Extension
5. Consumer contexts for measuring pH include fish tanks and hot tubs. ,ndicator solutions and
pH test strips are the most appropriate technology because they are inexpensive and give
results that are sufficiently accurate for these contexts.
Commercial contexts for measuring pH include cleaning products, soaps, shampoos,
deodorants, and other personal hygiene products. Indicator solutions and pH test strips are the
most appropriate technology because they are inexpensive and give adequate result for these
contexts.
Industrial contexts for measuring pH include the manufacture of fertilizer (the Haber process)
and soda ash for products such as soaps (the Solvay process). pH meters are the most
appropriate technology in this context because, although they are more expensive, they
provide the manufacturer with a more accurate and precise measurement that can be carried
out automatically.
6. Several hundred randomly selected volunteers are studied over a period of several months.
The hair and scalp of each volunteer is assessed by a hairdresser and also by a personal
questionnaire. On the basis of these assessments, three groups with a similar range of hair
properties are constructed. In this double-blind study, one group receives a pH neutral
shampoo, one a slightly acidic shampoo, and the other a slightly basic shampoo. After the
study period, all volunteers are assessed again by a hairdresser and by personal questionnaire.
6.4 EXPLAINING ACIDS AND BASES
Investigation 6.2: Testing Arrhenius’ Acid–Base Definitions
(Pages 248, 260)
Purpose
The purpose of this investigation is to test Arrhenius’ definitions of an acid and a base.
Problem
Which of the substances tested may be classified as an acid, a base, or neutral, using Arrhenius’
definitions?
Prediction
According to Arrhenius’ definitions, of the substances to be tested, the acids are HCl(aq) and
CH3COOH(aq), the bases are NaOH(aq) and Ca(OH)2(aq), and all other substances are neutral.
The reasoning behind this prediction is grounded in the theory that acids are hydrogen
compounds that ionize to produce H+(aq) ions and that bases are ionic hydroxides that dissociate
to produce OH–(aq) ions. The other substances tested are classified as neutral compounds that do
not yield hydrogen or hydroxide ions when they dissociate in water.
Acids:
HCl(g) o H+(aq) + Cl–(aq)
CH3COOH(l) o H+(aq) o CH3COO–(aq)
Bases:
NaOH(s) o Na+(aq) + OH–(aq)
Ca(OH)2(s) o Ca2+(aq) + 2 OH–(aq)
Neutral:
NH3(g) o NH3(aq)
Na2CO3(s) o 2 Na+(aq) + CO32–(aq)
NaHCO3(s) o Na+(aq) + HCO3–(aq)
NaHSO4(s) o Na+(aq) + HSO4–(aq)
CaO(s) o Ca2+(aq) + O2–(aq) or CaO(s)
CO2(g/aq) o CO2(aq)
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Al(NO3)3(s) o Al3+(aq) + 3 NO3–(aq)
NaNO3(s) o Na+(aq) + NO3–(aq)
Design
Equally concentrated samples of the solutions are tested for electrical conductivity and with
litmus to determine if they exhibit properties of acids and bases. The controlled variables are
concentration and temperature. The manipulated variable is the substance tested, and the
responding variables are conductivity and litmus colour. The control used in this experiment is
pure water.
Procedure
1. Test and record the conductivity of pure water.
2. Test pure water with both red and blue litmus paper.
3. Repeat steps 1 and 2 for all the solutions provided.
Evidence/Analysis
Diagnostic Test Results and Analysis
Substance
H2O(l)
HCl(aq)
Na2CO3(aq)
NaHCO3(aq)
NaHSO4(aq)
NaOH(aq)
Ca(OH)2(aq)
CO2(aq)
CaO(aq)
NH3(aq)
CH3COOH(aq)
Al(NO3)3(aq)
NaNO3(aq)
Conductivity
very slight
high
high
high
high
high
low
low
low
low
low
high
high
Red litmus
no change
no change
blue
blue
no change
blue
blue
no change
blue
blue
no change
no change
no change
Blue litmus
no change
red
no change
no change
red
no change
no change
red
no change
no change
red
red
no change
Analysis
neutral
acid
base
base
acid
base
base
acid
base
base
acid
acid
neutral
On the basis of the evidence gathered in this investigation, most of the substances tested are acids
and bases, as listed below. The only two neutral substances are water (the control) and sodium
nitrate.
Acids
HCl(aq)
NaHSO4(aq)
CO2(aq)
CH3COOH(aq)
Al(NO3)3(aq)
Bases
Na2CO3(aq)
NaHCO3(aq)
NaOH(aq)
Ca(OH)2(aq)
CaO(aq)
NH3(aq)
Neutral
H2O(l) (the control)
NaNO3(aq)
Evaluation
The design of this experiment appears adequate since the problem was answered with no obvious
flaws. Conductivity was not a useful diagnostic test for the classification of the substances and
therefore could be eliminated from the design and procedure. The materials were adequate, but a
pH meter would be more efficient and precise than litmus (although this would be a more
expensive alternative). The procedure also appears adequate because there were no difficulties in
performing the tasks and the results were certain. Some of the reactions of litmus were slower
than others (CaO(aq)) and may have been overlooked. A minimum observation time for each test
(at least two minutes) should be added to the design as a controlled variable. Technological skills
were minimal. On the basis of my evaluation of the experiment, I am reasonably certain of the
results obtained. The sources of uncertainty include the concentrations of the solutions and some
uncertainty regarding the litmus tests that were very slow in developing.
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183
Of the 12 solutions tested, 4 agreed with the predictions based on previous knowledge of
acid–base nomenclature. The remaining 8 solutions did not agree with the prediction. Therefore,
the prediction is judged to be falsified. Arrhenius’ definitions are deemed unacceptable since the
prediction was mostly falsified by the investigation.
The purpose was accomplished because we had sufficient examples to test the Arrhenius
theory clearly. Additional investigations could be done to obtain more examples, which may help
in the development of a modification or replacement of Arrhenius’ definitions that can explain the
results.
Practice
(Page 251)
1. (a) HI(aq) + H2O(aq) o H3O+(aq) + I–(aq)
(b) HOCl(aq) + H2O(aq) o H3O+(aq) + OCl–(aq)
(c) H3PO4(aq) + H2O(aq) o H3O+(aq) + H2PO4–(aq)
2. (a) Na2SO4(aq) o 2 Na+(aq) + SO42–(aq)
SO42–(aq) + H2O(l) o HSO4–(aq) + OH–(aq)
(b) NaCH3COO(aq) o Na+(aq) + CH3COO–(aq)
CH3COO–(aq) + H2O(l) o CH3COOH(aq) + OH–(aq)
(c) Sr(OH)2(aq) o Sr2+(aq) + 2 OH–(aq)
Case Study: Acid Deposition
(Page 252)
1. Possible technologies include improving the efficiency of the fuels we already burn, using
alternative energy sources, restoring the damaged environment rather than preventing the
problem, and taking action as individuals to reduce the problem.
2. Almost all of the electricity that powers modern life comes from burning fossil fuels, such as
coal, natural gas, and oil. Acid deposition is caused by two pollutants that are released into
the atmosphere, or emitted, when these fuels are burned: sulfur dioxide (SO2) and nitrogen
oxides (NOx). The following are examples of technologies that exist to help combat the
problem:
1) Improving the efficiency of the fuels we already burn: Coal accounts for most sulfur
dioxide (SO2) emissions and a large portion of NOx emissions. Sulfur is present in coal as
an impurity, and it reacts with air when the coal is burned to form SO2. In contrast, NOx
is formed when any fossil fuel is burned. There are several options for reducing SO2
emissions, including using coal containing less sulfur, washing the coal, and using
devices called scrubbers to chemically remove the SO2 from the gases leaving the
smokestack. Power plants can also switch fuels; for example, burning natural gas creates
much less SO2 than burning coal. Finally, power plants can use technologies that do not
burn fossil fuels. Similar to scrubbers on power plants, catalytic converters reduce NOx
emissions from cars. Also, changes to gasoline are constantly being implemented that
allow it to burn more cleanly.
2) Using alternative energy sources such as nuclear power, hydropower, wind energy,
geothermal energy, and solar energy. There are also alternative energies available to
power automobiles, including natural gas powered vehicles, battery-powered cars, fuel
cells, and combinations of alternative and gasoline-powered vehicles. These alternatives,
although good in theory, are still in the infancy stage of development and are, on average,
too expensive to produce and for the average consumer to buy.
3) Restoring the damaged environment rather than preventing the problem: Limestone or
lime (a naturally occurring basic compound) can be added to acidic lakes to neutralize the
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acidity. Liming tends to be expensive, has to be done repeatedly to keep the water from
returning to its acidic condition, and is considered a short-term solution.
4) Taking action as individuals to reduce the problem: Simple examples include:
Ɣ turning off lights, computers, and other appliances when not in use;
Ɣ using energy-efficient appliances, such as lighting, air conditioners, heaters,
refrigerators, and washing machines;
Ɣ insulating your home as best you can;
Ɣ carpooling, using public transportation, or better yet, walking or bicycling whenever
possible; and
Ɣ buying vehicles with low NOx emissions, and maintaining vehicles well
3. Of the four technologies listed, the individual action (#4) technology is the least expensive
and the easiest to implement.
4. [The presentation of the information gathered will depend on the teacher’s instructions.]
Section 6.4 Questions
(Page 253)
1. (i) Original Arrhenius theory: hydrogen ions
Modified Arrhenius definition: hydronium ions
(ii) Original Arrhenius theory: Acids ionize in water to produce hydrogen ions. Arrhenius did
not know how this ionization occurred.
Modified Arrhenius definition: Acids react with water to produce hydronium ions.
2. In the original Arrhenius theory, bases were restricted to those ionic compounds that already
contained hydroxide ions. Hydroxide ions were produced by a simple dissociation of the
ionic compound. In the modified Arrhenius theory, ionic hydroxides still dissociate into
hydroxide ions but the many other bases are now explained as a reaction with water to
produce hydroxide ions. The end result, hydroxide ions in solution, is the same for both
theories.
3. (a) HCl(g) + H2O(l) o H3O+(aq) + Cl–(aq)
CH3COOH(l) + H2O(l) o H3O +(aq) + CH3COO–(aq)
NaOH(s) o Na+(aq) + OH–(aq)
Ca(OH)2(s) o Ca2+(aq) + 2 OH–(aq)
NH3(g) + H2O(l) o NH4+(aq) + OH–(aq)
Na2CO3(s) o 2 Na+(aq) + CO32–(aq)
CO32–(aq) + H2O(l) o HCO3–(aq) + OH–(aq)
NaHCO3(s) o Na+(aq) + HCO3–(aq)
HCO3–(aq) + H2O(l) o H2CO3(aq) + OH–(aq)
NaHSO4(s) o Na+(aq) + HSO4–(aq)
HSO4–(aq) + H2O(l) o SO42–(aq) + H3O+(aq)
CaO(s) o Ca2+(aq) + O2–(aq)
O2–(aq) + H2O(l) o 2 OH–(aq)
CO2 + 2 H2O(l) o HCO3–(aq) + H3O+(aq)
(b) The modified Arrhenius theory could not explain the acidity of Al(NO3)3(aq). Why
NaNO3(aq) is neutral while so many other anions react with water to produce acidic or
basic solutions is also not answered by the modified Arrhenius theory. Knowing that
NaNO3(aq) is neutral means that neither sodium nor nitrate ions react with water.
Therefore, the acidity of Al(NO3)3(aq) must be due to the aluminium ion. Why does this
ion react with water and not other metal ions? How do aluminium ions react with water to
produce hydronium ions?
[Note: All of these questions can be answered with more modern acid-base theories.]
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(c) In most cases, the modified Arrhenius theory is able to explain why solutions are acidic
or basic, even though their acidic or basic nature cannot generally be predicted from their
chemical formulas. An acceptable theory must not only explain known evidence, but also
predict the results of new evidence. The modified Arrhenius theory is unable to predict
and therefore cannot be given a “passing grade,” but it is an improvement over the
original Arrhenius theory.
4. (a) HBr(g) + H2O(l) o H3O+(aq) + Br–(aq)
(b) Na3PO4(s) o 3 Na+(aq) + PO43–(aq)
PO43–(aq) + H2O(l) o HPO42–(aq) + OH–(aq)
(c) NaHSO3(s) o Na+(aq) + HSO3–(aq)
HSO3–(aq) + H2O(l) o SO32–(aq) + H3O+(aq)
(d) Na2HPO4(s) o 2 Na+(aq) + HPO42–(aq)
HPO42–(aq) + H2O(l) o H2PO4–(aq) + OH–(aq)
(e) Na2O(s) o 2 Na+(aq) + O2–(aq)
O2–(aq) + H2O(l) o 2 OH–(aq)
(f) SO3(g) + 2 H2O(l) o HSO4–(aq) + H3O+(aq)
(g) KOH(s) o K+(aq) + OH–(aq)
5. H3O+(aq)
+
OH–(aq)
o
2 H2O(l)
from nitric acid
from potassium hydroxide
6. Because the modified Arrhenius theory defines acids as substances that react with water to
produce hydronium ions and bases as substances that react with water to produce hydroxide
ions, every neutralization reaction can be written as H3O+(aq) + OH–(aq) o 2 H2O(l).
7. Once a theory has been found to explain evidence collected, it must then be tested to
determine if it can predict results before evidence is collected.
8.
Neutralization
Dilution
Pro
Ɣ the resulting solution would be
chemically neutral, with no
risk of increasing or
decreasing the pH of the
surrounding environment
Ɣ soil, which is slightly acidic,
would act as a buffer
Ɣ no new chemical is being
introduced to the environment,
only water, so no new
products are produced that
could potentially harm the
environment
Ɣ water is easy and safe to
transport
Con
Ɣ the process involves using one chemical to
clean up another chemical
Ɣ the salt produced in the neutralization
reaction might be harmful to the
environment in large doses
Ɣ this method involves transporting an acid,
risking another dangerous spill
Ɣ neutralization is a potentially dangerous
exothermic reaction because of the heat
released
Ɣ even diluted solutions can be harmful to the
environment
Ɣ the solute can stay in the soil for many years
and become concentrated again when the
water used to dilute it evaporates
Ɣ it has been repeatedly shown that the
“solution to pollution is not dilution”
Extension
9. The process of brain tanning is the most popular skin-tanning method used by North
American Aboriginal peoples. The process involves soaking the animal skin in a solution of
brain (containing glycolic acid), liver, and/or tannins (acidic compounds from plants). Before
being soaked in the solution, the skin is soaked in a solution made from wood ash (a basic
substance of pH 12–13). This process is repeated many times until the skin is tanned. The
modern technique of tanning leather involves soaking the hide in a brine (sodium chloride)
solution to kill all protein-destroying organisms. It is then exposed to tanning agents, most
often trivalent chromium salts. These salts are used to speed up processing time, and produce
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a very attractive product. The modern method produces a consistent product, but the
environmental impacts are much greater because the chromium salts are toxic. There are also
claims that the brain-tanned leather is stronger and lasts longer.
6.5 THE STRENGTH OF ACIDS AND BASES
Investigation 6.3: Comparing the Properties of Acids (Demonstration)
(Pages 254, 261)
Purpose
The purpose of this demonstration is to create the concept of strengths of acids.
Problem
How do the properties of two common acids compare?
Design
The properties of aqueous solutions of two acids, hydrochloric acid and acetic acid, are observed
and compared. Diagnostic tests include the litmus test, conductivity test, and reaction with an
active metal. Two important controlled variables are the temperature and concentration of each
acid.
Evidence
Acid
HCl(aq)
CH3COOH(aq)
Conductivity test
high
low
Litmus test
blue to red
blue to red
Reaction with Mg(s)
vigorous bubbling; solid disappeared quickly
slow bubbling; solid disappeared after a much longer
time
Analysis
According to the evidence, both solutions tested as being acidic. The 1.0 mol/L HCl(aq) appears
to conduct electricity better and react much faster with Mg(s) than the 1.0 mol/L CH3COOH(aq).
Evaluation
The design was adequate in comparing the properties of the two acids and there were no obvious
flaws. Improvements to the design could include using a pH test instead of a litmus test. This
change would add more detailed information about the acidity of the solutions. The controlled
variables were adequate. The materials and procedure were also adequate for the given design but
would need to be altered if the experiment were repeated, with improvements made to the design.
No special technological skills were required. On the basis of my evaluation of the experiment, I
am very certain of the evidence collected. The sources of uncertainty include the concentrations
and purity of the solutions.
Overall, the purpose was accomplished. Additional investigations using the suggestion
for an improved design and other examples of acids would be useful.
Practice
(Page 255)
1. If a conductivity test is performed on the acid and the acid is a good conductor, then the acid
is a strong acid.
If a conductivity test is performed on the acid and the acid is a poor conductor, then the acid
is a weak acid.
If an active metal is combined with an acid and the resulting reaction rate is fast, then the acid
is a strong acid.
If an active metal is combined with an acid and the resulting reaction rate is slow, then the
acid is a weak acid.
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