Unit 13: Acids & Bases Chemistry Chapter 19 Welcome To The GowerHour I. Bronsted-Lowry Acids and Bases: A. Acid: Substance that _________ donates a ___________ H+, proton to another substance. accepts a __________ H+, proton from another B. Base: Substance that ________ substance. H+ is transferred from the ____ acid to the C. Acid – Base reaction: ___ _____. base Ex. HX + Y– ↔ HY + X– Acid Base Acid Base D. Conjugate acid – base pair: The pair of acid and base that differ by a ___. H+ Ex. ACID BASE – HF F Conjugate Acid add H+ HNO2 NO2– Conjugate Base subtract H+ HC2H3O2 C2H3O2– Memorize: NH3 = Base Memorize: NH4+ = Acid E. Amphoteric substance: Substance that can either _______ donate or Generally: H and _______ accept a hydrogen ion. – H+ + H+ Ex. H2O OH– ↔ H2O ↔ H3O+ ★Memorize ★ hydroxide ★ hydronium + + +H –H – H+ + H+ 2– – SO4 ↔ HSO4 ↔ H2SO4 CO32– ↔ HCO3– ↔ H2CO3 F. Example: Label the acids and bases, draw lines to connect the conjugate pairs. (1) NH3 + H2O NH4+ + OHBase Acid (CA) (CB) (2) HF + H2O F- + H3O+ Acid Base (CB) (CA) G. Categorize each of the following as an Acid, Base, or Amphoteric and below each, write the conjugate acid or base. H2SO4 H2PO4NH3 NO2HSO4Acid Ampho Base Base Ampho HSO4– H3PO4 NH4+ HNO2 H2SO4 HPO42– SO42– II. Water Dissociation Constant (Ion product) A. The acidic / basic properties of _________ aqueous solutions are dependent upon the ____________ equilibrium that involves the solvent, _______. water 1. Reaction: H2O (l) ↔ H+ (aq) + OH– (aq) Acid 2. Equilibrium expression: Base Keq = [H+][OH–] = Kw 1.0 x 10−14 @ 25 C. ___________ Reactants are favored. 3. Kw = ___________ H20 very little conc. of ions B. In pure water: H2O (l) ↔ H+ (aq) + OH– (aq) +][OH–] ICE Kw = 1.0 x 10−14 = [H x x 2 −14 x = 1.0 x 10 M x = 1.0 x 10−7 M [H+] = x = 1.0 x 10−7 M [OH-] = x = 1.0 x 10−7 M Kw = 1.0 x 10−14 = (1.0 x 10−7 M)(1.0 x 10−7 M) C. In impure water (contains an acidic or basic substance): Kw = 1.0 x 10−14 = [H+][OH–] As [H+] [OH–] If [H+] > 1.0 x 10-7 M, solution is _______. acidic basic or alkaline If [H+] < 1.0 x 10-7 M, solution is _________________. If [H+] = 1.0 x 10-7 M, solution is _________. neutral 1.0 x 10−7 = pH 7 pH < 7 = acid pH > 7 = base pH = 7 = neutral III. pH and pOH A. Because [H+] and [OH-] are generally very small numbers, a _____ log based system of measuring acidity is used. pH = – log [H+] pOH = – log [OH–] p H Practice: What is the pH of a solution with: [H+] = 10-5 M? 5 [H+] = 10-11 M? 11 unitless because we cannot take the 1. pH and pOH are _________, logarithm of a unit. 2. For each pH change of 1, the [H+] changes by a factor of ____. 10 Richter scale Ex. pH of 1 is 100 x’s more concentrated than pH of 3. (102 = 100) 104 = 10,000 pH = 1 pH = 5 3. [H+][OH-] = 1.0 x 10-14 Or pH + pOH = 14 B. Example calculations 1. Calculate the pH of the following solutions, and indicate if the solution is acidic or basic: (a) [H+] = 1.0 x 10-11 M pH = 11 basic or alkaline (b) [H+] = 2.11 x 10-2 M pH = – log [H+] pH = – log (2.11 x 10-2 ) (c) [OH-] = 3.98 x 10-7 M pOH = – log (3.98 x 10-7 ) pOH = 6.40 pH = 7.60 pH = 1.68 basic acidic 2. Calculate the [H+] and [OH-] of the following and indicate if they 10x = antilog (opposite of log) are acidic or basic: 10– 9.35 = [H+] 10x Key (a) pH = 9.35 pH = – log [H+] basic 9.35 = – log [H+] [H+] = 4.47 x 10– 10 M – 9.35 = log [H+] [H+][OH-] = 1.0 x 10-14 Divide both side by log [OH–] = 2.24 x 10– 5 M (b) pH = 1.10 [H+] = 7.94 x 10– 2 M acidic [OH–] = 1.26 x 10– 13 M (c) pOH = 2.98 [OH–] = 1.05 x 10– 3 M [H+] = 9.55 x 10– 12 M C. pH Scale: NaOH Lye N e u t r a l H2SO4 Alkaline/Basic Acidic 0 1 2 3 4 5 Acid Rain 6 7 High quality H2O 8 9 10 11 12 13 14 IV. Properties of Acids and Bases A. Properties of Acids Sour Vinegar 1. _________ taste (ex. _________) H2 (ex.___________) Magnesium 2. Reacts with some metals to form ___ Conducts electrolytes in solution) 3. __________electricity (__________ 4. React with _________ bases B. Properties of Bases Bitter Soap 1. _______taste (ex.________) Slippery Soap, NaOH 2. _____________ (ex._____________) Conducts electrolytes in solution) 3. ______________electricity (___________ V. Strong vs. Weak A. Strong Acids: Completely ______ ionize in solution (Not _________reaction). reversible Ex. HCl H2O + Cl(aq) H+(aq) (1) HCl (H3O+ = Hydronium ion) H3O+ + Cl (2) HCl + H2O Chemists use H+ & H3O+ interchangeably. H+ is often used for simplicity, but H3O+ more closely represents reality. *****MEMORIZE***** HCl H2SO4 hydrochloric sulfuric HNO3 nitric HBr hydrobromic HClO4 perchloric HI hydroiodic ionize in solution. (Group I and B. Strong Bases: Completely _______ II hydroxides) Ex. NaOH H2O H2O *****Memorize***** LiOH Ca(OH)2 + OH Na+ + OH NaOH + H2O Practice: Ca(OH)2 Na+ + H2O Ca2+ + 2 OH NaOH KOH Ba(OH)2 Sr(OH)2 C. Weak Acids: Ionize __________ partially (_________reaction). reversible Generally less than _____ 1 % of the molecules ionize. Ex. H2CO3 (1) H2CO3 H2O (2) H2CO3 + H2O H+ + HCO3 H3O+ + HCO3 Example of weak acids: HC2H3O2, H2CO3 If it is not a strong acid then it is a weak acid!! D. Weak Bases: Ionize __________ partially (_________reaction). reversible Partial dissociation of an weak base in water: NH4+ Ex. NH3 (1) NH3 + H2O Example of weak bases: NH3, NH2CH2CH3 (aminoethane) If it is not a strong base then it is a weak base!! + OH VI. Calculations with weak acids and weak bases. A. Weak Acids (Ka): As Ka decreases, less ____ H+ is formed in solution, so acid is _______. weaker HA (aq) H2O ↔ H+(aq) + A(aq) [H ][A - ] K eq K a [HA] Ka = acid-dissociation constant OH is formed in B. Weak Bases (Kb): As Kb decreases, less _____ weaker solution, so base is _______. B (aq) + H2O(l) ↔ BH+(aq)+ OH(aq) Kb = base-dissociation constant [BH ][OH - ] K eq K b [B] C. Examples 1. Example: Aspirin is a weak organic acid whose molecular formula is HC9H7O4. An aqueous solution of aspirin is prepared by dissolving 3.60 g/ L. The pH of this solution is found to be 2.60. Calculate Ka for aspirin. HC9H7O4 ↔ H+ + C9H7O4Initial (M) 2.00 x 10-2 0 0 Change (M) Equilibrium (M) 3.60 g L x 1 mol 180.15 g –x +x +x 0.0175 2.51 x 10-3 2.51 x 10-3 = 2.00 x 10-2 M = I (2.51 x 103 ) 2 Ka 3.61 x 10-4 (0.0175) pH = 2.60 [H+] = 2.51 x 10-3 - [H ][C9 H 7 O 4 ] Ka [HC9 H 7 O 4 ] 3. Compare the pH of 0.100 M HCl and 0.100 M HCN (Ka = 4.90 x 10-10) HCN ↔ H+ + CN HCl H+ + Cl [H+] = 0.100 M I 0.100 0 0 pH = 1.00 C -x E 0.100 - x +x +x x x 2 2 x [H ][CN - ] x 4.90 x 10-10 Ka 4.90 x 10-10 0.100 (0.100 - x) [HCN] Assume x is small x = 7.0 x 10–6 M pH = 5.15 VII. Multiple Equilibria: When two reactions are added to give a third (net reaction) the equilibrium constants are multiplied K1 • K2 = Knet (______________). Ex. HC2H3O2 Ka = 1.8 x 10 – 5 ↔ H+ + C2H3O2 C2H3O2- + H2O ↔ HC2H3O2 + OH H2O ↔ H+ + OH– ] [H+] [C2H3O2] [HC H O ] [OH Kw = _______________ ___________ 2 3 2 • [HC2H3O2] [C2H3O2] Kw = [H+][OH] Equation: Ka • Kb = K w Kb = 5.6 x 10 – 10 Kw = 1.0 x 10−14 A. As Ka increases, Kb ________ decrease (_______ inverse relationship). weaker its conjugate base. B. The stronger the acid, the ________ Ex. If Ka = 1.0 x 1030, determine Kb. Kb = 1.0 x 10 – 44 Ka = strong acid; Kb = weak base VIII. Polyprotic Acids: Certain weak acids contain more than one ionizable ___. H+ These acids dissociate in multiple steps. 2 hydrogen ions. A. Diprotic acid: Dissociates to form ___ H2O Ka1 = 4.2 x 10 – 7 H+ + HCO3 Ex. H2CO3 ↔ H2O Ka2 = 4.8 x 10 – 11 HCO3 H+ + CO32↔ 3 hydrogen ions. B. Triprotic acid: Dissociates to form ___ H2O H+ + H2PO4 H+ + HPO42H+ + PO43- –3 K = 7.5 x 10 a1 H↔ 2O Ka2 = 6.2 x 10 – 8 ↔ H2O – 13 K = 4.8 x 10 a3 ↔ HCO3 , H2PO4 C. The _________ substance formed in one step (e.g. _______________) dissociates in the next step. equilibrium constant becomes D. The dissociation constant (___________________) smaller with each successive step: Ka1 > Ka2 > Ka3 E. The acids formed in successive steps become progressively ________. weaker Ex. H3PO4 H2PO4 HPO42- F. Example: Write the dissociation reactions of sulfurous acid, H2SO3. H2SO3 HSO3 H2O H+ + HSO3 H2O H+ + SO32- ↔ ↔ G. Example: Write the dissociation reactions of citric acid, H3C5H5O7. H2O ↔ H+ + H2C5H5O7 H2O H+ + HC5H5O7 2- 2- H2O H+ + C5H5O7 3- H3C5H5O7 H2C5H5O7 HC5H5O7 ↔ ↔ IX. Acid / Base Properties of salt solutions: A. A salt is an ionic ____ compound not containing ___ H+ or____. OH B. When a salt dissolves in water, the ___ ion are formed. H2O Ex. NaCl(s) Na+ + Cl H2O Ex. K2CO3(s) 2 K+ + CO32C. Cations: Weak acids or “spectator” ions? 1. Cations derived from strong ___________ bases are _________ spectator ions. (Do not react with water, therefore have no _______ effect on pH.) - pair 2. Other cations are slightly acidic _____. (Lewis acid = e______acceptor). D. Anions: Weak bases or “spectator” ions? 1. Anions derived from strong __________are ________ ions. acids spectator 2. Other anions are slightly basic ____. Ex. HCl H+ + Cl– Spectator ions: Cations: Li+ ; Na+ ; K+ ; Ca2+ ; Sr2+ ; Ba2+ Anions: NO3 ; SO42- ; ClO4 ; Cl ; Br ; I E. 1. 2. F. (All others weak acids) (All others weak bases) Example: Consider water solutions of these four salts: (a) NH4I, (b) Zn(NO3)2, (c) KClO4, (d) Na3PO4 Classify each salt solution as acidic, basic, or neutral. (Show the dissociation reactions of each.) H2O a) NH4I NH4+ + I¯ Acidic (w. acid) b) Zn(NO3)2 c) KClO4 d) Na3PO4 H2O H2O Zn2+ (w. acid) K+ (Spec) H2O 3 Na+ (Spec) (Spec) + 2 NO3¯ Acidic (Spec) + ClO4¯ Neutral + PO43- Basic (Spec) (w. base) Which will undergo hydrolysis? The salts that react w/water to change the pH! X. Acid – Base Reactions H2O A. Strong acid + Strong base --> ____ salt + ____ Example: Example: HCl(aq) + NaOH(aq) --> 2 HNO3(aq) + Ca(OH)2(aq) --> Ca(NO3)2 (aq) + 2 H2O (l) 2 H+ + 2 NO3¯ + Ca2+ + 2 OH– Net reaction: NaCl (aq) + H2O (l) Ca2+ + 2 NO3¯+ 2 H2O (l) H+ + OH– (neutralization) H2 O B. Weak acid + Strong base ______+__________ water weak base Note: Strong Base weak base (Soln. will still be basic) H2O + C2H3O2 Example: HC2H3O2 + OHH2O + F Example: HF + OH- C. Strong acid + Weak base __________ weak acid Note: Strong acid weak acid (Soln. will still be acidic) NH4+ Example: H+ + NH3 HClO Example: H+ + ClO- D. Weak acid + Weak base ↔ weak acid weak base _________+__________ Example: HC2H3O2 + NH3 ↔ NH4+ + C2H3O2 Example: HF + ClO- ↔ HClO + F XI. Titration: A process in which one reagent is added to another with which it reacts; an ________ indicator is used to determine the point at which equal _____ quantities of the two reagents have been added. neutralization A. Equivalence point: the point at which the _____________ reaction is complete. nA = nB B. End point: The point at which the ________ indicator changes color. pH’s 1. Indicators change at different ______. 2. In doing a titration, one must choose an _________ indicator where the equivalence point and the _________ end point coincide. 3. Example Indicators: Indicator Color change pH at end point orange - red 5 Methyl Orange ___________ ___________ Bromothymol Blue Phenolphthalein yellow - blue ___________ clear - pink ___________ 7 ___________ 9 ___________ C. Acid – Base Indicators: Produce a _____ color change in an acid-base reaction. 1. Example: Phenolphthalein, bromothymol blue 2. Natural indicators: purple cabbage, hydrangeas 3. Reversible reactions: (Phenolphthalein) H-In H+ + InCLEAR PINK left solution turns _____. clear Add acid, rxn shifts ____, pink Add base, rxn shifts right _____, solution turns _____. D. Titration Curves: ~ pH 7 1. Strong acid – Strong Base (Equivalence point = _______) 0.100 M HCl is titrated with 0.100 M NaOH 8.8 2. Weak Acid – Strong Base (Equivalence point = ~pH _______) 0.100 M CH3COOH is titrated with 0.100 M NaOH pH 5.3 3. Strong Acid – Weak Base (Equivalence point = ~_______) 0.100 M NH3 is titrated with 0.100 M HCl E. Calculations with titrations: 1. STOICHIOMETRY 2. At equivalence point: nA = nB (mol acid = mol base) M A VA M B VB nA nB coefficients Problems: 1. Example titration problem: 35.00 mL of 0.2500 M sodium hydroxide is titrated with 0.4375 M HCl. (a) Write the balanced chemical equation. (b) Determine the volume of HCl added at the equivalence point (i) using stoichiometry and (ii) using the titration equation. (a) HCl + NaOH NaCl + H2 O (b) (i) 35.00 ml NaOH x 10-3 L 1 ml x 0.2500 mol NaOH x 1 L NaOH 1 mol HCl 1 mol NaOH x 1 L HCl x 1 ml 0.4375 mol HCl 10-3 L = 20.00 ml HCl (b) (ii) M A VA M B VB M V VA B B nA nB MA (0.2500 M)(35.00 mL) VA VA = 20.00 mL HCl (0.4375 M) 2. Example: A 15.0 mL sample of a solution of H2SO4 with an unknown molarity is titrated with 32.4 mL of 0.145 M NaOH to the bromothymol endpoint. What is the molarity of the sulfuric acid solution? H2SO4 + 2 NaOH M A VA M B VB nA nB Na2SO4 + 2 H 2O M B VB MA VA n B (0.145 M)(32.4 mL) MA 2(15.0 mL) MA = 0.157 M H2SO4 3. A Ca(OH)2 solution was used to titrate 15.0 mL of a 0.125 M H3PO4 solution. If 12.4 mL of Ca(OH)2 are used to reach the endpoint, what is the concentration of the Ca(OH)2? 2 H3PO4 + 3 Ca(OH)2 M A VA M B VB nA nB Ca3(PO4)2 + 6 H 2O M A VA n B MB VB n A 3(0.125 M)(15.0 mL) MB 2(12.4 mL) MB = 0.227 M Ca(OH)2 IQ 2 1. Show the dissociation rxn of each of the following salts and determine if each solution is acidic, basic, or neutral. a) Ca(ClO3)2 H2O Ca2+ (Spec) a) FeCl2 H2O Fe2+ (w. acid) 2. + 2 ClO3¯ Basic (w. base) + 2 Cl¯ Acidic (spec) Predict the products and balance the following: a. 2HNO3 + Sr(OH)2 Sr(NO3)2 + 2 H2O b. HClO2 + OH– H2O + ClO2¯ IQ 1 1. Write formulas for two salts that: (a) contain CO32- and are basic: Li2CO3 CaCO3 (b) contain Li+ and are neutral: LiBr Li2SO4 2. Predict the products and balance the following. Write the net rxn for each. Ca(NO3)2 + 2 H2O a. 2HNO3 + Ca(OH)2 Ca2+ NO3H+ NO3- Ca2+ OHH2 O Net Rxn: H+ + OHb. HClO2 + LiOH H+ ClO2- Li+ OHNet Rxn: HClO2 + OH- H2O + LiClO2 Li+ ClO2H2O + ClO2- Titration of a Strong Acid w/ a Strong Base 1. 2. 3. 4. 5. 6. 7. 8. 9. Clean burette w/ NaOH; Drain into “waste beaker”. Fill burette with NaOH to 0.00 mL (hundredths)(meniscus). Record initial vol. Measure out 10.00 mL (hundredths)(meniscus) of acid using the graduated cylinder (Use the pipet). Record volume of acid; add to the Erlenmeyer flask. Add 2 drops of phenolphthalein to the acid. IMPORTANT!!!! Add a little water to Erlenmeyer flask (Rinse acid off sides). Start adding NaOH slowly; swirl flask as you add. At the endpoint (when soln. remains light pink) stop adding base and record volume of base added. (Check: rinse flask) Dispose titrated solution in sink; rinse flask and repeat steps 3-8. The final vol. of the base is your initial vol. for the next trial. Reminders 1. 2. 3. 4. 5. 6. Wear safety glasses/goggles. Only use the pipette for acid! Base in burette Clean up after 3rd trial. Must get a clean up stamp before leaving. Leave excess acid and base in appropriate beakers. Titration of a Strong Acid w/ a Strong Base HCl + NaOH MA = ? VA = 10.00 mL nA = 1 M A VA M B VB nA nB NaCl + H2O MB = 0.0915 M VB = ? mL nB = 1 M B VB MA VA Titration of Vinegar Lab 1. 2. 3. 4. 5. 6. 7. 8. Fill burette with NaOH to 0.00 mL (hundredths)(meniscus). Record initial vol. Measure out 1.00 mL (hundredths)(meniscus) of vinegar using the graduated cylinder (Use the pipet). Record volume of vinegar; add to the Erlenmeyer flask. Add 2 drops of phenolphthalein to the vinegar. IMPORTANT!!!! Add a little water to Erlenmeyer flask (Rinse acid off sides). Start adding NaOH slowly; swirl flask as you add. At the endpoint (when soln. remains light pink) stop adding base and record volume of base added. Dispose titrated solution in sink; rinse flask and repeat steps 2-7. The final vol. of the base is your initial vol. for the next trial. Reminders 1. 2. 3. 4. 5. 6. Wear safety glasses/goggles. Only use the pipette for acid! Base in burette Clean up after 3rd trial. Must get a clean up stamp before leaving. Leave excess acid and base in appropriate beakers. Vinegar Calculations 1. 2. HC2H3O2 + NaOH H2O + C2H3O2– Vinegar = Acetic acid (solute) in water (solvent). Average number of moles of acetic acid: M A L vinegar mol HC 2 H3O2 Average [ ] of acid 3. Average vol. of vinegar (i.e. 1 mL) Mass (g) of acetic acid (HC2H3O2): molar mass mol HC 2 H 3O 2 x mass of acetic acid (g) of HC 2 H 3O 2 4. % mass of acetic acid: g solute % x 100 g solution g HC 2 H 3O 2 x 100 g vinegar Density of vinegar = 1.02 g/mL Vinegar Calculations 1. 2. HC2H3O2 + NaOH H2O + NaC2H3O2 Vinegar = Acetic acid (solute) in water (solvent). Average number of moles of acetic acid: mol HC 2 H 3O 2 MA x average vol in L L vinegar Average [ ] of acid 3. Average vol. of vinegar (i.e. 1 mL) Mass (g) of acetic acid (HC2H3O2): molar mass mol HC 2 H 3O 2 x mass of acetic acid (g) of HC 2 H 3O 2 4. % mass of acetic acid: g solute % x 100 g solution g HC 2 H 3O 2 x 100 g vinegar Density of vinegar = 1.02 g/mL Neutralization Capacity of an Antacid 1. Fill burette with 0.0953 M NaOH to 0.00 mL (hundredths)(meniscus). Record initial vol. 2. Weigh ~ 0.10 g of Tums (CaCO3). Record and add to Erlenmeyer flask. 3. Add water to E. flask and swirl to dissolve powder. 4. Measure out 5.00 mL (hundredths)(meniscus) of 0.336 M HCl using the graduated cylinder (Use the pipet). Record volume of acid; add to the E. flask containing antacid and swirl. 5. Add a little water to E. flask (Rinse acid off sides). 6. Add 2 drops of phenolphthalein to the acid. IMPORTANT!!!! 7. Start titrating; swirl E. flask as you add NaOH. 8. At the endpoint (when soln. remains light pink) stop adding base and record volume of base added. 9. Dispose titrated solution in sink; rinse flask and repeat steps 2-7. 10. The final vol. of the base is your initial vol. for the next trial. Reminders 1. 2. 3. 4. 5. 6. Wear safety glasses/goggles. Only use the pipette for acid! Base in burette Clean up after 3rd trial. Must get a clean up stamp before leaving. Leave excess acid and base in appropriate beakers. Antacid Calculations Acid (HCl) (VA) Antacid (CaCO3)(VA1) NaOH (VA2) VA1 = Volume of acid neutralized by the antacid. VA2 = Volume of acid neutralized by the NaOH. VA = Total volume of acid (5 mL). VA = VA1 + VA2 Step 1: Solve for VA2 M A VA2 M B VB nA nB Step 2: Solve for VA1 VA = VA1 + VA2 Antacid Calculations Step 3: Solve for VA1/ g. VA1 g Mass of antacid used in the trial (i.e. 0.10) Step 4: Solve for VA1/ Tablet Antacid : 1 Tablet = 1.30 g VA1 1.30 g x g Tablet VA1 Tablet Step 5: Solve for VA1/ cent Antacid : 72 Tablet = $5.49 VA1 Tablet x Tablet cents VA1 cents Labs Due: Tuesday 05/15 Acid Nomenclature Review 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. Acetic acid (w) Oxalic acid (w) Hydrocyanic acid (w) Cyanic acid (w) Sulfurous acid (w) Sulfuric acid (s) Hypochlorous acid (w) Bromous acid (w) Periodic acid (w) Phosphoric acid (w) 11. Carbonic acid (w) 12. Perchloric acid (s) 13. Hypobromous acid (w) 14. Nitric acid (s) 15. Chloric acid (w) 16. Hydrofluoric acid (w) 17. Phosphorous acid (w) 18. Hydroiodic acid (s) 19. Nitrous acid (w) 20. Hydrochloric acid (s)