2O(l) D OH–(aq) + HB+(aq)
The strength of an acid or base is the extent to which these reactions proceed to the
right. For acids of equal concentration, the one producing more hydronium ions, H3O+,
is the stronger acid; for bases of equal concentration, the one producing more hydroxide
ions, OH–, is the stronger base.
Because the concentrations of H3O+ and OH–are inversely proportional, as shown here
[H3O+] Í [OH–] = 1Í10–14
we can describe acidity and basicity in terms of the concentration of H3O+. The
concentration of H3O+ in a neutral solution is 1Í10–7 M. For an acidic solution, [H3O+]
is greater than 1Í10–7 M and for a basic solution, [H3O+] is less than 1Í10–7 M. The
concentration of H3O+ can vary over a large range; for this reason we report it as a pH
value where pH is defined as
pH = –log[H3O+]
The pH of a neutral solution is 7.00. For an acidic solution the pH is
less than 7.00, and for a basic solution the pH is greater than 7.00.
The figure to the right shows pH values for some solutions familiar
to you. Note that “pure” rain is slightly acidic due to the presence of
dissolved CO2, which makes it a very dilute solution of carbonic acid,
H2CO3. For the same reason, the deionized water in lab is slightly
acidic.
There are a variety of ways to measure a solution’s pH, including
sensors with signals that are proportional to pH (a glass pH
electrode is one example of such a sensor), organic dyes—such as
bromothymol blue and phenolphthalein—whose color depends on
pH (we call these compounds pH indicators), and pH paper whose color changes in
response to a solution’s pH (essentially just filter paper soaked in a solution of a pH
indicator and dried).
Using the method demonstrated in lab, determine the pH for each of the 1 M solutions
of acids and bases available to you (16 in total). In the space below, rank those
solutions that are acids from most acidic to least acidic, and rank those solutions that
are bases from most basic to least basic. If you cannot determine a difference in pH for
two or more solutions, simply group them together; thus, you might have a list that looks
like this: A (strongest acid) > B, C > D (weakest acid).
Acid–Base Reactivity: Strength vs. Concentration
When an acid or base reacts, we can characterize its reaction in two ways: how quickly
the reaction occurs and the extent to which the reaction occurs. We will explore these
two aspects of acid-base reactivity through two simple experiments.
Experiment 1: Place 5 mL of 1 M HCl in a test tube. Add a small piece of Mg to the
test tube and record how long it takes for the Mg to dissolve. If the reaction is not
complete after 10 min, estimate the amount of Mg that remains unreacted. Record
your observations in your lab notebook. Repeat using 1 M HNO3, 1 M HC2H3O2, 1 M
NH4Cl, 0.1 M HCl, and 0.1 M HC2H3O2. Try to use pieces of Mg of approximately
equal size. In the interest of time, you may wish to run all six reactions simultaneously.
Experiment 2: Place 30 drops of 1 M HCl in a test tube and add one drop of
phenolphthalein. Add 1 M NaOH to the test tube one drop at a time, mixing after each
addition, until the solution turns pink. Record in your lab notebook the number of drops
of NaOH used. Repeat using 1 M HNO3, 1 M HC2H3O2, 1 M NH4Cl, 0.1 M HCl, and
0.1 M HC2H3O2.
Based on your data, how does an acid’s strength and/or concentration affect how
quickly it reacts with Mg? How does an acid’s strength and/or concentration affect the
extent to which it reacts with 1 M NaOH?