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
What do you recall about Acid /Base Chemistry?
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-
The chemist-gardener:
how to make your hydrangeas Pink or Blue
Pink form = protonated anthocyanin, acid form AH
Need acid: gardeners use ferrous sulfate
Reaction:
The chemist-gardener:
how to make your hydrangeas Pink or Blue
Blue form = deprotonated anthocyanin, base form A-
Need base: gardeners use lime
Reaction:
The chemist-gardener:
how to choose reagents for your garden
Think about it: why use ferrous sulfate?
why not use HCl?
Reaction differences:
Equilibrium Constants in Acid Base Chemistry
Ka: acid dissociation constant
describes how much H+ (H3O+) formed
General Acid Reaction:
Equilibrium Constants in Acid Base Chemistry
Kb: base dissociation constant
describes how much OH- formed
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+]?
Takes Two to Tango! Acid Base Conjugate Species
AH
acid
+ H2O
base
A-
+
H3O+
conjugate conjugate
base
acid
Takes Two to Tango! Acid Base Conjugate Species
B
base
+ H2O
acid
BH+ +
HO-
conjugate conjugate
acid
base
Takes Two to Tango! Acid Base Conjugate Species
B
base
+ AH
acid
BH+ +
A-
conjugate conjugate
acid
base
Dual Personality of Water: both an acid and a base
Amphoteric (or amphiprotic)
The Autoionization reaction of water.
2 H20
Keq =
H3O+ + OH-
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
When [H3O+] > [OH-], [H3O+] > 10-7 M,
the solution is acidic
[H3O+] > 10-7 M
[OH-] < 10-7 M
Kw
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
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
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
The P-Function simplifies exponential numbers
Ka = 1.20 x 10-5 , pKeq = 4.92
[H3O+] = 4.20 x 10-5 M, pH = 4.376
[OH-] = 6.66 x 10-6 M, 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 10-8 M,
if pKa = 4.92,
Ka = 10-4.92 = 1.20 x 10-5
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
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
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
Now apply this equation: pKw = pH + pOH = 14
to this picture
[H3O+]
[OH-]
Kw =
-14
10
Now apply this equation: pKw = pH + pOH = 14
to this picture
pH
pOH
pKw = 14
When the solution is acidic
[H3O+] > 10-7 M, pH < 7 : pH is small
pH < 7
pOH > 7
pKw
Because pKw = pH + pOH must be 14
Fill in the
blanks!
pKw
pOH is
_______
pH is
_______
When the solution is ____________
[H3O+] __10-7 M, pH ___ 7 : pH is ________