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)
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 Conductivity Red litmus Blue litmus Analysis
H2O(l) very slight no change no change neutral
HCl(aq) high no change red acid
Na2CO3(aq) high blue no change base
NaHCO3(aq) high blue no change base
NaHSO4(aq) high no change red acid
NaOH(aq) high blue no change base
Ca(OH)2(aq) low blue no change base
CO2(aq) low no change red acid
CaO(aq) low blue no change base
NH3(aq) low blue no change base
CH3COOH(aq) low no change red acid
Al(NO3)3(aq) high no change red acid
NaNO3(aq) high no change no change 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 Bases Neutral HCl(aq) Na2CO3(aq) H2O(l) (the control)
NaHSO4(aq) NaHCO3(aq) NaNO3(aq)
CO2(aq) NaOH(aq)
CH3COOH(aq) Ca(OH)2(aq)
Al(NO3)3(aq) CaO(aq)
NH3(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.
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
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?
(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) + H
2O(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.
Pro Con
Neutralization 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 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
Dilution 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
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
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 Conductivity test Litmus test Reaction with Mg(s) HCl(aq) high blue to red vigorous bubbling; solid disappeared quickly
CH3COOH(aq) low blue to red 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.
