Chapter 17: The Chemistry of Acids and Bases

1
Chapter 17 The Chemistry of Acids and Bases
2
Acids Bases Brønsted-Lowry Definition

• A. An acid is any substance which can donate a
  proton to any other substance,
• HCl (aq) H2O(l) -----gt H3O(aq) Cl-(aq)
• acid hydronium ion
• B. A base is any substance which can accept a
  proton from any other substance.
• NH3(aq) H2O(l) -----gt NH4(aq) OH-
  (aq)
• base (note says nothing about hydroxides!)

3
Conjugate Acid-Base Pairs

• each of the above reactions involves the transfer
  of a proton.
• therefore, must have both an acid (H-donor) and
  base (H-acceptor) present.
• HCO3-(aq) H2O(l) ltgt CO3-2(aq)
  H3O(aq)
• bicarbonate carbonate
• bicarbonate and carbonate related to one another
  by the gain or loss of one H
• HCO3-(aq) ltgt CO3-2(aq) H(aq)
• called Conjugate Acid- Base Pair
• Every acid-base reaction involving H transfer
  has two acid-base conjugate pairs.
• HCO3-(aq) H2O(l) ltgt CO3-2(aq)
  H3O(aq)
• acid base conjugate base conjugate
  acid

4
Water the pH scale

• H OH-
• H2O ltgt H(aq) OH-(aq) Keq
• H2O
• in all aqueous solutions H2O 55M
• so rewrite Keq H2O Kw H OH-
  1.0 x 10-14
• in pure water H OH- 1.0 x 10-7
• DEFINE Any aqueous solution in which H
  OH- is called a neutral solution
• A neutral solution does not mean there are no H
  or OH- !!
• when H gt OH- acidic solution (H gt
  10-7 M)
• when H lt OH- basic solution (H lt
  10-7 M)
• Sørenson proposed that instead of using
  concentrations, use term called p, where pX
  -log X so pH -log H also
  10-pH H
• pH lt 7 acidic pH gt 7 basic
  pH 7 neutral
• pH pOH 14

5
Strong Acids and Bases

• ionize 100 in solution
• KNOW THESE STRONG ACIDS
• HCl hydrochloric acid HCl(aq) ---? H(aq)
  Cl-(aq)
• HBr hydrobromic acid
• HI hydroiodic acid
• HNO3 nitric acid
• H2SO4 sulfuric acid
• HClO4 perchloric acid
• KNOW THESE STRONG BASES
• Hydroxides of Group I metals LiOH, NaOH, KOH
• Hyroxides of Group II metals Mg(OH)2, Ca(OH)2

NO HCl in solution
LiOH(aq) ---? Li(aq) OH-(aq)
NO LiOH in solution
6
Weak Acids Bases

• only partially dissociate in solution
• HCN(aq) ltgt H(aq) CN-(aq)
• NH3(aq) H2O(l) ltgt OH-(aq) NH4(aq)
• notes
• 1. the stronger the acid, the weaker the
  conjugate base
• 2. the stronger the base, the weaker the
  conjugate acid
• 3. all are proton transfer reactions and run from
  the stronger pair towards the weaker pair.
• stronger
• weaker

All species in solution
7
strongest weak acids
weakest weak bases
strongest weak bases
weakest weak acids
8
Weak Acids Bases in Solution

• weak acids set up the following equilibrium
• H B-
• HB(aq) ltgt H(aq) B-(aq) Ka
  
• weak acid conjugate base HB
• weak bases set up the following equilibrium
• BaseH OH-
• Base(aq) H2O(l) ltgt BaseH(aq)
  OH-(aq) Kb
• conjugate acid Base
• Ka and Kb values found in Table 17.3
• Ks measure extent to which the acid/base
  dissociates in water
• (larger value stronger acid/base)
• can use just like other equilibrium expressions
• note not doing reaction here, just putting
  acid/base in water

acid dissociation constant
9
Kas the Quadratic Equation

• can always solve the quadratic equation, although
  many times this is not required
• Why? Kas only known to 2 Sig Figs.
• Therefore, can make mathematical approximation if
  doing so doesnt introduce an error in the 2 Sig
  Figs.
• HB(aq) ltgt H(aq) B-(aq)
• If HBo gt 100Ka , then use approximation
• H B- H2 H2
• Ka ?
• HBe HBo - H HBo
• H2
• Ka if HBo gt 100Ka

equilibrium conc.
initial conc.
The Approximation
10
Example

• 0.020 moles of acetic acid is diluted to 1.00L.
  What is the resulting pH?
• AcOH ltgt AcO- H
• start 0.020 0 0

11
Kbs the Approximation

• what is the pH if 0.010 moles of HCO3 - are
  dissolved in 1.00L?
• HCO3- H2O ltgt H2CO3 OH-
  
• start 0.010 0 0

12
Using Ka to Determine Starting Conditions

• How many moles of phosphoric acid should be
  placed in 1.0L of solution so that the resulting
  pH 3.00?
• H3PO4 ltgt H H2PO4-
• start x 0 0

13
Reactions Between Strong Acids Strong Bases

• in soln, each exists as
• HCl (aq) -----gt H(aq) Cl-(aq)
• .
• NaOH(aq) -----gt Na(aq) OH- (aq)
• mix together
• H(aq) Cl-(aq) Na(aq) OH- (aq) -----gt
  H2O(l) Na(aq) Cl-(aq)
• or
• H(aq) OH- (aq) -----gt H2O(l) neutralizat
  ion
• Cl-(aq) Na(aq) are spectator ions

14
Reactions between a Strong Base a Weak Acid

• First step is neutralization
• OH-(aq) HCOOH(aq) ------gt H2O HCOO-(aq)
• goes to completion but note that you are forming
  a weak base.
• What happens when a weak base is in solution?
• Second step is equilibration
• H2O HCOO-(aq) ltgt OH-(aq) HCOOH(aq)
  
• this is an equilibrium it does NOT go back to
  starting point
• if you know the concentration of the weak base
  formed and Kb, you can calculate the pH at the
  equivalence point.

100
15
Quantitative Example

• 0.1 mole NaOH and 0.1 mole acetic acid are placed
  in 1.0L of solution. What is the resulting pH?
• OH-(aq) CH3COOH(aq) ------gt H2O
  CH3COO-(aq)
• form 0.1mole of acetate in 1.0L solution 0.1M
• CH3COO-(aq) H2O ltgt CH3COOH(aq)
  OH-(aq)
• start 0.1 0 0

16
Reactions between a Strong Acid a Weak Base

• First step is neutralization
• H (aq) NH3(aq) ------gt NH4(aq)
• goes to completion but note that you are forming
  a weak acid.
• What happens when a weak acid is in solution?
• Second step is equilibration
• NH4(aq) ltgt NH3(aq) H (aq)
• this is an equilibrium it does NOT go back to
  starting point
• if you know the concentration of the weak acid
  formed and Ka, you can calculate the pH at the
  equivalence point.

100
17
Quantitative Example

• 0.1 mole HCl and 0.1 mole ammonia are placed in
  1.0L of solution. What is the resulting pH?
• H (aq) NH3(aq) ------gt NH4(aq)
• form 0.1mole of ammonium in 1.0L solution 0.1M
• NH4(aq) ltgt NH3(aq) H (aq)
• start 0.1 0 0

18
Reactions of Weak Acids Weak Bases

• example reaction of methylamine and hypochlorous
  acid
• CH3NH2(aq) HClO(aq) ltgt ClO-(aq)
  CH3NH3(aq)
• these reactions do not go to completion, but set
  up an equilibrium.
• will resulting solution be acidic or basic?
• look at products
• ClO-(aq) H2O(aq) ltgt HClO(aq)
  OH-(aq) Kb 2.9 x 10-7
• CH3NH3(aq) ltgt CH3NH2(aq) H(aq)
  Ka 2.0 x 10-10
• Kb gt Ka therefore, more OH- produced than H
  (pH gt 7)

weak base weak acid
weak base weak acid