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Aqueous Equilibria: Acids and Bases

Aqueous Equilibria: Acids and Bases. Chemistry 4th Edition McMurry/Fay. Acid–Base Concepts 01. Arrhenius Acid: A substance which dissociates to form hydrogen ions (H + ) in solution. HA( aq )  H + ( aq ) + A – ( aq )

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Aqueous Equilibria: Acids and Bases

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  1. Aqueous Equilibria: Acids and Bases Chemistry 4th Edition McMurry/Fay

  2. Acid–Base Concepts 01 • Arrhenius Acid:A substance which dissociates to form hydrogen ions (H+) in solution. HA(aq)  H+(aq) + A–(aq) • Arrhenius Base:A substance that dissociates in, or reacts with water to form hydroxide ions (OH–). • MOH(aq)  M+(aq) + OH–(aq) Chapter 15

  3. Acid–Base Concepts 02 • Brønsted–Lowry Acid:Substance that can donate H+ • Brønsted–Lowry Base:Substance that can accept H+ • Chemical species whose formulas differ only by one proton are said to be conjugate acid–base pairs. Chapter 15

  4. Acid–Base Concepts 03 Chapter 15

  5. Acid–Base Concepts 04 Chapter 15

  6. Acid–Base Concepts 05 • A Lewis Acid is an electron-pair acceptor. These are generally cations and neutral molecules with vacant valence orbitals, such as Al3+, Cu2+, H+, BF3. • A Lewis Base is an electron-pair donor. These are generally anions and neutral molecules with available pairs of electrons, such as H2O, NH3, O2–. • The bond formed is called a coordinate bond. Chapter 15

  7. Acid–Base Concepts 06 Chapter 15

  8. Acid–Base Concepts 07 • Write balanced equations for the dissociation of each of the following Brønsted–Lowry acids. (a) H2SO4 (b) HSO4– (c) H3O+ • Identify the Lewis acid and Lewis base in each of the following reactions: • (a) SnCl4(s) + 2 Cl–(aq) æ SnCl62–(aq) • (b) Hg2+(aq) + 4 CN–(aq) æ Hg(CN)42–(aq) • (c) Co3+(aq) + 6 NH3(aq) æ Co(NH3)63+(aq) Chapter 15

  9. Dissociation of Water 01 • Water can act as an acid or as a base. H2O(l) æ H+(aq) + OH–(aq) • This is called the autoionization of water. H2O(l) + H2O(l)æ H3O+(aq) + OH–(aq) Chapter 15

  10. Dissociation of Water 02 • This equilibrium gives us the ion product constant for water. Kw = Kc = [H+][OH–] = 1.0 x 10–14 • If we know either [H+] or [OH–] then we can determine the other quantity. Chapter 15

  11. Dissociation of Water 03 • The concentration of OH– ions in a certain household ammonia cleaning solution is 0.0025 M. Calculate the concentration of H+ ions. • Calculate the concentration of OH– ions in a HCl solution whose hydrogen ion concentration is 1.3 M. Chapter 15

  12. pH – A Measure of Acidity 01 • The pH of a solution is the negative logarithm of the hydrogen ion concentration (in mol/L). pH = –log [H+] pH + pOH = 14 Acidic solutions: [H+] > 1.0 x 10–7 M, pH < 7.00Basic solutions: [H+] < 1.0 x 10–7 M, pH > 7.00Neutral solutions: [H+] = 1.0 x 10–7 M, pH = 7.00 Chapter 15

  13. pH – A Measure of Acidity 02 • Nitric acid (HNO3) is used in the production of fertilizer, dyes, drugs, and explosives. Calculate the pH of a HNO3 solution having a hydrogen ion concentration of 0.76 M. • The pH of a certain orange juice is 3.33. Calculate the H+ ion concentration. • The OH– ion concentration of a blood sample is 2.5 x 10–7 M. What is the pH of the blood? Chapter 15

  14. pH – A Measure of Acidity 03 Chapter 15

  15. Strength of Acids and Bases 01 • Strong acids and bases: are strong electrolytes that are assumed to ionize completely in water. • Weak acids and bases: are weak electrolytes that ionize only to a limited extent in water. • Solutions of weak acids and bases contain ionized and non-ionized species. Chapter 15

  16. HClO4 HI HBr HCl H2SO4 HNO3 H3O+ HSO4– Strength of Acids and Bases 02 ACIDCONJ. BASE ACIDCONJ. BASE ClO4– I– Br – Cl – HSO4 – NO3 – H2O SO42– HSO4– HF HNO2 HCOOH NH4+ HCN H2O NH3 SO42– F – NO2 – HCOO – NH3 CN – OH – NH2 – IncreasingAcid Strength Increasing Acid Strength Chapter 15

  17. Strength of Acids and Bases 03 • Stronger acid + stronger base weaker acid + weaker base • Predict the direction of the following: • HNO2(aq) + CN–(aq) æ HCN(aq) + NO2–(aq) • HF(aq) + NH3(aq) æ F–(aq) + NH4+(aq) Chapter 15

  18. Acid Ionization Constants 01 • Acid Ionization Constant: the equilibrium constant for the ionization of an acid.HA(aq) + H2O(l) æ H3O+(aq) + A–(aq) • Or simply: HA(aq) æ H+(aq) + A–(aq) Chapter 15

  19. Acid Ionization Constants 02 ACIDKaCONJ. BASE Kb HF HNO2 C9H8O4 (aspirin) HCO2H (formic) C6H8O6 (ascorbic) C6H5CO2H (benzoic) CH3CO2H (acetic) HCN C6H5OH (phenol) 7.1 x 10 –4 4.5 x 10 –4 3.0 x 10 –4 1.7 x 10 –4 8.0 x 10 –5 6.5 x 10 –5 1.8 x 10 –5 4.9 x 10 –10 1.3 x 10 –10 F– NO2 – C9H7O4 – HCO2 – C6H7O6 – C6H5CO2 – CH3CO2 – CN – C6H5O – 1.4 x 10 –11 2.2 x 10 –11 3.3 x 10 –11 5.9 x 10 –11 1.3 x 10 –10 1.5 x 10 –10 5.6 x 10 –10 2.0 x 10 –5 7.7 x 10 –5 Chapter 15

  20. Strength of Acids and Bases 03 (a) Arrange the three acids in order of increasing value of Ka. (b) Which acid, if any, is a strong acid? (c) Which solution has the highest pH, and which has the lowest? Chapter 15

  21. Acid Ionization Constants 04 • Initial Change Equilibrium Table: Determine the pH of 0.50M HA solution at 25°C. Ka = 7.1 x 10–4. - + H + A æ HA (aq) (aq) (aq) Initial ( M ) : 0.50 0.00 0.00 Change (M): – x + x + x Equilib 0.50 – x x x (M): Chapter 15

  22. Acid Ionization Constants 05 • pH of a Weak Acid (Cont’d): • Substitute new values into equilibrium expression. • If Ka is significantly (>1000 x) smaller than [HA] the expression (0.50 – x) approximates to (0.50). • The equation can now be solved for x and pH. • If Ka is not significantly smaller than [HA] the quadratic equation must be used to solve for x and pH. Chapter 15

  23. Acid Ionization Constants 06 • The Quadratic Equation: • The expression must first be rearranged to: • The values are substituted into the quadratic and solved for a positive solution to x and pH. Chapter 15

  24. Acid Ionization Constants 07 • Calculate the pH of a 0.036 M nitrous acid (HNO2) solution. • What is the pH of a 0.122 M monoprotic acid whose Ka is 5.7 x 10–4? • The pH of a 0.060 M weak monoprotic acid is 3.44. Calculate the Ka of the acid. Chapter 15

  25. Acid Ionization Constants 08 • Percent Dissociation: A measure of the strength of an acid. • Stronger acids have higher percent dissociation. • Percent dissociation of a weak acid decreases as its concentration increases. Chapter 15

  26. Base Ionization Constants 01 • Base Ionization Constant: The equilibrium constant for the ionization of a base. • The ionization of weak bases is treated in the same way as the ionization of weak acids.B(aq) + H2O(l) æ BH+(aq) + OH–(aq) • Calculations follow the same procedure as used for a weak acid but [OH–] is calculated, not [H+]. Chapter 15

  27. Base Ionization Constants 02 BASEKbCONJ. ACID Ka C2H5NH2 (ethylamine) CH3NH2 (methylamine) C8H10N4O2 (caffeine) NH3 (ammonia) C5H5N(pyridine) C6H5NH2 (aniline) NH2CONH2 (urea) 5.6 x 10 –4 4.4 x 10 –4 4.1 x 10 –4 1.8 x 10 –5 1.7 x 10 –9 3.8 x 10 –10 1.5 x 10 –14 C2H5NH3+ CH3NH3+ C8H11N4O2+ NH4+ C5H6N+ C6H5NH3+ NH2CONH3+ 1.8 x 10 –11 2.3 x 10 –11 2.4 x 10 –11 5.6 x 10 –10 5.9 x 10 –6 2.6 x 10 –5 0.67 Note that the positive charge sits on the nitrogen. Chapter 15

  28. Diprotic & Polyprotic Acids 01 • Diprotic and polyprotic acids yield more than one hydrogen ion per molecule. • One proton is lost at a time. Conjugate base of first step is acid of second step. • Ionization constants decrease as protons are removed. Chapter 15

  29. Diprotic & Polyprotic Acids 02 ACIDKaCONJ. BASE Kb H2SO4 HSO4– C2H2O4 C2HO4– H2SO3 HSO3– H2CO3 HCO3– H2S HS– H3PO4 H2PO4– HPO42– Very Large 1.3 x 10 –2 6.5 x 10 –2 6.1 x 10 –5 1.3 x 10 –2 6.3 x 10 –8 4.2 x 10 –7 4.8 x 10 –11 9.5 x 10 –8 1 x 10 –19 7.5 x 10 –3 6.2 x 10 –8 4.8 x 10 –13 HSO4 – SO4 2– C2HO4– C2O42– HSO3 – SO3 2– HCO3– CO3 2– HS– S 2– H2PO4– HPO42– PO43– Very Small 7.7 x 10 –13 1.5 x 10 –13 1.6 x 10 –10 7.7 x 10 –13 1.6 x 10 –7 2.4 x 10 –8 2.1 x 10 –4 1.1 x 10 –7 1 x 10 –5 1.3 x 10 –12 1.6 x 10 –7 2.1 x 10 –2 Chapter 15

  30. Molecular Structure and Acid Strength 01 • The strength of an acid depends on its tendency to ionize. • For general acids of the type H–X: • The stronger the bond, the weaker the acid. • The more polar the bond, the stronger the acid. • For the hydrohalic acids, bond strength plays the key role giving: HF < HCl < HBr < HI Chapter 15

  31. Molecular Structure and Acid Strength 02 • The electrostatic potential maps show all the hydrohalic acids are polar. The variation in polarity is less significant than the bond strength which decreases from 567 kJ/mol for HF to 299 kJ/mol for HI. Chapter 15

  32. Molecular Structure and Acid Strength 03 • For binary acids in the same group, H–A bond strength decreases with increasing size of A, so acidity increases. • For binary acids in the same row, H–A polarity increases with increasing electronegativity of A, so acidity increases. Chapter 15

  33. Molecular Structure and Acid Strength 04 • For oxoacids bond polarity is more important. If we consider the main element (Y):Y–O–H • If Y is an electronegative element, or in a high oxidation state, the Y–O bond will be more covalent and the O–H bond more polar and the acid stronger. Chapter 15

  34. Molecular Structure and Acid Strength 05 • For oxoacids with different central atoms that are from the same group of the periodic table and that have the same oxidation number, acid strength increases with increasing electronegativity. Chapter 15

  35. Molecular Structure and Acid Strength 06 • For oxoacids having the same central atom but different numbers of attached groups, acid strength increases with increasing central atom oxidation number. • As shown on the next slide, the number of oxygen atoms increases the positive charge on the chlorine which weakens the O–H bond and increases its polarity. Chapter 15

  36. Oxoacids of Chlorine: Molecular Structure and Acid Strength 07 Chapter 15

  37. Molecular Structure and Acid Strength 08 • Predict the relative strengths of the following groups of oxoacids: • a) HClO, HBrO, and HIO. • b) HNO3 and HNO2. • c) H3PO3 and H3PO4. Chapter 15

  38. Acid–Base Properties of Salts 01 • Salts that produce neutral solutions are those formed from strong acids and strong bases. • Salts that produce basic solutions are those formed from weak acids and strong bases. • Salts that produce acidic solutions are those formed from strong acids and weak bases. Chapter 15

  39. Acid–Base Properties of Salts 02 • Calculate the pH of a 0.15 M solution of sodium acetate (CH3COONa). What is the percent hydrolysis? • Calculate the pH of a 0.24 M sodium formate solution (HCOONa). Chapter 15

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