Arrhenius Definition

 Acids
 Produce H+ ions when dissolved in water
 Ionize into H+ ions and negative ion
 (Ex. HCl, HBr)
 Bases
 Produce OH- ions when dissolved in water
 Ionize into OH- ion and positive ion
 (Ex. NaOH, Mg(OH)2 )
 Does not apply to ALL acids and bases! !
Examples

 HCl
 NaOH
Bronsted-Lowry
Definition

 Focus on hydronium (H O ) ion
3
+
 Acids
 H+ ion donator/proton donator
 Produces hydronium ion
 Donates hydrogen ions to molecule acting as base
 Bases
 H+ ion acceptor/proton acceptor
 Accepts hydrogen ions from donating compound
 Contains compounds with NONBONDING valence
electrons to accept proton
Examples

 CH3CO2H + H2O  CH3CO2- + H3O+
 C6H5NH2 + HNO3  C6H5NH3+ + NO3-
Lewis Definition

 Deals with electron pairs
 Acids
 Electron pair acceptor
 Bases
 Electron pair donor
 Covalent bond forms between acid and base
 ALL Bronsted-Lowry bases are Lewis bases, NOT all
Bronsted-Lowry acids act like Lewis acids
CaO +
Example

SO  CaSO
2
3
Monoprotic vs.
Polyprotic Acids

 Monoprotic acids
 Acid contains only ONE hydrogen ion that can be
donated
 Polyprotic acids
 More than one hydrogen ion can be donated from acid
 Amphiprotic
 Chemical compound acting as either acid or base
Conjugate Acids/Bases
to each other through the
 Acids and bases are related
addition/loss of hydrogen ions
 Conjugate acid-base pairs
 Acids produce conjugate bases
 Bases produce conjugate acids
Conjugate Examples

 HClO2 + KOH  H2O + KClO2
 HNO3 + NH3  NH4+ + NO3-
Conjugate Acid/Base
Strength

 Stronger the acid, the weaker the conjugate base
 Stronger the base, the weaker the conjugate acid
 Weak acids/bases have strong conjugate bases/acids
 Acid/Base reactions favor direction from
 Stronger-----------weaker of each conjugate acid/base
Strength of Acids and
Bases

Based on the concentration of H+ or
OH- ions in a solution.
Strong Acids/Bases: completely
dissociate into ions in a solution.
Weak Acids/Bases: do NOT
completely dissociate into ions in a
solution.
Strong Acid Example:
HCl

Weak Acid Example:
CH3CO2H

“BIG 6”---Strong Acids
(Know them!!)

 HClO4
 HI
 HCl
 HNO3
 HBr
 H2SO4
Strong Bases
(Know them!!)

 Group I metal hydroxides (NaOH, KOH, etc.)
 Soluble/Slightly soluble Group II metal hydroxides (
Ca(OH)2, Sr(OH)2, Ba(OH)2 )
 Soluble metal oxides ( Li2O, Na2O, K2O, CaO )

Strong Acids/Bases
COMPLETELY dissociate with
water.
Weak Acids/Bases PARTIALLY
dissociate with water.
Example 1:

HBr + H2O 
H3O+ + Br –
NH4+ + H2O  H3O+ + NH3
Structural Factors
Influencing Acidity

1) Polarity
2) Size of Anion
3) Acid/Base Charge
Structural Factors
Influencing Acidity

1) Polarity—How polar is H-X bond?
  acid strength,  polarity of chemical bond
 If anion (X) is very electronegative, less electron
density around hydrogen ion
 Ex. HF > H2O > NH3 > CH4 (similar in size)
Structural Factors
Influencing Acidity

2) Size of Anion (H-X)
  acid strength as go down a column
 Sizes affects H-X bond
 Larger anions hold onto hydrogen ions LESS and hydrogen
can be easily donated
 Strong acids have weak H-X bonds
 Bond strength greater influence than polarity
 Example: HI and HF

Structural Factors
Influencing Acidity

3) Acid/Base Charge
  negative charge on compound,  acidity (more
basic)
 Greater charge separation with opposite charges
so greater attraction between positive H ion
charge and anion—less likely to lose H+ ion
 Ex. H3PO4
H2PO4HPO4-2
PO4-3
Oxyacids

1) Hydroxide group bonded to element with NO other oxygen
atoms attached.
H—O—Y
2) Hydroxide group bonded to element WITH other oxygens
attached.
H—O—Y—O
Oxyacids—how strong are
they?

  electronegativity of central atom,  acid strength
  # of oxygens surrounding central atom,  acid
strength
 Oxygen is very electronegative—pulls electron density to
it
 Electron density moves away form O-H bond, bond
weakens, H+ ions easily pulled off
 Ex. H2SO4 > H2SO3 , HNO3 > HNO2
Carboxylic Acids

 Weak organic acids
 General Form—
  electronegativity of “R” elements,  acid strength
 Examples: CH3COOH (acetic acid) and CF3COOH
Water Dissociation

 Water acts as both acid and base
 Dissociates into H3O+ and OH- ions
 2H2O(l) 
H3O+(aq) + OH-(aq)
 Rewritten,
 H2O(l) + H2O(l)  H3O+(aq) + OH-(aq)
Water Dissociation
(cont.)

 Rate of reaction reaches equilibrium and allows
[H3O+] and [OH-] to be determined
 2H2O(l) 
H3O+(aq) + OH-(aq)
 Kw = [H3O+] [OH-]
 Kw = water dissociation constant = 1x10-14for pure
water
 [H3O+] = 1x10-7 M in pure water
 [OH-] = 1x10-7 M in pure water
Acid/Base Addition

 Kw = [H3O+] [OH-] = 1 x 10-14 regardless of where
ions come
 More acid, increase [H3O+] /decrease [OH-]
 2 sources for [H3O+] , so less water dissociation
 More base, decrease [H3O+] /increase [OH-]
pH

 Measure of the concentration of [H3O+] ions in an
acidic/basic solution
 Uses logarithmic scale
 pH = -log[H+] or –log[H3O+]
 [H3O+] = 10-pH
 pOH = -log[OH-]
 pH + pOH = 14
pH Scale

Logarithmic scale.
Measures the concentration of hydrogen ions
[H+] in a solution.
Range from 0-14.
NEUTRAL, pH=7. (pure water)
BASE, pH > 7. (ocean water, milk of
magnesia, baking sodea)
ACID, pH < 7. (stomach acid/HCl, vinegar,
soft drinks)
pH scale (cont.)

 Increase [H3O+], decrease pH value, decrease [OH-]
 Decrease [H3O+], increase pH value, increase [OH-]
pH Scale

How do we measure the
pH of a solution?

Acid-base indicators
Weak acids/bases
(ex. litmus paper)
pH meter
Equations

pH = -log[H+]
pOH = -log[OH-]
pH + pOH = 14
[H+][OH-] = 1x10 -14 M2
pH and strong acids

 Strong acids completely dissociate in solution
 SOOO solution’s [H3O+] ~ [H3O+] in acid
 All [H3O+] ions resulting from acid
 Dissociation of water is not a factor due to its small value
Example 1

 What is the pH of Pepsi Cola if the [H3O+] in the solution
is 0.0035M?
Example 2

 Find the pH of a 2.5M HCl solution.
Example 3

 Find the pH of a 0.05M H2SO4 solution
+
O]
Why is the [H3
from water
dissociation not a factor?

1) [H3O+] = 1x10-7 M in water
 not a big number
2) LeChatlier’s Principle
 As more H3O+ ions dissociate from water, reaction shifts to
LEFT to compensate (reforms water)
 If more H3O+ ions added to solution, water reforms to try
and compensate SOOO decrease in [H3O+] from water
dissociation
Homework

 Acid/Base Introductory Packet—due Wednesday