Previously in Chem104:
• plant pigments do acid/base chemistry
•it’s just equilibrium
• new names: Ka, Kb for same K expressions• the concept of Kw
• the concept of the Kw
circle
• p-functions (pH, pKa, pKw)
Today in Chem104:
• pH scale
•How Ka relates to Kb and pKa to pKb
•More ways to use the Kw circle
•Group worksheet on The Most Important Equilibrium on the Planet (Part 1)
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
[H3O+], M
-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
converts to a simpler scale
Apply the P-function to each side
p of Kw = p of [H3O+][OH-] = p of 10-14
Working in P-Functions can simplify problems
Recall Kw = [H3O+][OH-] = 10-14
-log Kw = -log ( [H3O+][OH-] )= -log 10-14
pKw = pH + pOH = 14
-log Kw = -log [H3O+] + ( -log [OH-] ) = -log 10-14
Kw = 10-14
Recall how we used this picture and this relationship: Kw = [H+] x [OH-] = 10-14
[H3O+] [OH-]
pKw = 14
Now apply this equation: pKw = pH + pOH = 14
to this picture
pH pOH
pKw
When the solution is acidic[H3O+] > 10-7 M, pH < 7 : pH is a small number
Because pKw = pH + pOH must be 14
pH < 7 pOH > 7
pKw
When the solution is ____________[H3O+] __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-OH2]2+ + H2O [Fe-OH]+ + H3O+
Keq = 10-10
When is the conjugate base (or acid) important in acid / base equilibria?
HCl + H2O Cl- + H3O+
acid conjugate base
base conjugate acid
Here?
AH + H2O A- + H3O+
acid conjugate base
base conjugate acid
Write the Ka expression for AH andthe Kb expression for A- .
Kw = 10-14
Alright, now we can understand why Cl- isn’t basic:We proved Kw = Ka x Kb
Use the Kw circle!
Ka Kb
Kw
If AH has a larger Ka, like 10-4
then A- must have a smaller Kb like 10-10
KaKb
The stronger the acid (Ka large), the weaker the conjugate base, (Kb small)
Because Kw = Kax Kb must = 10-14
Kw
KaKb
If A- has a larger Kb, like 10-3
then AH must have a smaller Ka like 10-11
The stronger the base (Kb large), the weaker the conjugate acid, (Ka small)
Because Kw = Kax Kb must = 10-14
Now do the same withKw = Ka x Kb = 10-14
p of Kw = p of [Ka x Kb ] = p of 10-14
Let’s apply P-Functions
We already did this one:Kw = [H3O+][OH-] = 10-14 pKw = pH + pOH = 14
-log Kw = -log (Ka x Kb )= -log 10-14
pKw = pKa + pKb = 14
-log Kw = -log Ka+ ( -log Kb ) = -log 10-14
pKw
Now apply this equation: pKw = pKa + pKb = 14
to this picture
pKa pKb