Updates

1
Updates

• Assignment 05 is is due Monday, Mar. 05 (in
  class)
• Midterm 2 is Thurs., March 15
• Huggins 10, 7-8pm
• For conflicts ELL 221, 6-7pm (must arrange at
  least one week in advance)

2
Acids and Bases

• Chapter 16

3
Binary acids (HX, H2X, H3X, H4X)

• Bond strength determines acidity within the same
  group (column), size
• Bond polarity determines acidity within the same
  period (row), electronegativity

4
Rationalizing acidity

• Two considerations
• Stability of reactants
• Stability of products
• Stable
• Happy
• Low energy
• Low number of charges
• Low charge density on atom(s)
• Charge distributed over multiple atoms
• Strong bonds
• Unstable
• Unhappy
• High energy
• High number of charges
• High charge density on atom(s)
• Weak bonds

5
Rationalizing acidity

• Two considerations
• Stability of reactants
• Stability of products
• Stable
• Happy
• Low energy
• Low number of charges
• Low charge density on atom(s)
• Charge distributed over multiple atoms
• Strong bonds
• Unstable
• Unhappy
• High energy
• High number of charges
• High charge density on atom(s)
• Weak bonds

6
Oxyacids Central atoms derived from same group
(same oxidation state)

• More electronegative central atom polarizes the
  OH bond more, facilitating ionization
• More electronegative central atom better able to
  stablize resulting negative charge following
  ionization, making a happier (more stable)
  conjugate base

7
Oxyacids Identical central atoms with different
oxidation states

• Acidity increases as oxidation number of central
  atom increases
• Said another way for oxyacids with the same
  central atom, acidity increases as the number of
  oxygens attached increases
• Additional electronegative oxygen atoms pull
  electron density from the O-H bond, further
  increasing its polarity, facilitating ionization
• Increasing the number of oxygens also helps
  stabilize the conjugate base by increasing its
  ability to spread out its negative charge (a
  happier conjugate base)

8
Carboxylic acids

• Portion in blue known as carboxyl group and is
  often written as COOH
• Acids that contain a carboxyl group are called
  carboxylic acids, and they form the largest
  category of organic acids (organic referring to
  carbon-containing compounds, based on C-C bonds)

9
Carboxylic acids

• Acetic acid is a weak acid (Ka 1.8 x 10-5),
  whereas methanol is not an acid in water
• Additional oxygen increases polarity of O-H bond
  and helps to stabilize the conjugate base
• The conjugate base (called a carboxylate anion)
  can exhibit resonance (p. 284), which contributes
  further to the stability of the anion by
  spreading the negative charge over several atoms
• When the three hydrogens are replaced with
  fluorines, the acidity increases (Ka 5.0 x
  10-1)

10
Acid-base properties of salt solutions

• Salt solutions can be neutral, acidic, or basic,
  owing to the reaction of a cation or anion (or
  both) with water
• These three solutions contain the acid-base
  indicator bromthymol blue.
• (a) The NaCl solution is neutral (pH 7.0)
• (b) The NH4Cl solution is acidic (pH 3.5)
• (c) The NaClO solution is basic (pH 9.5)

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Acid-base properties of salt solutions

• Anions
• Conjugate base of strong acid DOES NOT react with
  water (Cl-)
• Conjugate base of weak acid reacts with water
  (CH3COO-)
• Cations
• Conjugate acid of weak base reacts with water
  (NH4)
• Most metals can react with water (Al3, Cr3,
  Fe3, Bi3, Be2)
• Ions of alkali metals and heavier alkaline earth
  metals DO NOT react with water (Na)

12
Acid-base properties of salt solutions
(a) The NaCl solution is neutral (pH
7.0) (b) The NH4Cl solution is acidic (pH
3.5) (c) The NaClO solution is basic (pH
9.5)

• Anions
• Conjugate base of strong acid DOES NOT react with
  water (Cl-)
• Conjugate base of weak acid reacts with water
  (acetate)
• Cations
• Conjugate acid of weak base reacts with water
  (NH4)
• Most metals can react with water (Al3, Cr3,
  Fe3, Bi3, Be2)
• Ions of alkali metals and heavier alkaline earth
  metals DO NOT react with water (Na)

13
Acid-base properties of salt solutions
(a) The NaCl solution is neutral (pH
7.0) (b) The NH4Cl solution is acidic (pH
3.5) (c) The NaClO solution is basic (pH
9.5)

• Examples where either the cation or the anion of
  the salt reacts -but not both!

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Combined effect of cation and anion in solution

• If salt solution contains an anion that reacts
  with water, we expect solution to be basic
• If salt solution contains a cation that reacts
  with water, we expect solution to be acidic
• What happens when salt is made from a cation and
  anion that both react with water?
• Whether solution is basic, neutral, or acidic
  depends on the relative abilities of the ions to
  react with water
• When a solution contains both the conjugate base
  of a weak acid and the conjugate acid of a weak
  base, the ion with the larger equilibrium
  constant, Ka or Kb, will have the greater
  influence on the pH
• Stated another way When a solution contains a
  reactive anion and a reactive cation (toward
  water), the ion with the larger equilibrium
  constant, Ka or Kb, will have the greater
  influence on the pH

15
Combined effect of cation and anion in solution

• When a solution contains both the conjugate base
  of a weak acid and the conjugate acid of a weak
  base, the ion with the larger equilibrium
  constant, Ka or Kb, will have the greater
  influence on the pH
• Take NH4F as an example
• Both NH4 and F- will react with water
• Ka for NH4 is 5.6 x 10-10
• Kb for F- is 1.4 x 10-11
• Therefore, Ka is larger, and solution will be
  acidic

16
Anions that act as both an acid and a base

• Bicarbonate ion can ionize (rxn 1) or undergo
  hydrolysis (rxn 2)
• Ionization involves bicarbonate acting as an
  acid hydrolysis involves bicarbonate acting as a
  base
• Because Kb is larger, we predict that hydrolysis
  will predominate, yielding a basic solution

17
Acid-Base Properties of Salts
16.10
18
Acidic, Basic and Amphoteric Oxides, p. 264
(shown in highest oxidation states)
16.11
19
Lewis Acids

• Lewis acids are defined as electron-pair
  acceptors.
• Atoms with an empty valence orbital can be Lewis
  acids.

20
Lewis Bases

• Lewis bases are defined as electron-pair donors.
• Anything that could be a BrønstedLowry base is a
  Lewis base.
• Lewis bases can interact with things other than
  protons, however (broadest definition).

21
Chemistry In Action Antacids and the Stomach pH
Balance