Acid and Base Equilibrium
CHM1025C
Arrhenius
• Acid: A substance that make H+ (H3O+)
when dissolved in water.
• Base: A substance that makes OH- when
dissolved in water.
• An acid/base reaction occurs when and H+
from an acid reacts with an OH- from a
base.
Acids
• Strong acids: Dissociate completely when
dissolved in water.
– HCl, HNO3
• Weak acids only dissociate a little bit.
– CH3CO2H
Base
• Strong base: dissociates completely when
dissolved in water.
– NaOH, KOH
• Weak base: Makes only a little bit of OH– NH3
Electrolyte
• Electrolyte: A substance that produces ions
when dissolved in water.
• NaCl, HCl and NaOH are all strong
electrolytes because they dissociate
essentially completely into ions. Strong
acids and strong bases are strong
electrolytes.
• CH3CO2H, HF and NH3 are examples of
weak electrolytes because they produce
only a few ions.
1.00 M Acetic acid, a weak acid
• CH3COOH (aq)+ H2O(l)  H3O+(aq) + CH3COO-(aq)

H O CH CO 
K


3
3
2
CH3CO2H
• The Ka for this reaction is 1.8 x 10-5. Since little
of the CH3CO2H dissociates, we can call it 1 M.
• For every H3O+ there will be one CH3COO-. Let
these concentrations = x
• x=[H+] = [CH3COO-] = .00042
Brønsted-Lowry Definition
• Acid: A proton donor.
• Base: A proton acceptor.
• An acid base reaction is one where there is a
proton transfer.
• A broader definition than the Arrhenius
definition.
• conjugate base: The acid without an H+.
• conjugate acid: The base with an H+.
TABLE: Relative Strengths of Acids and Their Conjugate Bases
Strong
Acids
Weak
Acids
Acid
HI
H2SO4
HC1
HNO3
H3 O +
HSO4H3PO4
HF
CH3CO2H
H2CO3
H2 S
H2PO4NH4+
C6H5OH
HCO3HPO42H2 O
C2H5OH
Hydroiodic acid
Sulfuric acid
Hydrochloric acid
Nitric acid
Hydronium ion
Hydrogen sulfate ion
Phosphoric acid
Hydrofluoric acid
Acetic acid
Carbonic acid
Hydrogen sulfide
Dihydrogen phosphate ion
Ammonium ion
Phenol
Bicarbonate ion
Hydrogen phosphate ion
Water
Ethanol
Conjugate Base
IHSO4 ClNO3 H2O
SO42H2PO4FCH3CO2HCO3HSHPO4 2NH3
C6H5OCO32PO43OHC2H5O-
Iodide ion
Hydrogen sulfate ion
Chloride ion
Nitrate ion
Water
Sulfate ion
Dihydrogen phosphate ion
Fluoride Ion
Acetate ion
Bicarbonate ion
Hydrogen sulfide ion
Hydrogen phosphate ion
Ammonia
Phenoxide ion
Carbonate ion
Phosphate ion
Hydroxide ion
Ethoxide ion
Given on test
Weak
Bases
Strong
Bases
A standard acid/base reaction
Acid + Base  c. base + c. acid
ex:
CH3COOH + H2O  CH3COO- + H3O+
NH3+ H2O NH4+ + OH-
More on Acid Base reactions
• Water is amphoteric or amphiprotic, it can
act as both an acid and a base.
• All acid base reactions are equilibrium
reactions
• The equilibrium lies to the side of the
weaker acid.
• When the equilibrium lies to the right, a lot
of reaction occurs and there is often heat
released or there is a color change….
• When the equilibrium lies to the left very
little reaction occurs. (no heat…)
Water
• Water auto-ionizes
H2O + H2O  H3O+ + OHacid
base
[10-7M]
[10-7M]
éëH3O+ùûéëOH-ùû
éëH3O+ùûéëOH -ùû=1´10-14
K=
=
2
[H 2O]
Acids and pH
[H+] or
[H3O+]
[OH-]
0.1 M HCl
pure water 0.1 M NaOH
0.100 or
1.00 x10-1
1.00 x10-13
1x10-7
1.00 x10-13
1x10-7
0.100 or
1.00 x10-1
13
pH
1
7
pOH
13
7
acid or
basic
1
neutral
Acid
Basic
[H3O+]>[OH-] [H3O+]=[OH-] [H3O+]<[OH-]
pH scale
Lemon juice
Soda
vinegar
ammonia
Comet cleanser
drano
The big six
1.
2.
3.
4.
5.
6.
[H3O+] [OH-]= 1 x 10-14
pH= -log[H3O+]
pOH = -log[OH-]
[H3O+] = 10-pH
[OH-]=10-pOH
pH + pOH = 14
The
•
•
•
•
+
[H ]
or [H3
+
O]
of 0.1 M NH3
The pH is 11.13. What is the [H3O+] ?
pH =-log [H3O+]
[H3O+] =10-pH
[H3O+] =10-11.13=7.41 x 10-12
Filling out a table
+
-
[H3O ] [OH ]
.0800 M
HCl
.0800 M
CH3CO2H
pH
.0800
2.92
pOH
Buffer solution
A buffer solution keeps the pH approximately
the same even upon the addition of a strong
acid or strong base.
• Need a weak acid.
• Its conjugate base.
• Present in a large enough quantity to resist
the pH changes.
CH3CO2H/CH3CO2• If you add an acid, H+, The base of the
buffer reacts.
– H+ + CH3CO2-  CH3CO2H
– H3O+ + CH3CO2-  H2O + CH3CO2H
• Of you add a base, OH-, the acid of the
buffer system reacts.
– CH3CO2H + OH-  H2O + CH3CO2-
Titrations
• How many mL of 0.660M H2SO4 are
required to react completely with 72.5 mL
of 0.137M NaOH?