Review for Exam 2 Chem 173

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1 Review for Exam 2 Chem 173 The following are the major concepts with which you should be well acquainted from Chapters 15, 16, 17, and 18: Chapters 15 and 16: Acids and Bases Fundamentals Bronsted-Lowry definitions of acids and bases as H + donors and acceptors Acid and base ionization (dissociation) equations including identification of conjugate pairs K a and K b expressions relationship of acid strength to: K a value, [H 3 O + ], [HA] eq, equilibrium position, strength of conjugate base relationship of base strength to: K b value, [OH ], [B] eq, equilibrium position, strength of conjugate acid relative [H 3 O + ] and [OH ] in acidic, basic, and neutral solutions identification of strong acids and bases (only a small number so KNOW THEM!!) Water Water is amphoteric (or amphoprotic). What does that mean? Auto-ionization of water, and water auto-ionization constant K w K w = 1.0 x at 25 C; K w = [H 3 O + ][OH ] ph (and poh, pk a, pk b ) scale logarithmic scale ph = log[h 3 O + ] and [H 3 O + ] = 10 ph ; poh = log[oh ] and [OH ] = 10 poh pk a = logk a and K a = 10 pka ; pk b = logk b and K b = 10 pkb ph + poh = pk w Where do acidic, basic, and neutral solutions fall on the ph scale? Polyprotic acids dissociate in a stepwise manner there is a dissociation equation and K a value corresponding to each step acids are progressively weaker (smaller K a ) in each successive dissociation step Salts ionic compounds dissociate into constituent ions ions can produce acidic, basic or neutral soltuions ionization equations corresponding to acid or base behavior of cations or anions relationship between K a and K b for a conjugate acid/base pair Structure/Strength relationships for acids Lewis acid/base definitions Calculations based on these concepts include: ph of strong acid and strong base solutions ph of weak acid and weak base solutions: this includes identifying the major species in solution, identifying the major source of H 3 O + or OH in solution and setting up an equilibrium calculation ph of mixtures of weak acids or weak bases ph of polyprotic acid solutions ph of salt solutions Calculation of % dissociation or % ionization or weak acids and weak bases Determination of K a or K b using ph or % ionization values

2 Chapters 16 and 17: Applications of Aqueous Equilibria Common Ion Effect and Buffer Solutions understand how the presence of a common ion effects the solution with respect to equilibrium position, % ionization and ph buffer solutions are common ion solutions usually composed of a conjugate acid/base pair buffer solutions resist changes in ph when small amounts of strong acid (H + ) or strong base (OH ) are added upon addition of strong acid or strong base a neutralization reaction occurs resulting in changing [HA] and [A ] Henderson-Hasselbalch equation for calculation of ph of buffer solutions effective buffering occurs when ph of solution = pk a + 1 other terms to understand: buffer capacity, buffer failure Titrations 3 classes of titrations: strong acid + strong base, weak acid + strong base, and weak base + strong acid calculation of ph before (including half-way point), at, and beyond stoichiometric point characteristic profiles of ph or titration curves indicators chosen appropriately based on relationship of pk a of indicator to ph at stoichiometric point Solubility Equilibria ionic solid in equilibrium with constituent ions in solution (saturated solution); equilibrium constant is the solubility product constant, K sp solubility quantified in mol/l or g/l relative solubilities of ionic compounds when can you compare K sp s directly and when is a calculation required? presence of a common ion, ph of solution, and complex ion formation may effect a compound s solubility precipitation of an ionic compound occurs from a supersaturated solution with Q (ion product) > K sp dissolution of solid occurs in an unsaturated solution with Q < K sp Calculations based on these concepts include: ph of common ion and buffer solutions ΔpH of buffer solution after the addition of strong acid or strong base ph of solution at any given point during an acid + base titration ph corresponding to visible indicator color change calculation of K sp given solubility or ion concentration data calculation of molar solubility or g/l solubility of ionic compounds in water or a common ion solution calculation of ion product (Q) for prediction of precipitation calculation of mass or concentration of ion required to initiate precipitation Chapter 18: Thermodynamics and Equilibrium * you should review the fundamental concepts of thermochemistry that you learned in Chapter 6 * 1 st Law of Thermodynamics: the energy of the universe is constant Entropy and the 2 nd Law of Thermodynamics the entropy of the universe is increasing; ΔS univ = ΔS sys + ΔS surrr entropy is the thermodynamic quantity related to randomness and disorder qualitative prediction of the sign of ΔS for physical or chemical change relationship between spontaneity and temperature given by: ΔS = q/t or ΔS = ΔH/T ΔS can be calculated using standard molar entropy data: ΔS = Σn S prod Σn S react Gibbs Free Energy thermodynamic property of the system that is related to spontaneity for a spontaneous process ΔG< 0 calculation of ΔG: ΔG = ΔH TΔS; ΔG = Σn ΔG f prod Σn ΔG f react ; Hess s Law approach ΔG is dependent on reaction composition: ΔG = ΔG + RTlnQ where Q = reaction quotient ΔG is related to the equilibrium position of a process: at equilibrium ΔG = 0 so... ΔG is related to the equilibrium constant: ΔG = RTlnK A process will proceed spontaneously in the direction that maximizes the entropy of the universe and minimizes the free energy of the system; this is where the process reaches equilibrium.

3 Some Review Problems, Chapters 15, 16, 17, and 18 (See App. 4 & 5 as necessary for K a, K b, K sp, ΔH f, ΔS, ΔG f data) 1. Write the balanced equation for the reaction of CN with water. Identify all species as acid, base, conjugate acid, or conjugate base. 2. For each of the following pairs, give the stronger acid: H 2 SO 3 or H 2 SeO 3, HIO 4 or HIO 3, H 2 S or H 2 O 3. Phthalic acid, H 2 C 8 H 4 O 4 is a diprotic acid used in the synthesis of phenolphthalein indicator. K a1 = 1.2 x 10 3 and K a2 = 3.9 x Calculate [H 3 O + ] and [C 8 H 4 O 4 2 ] in a M H 2 C 8 H 4 O 4 (aq). 4. For each of the following salts indicate whether the aqueous solution will be acidic, basic or neutral: a. Fe(NO 3 ) 3, b. Na 2 CO 3 c. Ca(CN) 2 d. NH 4 ClO 4 5. a. Calculate the value of K b for NO 2. b. Calculate K a for C 5 H 5 NH + (pyridinium ion). 6. Calculate the concentration of pyridine (C 5 H 5 N) in and the ph of 0.15 M pyridinium bromide (C 5 H 5 NHBr). 7. Calculate the percent ionization of each of the following solutions: a M HF b. a mixture of 0.80 M HF and 0.10 M HCl. 8. Calculate the ph of a sol n that is 0.10 M CH 3 NH 2 (methylamine) and 0.15 M CH 3 NH 3 Cl (methylammonium chloride). 9. Calculate the ph of a buffer solution that is 0.10 M NH 3 and 0.10 M NH 4 +. Calculate the change in ph that occurs when 12.0 ml of 0.20 M HCl is added to ml of this buffer. 10. Calculate the ph of a buffer solution that is 0.10 M monochloroacetic acid and 0.15 M sodium chloroacetate. 11. How many grams of sodium acetate must be added to 2.0 L of 0.10 M acetic acid to give a solution with a ph of 5.00? Ignore the volume change due to the addition of sodium acetate. 12. Calculate the ph of the solution that forms when 20.0 ml of 0.10 M NaOH is added to 25.0 ml of 0.10 M HCl. 13. A 1.24 g sample of benzoic acid (C 6 H 5 COOH) is dissolved in water resulting in 50.0 ml of solution. This solution is titrated with M NaOH. Calculate the ph of the solution (a) after the addition of 35.0 ml of NaOH, (b) at the stoichometric point, and (c) after the add n of 60.0 ml of NaOH ml of M ethylamine is titrated with 0.15 M HCl. Calculate the ph of the solution (a) after the add n of 15.0 ml of HCl, (b) at the stoichiometric pt, and (c) after the add n of 22.5 ml of HCl. 15. The solubility of silver bromate, AgBrO 3, in water is g/l. Calculate K sp. 16. Celestite (strontium sulfate) is an important mineral of Sr. Calculate the molar solubility of SrSO 4 (K sp = 2.5 x 10 7 ). 17. The solubility of magnesium fluoride, MgF 2, in water is g/l. Calculate the solubility (in g/l) of magnesium fluoride in M sodium fluoride. 18. Lead (II) chromate, PbCrO 4, is used as a yellow paint pigment. A solution is prepared that is 5.0 x 10 4 M in lead ion and 5.0 x 10 5 M in chromate ion. Do you expect precipitation of lead chromate from this solution? 19. How many moles of calcium chloride can be added to 1.5 L of M potassium sulfate before a precipitate is expected? Assume that the volume of the solution is not changed significantly by the addition of the calcium chloride. 20. Calculate the ph of the following solutions: a M C 2 H 5 NH 3 Br b M Ca(NO 2 ) 2, c M CoCl 2 (for Co(H 2 O) 6 2+ K a = 1.3 x 10 9 ) 21. What mass (in g) of sodium chloride can be added to 785 ml of M silver nitrate before a precipitate forms?

4 22. Sorbic acid (HC 6 H 7 O 2, pk a = 4.77) is widely used in the food industry as a preservative. For example, its potassium salt (potassium sorbate) is added to cheese to inhibit the formation of mold. Calculate the ph of 0.37 M KC 6 H 7 O 2 (aq). 23. Calculate [H 3 O + ] in the following solutions: a M HCl and M HOCl b M NaNO 2 and M HNO You wish to prepare a buffer solution with ph = How many grams of (NH 4 ) 2 SO 4 would you add to 425 ml of M NH 3 to do this? ml of 1.25 M HNO 3 is titrated with 0.95 M KOH. Calculate the ph of the solution (a) after the add n of 20.0 ml of KOH, (b) at the stoichiometric pt., (c) after the add n of 40.0 ml of KOH. 26. Write the solubility product expression for each of the following salts: a, CrF 3 b. Cd 3 (PO 4 ) 2 c. SrF Chloroform, CHCl 3, is a solvent that has been used as an anesthetic. The heat of vaporization of chloroform at its boiling point (61.2 C) is 29.6 kj/mol. What is the entropy change when 1 mol CHCl 3 vaporizes at its boiling point? 28. Predict the sign of ΔS if possible for each of the following reactions: a. C 2 H 2 (g) + 2 H 2 (g) C 2 H 6 (g) b. N 2 (g) + O 2 (g) 2 NO (g) c. 2 C 2 H 2 (g) + 3 O 2 (g) 4 CO (g) + 2 H 2 O (g) d. 2 C (s) + O 2 (g) 2 CO (g) 29. Calculate ΔS for the following reaction using S data from Appendix 4: CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O (l) 30. Using enthalpies of formation calculate ΔH for the following reaction at 25 C. Also, calculate ΔS for this reaction from standard entropies at 25 C. Then use these values to calculate ΔG. 2 CH 3 OH (l) + 3 O 2 (g) 2 CO 2 (g) + 4 H 2 O (l) 31. Calculate the standard free energy of the following reactions at 25 C using standard free energies of formation. a. CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O (g) b. CaCO 3 (calcite) + 2 H + (aq) Ca 2+ (aq) + H 2 O (l) + CO 2 (g) (for Ca 2+ ΔG f = 554 kj) 32. Consider: H 2 (g) + Cl 2 (g) 2 HCl (g). Calculate ΔG and K for this reaction at 25 C. 33. Calculate ΔG and K for the following reaction at 25 C: CO (g) + 3 H 2 (g) CH 4 (g) + H 2 O (g). 34. Calculate ΔG at 25 C for the equilibrium: CaF 2 (s) Ca 2+ (aq) + 2 F (aq). Calculate K sp for this reaction at 25 o C. For CaF 2 (s) ΔG f = 1162 kj/mol; see problem 31 for ΔG f of Ca Calculate ΔH f for CH 4 (g) using the following reactions: CO 2 (g) + 2 H 2 (g) CH 2 O (g) + H 2 O (g) ΔH = 35.5 kj CO 2 (g) C (s) + O 2 (g) ΔH = 393 kj CH 2 O (g) + 2 H 2 (g) CH 4 (g) + H 2 O (g) ΔH = 210 kj 2 H 2 (g) + O 2 (g) 2 H 2 O (g) ΔH = 484 kj From appropriate standard entropy data calculate ΔS for the formation of methane from its elements. Then, using ΔH and ΔS, calculate ΔG f for methane at 25 C.

5 Answers to Review Problems: 1. CN + H 2 O HCN + OH ; CN base, H 2 O acid, HCN conj. acid, OH conj. base 2. H 2 SO 3, HIO 4, H 2 S 3. a. 3.7 x 10 3 M, b. 3.9 x 10 6 M 4. a. acidic, b. basic, c. basic, d. acidic 5. a. 2.5 x 10 11, b. 5.9 x x 10 4 M, %, 0.73% , ΔpH = 0.17 (ph = 9.09) g (a) 4.40, (b) 8.59, (c) (a) 10.11, (b) 6.01, (c) x x x 10 3 g/l 18. ppt'n occurs x 10 3 mol 20. (a) 5.43, (b) 8.94, (c) x 10 6 g a , b. 2.2 x g 25. (a) 0.56, (b) 7.00, (c) a. K sp = [Cr 3+ ][F ] 3, b. K sp = [Cd 2+ ] 3 [PO 4 3 ] 2, c. K sp = [Sr 2+ ][F ] J/K mol 28. a. ; b. can't predict; c. +; d J/K 30. ΔH = 1453 kj; ΔS = 161 J/K; ΔG = 1405 kj 31. a. 801 kj; b. 56 kj kj; 2.0 x kj; 1.2 x ΔG = 50 kj, Ksp = 1.7 x ΔH = 83.5 kj; ΔS = 82.0 J/K; ΔG = 59.1 kj