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Acids & Bases

This warm-up activity introduces the concepts of Arrhenius and Bronsted-Lowry acids and bases, Lewis definitions, and conjugate pairs. It also covers the strength of acids and bases, including strong and weak acids, concentrated vs. strong, and calculating pH of strong acid solutions.

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Acids & Bases

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  1. Acids & Bases

  2. Warm Up • Give you 5 minutes to discuss with your peers before checking your answers.

  3. Conceptual-Use it until you can’t! • Arrhenius Acid - anything that can give off a H+ ion. • Arrhenius Base - anything that can give off an OH- ion.

  4. Conceptual • Bronsted-Lowry Acid (1923) - anything that will donate a proton (H+) • Bronsted-Lowry Base - anything that will accept a proton

  5. ACID-BASE THEORIES Lewis Definition • Acid – accepts an electron pair • Base – donates an electron pair • This terminology is not as widely used as it used to be because it is too general. Instead, we focus on coordination complexes, which are products of Lewis Acid-Base reactions.

  6. Conjugate Pairs • Acids LOSE an H+ and have conjugate bases • Bases GAIN an H+ and have conjugate acids

  7. Conjugate Pairs • HC2H3O2 + H2O  • CH3CH3NH2 + H2O 

  8. Bronsted-Lowry • NH3 + H2O ==> NH4+ + OH- base acid conjugate conjugate acid base Know how to label!

  9. Warm Up-Have article on desk For questions 1 and 2 identify the acid, base, conjugate acid, and conjugate base. 1. OH- + HPO42- → H2O + PO43- 2. HF + NH3 → NH4+ + F- For questions 3-7 Name the acid or base. • H3PO4 6. HNO2 • HBr 7. LiOH • Mg(OH)2 • Given a pH of 9, find the [H+], pOH, and [OH-]. Is the solution acidic or basic? (show all work) • If [H+] = 2.5 x 10-5, find the pH, pOH, and [OH-]. Is the solution acidic or basic? (show all work)

  10. Motivation • https://www.youtube.com/watch?v=PZeAcWhCrls

  11. CONCENTRATED VS. STRONG • “Concentrated” – refers to the amount dissolved in solution. • “Strong” – refers to the fraction of molecules that ionize.   • For example, if you put a lot of ammonia into a little water, you will create a highly concentrated solution. However, since only 0.5% of ammonia molecules ionize in water, this basic solution will not be very strong.

  12. ACID STRENGTH • A strong acid completely ionizes in solution • A weak acid only partially ionizes. • The strength of an acid depends on the equilibrium: HA(aq) + H2O(l) ⇄ H3O+(aq) + A– (aq) If the equilibrium lies far to the left, the acid is weak (only a small percentage of the acid molecules ionize) If the equilibrium lies far to the right, the acid is strong (completely ionizes)

  13. Strong Acids • Ionize(dissociate) completely • HCl, HBr, HI, HClO3, HClO4, HNO3, H2SO4 • Only do it one at a time.

  14. Strong Bases • Ionize (dissociate) completely • Alkali metal hydroxides and heavy alkaline earth hydroxide. (Ca, Sr & Ba) • Facts: 1) No rule about breaking apart 2) Why can we eat some strong bases?

  15. Bronsted-Lowry • NH3 + H2O ==> NH4+ + OH- base acid conjugate conjugate acid base Understand: It is a competition between bases for the proton H+

  16. Finish the statement! • NH3 + H2O ==> NH4+ + OH- base acid conjugate conjugate acid base The more readily a substance gives up a proton…….

  17. Finish the statement! • NH3 + H2O ==> NH4+ + OH- base acid conjugate conjugate acid base The more readily a substance accepts a proton…..

  18. Finish the statement! • HCl + H2O ==> Cl- + H3O+ acid base conjugate conjugate base acid Strong acids have _______conjugate bases.

  19. Finish the statement! • NH3 + H2O ==> NH4+ + OH- base acid conjugate conjugate acid base Weak acids always have _________ conjugate bases.

  20. Conjugates and Strength

  21. Finish the statement! • Water can act as an acid or a base. Water is called __________.

  22. Warm Up • Define concentrated vs. strong • Clarification: Why is HI stronger then HCl? • Which statement is correct? Why? • HClO2 is stronger then HClO3 • HI is a weaker acid then HCl • CH3COOH is a stronger acid then CH2BrCOOH • HNO3 is stronger then HNO2

  23. STRONG ACIDS Single arrow indicates complete ionization. • An HCl solution contains virtually no intact HCl. • The HCl has essentially all ionized to form H3O+(aq) and Cl–(aq). HCl(aq) + H2O(l) → H3O+(aq) + Cl– (aq)

  24. Calculating pH of Strong Acid Solutions • Because a strong acid completely ionizes, or dissociates ions, [HA] = [H+] for the first hydrogen ion released by the acid. • Monoprotic acids only release one hydrogen ion. • Polyprotic acids (di- and tri-protic) completely dissociate from their first hydrogen ion, but the second hydrogen ion is much less likely to dissociate.

  25. Finding the pH of a Strong Acid • Strong acids • Ionize completely • HA  H+ + A- • Do it only one at a time • So?? • The concentration of the strong acid is EXACTLY the same as the [H+]

  26. Finding the pH of a Strong Acid • Example – Find the pH of 1.0 M HCl. • What contributes to H+? • HCl is a strong acid • pH = - log [H+] = - log [1.0] = 0.00

  27. Finding the pH of a Strong Acid • Example – Find the pH of 0.10 M HNO3. • What contributes to H+? • HNO3 is a strong acid • [HNO3] = 0.10 M  [H+] = 0.10 M • pH = - log [H+] = - log [0.10] = 1.00

  28. Finding the pH of a Strong Acid • Sometimes an acid is so dilute that water is actually better • Example – Find the pH of 1.0 x 10-10 M HBr. • What contributes to H+? • HBr is an acid, water could be an acid • [HBr] = 1.0 x 10-10 M  [H+] = 1.0 x 10-10 M • [H+] in water = 1.0 x 10-7 M • So WATER is actually a better acid • pH = - log [H+] = - log [1.0 x 10-7] = 7.00

  29. Strong Bases • Strong bases (soluble in water): NaOH, KOH, LiOH, RbOH, CsOH • Strong bases (not very soluble in water): Ca(OH)2, Ba(OH)2, Sr(OH)2 • Strong bases are OH- combined with group I or II cations. (NEED TO KNOW!!) • When a strong base completely ionizes, or dissociates ions, [MOH] = [OH-] for the hydroxide ion released by the base. (Note: M is for metal)

  30. Work the same way =] • Strength of bases • Strong = group 1 hydroxide group 2 (Ca and below) hydroxide • Weak  based on Kb • Bigger the Kb, the better the base • Some bases may not be on there because they are conjugates of weak acids! • Remember Ka x Kb = 1 x 10-14

  31. Work the same way =] • Strong Base Find the pH of 5.0 x 10-2 M NaOH. 2 Ways [H+] = 1.0 x 10-14 -log [5.0 x 10-2] 5.0 x 10-2 pOH = 1.30 -log [2.0 x 10-13] pH = 14-1.30 pH = 12.70

  32. Remember! • There’s no one at a time rule for bases! • pH of 3.0 M Ca(OH)2? • [OH-] = 6.0 M • pOH = - 0.78 • pH = 14.78

  33. WEAK ACIDS Double arrow indicates partial ionization. • An HF solution contains a large number of intact (or un-ionized) HF molecules. • It also contains some H3O+(aq) and F –(aq). HF(aq) + H2O(l) ⇄ H3O+(aq) + F – (aq)

  34. ACID IONIZATION CONSTANT (Ka) • A way to quantify the relative strength of a weak acid • The equilibrium constant of the weak acid HA(aq) ⇄ H+(aq) + A–(aq)

  35. Ka Hydrocyanic acid (HCN) Ka = 4.9 x 10-10 Nitrous acid (HNO2) Ka = 4.6 x 10-2 Acetic acid (HC2H3O2) Ka = 1.8 x 10-5 Hydrofluoric acid (HF) Ka = 3.5 x 10-4 Chlorous acid (HClO2) Ka = 1.1 x 10-2

  36. Practice…. If 0.100 mol of HF is diluted with distilled water to a volume of 500 mL, what is the pH of the solution? Ka = 6.6x10-4

  37. Work….. • Write the reaction • HF + H2O ↔ F- + H3O+ • Write your equilibrium expression(should we include water in our equilibrium expression?) • Set up your ICE box

  38. Write your Ka expression and solve for x

  39. BASE IONIZATION CONSTANT • The equilibrium expression for a base is known as Kb. • This expression is used to quantify the relative strength of a weak base. B + H2O → HB+ + OH- Kb = [HB+] [OH-] [B]

  40. What is the pH of a 0.100 M solution of ammonia? Kb = 1.77 x 10-5

  41. Something better? • H2O (l) + H2O (l) <==> H3O+(aq) + OH-(aq) Auto-ionization of water=one molecule donates an electron pair while another accepts it. Kw =

  42. Something better? • H2O(l) + H2O(l) <==> H3O+(aq) + OH-(aq) Kw = 1 x 10-14 at 25⁰C Kw = 1 x 10-12 at 95⁰C Temperature affects K – so only a scale of 14 at standard lab conditions

  43. What about the [ ] of H+’s and OH-’s • Developed by a brewmaster(Sorenson) to test acidity of beer. • pH scale pH = -log[ H+ ] • pOH scale pOH = -log [OH-]

  44. What about the [ ] of H+’s and OH-’s • Problem. What is the pH and pOH of water at 25⁰C? • H2O(l) + H2O(l) <==> H3O+(aq) + OH-(aq) Kw = [H+] [OH-] 1 x 10-14 = [x][x] 1 x 10-7 = x pH = -log1 x 10-7 pOH = -log 1 x 10-7 pH = 7 pOH = 7

  45. Acidic/Basic/Neutral • Can look at pH • Less than 7 = ACIDIC • More than 7 = BASIC • Neutral = 7.00 @ 25⁰C

  46. Acidic/Basic Neutral • Can also compare [H+] and [OH-] • If [H+] > [OH-] then it is ACIDIC • If [H+] < [OH-] then it is BASIC • Neutral is [H+] = [OH-]  at ANY temp!!

  47. Problems? • 1) What is the pH if the [H+] = 4.5 x 10-6?

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