Generic Intro to AB chem. (Sect. J)

1
Generic Intro to AB chem. (Sect. J)

• Arrhenius theory an acid forms H in water and
  a base forms OH in water.
• But not all acidbase reactions involve water,
  and many bases (NH3, carbonates) do not contain
  any OH.
• BrønstedLowry theory defines acids and bases in
  terms of proton (H) transfer.
• A BrønstedLowry acid is a proton donor.
• A BrønstedLowry base is a proton acceptor.
• The conjugate base of an acid is the acid minus
  the proton it has donated.
• The conjugate acid of a base is the base plus the
  accepted proton.

2
Examples of Conjugate AB pairs
3
Couple of example problems

• Write a balanced chemical equation for the
  dissociation of each of the following (in H2O)
• H2SO4, H2PO4-,
• What is the conjugate base of each of the
  following?
• HCO3-, CO32-, OH-, H2PO4-

4
Describing strength of Acids/Bases

• Some acids are stronger than othersyou know this
• Where do we draw the line?
• I suppose that depends on what line we draw
• Strong Acids/Bases dissociate COMPLETELY100
• Use an arrow to designate the reaction
• Kc values well above 103
• Weak Acids/Bases dissociate very little (1 or
  less)
• Use equilibrium arrows
• Kc values well below 10-3

5
Ionization of HClstrong/weak acids
H2O is a base in this reaction because it accepts
the H
Conjugate acid of H2O
HCl acts as an acid by donating H to H2O
Conjugate base of HCl
6
Ionization of Ammonia
This example also illustrates another Acid/Base
definitionLewis ABs Electron pair
acceptorLewis Acid E pair donorbase.
7
Lewis Acid/Base interaction (2)
A couple of things worth noting here. Arrow
indicates movement of electrons! Helpful hint in
Organic C of CO2 electron starved due to pull
of oxygens Illustrates the reaction of an oxide
with water. In this case, formation of an acidic
oxide (most elements on right form acidic
oxides Metal oxides react to form bases
(Na2O?) Amphoteric (goes both ways)
Amphoteric oxides
8
Water Is Amphiproticgoes both ways
H2O acts as an acid when it donates H, forming
the conjugate base ___
H2O acts as a base when it accepts H, forming
the conjugate acid ___
Amphiprotic Can act as either an acid or as a
base
9
Self-Ionization of Water

• Even pure water conducts some electricity. This
  is due to the fact that water self-ionizes

• The equilibrium constant for this process is
  called the ion product of water (Kw).
• At 25 C, Kw 1.0 x 1014 H3OOH
• This equilibrium constant is very important
  because it applies to all aqueous
  solutionsacids, bases, salts, and
  nonelectrolytesnot just to pure water.

10
The pH Scale

• Concentration of H3O can vary over a wide range
  in aqueous solution, from about 10 M to about
  1014 M.
• A more convenient expression for H3O is pH.
• pH log H3O and so H3O 10pH
• The negative logarithm function of pH is so
  useful that it has been applied to other species
  and constants.
• pOH log OH and so OH
  10pOH
• pKw log Kw
• At 25 C, pKw 14.00
• pKw pH pOH 14.00

11
The pH Scale
Since pH is a logarithmic scale, cola drinks (pH
about 2.5) are about ____ times as acidic as
tomatoes (pH about 4.5)
12
Strength of Conjugate AcidBase Pairs

• A stronger acid can donate H more readily than a
  weaker acid.
• The stronger an acid, the weaker is its conjugate
  base.
• The stronger a base, the weaker is its conjugate
  acid.
• An acidbase reaction is favored in the direction
  from the stronger member to the weaker member of
  each conjugate acidbase pair.

13
Ka and KbYOUVE SEEN THIS!!

• The equilibrium constant for a Brønsted acid is
  represented by Ka, and that for a base is
  represented by Kb.
• Nothing more than a simple extension of what you
  already know!!

H3OCH3COO Ka
CH3COOH
Notice that H2O is not included in either
equilibrium expression.
NH4OH Kb
NH3
14
Whats happening in solution?

• A solution of a weak acid or base
• Mostly acid molecules (or ions, like the
  phosphate example)
• Very small of H, and even SMALLER amounts of
  OH
• Molecules or ions in ceaseless dynamic equilibrium

15
More on the acids
Note the higher values of pKa. Higher values
correspond to weaker acids. pKas more commonly
used for acid strength
16
A graphic to relate Ka/weak/strong acids
17
Strong Acids

• The strong acidsHCl, HBr, HI, HNO3, H2SO4,
  HClO4are considered strong because they ionize
  completely in water.
• The strong acids all appear above H3O in Table
  15.1.
• The strong acids are leveled to the same
  strengthto that of H3Owhen they are placed in
  water.

18
Weak acids, and how structure determines relative
strength

• Yesterday morning, we touched on weak acids/bases
• Some stronger than otherswhy?
• Know theyre stronger b/c pKa values differ
• Well cover the following situations for weak
  acids
• Oxo acids
• Binary Hydrides (not given much love in text)
• Carboxylic Acids (pay attentioncrops up again
  next year)
• ProblemNo ONE thing useful for predicting
  acidity. MUST use all of your knowledge
  (structure, trends etc)

19
Periodic Trends Binary Hydrides (HnX)

• The greater the tendency for HX (general acid) to
  transfer a proton to H2O, the more the forward
  reaction is favored and the stronger the acid.
• ANY factor that makes it easier for the H to
  leave will increase the strength of the acid.
• Acid strength is inversely proportional to HX
  bond-dissociation energy. Weaker HX bond gt
  stronger acid.
• Acid strength is directly proportional to anion
  radius. Larger X radius gt stronger acid.

20
Periodic Trends in Acid Strength
21
Strength of Oxoacids

• Acid strength increases with the
  electronegativity of the central atom, and with
  the number of terminal oxygen atoms.

22
Other ways to see this
23
Strength of Carboxylic Acids

• Carboxylic acids all have the COOH group in
  common.
• Differences in acid strength come from
  differences in the R group attached to the
  carboxyl group.
• In general, the more that electronegative atoms
  appear in the R group, the stronger is the acid.

24

• Select the stronger acid in each pair Problems
  like this will obviously be on an exam
• (a) nitrous acid, HNO2, and nitric acid, HNO3
• (b) Cl3CCOOH and BrCH2COOH

25
Weak BasesAgainnot much love

• We know many common strong bases (MetalOH)
• Weak bases? Not manymostly R3N compounds
• Why? They act as good Lewis Bases (lone pair on
  N)
• Two main typesR group just a typical organic
  group
• When R group involves a benzene group.

26
Strengths of Amines as Bases

• Aromatic amines are much weaker bases than
  aliphatic amines.
• This is due in part to the fact that the p
  electrons in the benzene ring of an aromatic
  molecule are delocalized and can involve the
  nitrogen atoms lone-pair electrons in the
  resonance hybrid.
• As a result, the lone-pair electrons are much
  less likely to accept a proton.
• Electron-withdrawing groups on the ring further
  diminish the basicity of aromatic amines relative
  to aniline.

27

• Select the weaker base in each pair (Good Primer
  for your Organic Class this fall)

28
Equilibrium in Solutions of Weak Acids and Weak
Bases

• These calculations are similar to the
  equilibrium calculations performed in Chapter 9.
• An equation is written for the reversible
  reaction.
• Data are organized, often in an ICE format.
• Changes that occur in establishing equilibrium
  are assessed.
• Simplifying assumptions are examined (the 5
  rule).
• Equilibrium concentrations, equilibrium constant,
  etc. are calculated.

29

• What is the pH of 0.00200 M ClCH2COOH(aq)?

30
Several Textbook sample problems

• What is the pH of 0.500 M NH3(aq)?
• The pH of a 0.164 M aqueous solution of
  dimethylamine is 11.98. What are the values of Kb
  and pKb? The ionization equation is
• (CH3)2NH H2O (CH3)2NH2
  OH Kb ?
• Dimethylamine
  Dimethylammonium ion
• A Conceptual Example
• Without doing detailed calculations, indicate
  which solution has the greater H3O, 0.030 M
  HCl or 0.050 M CH3COOH.