1. Previously in Chem104: plant pigments do acid/base chemistry it’s just equilibrium new names: K a, K b for same K expressions the concept of K w the concept of the K w circle p-functions (pH, pK a, pK w ) Today in Chem104: pH scale How K a relates to K b and pK a to pK b More ways to use the K w circle Group worksheet on The Most Important Equilibrium on the Planet (Part 1)

2. P-Function simplifies a large range of numbers: graphically 10 1 10 -1 10 -3 10 -5 10 -7 10 -9 10 -11 10 -13 10 -14 [H 3 O+], M -1 0 1 3 5 7 91113pH Note that on a p-scale, the smaller the p-number, the larger the actual number converts to a simpler scale

3. Apply the P-function to each side p of K w = p of [H 3 O+][OH-] = p of 10 -14 Working in P-Functions can simplify problems Recall K w = [H 3 O+][OH-] = 10 -14 -log K w = -log ( [H 3 O+][OH-] )= -log 10 -14 pK w = pH + pOH = 14 -log K w = -log [H 3 O+] + ( -log [OH-] ) = -log 10 -14

4. K w = 10 -14 Recall how we used this picture and this relationship: K w = [H + ] x [OH - ] = 10 -14 [H 3 O+] [OH-]

5. pK w = 14 Now apply this equation: pK w = pH + pOH = 14 to this picture pH pOH

6. pK w When the solution is acidic [H 3 O+] > 10 -7 M, pH < 7 : pH is a small number Because pK w = pH + pOH must be 14 pH < 7 pOH > 7

7. pK w When the solution is ____________ [H 3 O+] __10 -7 M, pH ___ 7 pOH is _______ pH is _______ Fill in the blanks!

8. Let’s do some problems !!

9. Example problems to be used with reaction: [Fe-OH 2 ] 2+ + H 2 O[Fe-OH] + + H 3 O+ K eq = 10 -10

10. When is the conjugate base (or acid) important in acid / base equilibria? HCl + H 2 OCl - + H 3 O+ acid conjugate base conjugate acid Here?

11. AH + H 2 OA - + H 3 O+ acid conjugate base conjugate acid Write the K a expression for AH and the K b expression for A-.

12. K w = 10 -14 Alright, now we can understand why Cl- isn’t basic: We proved K w = K a x K b Use the K w circle! KaKa KbKb

13. KwKw If AH has a larger K a, like 10 -4 then A- must have a smaller K b like 10 -10 KaKa KbKb The stronger the acid (K a large), the weaker the conjugate base, (K b small) Because K w = K a x K b must = 10 -14

14. KwKw KaKa KbKb If A- has a larger K b, like 10 -3 then AH must have a smaller K a like 10 -11 The stronger the base (K b large), the weaker the conjugate acid, (K a small) Because K w = K a x K b must = 10 -14

15. Now do the same with K w = K a x K b = 10 -14 p of K w = p of [K a x K b ] = p of 10 -14 Let’s apply P-Functions We already did this one: K w = [H 3 O+][OH-] = 10 -14  pK w = pH + pOH = 14 -log K w = -log (K a x K b )= -log 10 -14 pK w = pK a + pK b = 14 -log K w = -log K a + ( -log K b ) = -log 10 -14

16. pK w Now apply this equation: pK w = pK a + pK b = 14 to this picture pK a pK b