Chapter 16 - Acid Base Equilibria #1
Acids & Bases
Acids: taste sour
Bases: taste bitter and feel soapy
Arrhenius Acids & Bases (1st year chem definition!)
An Arrhenius acid is a substance that, when dissolved in water, increases the concentration of
H+ (needs H+ in it)
Example: HCl (monoprotic)
H2SO4 (diprotic)
An Arrhenius base is a substance that, when dissolved in water, increases the concentration of
OH– (needs OH- in it)
Example: NaOH (monobasic)
Ca(OH)2 (dibasic)
Brønsted-Lowry Acids and Bases
Brønsted-Lowry acid is a species that _________________________________________________
Brønsted-Lowry base is a species that _________________________________________________
Therefore a Brønsted-Lowry base does not need to contain OH– and the reaction does not need
to be aqueous.
The H+ Ion in Water
The
ion is simply a proton with no surrounding valence electrons.
In water, clusters of hydrated H+(aq) ions form.
The simplest cluster is H3O+(aq) We call this a hydronium ion.
Larger clusters are also possible (such as H5O2+ and H9O4+).
Generally we use H+(aq) and H3O+(aq) interchangeably.
H+(aq)
Proton-Transfer Reactions
NH3(aq) + H2O(l)  NH4+(aq) + OH–(aq)
H2O donates a proton to ammonia. Therefore, water is acting as an acid.
NH3 accepts a proton from water. Therefore, ammonia is acting as a base
Amphoteric substances
Can behave as acids and bases.
Water is an example of an amphoteric species.
Water as an acid (proton donor)
H2O + NH3  NH4+ + OHWater as a base (proton acceptor)
H2O + HNO2  H30+ + NO2Conjugate Acid-Base Pairs
A conjugate acid is the substance formed by adding a proton to the base.
A conjugate base is the substance left over after the acid donates a proton
HA(aq) + H2O(l)  H3O+(aq) + A–(aq):
A– is the conjugate base of HA
•
HA and A-1 are a conjugate acid-base pair.
H3O+ is the conjugate acid of H2O
•
H2O and H3O+ are a conjugate acid-base pair.
Within a pair the _____________________________________________________.
The equilibrium reactions can be read in either direction
Strong Acids & Bases
Strong Acids
Sulfuric acid
Strong Bases
Lithium hydroxide
Nitric acid
Sodium hydroxide
Perchloric acid
Potassium hydroxide
Chloric acid
Rubidium hydroxide
Hydrochloric acid
Cesium hydroxide
Hydrobromic acid
Calcium hydroxide
Hydroiodic acid
Strontium hydroxide
Barium hydroxide
All other acids and bases are weak!
Ions
Most anions are weak bases
Most cations are weak acids
Anions of strong acids and cations of strong bases are neutral
Neutral anions
Hydrogen sulfate
Neutral cations
Lithium
nitrate
Sodium
perchlorate
Potassium
chlorate
Rubidium
chloride
Cesium
bromide
Calcium
iodide
Strontium
Barium
Strengths of Acids and Bases
Strong acids completely ionize in water.
HCl + H2O  Cl- + H3O+
HCl  Cl- + H+
Essentially no un-ionized molecules remain in solution so the equation usually does not contain
equilibrium arrows. Keq >>1
Their conjugate bases have negligible tendencies to become protonated
Cl- + H+ 
The conjugate base of a strong acid is a neutral anion.
Strong bases completely dissociate in water
NaOH(aq)  Na+(aq) + OH-(aq)
Essentially no undissociated compound remains in solution so the equation usually does not
contain equilibrium arrows. Keq >>1
The ions have negligible tendencies to attract in solution
Na+(aq) + OH-(aq) 
The cation of a strong base is a neutral cation.
All other acids are Weak acids. They only partially dissociate in aqueous solution.
HC2H3O2(aq) + H2O(l)  H3O+(aq) + C2H3O2-(aq)
They exist in solution as a mixture of molecules and component ions. (mostly molecules in
equilibrium)
Their conjugate bases are weak bases.
Weak bases tend to be nitrogen containing organic compounds and non-neutral anions
Example: Acetic acid is a weak acid; acetate ion (conjugate base) is a weak base
The stronger an acid is, the weaker its conjugate base will be.
(see rankings on p. 657)
In acid-base reactions, the position of the equilibrium favors the transfer of a proton from the
stronger acid to the stronger base.
Where does that ranking come from?
Since weak acids and bases are in equilibrium…we can write equilibrium constant expressions!
When looking at the reaction of a weak acid with water we label the equilibrium constant K a:
HF(aq) + H2O(l)  H3O+(aq) + F-(aq)
When looking at the reaction of a weak base with water we label the equilibrium constant K b:
NH3(aq) + H2O(l)  NH4+(aq) + OH-(aq)
Ka and Kb
The value of Ka or Kb indication the extent to which the weak acid or base ionizes/dissociates.
Larger Ka or Kb means more products!
Weak acids with larger Ka values are stronger.
Weak bases with larger Kb values are stronger.
Percent Ionization
Percent ionization is another method to assess acid strength.
For the reaction: HA(aq)  H+(aq) + A–(aq)
The higher the percent ionization, the stronger the acid.
Percent ionization of a weak acid actually decreases as the molarity of the solution increases.
Polyprotic Acids
Polyprotic acids have more than one ionizable proton.
The protons are removed in successive steps.
Consider the weak acid, H2SO3 (sulfurous acid):
H2SO3(aq)  H+(aq) + HSO3–(aq) Ka1 = 1.7 x 10–2
HSO3–(aq)  H+(aq) + SO32–(aq)
Ka2 = 6.4 x 10–8
It is always easier to remove the first proton in a polyprotic acid than the second.
Ka1 > Ka2 > Ka3, etc
The Autoionization of Water
In pure water the following equilibrium is established:
H2O(l) + H2O(l)  H3O+(aq) + OH–(aq)
acid
base
acid
base
This process is called the autoionization of water.
We can write an equilibrium constant expression for the autoionization of water:
Kw = [H3O+] [OH–] = 1.0*10-14 @25°C
Kw is called the “ion-product constant”
The Ion Product Constant
This applies to pure water as well as to aqueous solutions.
A solution is neutral if [OH–] = [H3O+].
If the [H3O+] > [OH–], the solution is acidic.
If the [H3O+] < [OH–], the solution is basic.
In a neutral solution at 25oC,
[H+] = [OH-] = 1.0 x 10-7 M
Example 1: Give the conjugate base of the following Bronsted-Lowry acids:
(a) HIO3 (b) NH4+1 (c) H2PO4-1 (d) HC7H5O2
Example 2: Designate the Bronsted-Lowry acid and the Bronsted-Lowry base on the left side of
each of the following equations, and also designate the conjucate acid and base on the right
side:
(a) NH4+1(aq) + CN-1(aq)  HCN(aq)
+
NH3(aq)
(b) (CH3)3N(aq) +
(c) HCHO2(aq) +
H2O
 OH-1(aq) +
PO4-3(aq) 
HPO4-2(aq)
(CH3)3NH+1(aq)
+
CHO2-1(aq)
Example 3:
(a) The hydrogen oxalate ion (HC2O4-1) is amphoteric. Write a balanced chemical equation
showing how it acts as an acid toward water and another equation showing how it acts as a
base towards water.
(b) What is the conjugate acid and base of HC2O4-1?
Example 4: Label each of the following as being a strong acid, a weak acid, or a species with
negligible acidity. In each case write the formula of its conjugate base, and indicate whether
the conjugate base is a strong base, a weak base or a species with negligible basicity:
(a) HNO2
(b) H2SO4
(c) HPO4-2
(d) CH4
(e) CH3NH3+1
Example 5: Which of the following is the stronger acid, HBrO or HBr?
Which is the stronger base, F-1 or Cl-1?
Briefly explain your choices.
Example 6: Predict the products of the following acid-base reactions:
(a) O-2(aq) + H2O(l) 
(b) CH3COOH(aq) + HS-1(aq) 
(c) NO3-1(aq) + H2O(l) 
HW #1 16(2, 16, 18, 20, 22, 26, 42, 49)