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a new chapter: 17 Acids & Bases The basics: it’s just equilibrium

a new chapter: 17 Acids & Bases The basics: it’s just equilibrium same K expressions, with new names: K a , K b same equilibrium calculations What’s new: the concept of K w p -functions (pH, pK a , pK w ) pH scale. What do you recall about Acid /Base Chemistry?.

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a new chapter: 17 Acids & Bases The basics: it’s just equilibrium

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  1. a new chapter: 17 Acids & Bases • The basics: • it’s just equilibrium • same K expressions, with new names: Ka, Kb • same equilibrium calculations • What’s new: • the concept of Kw • p-functions (pH, pKa, pKw) • pH scale

  2. What do you recall about Acid /Base Chemistry?

  3. Acid /Base Chemistry in the Garden Soil Acidity determines Flower color Plant pigments: anthocyanin Anthocyanin, red form Acid form: AH Anthocyanin, blue form Base form, A-

  4. The chemist-gardener: how to make your hydrangeas Pink or Blue Pink form = protonatedanthocyanin, acid form AH Need acid: gardeners use ferrous sulfate Reaction:

  5. The chemist-gardener: how to make your hydrangeas Pink or Blue Blue form = deprotonatedanthocyanin, base form A- Need base: gardeners use lime Reaction:

  6. The chemist-gardener: how to choose reagents for your garden Think about it: why use ferrous sulfate? why not use HCl? Reaction differences:

  7. Equilibrium Constants in Acid Base Chemistry Ka: acid dissociation constant describes how much H+ (H3O+) formed General Acid Reaction:

  8. Equilibrium Constants in Acid Base Chemistry Kb: base dissociation constant describes how much OH- formed

  9. Equilibrium Constants in Acid Base Chemistry The magnitude of Keq( so either Ka or Kb) determines the strength of acid or base HCl is a strong acid: so Ka is very large, Ka = 108, what is [H3O+]? Ferrous ion, Fe(2+) is a weak acid: so Ka is small, Ka = 10-10. Now what is [H3O+]?

  10. Takes Two to Tango! Acid Base Conjugate Species AH + H2O A- + H3O+ acid base conjugate base conjugate acid

  11. Takes Two to Tango! Acid Base Conjugate Species B + H2O BH++ HO- base acid conjugate acid conjugate base

  12. Takes Two to Tango! Acid Base Conjugate Species B + AH BH++ A- base acid conjugate acid conjugate base

  13. Dual Personality of Water: both an acid and a base Amphoteric (or amphiprotic) The Autoionization reaction of water. 2 H20 H3O+ + OH- Keq =

  14. Because Kw = [H3O+][OH-] = 10-14 and Kw is a constant: then When [H3O+] = [OH-] = 10-7 M, the solution is neutral [H3O+] [OH-] Kw

  15. When [H3O+] > [OH-], [H3O+] > 10-7 M, the solution is acidic [H3O+] > 10-7 M [OH-] < 10-7 M Kw

  16. When [OH-] > [H3O+], [OH-] > 10-7 M, the solution is basic [H3O+] < 10-7 M [OH-] > 10-7 M Kw Because Kw = [H3O+] x [OH-] must be 10-14

  17. Because all these exponential numbers are a pain: i.e. Kw = [H3O+][OH-] = 10-14 [H3O+] = [OH-] = 10-7 M (in neutral water) Ka = 10-10 we will use: The P-Function p of (a number) = -log10(a number) so “pH” means p of [H3O+], or -log10[H3O+] so the pH of neutral water, where [H3O+] = 10-7 M is pH 7

  18. The P-Function and K’s p of (Keq) = -log10Keq = pKeq p of (Ka) = -log10Ka = pKa p of (Kw) = -log10Kw = pKw Examples: Keq = 10-10 the pKeq = 10 Ka = 1.20 x 10-5 the pKeq = 4.92 Kw = 10-14 the pKw = 14

  19. The P-Function simplifies exponential numbers Ka = 1.20 x 10-5 , pKeq = 4.92 [H3O+] = 4.20x 10-5M, pH = 4.376 [OH-] = 6.66x 10-6M, pOH = 5.176 (note number of sig figs) Examples converting from p-function: if pH = 7.47, [H3O+] = 10-7.47 M= 3.39 x 10-8M, if pKa = 4.92, Ka = 10-4.92 = 1.20 x 10-5

  20. P-Function simplifies a large range of numbers: graphically 10 1 10-110-310-5 10-7 10-9 10-11 10-13 10-14 [H3O+], M converts to a simpler scale -1 0 1 3 5 7 9 11 13 pH Note that on a p-scale, the smaller the p-number, the larger the actual number

  21. Working in P-Functions can simplify problems Recall Kw = [H3O+][OH-] = 10-14 Apply the P-function to each side p of Kw = p of [H3O+][OH-] = p of 10-14 -log Kw = -log ( [H3O+][OH-] )= -log 10-14 -log Kw = -log [H3O+] + ( -log [OH-] ) = -log 10-14 pKw = pH + pOH = 14

  22. Now apply this equation: pKw = pH + pOH = 14 to this picture [H3O+] [OH-] Kw= 10-14

  23. Now apply this equation: pKw = pH + pOH = 14 to this picture pH pOH pKw= 14

  24. When the solution is acidic [H3O+] > 10-7 M, pH < 7 : pH is small pH < 7 pOH > 7 pKw Because pKw = pH + pOHmust be 14

  25. Fill in the blanks! pOH is _______ pKw pH is _______ When the solution is ____________ [H3O+] __10-7 M, pH ___ 7 : pH is ________

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