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)

P-Function simplifies a large range of numbers: graphically [H 3 O+], M pH Note that on a p-scale, the smaller the p-number, the larger the actual number converts to a simpler scale

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

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

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

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

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

Let’s do some problems !!

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

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?

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-.

K w = 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

KwKw If AH has a larger K a, like then A- must have a smaller K b like 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 =

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

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

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