Unit 4: Equilibrium, Acids

1
Unit 4 Equilibrium, Acids BasesPart 2
Acids and Bases

• Bronsted Lowry Acids and Bases
• Autoionization of Water
• pH
• Strong Acids and Bases
• Weak Acids and Bases
• Ionization Constants
• Buffers
• Titrations
• Lewis Acids and Bases

2
Review

• Arrhenius Acid Substance that increases the
  concentration of H ions when dissolved in water
• HCl (g) H (aq) Cl- (aq)
• Arrhenius Base Substance that increases the
  concentration of OH- ions when dissolved in
  water.
• NaOH (s) Na (aq) OH- (aq)

H2O
H2O
3
Review

• The Arrhenius definition of acids and bases is
  limited to aqueous solutions
• Two other common definitions for acids and bases.
• Bronsted-Lowry acids and bases
• Lewis acids and bases

4
Bronsted-Lowry Acids Bases

• Bronsted-Lowry Acid
• any substance that can transfer a proton (H ion)
  to another substance
• a proton donor
• HCl (g) H2O (l) H3O (aq) Cl- (aq)
• A Bronsted-Lowry acid must have a hydrogen that
  can be lost as H

B-L acid
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Bronsted-Lowry Acids Bases

• Bronsted-Lowry Base
• any substance that can accept a proton (H ion)
  from another substance
• A proton acceptor
• HCl (g) H2O (l) H3O (aq) Cl- (aq)
• A B-L base must have an atom with a lone pair of
  electrons that can form a new bond to a hydrogen
  ion.

B-L acid
B-L base
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Bronsted-Lowry Acids Bases

• The H ion interacts strongly with the nonbonding
  pairs of electrons on water molecules, forming
  the hydronium ion
• The hydronium ion is responsible for the
  characteristic properties of aqueous solutions of
  acids.

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Bronsted-Lowry Acids Bases

• H3O is a more realistic depiction of the
  hydrogen ion in solution but for convenience we
  often use H (aq) to depict the hydrated hydrogen
  ion.

HCl (g) H20 (l) ? H3O (aq) Cl- (aq) HCl
(aq) ? H (aq) Cl- (aq)
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Bronsted-Lowry Acids Bases

• Some substances like water are amphoteric
• Capable of acting as either an acid or a base
• H2O (l) HCl (g) ? H3O (aq) Cl- (aq)
• NH3 (g) H2O (l) ? NH4 (aq) OH- (aq)

base
acid
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Bronsted-Lowry Acids Bases

• Examples of other amphoteric substances include
• NaHCO3
• NaH2PO4
• NaHSO4
• Alcohols such as ethanol

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Bronsted-Lowry Acids Bases

• In any acid-base equilibrium, both forward and
  reverse reactions involve proton transfers.
• HNO2 (aq) H2O (l) NO2 (aq) H3O
  (aq)

• Forward Reaction
• B-L acid
• B-L base

• Reverse Reaction
• acid
• base

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Bronsted-Lowry Acids Bases

• The reaction between a BL acid and base produces
  a new acid (the conjugate acid) and a new base
  (the conjugate base).
• HNO2 (aq) H2O (l) NO2 (aq) H3O
  (aq)
• HNO2 and NO2- are called a conjugate acid-base
  pair.
• H2O and H3O are also a conjugate acid-base pair.

BL acid
conjugate base
BL base
conjugate acid
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Bronsted-Lowry Acids Bases

• Conjugate acid
• The acid formed when a base gains a proton
• Conjugate acid of H2O
• H3O
• Conjugate acid of SO4 2-
• HSO4 -
• NOTE The conjugate acid is always shown on the
  product side.

13
Bronsted-Lowry Acids Bases

• Conjugate base
• The base formed by removing a proton from an acid
• Conjugate base of H2O
• OH -
• Conjugate base of H2SO4
• HSO4 -
• NOTE The conjugate base is always shown on the
  product side.

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Bronsted-Lowry Acids Bases

• Conjugate acid-base pair
• An acid and a base that differ only in the
  presence or absence of a single proton
• HNO2 and NO2-
• H3O and H2O
• HCO3 - and CO32-
• NH4 and NH3

15
Bronsted-Lowry Acids Bases

• Example Identify the acid, base, conjugate acid
  and conjugate base for the following reaction.

HSO4 (aq) CO32- (aq) SO42- (aq) HCO3-
(aq)
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Bronsted-Lowry Acids Bases

• Example Identify the BL acid, BL base,
  conjugate acid and conjugate base in the
  following reactions.

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Bronsted-Lowry Acids Bases

• In any acid-base equilibrium, both forward and
  reverse reactions involve proton transfers.
• HNO2 (aq) H2O (l) NO2 (aq) H3O
  (aq)

Conjugate acid
Conjugate base
base
acid

• How can we predict the position of the chemical
  equilibrium?

18
Bronsted Lowry Acids Bases

• The relative strengths of the acid and the
  conjugate acid can be used to predict the
  position of the equilibrium.
• Equilibrium favors the formation of the weaker
  acid.
• The stronger acid more effectively loses a proton
  than its conjugate acid

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Bronsted Lowry Acids Bases

• Every substance can be categorized as either
• strong acid
• ionizes completely
• weak acid
• ionizes partially
• solutions contain mixture of acid molecules,
  hydronium ion, and conjugate base
• negligible acidity
• no tendency to form H in solution

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Bronsted-Lowry Acids Bases

• The seven most common strong acids
• HCl hydrochloric acid
• HBr hydrobromic acid
• HI hydroiodic acid
• HNO3 nitric acid
• HClO3 chloric acid
• HClO4 perchloric acid
• H2SO4 sulfuric acid

You must know these acids by name and formula.
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Bronsted-Lowry Acids Bases

• Examples of common weak acids
• acetic acid,
• citric acid,
• phosphoric acid
• Examples of substances with negligible acidity
• CH4
• H2
• OH
• NH3

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Bronsted-Lowry Acids Bases

• An inverse relationship exists between the
  strength of an acid and its conjugate base or
  between a base and its conjugate acid.
• Strong acids form very weak conjugate bases
  (usually have negligible basicity).
• Weak acids form stronger (but still fairly weak)
  conjugate bases
• Substances with negligible acidity form very
  strong bases.

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Bronsted-Lowry Acids and Bases

• The stronger the acid, the weaker the conjugate
  base
• The stronger the base, the weaker the conjugate
  acid

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Bronsted-Lowry Acids Bases

• Example Does the following acid/base
  equilibrium favor the reactants or products?