John A. Schreifels

1
Chapter 16

• Acids and Bases

2
Overview

• Acid Base Concepts
• Arrhenius
• Brønsted Lowry
• Lewis
• Acid and Base Strengths
• Relative Strengths of Acids and Bases
• Molecular Structure and Acid Strength
• Self Ionization of Water and pH
• Self Ionization of Water
• Solutions of a Strong Acid or Base
• The pH of a Solution

3
Acid base concepts

• Arhennius Concept
• acid provides H ions (called a proton) in
  water.
• HA(aq) ? H(aq) A?(aq)
• base provides OH? in water.
• BrønstedLowry Concept
• acid donates H.
• base accepts H.
• No water or hydroxide required.
• Base converted to its conjugate acid and develops
  properties that are those of an acid.
• E.g. Identify each as either an acid or base and
  determine its conjugate and
  , F?, H2CO3 and
• E.g. Identify the conjugate acids and bases in
  the reaction below
• Amphoteric substance a substance that can act as
  either an acid or base. (E.g. H2O in examples)

4
Acid base concepts 2

• Lewis acid electron pair acceptor
• Lewis base electron pair donor. .
• Lewis definition most general, then
  Brønsted-Lowery and finally Arhennius
• E.g.
• E.g.2 determine the Lewis acid and base in BF3
  and NH3 in the reaction
• E.g.3 determine the Lewis acid and base in the
  following reaction
• Co3(aq) 6F?(aq) ? CoF63?.

5
Acid/Base Strength

• Extent of acid/base reaction variable and depends
  upon the relative strengths of the acids and
  conjugate bases..
• Strong acids and bases react with other reactant
  to produce all product.

Stronger acids and bases react to form weaker
conjugate bases and acids.
Common Strong Acids and Bases Common Strong Acids and Bases
Acids Bases
HClO4 LiOH
H2SO4 NaOH
HI KOH
HBr Ca(OH)2
HCl Sr(OH)2
HNO3 Ba(OH)2
6
Relative Strengths of Acids and Bases and Extent
of Reaction

• The table of relative strengths of acids and
  conjugate bases can be used to predict if a
  reaction will produce product.
• E.g. Which will produce product?
• HNO3 CN? or HCN

7
Factors Affecting Acid Strength

• Binary acids
• Bond strength is directly related to the acid
  strength (bond size).
• HI and HBr have larger bonds lengths and are more
  acidic than HF and HCl, even though fluorine is
  most electronegative.
• For bonds of similar size the acid strength is
  related to electronegativity difference.
• E.g. bonds across a row more acidic towards right
  side of periodic table.
• Oxyacids are acidic substances that contain
  oxygen and some other nonmetal, e.g. HNO3, HOCl,
  etc.. Anything that affects the polarity of the
  OH bond will affect the strength of the acid.
• An increase in the electronegativity of an atom
  bound to oxygen increases in polarity of the bond
  and makes it more acidic.
• More oxygen more polar.
• E.g. determine relative acidity of HOI, HOBr and
  HOCl.
• E.g. HClO4 is strongest acid in its oxyacid
  series.

8
Autoionization of Water

• Water can act as both an acid and a base
  equilibrium is
• H2O H2O ? H3O OH?. Kw OH?H3O 1.00
  x 10?14M2.
• Since OH? H3O? H3O 1 x 10?7 M (called
  a neutral solution)
• Acidic H3O gt 1.00x10?7M
• Neutral H3O 1.00x10?7M
• Basic H3O lt 1.00x10?7M
• All acids/bases dissolved in water must obey
  equation for the ionization of water.
• They either add H3O or OH? to water.
• Most of the acids in this chapter will be
  stronger than water and add significantly to the
  hydronium ion concentration.
• E.g. the hydronium ion concentration of an
  acidic solution was 1.00x10?5 M. What was the
  OH??
• E.g. what is the hydronium ion concentration if
  the hydroxide concentration was 2.50x10?3 M?

9
Strong acids and Bases

• A strong acid is completely dissociated in water.
  This leads to H3O and OH?.
• Eg Calculate the H3O,OH? and Cl? for a
  0.048 M HCl solution. Assume the contribution
  from water is negligible.
• E.g.2 Calculate the conc. of all ionic species
  as well as the pH if CNaOH 0.080 M.
• E.g. 3 what is pH and OH? of 0.125 M Ba(OH)2.
• H3O of water is small compared to added H3O
  from the acid and ignored in the calculation.
• A more rigorous treatment is
• The last term is very small except when the
  concentration of the strong acid is very small.
• When the concentration of added acid is small
  compared with pure water, include the
  contribution from water.
• E.g.4 estimate the H3O of 10?8 M HCl.

10
The pH of a Solution

• pH ?logH3O and H3O 10?pH
• Acidic pH lt 7.00
• Neutral pH 7.00
• Basic pH gt 7.00
• E.g. determine the pH of a solution in which
  H3O 5.40x10?6 M
• E.g.2 determine the pH of a solution in which the
  OH? 3.33x10?3 M
• E.g.3 determine the pOH of a solution in which
  the OH? 3.33x10?3 M
• E.g.4 Determine the H3O if the pH of the
  solution is 7.35.
• The term pX is defined in exactly the same way as
  pH.
• Eg.5 What is the pCa if Ca2 6.44x10-4

11
Methods of Measuring pH

• pH paper is used that has compounds in it which
  are change to different colors for different pH
  ranges.
• An colored indicator can be placed in the
  solution and its color correlated with pH.
• HIn(aq) H2O(l) ? H3O(aq) In?(aq).
• E.g. phenolphthalein is colorless in acid form
  but pink in basic form.
• The pH at which they change color depends on
  their equilibrium constant.
• More accurate and precise measurements are made
  with a pH meter. A combination of voltmeter and
  electrodes.