Chemactivity 28 Critical Thinking Questions For Children



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Unformatted text preview: 246 i ChemActivity 42 Acids and Bases Model 1: Two Definitions of Acids and Bases. Arrhenius Definitions Ari acid is a substance that produces hydronium ions, H30+(aq), when it is added to water. A base is a substance that produces hydroxide ions, OH—(aq), when it is added to water. Bronsted-Lowry Definitions ii An acid is a substance that donates a proton, HJ“, to another species. j- A base is a substance that accepts a proton, Hi", from another species. Acrd—base reactions are one of the most important types of chemical reactions. Table 1. E uilibrium constants for some acid-base reactions. it it it HCl(g) + H2003) 3 H30+(aq) + Cit (aq) 2 ><104 (1) NH3(aq) + H2003) «1” NH4+(aq) + OH-(aq) 3.3 x104 (2) HONqu + 1320(2) :1 H30+(aq) + CNWaq) 1.1x10—11 (3) 1. a) What chemical species are the Arrhenius acids in the forward reactions (i)—(3)? b)7 What chemical species are the Arrhenius bases in the forward reactions (1)43)? c) What chemical species are the Bronsted—Lowry acids in the forward reactions (1H3)? d) What chemical species are the Bronsted-Lowry bases in the forward reactions (1H3)? ChemActivity 42 Acids and Bases 247 2. Is it possible for a substance to act as both an acid and a base? Explain your reasoning. 3. Based on the data in Table 1, which do you think is considered the stronger acid, HCl or HCN? Explain your reasoning. 4. Consider reaction (1). _ a) What species results from the loss of a proton from the Bronsted—Lowry acid in the forward reaction? b) Does the species indicated in part a) (the answer that you gave) act as an acid or a base when the reverse of reaction (1) occurs? c) What species results from the gain of a proton by the Bronsted—Lowry base in the forward reaction? 4 d) Does the species indicated in part c) act as an acid or a base when the reverse of reaction (1) occurs? e) Answer parts a) — d) for reactions (2) and (3) also. Describe any general relationship that you observe using a grammatically correct English sentence. 248 ChemActivity 42, Acids and Bases Model 2: Conjugate Pairs. Within the Bronsted-Lowry model, certain pairs of molecules are described as a conjugate acid—base pair. The two species in a conjugate acid-base pair differ by a proton only. A base is said to have a conjugate acid, and an acid is said to have a conjugate base. _ 2_ HCO3 CO3 remove H+ _.__v Q—'— add H+ Tableiz. Examples of conjugate acid-base pairs. Acid Base H2CO3 HCO§ HCOgl CO3— H30+ H20 H23 HS— A conjugate acid—base pair differs by a proton, If“. The species with more protons is the acid. Critical Thinking Questions 5. Why is the charge on the hydrogen sulfide ion in Table ’2 given as —1? 6. Answer and explain each of the following: a) What is the conjugate acid of N113? b) What is the conjugate base of H20? c) Define a conjugate acid-base pair. ChemActivity 42 Acids’and Bases 249 Exercises 1. Give the conjugate base of each of the following: HSOZ- ; HCOE ; H20 ; OPT; H30+ ; NH4+; CH3NH3+; HF ; CH3000H. 2. Giverthe conjugate acid of each of the following: 80%- ; 003W ; H20 ; OH‘; 02—; N113 ; CH3NH2 ; CW; CH3COO“ ; F‘; HCng; NH;— . .——) a For each of the following reactions: £12804an + H20 2 H3O+(aq) + HSOZ(aq) Hso:4‘(aq) + H20 2 SOETaq) + H30+(aq) H20 + H20 2 H30+(aq) + OH“(aq) HCN(aq) + co§_(aq) Z HCO§(aq) + CN‘"(aq) _ H23(s) + NH3(E) 2 11313111) + NH4+(am) ' (am) = dissolved in liquid ammonia a) Which reactant is the acid? b) Which reactant is the base? c) Find the two conjugate pairs present in the reaction. a n M ON. 41* “A 0 “LL ‘Hu. o‘i-imr 4. Ammonia can react as an acid or a base. a) What is the conjugate acid of ammonia? b) What is the conjugate base of ammonia? c) Complete the following acid—base reaction in which NH3(£) acts as both an acid and a base: NH3(£’) + NH3(£) -—>— .—-—> [a Complete the following table of conjugate acids and bases: 6. J. N. Spencer, G. M. Bodner, and L. H. Rickard, Chemistry: Structure & Dynamics, Fourth Edition, John Wiley 8:, Sons, 2009. Chapter 11: Problems: 5—7, 9, 15, 16, 19-22, 156. 264 Grimmemfifictifii 4.5 pH (What Is pH Good pHor?) Model: pH Is Defined as — log[H30+]. H20 molecules are not shown The water-dissociation equilibrium constant, KW, is (at 25 °C) Kw = [H30+] [one] = 1.0 x 1044 Several definitions have been found to be useful: pH e -10s [H30+} ifs—iwll‘j W M i“ t' 4‘33 CW] pOH 5 Vlog [OH‘] pKw E —log Kw In general, pX E ~logX For pX expressions involving concentrations, the concentration units are always mole/liter, but they are omitted in the calculation. Thus, for example, values for pH are unitless. Note on significant figures. For logarithms, the number of significant figures is determined by the number of digits to the right of the decimal point. The value to the left of the decimal point indicates only the power of ten by which the number is to be multiplied. For example, if pH = 2.15, then the corresponding value for {HgOfi has only - 2 significant figures (7.1 x 10—3). The "2" in the value "2.15" is not considered a significant figure in the conversion to concentration. ChemActivity 45 pH 265 Critical Thinking Questions 1. Show that the pH reading in the model is correct (given that the hydronium ion concentration is 5.0 x 10—4 M). 2. Consider a neutral aqueous solution: {QM 0450*] = G) H “J a) What is the pH of a neutral aqueous solution? ail 35°C b) What is the pOH of a neutral aqueous solution? air 615°C 3. What values of pH characterize: a) anacidic solution? h) a basic solution? 4. What is the numerical value of pKW? all 950C. 5. Recall that log (A x B) = log A + log B. What is the relationship between pH, . pOH, and pKw? ' 6. For a Weak acid, HA: HA(aq) + H20 2 H30+(aq) + A-(aq) a) Write the equilibrium expression for K3. A b) Show that pH = pKa — log [H‘F‘l HF] 266 ChemActivity 45 pH Exercises 1. Determine the pH and pOH of each of the following solutions, and indicate whether each is acidic, basic, or neutral: a) Milk, [H30+} = 3.2 x 1077 M b) Pickle juice , [H3O+} = 2.0 x 10*4 M 0) Beer, {H3O+] = 3.2 x 10—5 M d) Blood, [H3O+] = 4.0 x 10—8 M 2. Determine the [H3O+] and [OH‘] of each of the following solutions, and indicate Whether each is acidic, basic, or neutral: " a) Lime juice, pH = 1.9 ’ 1)) Tomato juice, pH = 4.2 c) Saliva, pH = 7.0 d) Kitchen cleanser, pH = 9.3 3. Rank the following aqueous solutions in order of increasing pH Without referring to a table of acid/base constants. Explain your reasoning. a) pure H20 13) x molar NaOH 0) x molar HCl d) x molar acetic acid 4. Rank the following aqueous solutions in order of increasing pH (you will need to refer to a table of acid/base constants). Explain your reasoning. a) pure H20 b) x molar NaOH 0) 3: molar C5H5N d) x molar NH3 5. The pH of a 0.040 M solution of HOBr is 5.01. Determine K3 for the weak acid HOBr. 6. The pH of 0.300 M formic acid is 2.13. What is the Ka of formic acid? 7. The pH of a 0.15 M hydrazine (H2NNH2; a weak base) is 10.68. What is the Kb of ’ hydrazine ? For the following problems, you will need. to refer to a table of acid/base constants. 8. How many moles of NH3 must be dissolved in 1.00 liters of aqueous solution to produce a solution with a pH of 11.47? 9. What are the pH and the pOH of 0.125 M HClO4? 10. What are the pH and the pOH of 0.125 M HCl? 11. l2. 13. 14. 15. 16. 267 ChemActivity 45 pH What are the H30”r and OH“ concentrations in a 125 Hi solution prepared with 0.100 mol of H1 and water? What are the H3O+ and OH— concentrations in a 125 rnL solution prepared with 0.100 mol ofNaOH and water? What is the pH of a solution prepared from 6.50 g of benzoic acid (a weak acid), C5H5COOH, and 500 mL of water? What is the pH of a solution prepared from 3.52 g of aniline (a weak base), C6H5NH2, and 200 mL of water? Calculate the pH for each of the following solutions: a) 0.45 M NaOH b) 0.45 M HCl 0) 0.45 M CH3COOH d) r 0.45 M CH3NH2 J. N. Spencer, G. M. Bodner, and L. H. Rickard, Chemistry: Structure & Dynamics, Fourth Edition, John Wiley & Sons, 2009. Chapter 11: Problems: 34-36, 40, 74. 75, 79, 103, 132, 136. Problem When 5000 mL of 0.10 M NaOH solution (containing Na+ and 0H" ions) is mixed with 500.0 mL of 0.10 M Mg(NO3)2 solution (containing Mg2+ and N03' ions) a precipitate of solid Mg(OH)2 forms, and the resulting aqueous solution has pH = 9.43. Based on this information, determine the value of Ksp for Mg(OH)2. Show your reasoning clearly. 5 250 ChemActivity 43' Siren and Acids I (How Strong Is an Acid?) Model 1: Strong and Weak Acids. HA molecule H20 molecules are not shown Acid 1 Acid 2 A strong acid is one that is essentially 100% dissociated in water; that is, when 1.0 mole of the acid is added to enough water to make 1.0 liter of solution, the resulting [H3O+] is essentially 1.0 M. A weak acid is one that is significantly less than 100% dissociated in water; that is, when 1.0 mole of a weak acid is added to enough water to make 1.0 liter of solution, the resulting 3130+} is significantly less than 1.0 M. Critical Thinking Questions - - E 1. Is Acid 1, shown in Model 1, a strong acid or a weak acid? bhw" ‘5 'flm‘" ammo)“ ' 2. Is Acid 2, shown in Model 1, a strong acid or a weak acid? wad» is abour «evidence—i Model 2: Kc and Ka for Acids. When an acid HA is placed in water, hydroniurn ions are produced according to the reaction HA(aq) + H20 3 H3o+(aq) + Ajaq) The equilibrium constant, KC , for this type of reaction is: [H3 0+] [A‘] Kc = W (i) Most solutions are sufficiently dilute that the concentration of water is the same before and after reaction with the acid. The concentration of the water is incorporated into the value of Kc and the equilibrium expression is given a special name and symbolw the acid-dissociation constant, Ka. ChemActivity 43 Strong and Weak Acids 251 [H20] z ggggf : 55 M (2) [H300 {Ad Kc : W ‘3) + A ch55=Ka=LW (4) Table 1. The names and formulas of acids commonly encountered in general chemistry courses and the values of K6 and Ka for each. Acid Name Molecular K0 K3 Formula hydroiodic acid HI 5 x 107 3 x 109 hydrobromic acid HBr 2 x 107 1 $< 109 ‘33 hydrochloric acid HCi 2 x 104 I x 106 10* perchloric acid HOCiO3 2 x 105 1 x 108 k3 sulfuric acid H2804 2 x 101 1 x 103 hydroniurn ion H3 0+ 1 55 nitric acid HNO3 z 0.5 28 acetic acid CH3COOH 3.2 x 104 1.75 x 10*5 carbonic acid H2C03 8.2 x 10’9 4.5 x 10*7 b hydrofluoric acid HF 1,3 x 10*5 7 .2 x 104 \0 hydrosulfuric acid HES 1.8 x 10‘9 1.0 x 10“7 3 nitrous acid HNOZ 9.2 x 10’6 5.1 x 104 phosphoric acid H3PO4 1.3 x 10‘4 7.1 x 10—3 water H20 3.3 x 10—13 1.8 X 10—16 The relative strength of an acid is determined by the relative H30+ concentration produced at equilibrium for a given molarity of the acid. For example, if a 0.5 M solution of HA has [H3O+] = I x 10%4 M and a 0.5 M solution of HX has [H3O+] = 1 x 10’3 M, then HX is a stronger acid than HA (even though both are considered to be weak acids). Critical Thinking Questions an 3. The Ka for acetic acid, given in Tabie 1, is 1.75 x 10*5 . Show that the vaiue given or c o acetic a01 , glven 1n a e , 13 correct. f K f ' 'd ' ' T bi 1 ' “$4. In a solution of nitrous acid: {HN02] = 1.33 M; [H30+} = 0.026 M; [N02—} 2 0.026 M. Show that Ka is correct in Table 1. 252 ChemActivity 43 Strong and Weak Acids 5. Nitric acid and the acids above it in Table l are considered to be strong acids. Explain why. ' 6. Acetic acid and the acids below it in Table l are considered to be weak acids. Explain why. 7. ,Of the weak acids in Table l (ignore H20), which one will produce the highest {H30+] in a solution of a given molarity of acid? Explain your reasoning. (You should answer this question Without doing extensive equilibrium calculations.) 8. Of the weak acids in Table I (ignore H20), which one will produce the lowest [H3O+] in a solution of a given molarity of acid? Explain your reasoning. (You should answer this question without doing extensive equilibrium calculations.) 9. Are all of the strong acids of equal strength? If not, what is the strongest strong acid? The weakest strong acid? Exercises 1. What are the names and the chemical formulas for the conjugate bases of the acids listed in Table l? @ Rank the weak acids in Table 1 in order from strongest to weakest. 3. Write the balanced chemical equation for the reaction of HF with water. What is the expression for K3? Choose three additional acids from Table 1 and provide the chemical reaction with water and the expression for K3. Q Indicate whether the following statement is true or false and explain your reasoning: A 0.25 M solution of acetic acid has a higher {H30+] than does a 0.25 M solution of nitrous acid. i 255 ME 4d sols Acid/ass Dissociation (How Much Acid or Base Reacts?) Model 1: A Weak Acid Increases the Hydronium Concentration of a Solution, but the Amount of Dissociation is Small. H20 moiecules H20 molecuies are not shown are not shown A solution of A solution of a strong acid, HCi a weak acid, HA Table 1. Characteristics of solutions of various amounts of hypochlorous acid, HOCI, dissolved in water to make 1.00 L of solution at 25°C. Moles of HOCl added [H3041 [OI—1”] (M) (M) 0.00 l.0 ><10—7 1.0 x 10—7 0.30 9.3 x 10‘5 1.1 x 10—10 0.75 1.5 >< 10“4 6.8 x 10—11 1.00 1.7 x10—4 5.9 x 10—11 ____#________~——.———-——-—— Critical Thinking Questions 1. Complete and balance the following reaction: HOCl(aq) + H2009) 2 Write the equilibrium expression for the Ka of HOCI. ChemActivity 44 Weak Acid/Base Dissociation 257 2. Complete the following table assuming that 0.30 moles of HOCl are added to sufficient water to make 1.0 L of solution at 25 "C: — Heel _— —_ 1.0 3. The KB of HOCI, at 25 °C, is 2.9 x 10‘3. Find "x" (CTQ 2), and enter the equilibrium concentration values in the last row of the following table. ‘ initial moles . change in moles e uilibriutn moles equilibrlum cone value Verify that your equilibrium concentrations are correct. .2 4. Add the missing values for [H001] and [OCl‘] to Table la. Table 1a. Characteristics of solutions of varous amounts of hypochlorous acid, HOCI, dissolved in water to make 1.00 L of solution at 25 °c. Moles of [H3041 [OH—jg [H001] [OCH HOCl added (M) A (M) (M) (M) 0.00 1.0 x 107 1.0 x 10—7 0 0 0.30 9.3 x 10—5 1.1 x 10“10 0.75 1.5 x 104 6.8 x 1011 0.75 1.5 x 10—4 1.00 1.7 x104 5.9 x10‘11 1.0 1.7 ><104L M 258 ChemActivity 44 Weak Acidease Dissociation 5. Explain why the number of moles of HOCI added is equal to the number of moles of HOCi at equilibrium in spite of the fact that some of the HOCI reacts! 6. Explain why the equilibrium concentration of £130“ is equal to the equilibrium concentration of OCl‘. Model 2: A Weak Base increases the Hydroxide Concentration of a Solution, but the Amount of Reaction ls Small. 3-) Na+ B molecule 09 OH— BH+ i011 H20 molecules are not shown H20 molecules are not shown A solution of a A solution of a strong base, NaOH weak base, B When a base B is placed in water, hydroxide ions are produced according to the reaction B(aq) + H20 :1 BH+(aq) + OH“(aq) The equilibrium constant, Kc , for this type of reaction is: K _ [BH+] [one] C # [B] [H20] that the concentration of water is the same Most solutions are sufficiently dilute before and after reaction with the base. The concentration of the water is incorporated into the value of K; and the equilibrium expression is given a special name and symbol— the base-dissociation constant, Kb. IBH+| ]OH—| KC : [B] [55] B + e Kc x55 = Kb :- Hmofl ChemActivity 44 Weak Acidease Dissociation 259 Table 2. Characteristics of solutions of various amounts of the base pyridine, CSHSNG) dissolved in water to make 1.00 L of solution at 25 °C. Moles of C5H5N added [H3041 {OPP} (M) (M) 0.00 1.0 x 10—7 1.0 KID—7 0.30 4.4 x 10—10 2.3 x 10—5 0.75 2.8 x 10—10 3.6 x 10os 1-00 2.4 x 10—10 4.1 x 10'5 Critical Thinking Questions 7. How was the species BH+, , produced in Model 2? E 8. Determine the value of KW from the data given in Table 2. Does this value agree with your value calculated from Table 2 of ChemActiVity 43: Strong and Weak Acids? 9. Complete and balance the following reaction for the weak base pyridine: C5H5N(aq) + H20 2 Write the equilibrium expression for the [(5 of C5H5N. . 10. Complete the following table assuming that 0.30 moles of C5H5N are added to sufficient water to make 1.0 L of solution at 25 °C: — “n — _ 260 ChemActivityfii-i Weak AcidlBase Dissociation ii. The Kb of C5H5N, at 25 °C, is 1.7 x 10-9. Find "x" (CTQ 10), and enter the equilibrium concentration vaiues in the last row of the following table. _ equilibrium conc - equilibrium cone value Verify that your equilibrium concentrations are correct. 12. Add the missing values for [C5H5N} and [C5H5NH+] to Table 2a. Table 2a. Characteristics of solutions of various amounts of the base pyridine, CSHSNU), dissoived in water to make 1.00 L of solution at 25 °C. W M0168 0f 3130+] [one] [€5H5N] [CsHsNflti C5H5N added (M) (M) (M) (M) 0.00 1.0 x 1077 1.0 x roe7 0 0 0.30 4.4 x 10—10 2.3 x 10—5 0.75 2.8 x 1040 3.6 x ioefi 0.75 3.6 X 1&5 1.00 2.4 x 10-10 4.1 x 10—5 1.00 4.1 x 10-5 13. Explain why the moles of C5H5N added are equal to the moles of €5H5N at equilibrium in spite of the fact that some of the C5H5N reacts! . 14. Explain Why the equilibrium concentration of OH‘ is equai to the equilibrium concentration of C5H5NH+. 262 ChemActivity 44 Weak AcidlBase Dissociation 17. A 0.50 M solution of CH3C00H has {CH3COO—] = 3.0 x 10"3 M. What are the values of: (CH3000H)0 : [CH3COOH] = [H3040 = Ka Ii 18. A 0.50 M solution ofhydrozoic acid contains 3.1 x 10—3 M $130+. W " 0 ca cu a e to calculate the value of K3 for hydrozeic acid. Use—mam WWW-aid? usedQ—E-xpi-am— Exercises 1. The hydronium ion concentration of a 0.30 M solution of a weak acid is 5.7 x 10—4 M. What is the value of Ka for this acid? 2. The hydroxide ion concentration of a 0.200 M solution of a weak acid is 7.0 x 10—10 M. What is the value of Ka for this acid? ...
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