16.2 THE RØNSTED LOWRY ACID ASE CONCEPT Practice (Page 724) 1. (a) According to Arrhenius original theory, acids are substances that ionize in an aqueous solution, to produce aqueous hydrogen ions. (b) According to the modified Arrhenius theory, acids are substances that react with water to form aqueous hydronium ions. (c) According to the rønsted Lowry concept, acids are reactant entities that donate protons in proton transfer reactions. 2. The definition of a base using the modified Arrhenius theory is specifically that the base will react with water, while the rønsted Lowry concept is more general and describes what will happen when one entity in a reaction donates a proton to another entity in an acid base reaction. According to the modified Arrhenius theory, a base is any ion or chemical that reacts with water to form hydroxide ions in solution. The rønsted Lowry concept states that a base is a proton acceptor, without being specific as to what is donating the proton. oth theories predict the production of hydroxide ion, when a base is present in water. 3. (a) HF(aq) + SO 2-3 (aq) F (aq) + HSO 3 (aq) acid base (b) CO 2 3 (aq) + CH 3 COOH(aq) CH 3 COO (aq) + HCO 3 (aq) base acid (c) H 3 PO 4 (aq) + OCl (aq) H 2 PO 4 (aq) + HOCl(aq) acid base (d) HCO 3 (aq) + HSO 4 (aq) SO 2 4 (aq) + H 2 CO 3 (aq) base acid 4. (a) HSO 3 (aq) + OH (aq) SO 2 3 (aq) + H 2 O(l) acid base (b) HSO 3 (aq) + H 3 O + (aq) H 2 SO 3 (aq) + H 2 O(l) base acid 5. The rønsted Lowry concept removes two important restrictions that the modified Arrhenius theory could not explain. The first is that the rønsted Lowry concept allows chemists to describe reactions between acids and bases, that are not in aqueous solution. Secondly, the rønsted Lowry concept allows for the possibility of entities that are amphiprotic, which means that they can accept or donate protons depending on what other entities are present. 6. The rønsted Lowry concept is labelled a theoretical definition, rather than a theory, because it does not explain why a proton is donated or accepted, and it cannot predict theoretically which reaction will occur for a given entity in a new situation. Practice (Page726) 7. acid base pairs (a) HCO 3 (aq)/co 3 2 (aq) and HS (aq)/s 2 (aq) (b) H 2 CO 3 (aq)/hco 3 (aq) and H 2 O(l)/OH (aq) (c) HSO 4 (aq)/so 4 2 (aq) and H 2 PO 4 (aq)/hpo 4 2 (aq) (d) H 2 O(l)/OH (aq) and H 3 O + (aq)/h 2 O(l) 8. HCO 3 (aq)/co 3 2 (aq) and H 2 CO 3 (aq)/hco 3 (aq) 630 Unit 8 Solutions Manual Copyright 2007 Thomson Nelson
Investigation 16.1: Creating an Acid ase Strength Table (Pages 727, 768) Purpose The purpose of this investigation is to test an experimental design for using indicators to create a table of relative strengths of acids and bases. Problem Can the indicators that are available be used to rank the acids and bases provided, in order of strength? Prediction According to the acid base table in Appendix I, the relative strengths of the provided acids and bases are expressed in the table below. Solution Acid Conjugate base HCl(aq) SA H 3 O + (aq) Cl (aq) NaHSO 4 (aq) HSO 4 (aq) SO 2-4 (aq) CH 3 COOH(aq) CH 3 COOH(aq) CH 3 COO (aq) NaHSO 3 (aq) HSO 3 (aq) SO 2 3 (aq) Na 2 CO 3 (aq) HCO 3 (aq) CO 2 3 (aq) NaOH(aq) H 2 O(l) OH (aq) The reasoning is a referenced way of knowing. Design The solutions are tested with indicators to determine the ph of the solutions from the indicator colour. controlled variables concentration, temperature, time, mixing manipulated variable a variety of acids and bases responding variable indicator colour Procedure 1. Add a small quantity of each acid/base to a test tube (or spot-plate well). 2. Add a few drops of the same indicator to each of the test tubes and mix well, if necessary. 3. Note and record the colour of each solution. 4. Repeat steps 1 to 3 using each of the other indicators. Rinse the test tubes or spot plate. 5. Dispose of the solutions into the sink with plenty of water. Evidence Indicator Colours in Provided 0.10 mol/l Acids/ases Solution Methyl orange Methyl violet romothymol Phenolphthalein blue HCl(aq) red green yellow colourless NaHSO 4 (aq) red blue-green yellow colourless CH 3 COOH(aq) red blue yellow colourless NaHSO 3 (aq) orange blue yellow colourless Na 2 CO 3 (aq) yellow blue blue red NaOH(aq) yellow blue blue red Analysis ph Values of Provided 0.10 mol/l Acids/ases Solution Methyl Methyl romothymol Phenolphthalein Possible Order orange violet blue ph HCl(aq) < 3.2 0.0 0.16 < 6.0 < 8.2 0.0 0.16 1 NaHSO 4 (aq) < 3.2 0.0 0.16 < 6.0 < 8.2 0.0 0.16 1 CH 3 COOH(aq) < 3.2 > 1.6 < 6.0 < 8.2 1.6 3.2 3 NaHSO 3 (aq) 3.2 4.4 > 1.6 < 6.0 < 8.2 3.2 4.4 4 Na 2 CO 3 (aq) > 4.4 > 1.6 > 7.6 > 10.0 > 10.0 5 NaOH(aq) > 4.4 > 1.6 > 7.6 > 10.0 > 10.0 5 Copyright 2007 Thomson Nelson Unit 8 Solutions Manual 631
According to the evidence, the relative strengths of the provided acids and bases expressed in an acid base table are displayed below. Chemical Acid Conjugate base HCl(aq) SA H 3 O + (aq) Cl (aq) NaHSO 4 (aq) HSO 4 (aq) SO 2-4 (aq) CH 3 COOH(aq) CH 3 COOH(aq) CH 3 COO (aq) NaHSO 3 (aq) HSO 3 (aq) SO 2 3 (aq) Na 2 CO 3 (aq) HCO 3 (aq) CO 2 3 (aq) NaOH(aq) H 2 O(l) OH (aq) Evaluation (Part 1) The Design is judged to be inadequate because the selection of indicators provided did not allow all the solutions to be sequentially ranked: there was a tie for strongest acid and another for strongest base. A greater selection of indicators with very low and very high ph colour change ranges might have solved this problem. Lab Exercise 16: Predicting Acid ase Equilibria (Page 727) Purpose The purpose of this investigation is to develop a generalization for predicting the position of acid base equilibria. Problem How do the positions of the reactant acid and base in the acid base table relate to the position of equilibrium? Analysis Reactions 1, 3, and 5 all have <50% reaction. According to the acid base table, the reactant acid is listed below the reactant base for each of these equilibria. Reactions 2, 4, and 6 all have >50% reaction (#6 is quantitative). According to the acid base table, the reactant acid is listed above the reactant base for each of these equilibria. As a generalization, we might conclude that in acid base reactions where the acid is listed higher on the table than the base, it will favour products, and reactions where the base is listed higher on the table than the acid will favour reactants. Web Activity: Web Quest Pool Chemistry (Page 728) How is hypochlorous acid produced when a chlorine-containing compound is added to water? Ca(OCl) 2 (s) + H 2 O(l) 2 HOCl(aq) + CaO(aq) What is the chemical equation for the equilibrium that exists between aqueous hypochlorous acid and its conjugate base, hypochlorite ion? HOCl(aq) + H 2 O(l) H 3 O + (aq) + OCl - (aq) Use Le Châtelier s principle to explain which chlorine-containing entity, HClO or ClO, predominates in pool water below and above ph 7. When pool water becomes acidic, the increased concentration of hydrogen ions causes the above equilibrium to shift to the left, resulting in an increase in the hypochlorous acid concentration. As pool water becomes slightly basic, the removal of hydrogen ions from the equilibrium results in a shift to the right, favouring the formation of the hypochlorite ion. As 632 Unit 8 Solutions Manual Copyright 2007 Thomson Nelson
pool water approaches ph 8, the HClO/ClO equilibrium consists of almost 80% hypochlorite. What effect does increasing ph have on the sanitizing ability of chlorine-containing compounds in pool water? Why? Hypochlorous acid is about 1000 times more powerful than hypochlorite as a sanitizer. Consequently, as the ph of the water increases, the sanitizing ability of the water decreases because of the net conversion of hypochlorous acid to its conjugate base. How do free chlorine and combined chlorine differ? Free chlorine refers to all the dissolved chlorine gas, hypochlorous acid, and hypochlorite in water. Combined chlorine refers to chlorine that is combined with organic compounds or with nitrogen compounds like ammonia, to form chloramines. Why can the formation of combined chlorine in pool water be a nuisance? Combined chlorine is a nuisance because it is less potent as a sanitizing agent than is free chlorine. Furthermore, increased levels of combined chlorine can result in many of the symptoms that swimmers associate with too much chlorine in the water. These include redness and burning of the eyes, irritation of the nose and throat, dry skin and hair, and breathing difficulties. What are the sources of ammonia and other nitrogen-containing compounds in pool water? Skin secretions and urine are the most common sources of the ammonia and other nitrogencontaining compounds in pools. What are chloramines? Which chloramine is responsible for the chlorine smell of indoor pools? Write a series of chemical equations to show the formation of this compound. Chloramines are nitrogen/chlorine compounds produced by the reaction of ammonia and related compounds with hypochlorous acid. Nitrogen trichloride (otherwise known as trichloramine, NCl 3 (aq) is responsible for the chlorine odour of pools. The accumulation of trichloramine is a particular problem in indoor pools that have inadequate ventilation systems. Trichloramine can be produced in pool water by this series of chemical reactions: NH 3 (g) + HOCl(aq) NH 2 Cl(aq) + H 2 O(l) NH 2 Cl(aq) + HOCl(aq) NHCl 2 (aq) + H 2 O(l) NHCl 2 (aq) + HOCl(aq) NCl 3 (aq) + H 2 O(l) How does superchlorination keep chloramines under control? Superchlorination, also called shock treatment, involves the addition of a large dose of oxidizing chemicals to the pool to destroy organic contaminants and oxidize ammonia and other nitrogen-containing compounds, so that they cannot react with hypochlorous acid to produce chloramines. Chlorine-based compounds are most commonly used for shock treatments. However, many other oxidizing agents are also available. What alternatives are there to disinfecting pool water with chlorine? Recommend the best alternative. Alternatives include bromine, persulfates, and ozone. Ozone, O 3, is currently the most widely used alternative to chlorine. Ozone is a much more powerful oxidizing agent than chlorine and, therefore is a better sanitizer. On the downside however, ozone has a very short shelf life, and it must be generated on site as a result. This requires specialized equipment. Large pools that use ozone disinfect their water in smaller treatment pools first, before releasing the water into the main pool. This increases the initial construction cost of the pool. However, since ozone is far less corrosive to pool plumbing and ventilation systems, the initial construction costs may be recovered over the life of the pool. Some experts maintain, however, that ozone alone does not do an adequate job of removing micro-organisms and organic matter from the water, and they recommend using low doses of chlorine in addition to ozone. Copyright 2007 Thomson Nelson Unit 8 Solutions Manual 633
Career Connection: Chemical Researcher (Page728) One part of Dr. Jalilehvand s research has determined that sulfur is absorbed slowly by longsubmerged wooden ships. The absorbed sulfur then slowly converts to sulfuric acid after the ancient hull is raised and placed in a museum. It is hoped that this will lead to the development of methods to remove this element from wood, to prevent gradual deterioration of these historical objects. Dr. Jalilehvand has studied and worked at universities in Tehran, Tokyo, Stockholm, and Calgary. Practice (Page 731) 9. SA A HF(aq), Na + (aq), SO 2 4 (aq), H 2 O(l) HF(aq) + SO 2 50% 4 (aq) F (aq) + HSO 4 (aq) 10. SA A H 3 O + (aq), ClO 4 (aq), Na + (aq), OH (aq), H 2 O(l) H 3 O + (aq) + OH (aq) 2 H 2 O(l) (Note: question specifies quantitatively, therefore use ) 11. SA A A HCOOH(aq), Na + (aq), HS (aq), H 2 O(l) HCOOH(aq) + HS 50% (aq) HCOO (aq) + H 2 S(aq) 12. SA A NH + 4 (aq), Cl (aq), Na + (aq), NO 2 (aq), H 2 O(l) NH + 4 (aq) + NO 50% 2 (aq) NH 3 (aq) + HNO 2 (aq) 13. SA A H 3 O + (aq), NO 3 (aq), Na + (aq), CH 3 COO (aq), H 2 O(l) H 3 O + (aq) + CH 3 COO (aq) H 2 O(l) + CH 3 COOH(aq) 14. SA A Na + (aq), HSO 4 (aq), Na + (aq), OH (aq), H 2 O(l) HSO - 4 (aq) + OH 50% (aq) SO 2 4 (aq) + H 2 O(l) 15. A SA A NH + 4 (aq), NO 3 (aq), H 3 O + (aq), Cl (aq), H 2 O(l) H 3 O + (aq) + H 2 O(l) H 2 O(l) + H 3 O + (aq) (no reaction, so no observable change: products and reactants are identical) Therefore, NH 4 NO 3 (aq) cannot be used to neutralize HCl(aq). 16. SA A Na + (aq), ClO (aq), CH 3 COOH(aq), H 2 O(l) ClO 50% (aq) + CH 3 COOH(aq) HClO(aq) + CH 3 COO (aq) 634 Unit 8 Solutions Manual Copyright 2007 Thomson Nelson
17. Mixing bleach solution with an acidic toilet bowl cleaner causes a quantitative reaction between the hydronium ion, present in the acid solution, and the hydroxide ion, present in the + - bleach. H3O (aq) + OH (aq) 2 H2O(l). This causes a drop in the concentration of hydroxide ion in the bleach solution. Lower [OH (aq)] shifts the equilibrium (given in the Student ook) to the left, according to Le Châtelier s principle, which produces more highly toxic chlorine gas. Lab Exercise 16C: Aqueous icarbonate Ion Acid ase Reactions (Page 732) Purpose The purpose of this investigation is to test the five-step method for predicting reactions in acid base systems. Problem What are the products and position of the equilibrium for sodium hydrogen carbonate with stomach acid, vinegar, household ammonia, and lye, respectively? Prediction According to the five-step method for predicting acid base reactions, the following reactions are expected. 1. Reaction with hydrochloric acid, HCl(aq) A SA A Na + (aq), HCO 3 (aq), H 3 O + (aq), Cl (aq), H 2 O(l) H 3 O + (aq) + HCO 50% 3 (aq) H 2 O(l) + H 2 CO 3 (aq) 2. Reaction with vinegar, CH 3 COOH(aq) A SA A Na + (aq), HCO 3 (aq), CH 3 COOH(aq), H 2 O(l) CH 3 COOH(aq) + HCO 50% 3 (aq) CH 3 COO (aq) + H 2 CO 3 (aq) 3. Reaction with ammonia, NH 3 (aq) SA A Na + (aq), HCO 3 (aq), NH 3 (aq), H 2 O(l) HCO 50% 3 (aq) + NH 3 (aq) CO 2 3 (aq) + NH + 4 (aq) 4. Reaction with lye, NaOH(aq) SA A Na + (aq), HCO 3 (aq), OH (aq), H 2 O(l) HCO 3 (aq) + OH 50% (aq) CO 2 3 (aq) + H 2 O(l) Analysis 1. The evidence indicates that baking soda reacts appreciably with hydrochloric acid, producing gas bubbles (assumed to be carbon dioxide from the decomposition of carbonic acid, HCO 3 (aq) CO 2 (g) + H 2 O(l)) and increasing the ph of the solution. No evidence was obtained for the possible formation of water. 2. The evidence indicates that baking soda reacts appreciably with vinegar, producing gas bubbles (assumed to be carbon dioxide from the decomposition of carbonic acid), increasing Copyright 2007 Thomson Nelson Unit 8 Solutions Manual 635
the ph of the solution and causing the disappearance of the odour of vinegar. No evidence was obtained for the production of acetate ions. 3. The decrease in ph is an indirect indication of a possible reaction of baking soda with ammonia. The characteristic ammonia smell remaining after the reaction indicates that the reaction was not complete, agreeing with the less than 50% position of equilibrium predicted. No evidence for any predicted products was obtained. 4. The decrease in ph is evidence that the baking soda reacts with lye. No evidence is given for the position of the equilibrium or the presence of the predicted products. Evaluation The Prediction for the reaction of baking soda and hydrochloric acid, and baking soda and acetic acid appears to be verified since the evidence in each case does suggest a significant reaction with a clear indication of one major product. The Prediction for the other two reactions is inconclusive because the evidence is too limited. On the basis of this judgment, the five-step method remains acceptable. No evidence was obtained that definitely falsified the Prediction. The Purpose was accomplished, but further tests are needed in order to test the validity of this five-step method. Investigation 16.2: Testing rønsted Lowry Reaction Predictions (Pages 732, 768 769) [Evaluation, Part 1 should have been checked in the Report Checklist.] Purpose The purpose of this investigation is to test the rønsted Lowry concept and the five-step method for reaction prediction from a table of relative acid base strength. Problem What reactions occur when various pairs of substances are mixed? Prediction According to the rønsted Lowry concept of acids and bases and the five-step procedure, the following reactions and diagnostic test results are predicted: 1. SA A NH + 4 (aq), Cl (aq), Na + (aq), OH (aq), H 2 O(l) NH + 4 (aq) + OH 50% (aq) NH 3 (aq) + H 2 O(l) (Ammonia odour is predicted.) 2. SA A H 3 O + (aq), Cl (aq), Na + (aq), CH 3 COO (aq), H 2 O(l) H 3 O + (aq) + CH 3 COO 50% (aq) H 2 O(l) + CH 3 COOH(aq) (Vinegar odour is predicted.) 3. SA A Na + (aq), C 6 H 5 COO (aq), HSO 4 (aq), H 2 O(l) HSO 4 (aq) + C 6 H 5 COO 50% (aq) SO 2 4 (aq) + C 6 H 5 COOH(s) (A precipitate is predicted.) 4. SA A A H 3 O + (aq), Cl (aq), NH + 4 (aq), H 2 O(l) H 3 O + (aq) + H 2 O(l) H 2 O(l) + H 3 O + (aq) (No observable change is predicted.) 636 Unit 8 Solutions Manual Copyright 2007 Thomson Nelson
5. SA Na + (aq), Cl (aq), H 2 O(l) H 2 O(l) + H 2 O(l) H 3 O + (aq) + OH (aq) 6. SA Al 3+ (aq), SO 2 4 (aq), H 2 O(l) 50% (No reaction no litmus change is predicted.) H 2 O(l) + SO 2 4 (aq) OH (aq) + HSO 4 (aq) 7. SA Na + (aq), PO 3 4 (aq), H 2 O(l) H 2 O(l) + PO 3 50% 4 (aq) OH (aq) + HPO 2 4 (aq) 8. SA A Na + (aq), HSO 4 (aq), H 2 O(l) HSO 50% 4 (aq) + H 2 O(l) SO 2 4 (aq) + H 3 O + (aq) 9. A SA A Na + (aq), HCO 3 (aq), H 3 O + (aq), Cl (aq), H 2 O(l) (Red litmus should turn blue.) (Red litmus should turn blue.) (lue litmus should turn red.) H 3 O + (aq) + HCO 50% 3 (aq) H 2 O(l) + H 2 CO 3 (aq) (Gas bubbles are predicted: H 2 CO 3 (aq) H 2 O(l) + CO 2 (g)) (ph of HCl(aq) is expected to increase) 10. SA A Na + (aq), HCO 3 (aq), OH (aq), H 2 O(l) HCO 3 (aq) + OH 50% (aq) CO 2 3 (aq) + H 2 O(l) (ph of NaOH(aq) should decrease.) 11. A SA A Na + (aq), HCO 3 (aq), HSO 4 (aq), H 2 O(l) HSO 4 (aq) + HCO 50% 3 (aq) SO 2 4 (aq) + H 2 CO 3 (aq) (H 2 O(l) + CO 2 (g)) (ph of the solution should increase.) Materials lab apron eye protection two 18 150 mm test tubes 50 ml beaker wash bottle of distilled water samples of the following solutions, in equal concentration (e.g., 0.50 mol/l): NH 4 Cl(aq), NaOH(aq), HCl(aq), NaCH 3 COO(aq), NaHSO 4 (aq), NaC 6 H 5 COO(aq) samples of the following solids: NaCl(s), Al 2 (SO 4 ) 3 (s), Na 3 PO 4 (s), NaHSO 4 (s), NaHCO 3 (s) laboratory scoop vial of red litmus paper vial of blue litmus paper ph paper roll ph meter Copyright 2007 Thomson Nelson Unit 8 Solutions Manual 637
Procedure Problems 1 4 1. Rinse test tubes with distilled water. 2. Add each solution to a separate test tube to a depth of one-third of the test tube. 3. Pour one solution into the other and note evidence of a reaction. 4. Dispose of the final mixture in the sink. Caution: Do not stopper a test tube if a gas is being produced. Problems 5 11 5. Rinse beaker with distilled water. 6. Add about 10 ml of the liquid to the beaker. 7. Test the liquid with litmus or measure the ph. 8. Add a small quantity of solid and swirl to dissolve and observe any changes. 9. Test the final mixture with litmus or measure the ph. 10. Repeat steps 8 and 9 if no change is observed. 11. Dispose of the litmus or ph paper in a waste container and dispose of the liquid in the sink. Evidence/Analysis Summary of observations and analysis Reactants Observations Analysis: Did the predicted reaction occur? 1. NH 4 Cl(aq) + NaOH(aq) ammonia odour yes 2. HCl(aq) + NaCH 3 COO(aq) vinegar odour yes 3. NaHSO 4 (aq) + NaC 6 H 5 COO(aq) white precipitate yes 4. HCl(aq) + NH 4 Cl(aq) no change yes 5. NaCl(s) + H 2 O(l) no change in litmus yes 6. Al 2 (SO 4 ) 3 (s) + H 2 O(l) blue litmus turned red no 7. Na 3 PO 4 (s) + H 2 O(l) red litmus turned blue yes 8. NaHSO 4 (s) + H 2 O(l) blue litmus turned red yes 9. NaHCO 3 (s) + HCl(aq) ph increased, gas bubbles yes 10. NaHCO 3 (s) + NaOH(aq) ph decreased yes 11. NaHCO 3 (s) + NaHSO 4 (aq) ph increased, gas bubbles yes According to the evidence gathered in this investigation, most of the reactions occurred as predicted. Evaluation (Parts 1, 2 & 3) The experimental Design is barely adequate, since the evidence obtained for the products was limited and no direct evidence of the extent of reaction was obtained. The Materials provided were satisfactory for performing the procedure. The evidence is not sufficient to confirm the prediction and answer the problem. The Procedure adequately covered a very limited experimental design. More detailed diagnostic tests should be conducted to verify predicted products and to test the position of the acid base equilibrium. The technological skills were adequate for the procedure since the required skills were minimal. I am only moderately certain of the interpretations made from the evidence gathered. The major source of uncertainty is the lack of specific diagnostic tests. Although the evidence obtained was very limited, the experimental results of 10 out of 11 reactions generally agreed with and did not contradict the Prediction. The gas bubbles formed in reactions 9 and 11 indicated the presence of the predicted (decomposition of) carbonic acid product. Overall, the Prediction is judged to be inconclusive, since the evidence for reaction 6 clearly disagrees with the Prediction. However, the five-step procedure based on rønsted Lowry definitions and the empirical generalization on the position of acid based equilibria remains acceptable until it is clearly shown to be falsified. Overall, I am not very confident about my judgment. 638 Unit 8 Solutions Manual Copyright 2007 Thomson Nelson
The Purpose was not completely accomplished and the experiment should be repeated with a better design. It is possible that the rønsted Lowry definition may need to be revised or restricted, in order to deal with the result of reaction 6. Lab Exercise 16D: Creating an Acid ase Table (Page 733) Purpose The purpose of this investigation is to test an experimental design for using equilibrium position to create a table of relative strengths of acids and bases. Problem What is the order of acid strength for the first four members of the carboxylic acid family? Analysis According to the evidence provided, translated into acid base table position, the order of strength is shown in the following table of these carboxylic acids and their conjugate bases. Acid Conjugate base SA HCOOH(aq) HCOO (aq) CH 3 COOH(aq) CH 3 COO (aq) C 3 H 7 COOH(aq) C 3 H 7 COO (aq) C 2 H 5 COOH(aq) C 2 H 5 COO (aq) Case Study: Changing Ideas on Acids and ases The Evolution of a Scientific Theory (Pages 733 735) 1. Lavoisier s theory of acids assumed that acids were substances that combine with or contain oxygen (meaning that they appear in the category of acid maker ). Davy s theory of acids and bases proposed that hydrogen gives compounds acidic properties. Arrhenius theory proposed that acids are substances that ionize in a solution to form hydrogen ions. It also proposed that bases are substances that dissociate in solutions to form hydroxide ions. The rønsted Lowry concept proposed that acids are proton donors and bases are proton acceptors. The solvent has an important role in the concept, as the solvent may act as an acid or a base. Lewis theory proposes that acids accept electron pairs, while bases donate electron pairs. No hydrogen (or hydronium) ion and no solvent are required. 2. Lavoisier s oxygen theory was rejected, and then replaced, when evidence was obtained for acids not containing oxygen (such as muriatic acid, or HCl), and for chemicals that did contain oxygen not acting as acids (such as lime, CaO). Davy s theory was rejected, and then replaced, when it could not explain how some hydrogen compounds (such as methane, CH 4 ) did not have acidic properties but were, in fact, neutral. Davy s theory presented additional difficulties in that it could not predict how some compounds containing hydrogen actually had basic properties (such as ammonia, NH 3 ). Arrhenius theory was ultimately replaced for a number of reasons. Evidence showed that compounds that acted as acids by dissociating into hydrogen ions (and forming hydronium ions) in water did not do this in all solvents. Since hydroxide ion was central to identifying bases, chemicals such as ammonia (NH 3 ) were modified to include hydroxide (such as NH 4 OH), which was a misconception. Arrhenius theory proposed Copyright 2007 Thomson Nelson Unit 8 Solutions Manual 639
that salts produced in neutralization reactions should be neutral when, in fact, they are not always neutral. Lastly, bonding theory predicted that it was very unlikely that a lone proton could exist in an aqueous solution without being bonded to a water molecule. The rønsted Lowry concept was ultimately replaced because it depended too strongly on the acids and bases reacting in a solution (either gas or liquid). As well, the rønsted Lowry concept depends on compounds transferring a proton, yet there are chemicals that can make an acidic solution without donating a proton at all. Lewis acid base theory so far has not been improved upon or replaced, but it may be in the future. 3. The first theoretical definition of a base was developed by Arrhenius. He proposed that a base was a compound that, when placed into water, would dissociate into hydroxide ions. 4. Section 16.2 Questions (Pages 735 736) 1. (a) The ph of nitric acid should be much lower than that of a nitrous acid of the same concentration. (b) Nitric acid loses (donates) a proton to a water molecule much more easily than nitrous acid does, creating a higher concentration of hydronium ions in the solution and thus a lower ph. 2. List all entities. Identify all possible acids and bases. Identify the strongest acid (SA) and the strongest base (). Write an equation showing the SA reacting with the and transferring one H + from A to. Predict the equation position using the acid base table and the position generalization. 3. The position of equilibrium is determined by the result of the competition for protons. Of the forward and reverse reactions, the reaction involving the stronger acid and the stronger base is favoured. 4. If the acid is listed above the base in the table of acids and bases, then the products will be favoured. If the acid is listed below the base, then the reactants will be favoured. 5. H 2 SO 3 (aq) / HSO 3 (aq) and HSO 3 (aq) / SO 2 3 (aq) 6. The rønsted Lowry equations predict either hydronium or hydroxide ion formation in solution. (a) Hr(aq) + H 2 O(l) r (aq) + H 3 O + (aq) acidic (b) NO 2 (aq) + H 2 O(l) HNO 2 (aq) + OH (aq) basic (c) NH 3 (aq) + H 2 O(l) NH + 4 (aq) + OH (aq) basic (d) Since HSO 4 (aq) is amphiprotic, it will react both as an acid and as a base. There is no way to tell, given our current level of theory, which reaction would predominate. HSO 4 (aq) + H 2 O(l) SO 2 4 (aq) + H 3 O + (aq) acidic? HSO 4 (aq) + H 2 O(l) H 2 SO 4 (aq) + OH (aq) basic? (e) CO 2 (g) + 2 H 2 O(l) H 3 O + (aq) + HCO 3 (aq) acidic (f) CaO(s) + H 2 O(l) Ca 2+ (aq) + 2 OH (aq) basic (g) CH 3 COOH(aq) + H 2 O(l) H 3 O + (aq) + CH 3 COO (aq) acidic 640 Unit 8 Solutions Manual Copyright 2007 Thomson Nelson
7. Each substance is tested with both blue and red litmus paper. The substance tested is the manipulated variable and the colour change of the litmus is the responding variable. Temperature and concentration are controlled variables. 8. Solutions of equal concentration of several bases are prepared and the ph is measured for each solution. The manipulated variable is the base, and the responding variable is the ph. The controlled variables are temperature and concentration. The electrical conductivities of aqueous solutions of equal concentration are measured. The manipulated variable is the base, the responding variable is the electrical conductivity, and the controlled variables are temperature and concentration. 9. Ammonia and hydronium ions are both pyramidal in shape due to having a lone pair of electrons and three attached hydrogen atoms. However, a hydronium ion is positively charged overall and will repel protons, while an ammonia molecule is electrically neutral overall, and the lone pair will attract a proton. acid base conjugate base conjugate acid 2- >50% - - 10. (a) CH2OHCOOH(aq) + CO 3 (aq) CH2OHCOO (aq) + HCO 3 (aq) (b) Since glycolic acid solution is 3.9% ionized compared to the 1.3% ionization of acetic acid, glycolic acid would be a stronger weak acid. Since the compared ionizations are 3.9% and 1.3%, it is likely that the only difference a consumer might notice is the amount of time that the acid takes to react, and, possibly, how vigorous any signs of the reaction are (such as bubbles produced) etc. (c) Glycolic acid, which is 3.9% ionized in water at standard ambient temperature and pressure (SATP), would react more quickly than acetic acid, which is 1.3% ionized in water. The glycolic acid, due to its greater ionization in water, produces a larger hydronium ion concentration and a lower ph. Since the concentration of hydronium ion is larger, the rate of reaction with the rust is faster. (d) With equally concentrated glycolic acid and acetic acid solutions, they would react with the same amount of rust, because the same chemical amount of acid is present in both solutions. The glycolic acid would react slightly more quickly. 11. (a) CaCO 3 (s) CaO(s) + CO 2 (g) (b) CaO(s) + H 2 O(l) Ca(OH) 2 (s) 2 (c) O (aq) + H O(l) OH (aq) + OH (aq) 2 16.3 ACID ASE STRENGTH AND THE EQUILIRIUM LAW Practice (Page 743) 1. (a) Acid strength depends on the extent to which an acid ionizes in (reacts with) water. (b) The differing ph, rate of reaction, and conductivity of acids of equal concentration is evidence for differing acid strength. (c) The strength of an acid refers to how easily a proton can be removed from it. The stronger the acid, the easier it is to remove a proton from it. The concentration of the acid refers to how much acid is present in the solution relative to the quantity of water. A higher concentration of acid has a larger quantity of acid per unit volume of the solution. (d) The stronger of two acids does not necessarily make a more acidic solution. Every acid, when placed into water, reacts with the water to form a conjugate base and a hydronium ion. A stronger acid will produce more hydronium ions than a weaker acid if they are at the same initial concentration. It is possible to have a weaker acid produce more hydronium ions in total than a stronger acid, if the weaker acid is sufficiently more concentrated. Copyright 2007 Thomson Nelson Unit 8 Solutions Manual 641