-8-5. [H3O (aq)] = mol/l ph = -log [H3O (aq)] = -log mol/l = 4.25 According to the equilibrium law, the ph of the solution is 4.25.

Transcription

1 (c) -8-5 x = [H3O (aq)] = mol/l + -5 ph = -log [H3O (aq)] = -log mol/l = 4.25 According to the equilibrium law, the ph of the solution is Na 2HC6H5O 7(s) 2Na (aq) + HC6H5O 7 (aq) HC6H5O 7 (aq) + H2O(l) HO 3 (aq) + CHO (aq) (from question 9) The quadratic formula is not required, as hydrogen citrate ion is a weak acid and 2-2 [HC6H5O 7 (aq)] , which is greater than Ka [C6H5O 7 (aq)][h3o (aq)] Ka [HC6H5O 7 (aq)] + 3- At equilibrium: Let x = [H3O (aq)] = [C6H5O 7 (aq)] 2- Then [HC6H5O 7 (aq)] = ( x) = (using the assumption) (Optional) ICE Table for HC6H5O 7 (aq) + H2O(l) HO 3 (aq) + CHO (aq) [HC 6 H 5 O - 7 (aq)] [H 3 O + (aq)] [C 6 H 5 O 2-7 (aq)] Concentration (mol/l) (mol/l) (mol/l) Initial Change - x + x + x Equilibrium ( x) = x x 2-7 x = x = [H3O (aq)] = mol/l + -4 ph = -log [H3O (aq)] = -log mol/l = 3.85 According to the equilibrium law, the ph of the solution is INTERPRETING ph CURVES Practice (Page 754) 1. (a) A buffering region of a ph curve is a relatively level (slope is close to 0) part of the curve preceding or following the equivalence point. (b) The buffer zone represents the part of the reaction where the titrant that is added to the sample is either much less or much greater than the volume at the equivalence point. During this portion of the titration, the ph of the solution changes relatively little. (c) The region near the equivalence point of a ph curve is the steepest (slope is very large) part of the curve between two buffering regions. (d) The titrant volume at the equivalence point represents the point of the titration where the chemical amounts of titrant and sample reactants are in the same ratio as the stoichiometry of the chemical equation. At this point on a curve, a very small addition of titrant to the sample changes the ph of the solution significantly the rate of change of ph per volume of titrant is at a maximum. Copyright 2007 Thomson Nelson Unit 8 Solutions Manual 651

2 + 2. (a) HMo(aq) + H2O(l) H3O (aq) + Mo (aq) acid base acid base conjugate acid base pairs: HMo(aq)/Mo (aq), H 3 O + (aq)/h 2 O(l) + (b) HIc(aq) + H2O(l) H3O (aq) + Ic (aq) acid base acid base conjugate acid base pairs: HIc(aq)/Ic (aq), H 3 O + (aq)/h 2 O(l) + (c) HTb(aq) 2 + HO(l) 2 H3O (aq) + HTb (aq) acid base acid base conjugate acid base pairs: H 2 Tb(aq)/HTb (aq), H 3 O + (aq)/h 2 O(l) + 2 (d) HTb (aq) + H2O(l) H3O (aq) + Tb (aq) acid base acid base conjugate acid base pairs: HTb (aq)/tb 2 (aq), H 3 O + (aq)/h 2 O(l) + (e) HBb(aq) + H2O(l) H3O (aq) + Bb (aq) acid base acid base conjugate acid base pairs: HBb(aq)/Bb (aq), H 3 O + (aq)/h 2 O(l) - HTb (aq) can act as an acid by donating a proton or as a base by accepting a 3. In question 2, proton. The empirical term amphoteric may be used to describe a chemical substance that can act as both an acid and a base. Amphiprotic is a more theoretical term used to describe an entity that can either donate or accept a proton in a reaction. 4. (a) HTb(aq) 2 HMo(aq) HBb(aq) - HTb (aq) HIc(aq) (b) Since each sample to be titrated is identical, the starting ph will be the same and basic. Each indicator should start in its base form, and be converted to its acid form once enough hydrochloric acid titrant is added. Since indigo carmine, in its basic form (Ic (aq)), has a ph range of , which is more basic than all the other indicators, it would change colour once the addition of enough titrant causes the ph to drop below Practice (Page 759) 5. Buffering action is displayed by a relatively horizontal portion of a ph curve, indicating that the ph changes very little when small amounts of acid or base are added. 6. Quantitative reactions are displayed by a relatively steep portion of a ph curve, indicating an abrupt change in ph around the equivalence point. 7. A ph curve is used to determine the ph of the solution at the equivalence point of a titration reaction, so that a suitable indicatorone that will change colour at or very near the ph at that equivalence pointcan be chosen. 8. (a) The ph of the equivalence point of the titration of NaOH(aq) with CH 3 COOH(aq) is approximately 9. (b) An appropriate indicator should have the middle of its colour change ph range at or very close to 9, such as phenolphthalein (ph range ), or monoprotic thymol blue (ph range ). 652 Unit 8 Solutions Manual Copyright 2007 Thomson Nelson

3 (c) SA SB A + - CH3COOH(aq), Na (aq), OH (aq), H2O(l) B - - CH COOH(aq) + OH (aq) CH COO (aq) + H O(l) (The curve indicates that this reaction is quantitative.) (d) The curve at the very beginning of the titration rises abruptly because of the entities present in the sample. In the sample, before the titration begins, the weak acetic acid is the only entity present (other than water). When the first hydroxide ion titrant is added, it reacts with the acetic acid to produce some basic acetate ion, CH 3 COO (aq). Since the sample solution now contains some basic acetate ion and some acidic acetic acid mixed together, the ph of the sample abruptly increases. Further addition of titrant only results in a gradual change in the relative concentrations of acetate ion and acetic acid entities. The ph rise remains very gradual for a while until the titration nears the equivalence point because the kinds of entities present are not being changed. 9. (a) Only two equivalence points are evident because the third possible reaction is not quantitative. 10. (b) PO 3 4 (aq) + H 3 O + (aq) HPO 2 4 (aq) + H 2 O(l) (quantitative) HPO 2 4 (aq) + H 3 O + (aq) H 2 PO 4 (aq) + H 2 O(l) (quantitative) H 2 PO 4 (aq) + H 3 O + 50% (aq) H 3 PO 4 (aq) + H 2 O(l) (This reaction establishes an equilibrium that shifts gradually as more hydrochloric acid is added.). Web Activity: SimulationTitration of Polyprotic Acids and Bases (Page 762) [No written response is required.] Practice (Page 762) 11. (a) ph at equivalence point is < 7; strong acidweak base (SAWB) reaction (b) ph at equivalence point is > 7; weak acidstrong base (WASB) reaction (c) ph at equivalence point is > 7; weak acidstrong base (WASB) reaction (d) ph at equivalence point = 7; strong acidstrong base (SASB) reaction (e) ph at equivalence point is not predictable; weak acidweak base (WAWB) reaction (f) ph at equivalence point is < 7; strong acidweak base (SAWB) reaction 12. (Note that the only reliable way to choose an indicator is by using a ph curve.) Copyright 2007 Thomson Nelson Unit 8 Solutions Manual 653

4 (a) As this is a strong acidstrong base reaction, the ph endpoint is 7. Bromothymol blue, which has a pk a value of 7 (or K a = ), should be used. (b) As this is a strong acid weak base reaction, the ph endpoint is less than 7. Methyl orange, which has a pk a value of 4 (K a = ), is a likely choice. (c) As this is a weak acid strong base reaction, the ph endpoint is greater than 7. Phenolphthalein, which has a pk a value of 10 (K a = ), is a likely choice. 13. (a) The ph is predicted to be < 7, as the ammonium ion is a weak acid. (b) The ph is predicted to be > 7, as the sulfide ion is a weak base. (c) The ph is predicted to be ~ 7, as neither the K + (aq) nor the NO 3 (aq) ion acts as an acid or a base. (d) The ph is predicted to be < 7, as the bisulfate ion, while theoretically amphiprotic, can react only as an acid. Because it is an even weaker base than water, it cannot react as a base in aqueous solution. K a = mol/l and K b = negligibly small 14. (a) SA A H 3 O + (aq), ClO 4 (aq), Na + (aq), CO 2 3 (aq), H 2 O(l) B SB B 2 H 3 O + (aq) + CO 2 3 (aq) H 2 CO 3 (aq) + 2 H 2 O(l) (total of two quantitative proton transfer reactions) (b) SA A HNO 2 (aq), K + (aq), OH (aq), H 2 O(l) SB B HNO 2 (aq) + OH 50% (aq) H 2 O(l) + NO 2 (aq) (single proton transfer equilibrium reaction) (c) SA A H 3 PO 4 (aq), Na + (aq), OH (aq), H 2 O(l) SB B H 3 PO 4 (aq) + 2 OH (aq) HPO 2 4 (aq) + 2 H 2 O(l) (total of two quantitative proton transfer reactions) 15. (a) 2 HCl(aq) + Mg(OH) 2 (aq) 2 H 2 O(l) + MgCl 2 (aq) H 3 O + (aq) + OH (aq) 2 H 2 O(l) (b) NH 3 (aq) + H 2 SO 3 (aq) NH 4 HSO 3 (aq) NH 3 (aq) + H 2 SO 3 (aq) NH + 4 (aq) + HSO 3 (aq) (c) H 2 SO 4 (aq) + 2 NaOH(aq) 2 H 2 O(l) + Na 2 SO 4 (aq) H 3 O + (aq) + OH (aq) 2 H 2 O(l) Web Activity: Simulation: Preparation of Buffer Solutions (Page 765) [No written response is required.] Web Activity: Canadian AchieversMaud Menten (Page 766) Maud Menten, along with German biochemist Leonor Michaelis, developed an equation for calculating the rate of an enzyme-driven reaction. This formula is now known as the MichaelisMenten equation, and involves the concentrations of the substrates and the MichaelisMenten constant. 654 Unit 8 Solutions Manual Copyright 2007 Thomson Nelson

5 Practice (Page 766) 16. Empirically, a buffer is a mixture of substances in solution that maintains nearly constant ph when diluted or when small amounts of a strong acid or base are added. Theoretically, a buffer is a solution that contains significant chemical amounts of both a weak acid and its conjugate base. (These conjugate acidbase pairs establish an equilibrium that responds to the addition of small amounts of other acid or base entities by shifting to counteract any resulting change in hydronium ion concentration. The introduced change in hydronium ion concentration is counteracted almost completely, as long as neither conjugate entity of the buffer solution is completely depleted.) 17. H 2 PO 4 (aq) HPO 2 4 (aq), and H 2 CO 3 (aq) HCO 3 (aq) are both human body fluid buffers. 18. (a) A A A SA H 2 CO 3 (aq), HCO 3 (aq), H 2 O(l), H 3 O + (aq), Cl (aq) SB B B H 3 O + (aq) + HCO 3 (aq) H 2 O(l) + H 2 CO 3 (aq) (b) SA A A H 2 CO 3 (aq), HCO 3 (aq), H 2 O(l), Na + (aq), OH (aq) B B B SB H 2 CO 3 (aq) + OH (aq) HCO 3 (aq) + H 2 O(l) 19. A large amount of a strong acid would completely neutralize the conjugate base in the buffer, and the ph of the solution would drop dramatically at that point. A large amount of a strong base would completely neutralize the conjugate acid in the buffer, and the ph of the solution would rise abruptly. 20. (a) When a small amount of HCl(aq) acid is added to a C 6 H 5 COOH(aq)C 6 H 5 COO (aq) buffer, the increased concentration of the hydronium ion would react with the basic benzoate ion, shifting the equilibrium, - + C6H5COOH(aq) + H2O(l) C6H5COO (aq) + H3O (aq), to the left. (b) When a small amount of OH (aq) base is added to a C 6 H 5 COOH(aq)/C 6 H 5 COO (aq) buffer, the increased concentration of the hydroxide ion would react with the benzoic - acid, shifting the equilibrium, C H COOH(aq) + OH (aq) C H COO (aq) + H O(l), to the right. 21. (a) Nitric acid and sodium nitrate are not theoretically considered to be a buffer solution. Because nitric acid is a strong acid, the conjugate pair present in a nitric acid solution is actually H 3 O + (aq) and H 2 O(l), and these are not present in (even approximately) equal amounts. (Note that this solution will maintain a quite constant (very low) ph when small amounts of other acids or bases are added, however.) (b) C 6 H 5 COOH(aq)C 6 H 5 COO (aq) is an effective buffer, with the conjugate pair in the acidic part of the acidbase table. (c) NH 3 (aq)nh 4 + (aq) is an effective buffer, with the conjugate pair in the basic part of the acidbase table. (d) HCl(aq) and NaOH(aq) is not an effective buffer because the reaction of the strong acid and the strong base produces H 2 O(l) and NaCl(aq) only. (The net ionic equation will be H + (aq) + OH (aq) H 2 O(l). This mixture is essentially just a salt solution, with a ph of 7 that will change dramatically if even a tiny amount of excess acid or base is added.) Copyright 2007 Thomson Nelson Unit 8 Solutions Manual 655

6 Investigation 16.3: Testing a Buffer Effect (Pages 766, 769) (Students should complete all three parts of the Evaluation.) Purpose The purpose of this investigation is to test our concept of buffers. Problem How does the ph change when a strong acid and a strong base are slowly added separately to an H 2 PO 4 (aq) HPO 2 4 (aq) buffer? Prediction According to the empirical definition of a buffer, the addition of small amounts of a strong acid or strong base produces only a slight change in the ph of the mixture until the capacity of the buffer is exceeded. The reasoning is that the added hydronium ion or hydroxide ion is quantitatively removed by reacting with a component of the weak acid weak base buffer mixture, as shown below. H 3 O + (aq) + HPO 4 2 (aq) H 2 O(l) + H 2 PO 4 (aq) OH (aq) + H 2 PO 4 (aq) H 2 O(l) + HPO 4 2 (aq) On the basis of the evidence provided in Figure 9 (Student Book page 757) and Figure 13 (Student Book page 760), both of the above reactions are quantitative. Design An H 2 PO 4 (aq) HPO 4 2 (aq) buffer is prepared and then tested by adding small amounts of HCl(aq) and NaOH(aq), one drop at a time. Indicators are used to indicate a sudden change in ph. A NaCl(aq) solution is tested as a control. The manipulated variable is the volume (number of drops) of acid or base added. The responding variable is the colour of the indicator (the ph). The controlled variables are temperature, volume, and concentration of buffer used, and volume and concentration of added acid or base. Materials lab apron eye protection distilled or deionized water 0.10 mol/l solutions of the following: KH 2 PO(aq) NaOH(aq) NaCl(aq) HCl(aq) 150 ml beaker two 50 ml graduated cylinders four small test tubes or spot plate CAUTION: Some of the solutions are corrosive and must be handled with particular care. Evidence Colour of Bromocresol Green in Buffer Solution Volume of HCl(aq) (drops) Colour of buffer solution blue blue blue blue blue blue Colour of salt solution blue green yellow yellow yellow yellow 656 Unit 8 Solutions Manual Copyright 2007 Thomson Nelson

7 Colour of Phenolphthalein in Buffer Solution Volume of HCl(aq) (drops) Colour of buffer solution blue blue blue blue blue blue Colour of salt solution none pink red red red red Analysis On the basis of the evidence gathered in this investigation, the addition of small volumes of a strong acid or a strong base to H 2 PO 4 (aq) HPO 4 2 (aq) buffer does not change the ph significantly. Evaluation The Design is judged to be adequate because the problem was clearly answered and all necessary controls were employed. Using the salt solution as a control was even better than using distilled water. A universal indicator, ph paper, or a ph meter would have provided more precise information as to the ph change. The Procedure is judged to be adequate because it was simple and efficient. The droppers used to add the strong acid and strong base could be calibrated to give more quantitative evidence. The skills used carrying out the investigation are judged to be adequate because they are very simple. The precision of the measurements is not crucial. Overall, I am confident in the evidence gathered by the use of the experimental Design, Procedure, and skills employed. Therefore, I am sufficiently confident to continue and to judge the Prediction and the empirical definition of a buffer. The Prediction, based upon the empirical definition of a buffer, is judged to be verified because the evidence supported the predicted effects on the ph. The ph of the buffer solution did not change significantly when a small quantity of strong acid or base was added. The empirical definition of a buffer is judged to be acceptable because the Prediction was verified. The Purpose of this investigation would be much better accomplished by testing more buffers using more indicators. Career Connection: Microbiologist (Page766) (No written response is required.) Section 16.4 Questions (Page 767) 1. Copyright 2007 Thomson Nelson Unit 8 Solutions Manual 657

8 2. (a) Buffering is the maintenance of a relatively constant ph in a solution when small amounts of a strong acid or base are added. (b) Buffering is represented by the nearly level (plateau) portions of the line before and after an equivalence point. (This is most noticeable at a volume of titrant that is one-half of the volume at the first equivalence point or halfway between successive equivalence points for polyprotic acids). (c) Quantitative reactions are represented by steeply sloped portions of a ph curve. (d) The endpoint is the point at which an observationusually of indicator colour changesignals that the titration should be stopped. The equivalence point is the point in the titration where stoichiometrically equivalent amounts of the reactants have been combined. (e) The mid-point of the ph colour change range of a suitable indicator should nearly equal the ph at the equivalence point of the titration reaction, and the indicator should complete its colour change while the ph is changing abruptly. (f) Non-quantitative reactions do not have a distinct endpoint because the ph changes too gradually in the region where the equivalence point is expected. 3. See also Figure 14, Student Book page 760. See also Figure 12, Student Book page (a) yellow (b) red 658 Unit 8 Solutions Manual Copyright 2007 Thomson Nelson

9 (c) green (d) colourless (e) yellow 5. (a) one (b) H 3 O + (aq) + SO 3 2 (aq) H 2 O(l) + HSO 3 (aq) (c) The equivalence point occurs at a ph of about 4, when 23 ml of HCl(aq) has been added. (d) Either congo red (ph ) or methyl orange (ph ) would be suitable. (e) A buffering region on the graph occurs where about 5 to 20 ml of HCl(aq) has been added. The entities present before the equivalence point are Na + (aq), SO 3 2 (aq), HSO 3 (aq), Cl (aq), H 2 O(l). Another buffering region occurs after about 30 ml of HCl(aq) has been added. The entities present after the equivalence point are Na + (aq), HSO 3 (aq), Cl (aq), H 2 O(l) and H 3 O + (aq). 6. (a) HOOCCOOH(aq) + 2 OH (aq) 2 H 2 O(l) + OOCCOO 2 (aq) (assuming 2 nd equivalence point) (b) H 3 O + (aq) + PO 4 3 (aq) H 2 O(l) + HPO 4 2 (aq) (c) H 3 O + (aq) + HPO 4 2 (aq) H 2 PO 4 (aq) + H 2 O(l) (d) H 3 O + (aq) + OH (aq) 2 H 2 O(l) (e) (see 7(e) below) 7. (a) CH 3 COOH(aq) + NaOH(aq) H 2 O(l) + NaCH 3 COO(aq) CH 3 COOH(aq) + OH (aq) H 2 O(l) + CH 3 COO (aq) (b) 2 HNO 2 (aq) + Ba(OH) 2 (aq) 2 H 2 O(l) + Ba(NO 2 ) 2 (aq) HNO 2 (aq) + OH (aq) H 2 O(l) + NO 2 (aq) (c) Na 2 CO 3 (aq) + HBr(aq) NaHCO 3 (aq) + NaBr(aq) CO 3 2 (aq) + H 3 O + (aq) HCO 3 (aq) + H 2 O(l) (d) H 2 CO 3 (aq) + NaOH(aq) H 2 O(l) + NaHCO 3 (aq) H 2 CO 3 (aq) + OH (aq) H 2 O(l) + HCO 3 (aq) (e) H 2 SO 4 (aq) + 2 NaOH(aq) 2 H 2 O(l) + Na 2 SO 4 (aq) H 3 O + (aq) + HSO 4 (aq) + 2 OH (aq) 3 H 2 O(l) + SO 4 2 (aq) 8. Buffers are used in making cheese, yogurt, and sour cream, in preserving food, and in the production of antibiotics. 9. Dissolving a compound like sodium hydrogen sulfite, NaHSO 3 (s), into a solution of sulfurous acid would make an effective buffer. (The resulting buffer solution would contain the conjugate acid base pair of H 2 SO 3 (aq) and HSO 3 (aq), and would be effective because any added acid would react with the hydrogen sulfite ion and any added base would react with the sulfurous acid.) 10. (a) With a hydrogen carbonate solution, adding carbonic acid would create an H 2 CO 3 (aq) HCO 3 (aq) buffer with a ph lower than the original hydrogen carbonate solution. (b) With a hydrogen carbonate solution, adding sodium carbonate to the solution would create an HCO 3 (aq) CO 3 2 (aq) buffer with a ph higher than the original hydrogen carbonate solution. 11. Design The initial ph of the buffer is measured. A small volume of HCl(aq) is added to a sample of the buffer and, after mixing, the ph is recorded. This is repeated until the ph suddenly changes. The same procedure would be followed with a second sample of the buffer and adding NaOH(aq). Manipulated variable: volume of the HCl(aq) or the NaOH(aq) added to the buffer Responding variable: ph Controlled variables: starting volume of the buffer solution; starting amount concentration of the H 2 PO 4 and HPO 4 2 ; temperature of the solution; concentration of HCl(aq) and NaOH(aq); volume of HCl(aq) or NaOH(aq) added to the buffer solution. Copyright 2007 Thomson Nelson Unit 8 Solutions Manual 659

10 Extension 12. Recipe 1: Dissolve g potassium hydrogen phthalate in 226 ml of mol/l NaOH(aq), and dilute to 1.00 L with distilled water. Recipe 2: Combine ml 0.10 mol/l acetic acid with ml 0.10 mol/l sodium acetate solution. Chapter 16 SUMMARY Make a Summary (Page 771) (1) According to the BrønstedLowry concept, acids donate (lose) protons in reactions, while bases gain protons in reactions. There are some entities, such as anions containing hydrogen ions, that can either lose protons to stronger bases or gain protons from stronger acids. (2) Acidbase reactions can be predicted in a somewhat similar way to oxidationreduction reactions. Since acids donate protons (hydrogen ions) to bases and bases accept protons from acids, the two products can be predicted by removing a proton from the acid formula and adding it to the base formula. Unlike redox reactions, which are thought of as either occurring spontaneously or not, acidbase reactions are thought of as always occurring, to some extent. The extent of the reaction can be approximately predicted with acidbase reactions because if the reacting acid is relatively strong and the reacting base is relatively strong, the products will be favoured. This method is similar to (but not as simple as) predicting whether an oxidationreduction reaction will occur or not, based on the relative strengths of the oxidizing and reducing agents. (3) The ph curve for a weak acid weak base titration cannot predict an equivalence point as a ph curve for a strong acid with a strong base would. Weak acidweak base reactions are not quantitative and would not show a sharp change in ph during the titration. Only ph curves made from strong acid with strong base, strong acid with weak base, or weak acid with strong base titrations will normally show ph changes that are rapid enough to enable the identification of equivalence points. 660 Unit 8 Solutions Manual Copyright 2007 Thomson Nelson