6.4 EXPLAINING ACIDS AND BASES

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1 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 CH 3 COOH(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) H + (aq) + Cl (aq) CH 3 COOH(l) H + (aq) CH 3 COO (aq) Bases: NaOH(s) Na + (aq) + OH (aq) Ca(OH) 2 (s) Ca 2+ (aq) + 2 OH (aq) Neutral: NH 3 (g) NH 3 (aq) Na 2 CO 3 (s) 2 Na + (aq) + CO 2 3 (aq) NaHCO 3 (s) Na + (aq) + HCO 3 (aq) NaHSO 4 (s) Na + (aq) + HSO 4 (aq) CaO(s) Ca 2+ (aq) + O 2 (aq) or CaO(s) CO 2 (g/aq) CO 2 (aq) 182 Unit 3 Solutions Manual Copyright 2007 Thomson Nelson

2 Al(NO 3 ) 3 (s) Al 3+ (aq) + 3 NO 3 (aq) NaNO 3 (s) Na + (aq) + NO 3 (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 H 2 O(l) very slight no change no change neutral HCl(aq) high no change red acid Na 2 CO 3 (aq) high blue no change base NaHCO 3 (aq) high blue no change base NaHSO 4 (aq) high no change red acid NaOH(aq) high blue no change base Ca(OH) 2 (aq) low blue no change base CO 2 (aq) low no change red acid CaO(aq) low blue no change base NH 3 (aq) low blue no change base CH 3 COOH(aq) low no change red acid Al(NO 3 ) 3 (aq) high no change red acid NaNO 3 (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) Na 2 CO 3 (aq) H 2 O(l) (the control) NaHSO 4 (aq) NaHCO 3 (aq) NaNO 3 (aq) CO 2 (aq) NaOH(aq) CH 3 COOH(aq) Al(NO 3 ) 3 (aq) Ca(OH) 2 (aq) CaO(aq) NH 3 (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. Copyright 2007 Thomson Nelson Unit 3 Solutions Manual 183

3 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) + H 2 O(aq) H 3 O + (aq) + I (aq) (b) HOCl(aq) + H 2 O(aq) H 3 O + (aq) + OCl (aq) (c) H 3 PO 4 (aq) + H 2 O(aq) H 3 O + (aq) + H 2 PO 4 (aq) 2. (a) Na 2 SO 4 (aq) 2 Na + (aq) + SO 2 4 (aq) SO 2 4 (aq) + H 2 O(l) HSO 4 (aq) + OH (aq) (b) NaCH 3 COO(aq) Na + (aq) + CH 3 COO (aq) CH 3 COO (aq) + H 2 O(l) CH 3 COOH(aq) + OH (aq) (c) Sr(OH) 2 (aq) Sr 2+ (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 (SO 2 ) and nitrogen oxides (NO x ). 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 (SO 2 ) emissions and a large portion of NO x emissions. Sulfur is present in coal as an impurity, and it reacts with air when the coal is burned to form SO 2. In contrast, NO x is formed when any fossil fuel is burned. There are several options for reducing SO 2 emissions, including using coal containing less sulfur, washing the coal, and using devices called scrubbers to chemically remove the SO 2 from the gases leaving the smokestack. Power plants can also switch fuels; for example, burning natural gas creates much less SO 2 than burning coal. Finally, power plants can use technologies that do not burn fossil fuels. Similar to scrubbers on power plants, catalytic converters reduce NO x 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 184 Unit 3 Solutions Manual Copyright 2007 Thomson Nelson

4 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: turning off lights, computers, and other appliances when not in use; using energy-efficient appliances, such as lighting, air conditioners, heaters, refrigerators, and washing machines; insulating your home as best you can; carpooling, using public transportation, or better yet, walking or bicycling whenever possible; and buying vehicles with low NO x 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) + H 2 O(l) H 3 O + (aq) + Cl (aq) CH 3 COOH(l) + H 2 O(l) H 3 O + (aq) + CH 3 COO (aq) NaOH(s) Na + (aq) + OH (aq) Ca(OH) 2 (s) Ca 2+ (aq) + 2 OH (aq) NH 3 (g) + H 2 O(l) NH 4 + (aq) + OH (aq) Na 2 CO 3 (s) 2 Na + (aq) + CO 3 2 (aq) CO 3 2 (aq) + H 2 O(l) HCO 3 (aq) + OH (aq) NaHCO 3 (s) Na + (aq) + HCO 3 (aq) HCO 3 (aq) + H 2 O(l) H 2 CO 3 (aq) + OH (aq) NaHSO 4 (s) Na + (aq) + HSO 4 (aq) HSO 4 (aq) + H 2 O(l) SO 4 2 (aq) + H 3 O + (aq) CaO(s) Ca 2+ (aq) + O 2 (aq) O 2 (aq) + H 2 O(l) 2 OH (aq) CO H 2 O(l) HCO 3 (aq) + H 3 O + (aq) (b) The modified Arrhenius theory could not explain the acidity of Al(NO 3 ) 3 (aq). Why NaNO 3 (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 NaNO 3 (aq) is neutral means that neither sodium nor nitrate ions react with water. Therefore, the acidity of Al(NO 3 ) 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.] Copyright 2007 Thomson Nelson Unit 3 Solutions Manual 185

5 (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) + H 2 O(l) H 3 O + (aq) + Br (aq) (b) Na 3 PO 4 (s) 3 Na + (aq) + PO 3 4 (aq) PO 3 4 (aq) + H 2 O(l) HPO 2 4 (aq) + OH (aq) (c) NaHSO 3 (s) Na + (aq) + HSO 3 (aq) HSO 3 (aq) + H 2 O(l) SO 2 3 (aq) + H 3 O + (aq) (d) Na 2 HPO 4 (s) 2 Na + (aq) + HPO 2 4 (aq) HPO 2 4 (aq) + H 2 O(l) H 2 PO 4 (aq) + OH (aq) (e) Na 2 O(s) 2 Na + (aq) + O 2 (aq) O 2 (aq) + H 2 O(l) 2 OH (aq) (f) SO 3 (g) + 2 H 2 O(l) HSO 4 (aq) + H 3 O + (aq) (g) KOH(s) K + (aq) + OH (aq) 5. H 3 O + (aq) + OH (aq) 2 H 2 O(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 H 3 O + (aq) + OH (aq) 2 H 2 O(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 Dilution the resulting solution would be chemically neutral, with no risk of increasing or decreasing the ph of the surrounding environment soil, which is slightly acidic, would act as a buffer no new chemical is being introduced to the environment, only water, so no new products are produced that could potentially harm the environment water is easy and safe to transport the process involves using one chemical to clean up another chemical the salt produced in the neutralization reaction might be harmful to the environment in large doses this method involves transporting an acid, risking another dangerous spill neutralization is a potentially dangerous exothermic reaction because of the heat released even diluted solutions can be harmful to the environment the solute can stay in the soil for many years and become concentrated again when the water used to dilute it evaporates 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 186 Unit 3 Solutions Manual Copyright 2007 Thomson Nelson

6 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 CH 3 COOH(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 CH 3 COOH(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. Copyright 2007 Thomson Nelson Unit 3 Solutions Manual 187