Download Acids and Bases

Unit 4: Equilibrium, Acids & Bases
Part 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
Review
 Arrhenius Acid: Substance that increases the
concentration of H+ ions when dissolved in water
HCl (g)
H 2O
H+ (aq) + Cl- (aq)
 Arrhenius Base: Substance that increases the
concentration of OH- ions when dissolved in
water.
H 2O
NaOH (s)
Na+ (aq) + OH- (aq)
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
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)
B-L acid
 A Bronsted-Lowry acid must have a hydrogen
that can be lost as H+
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)
B-L acid
H3O+ (aq) + Cl- (aq)
B-L base
 A B-L base must have an atom with a lone
pair of electrons that can form a new bond
to a hydrogen ion.
Bronsted-Lowry Acids & Bases
 The H+ ion interacts strongly with the
nonbonding pairs of electrons on water
molecules, forming the hydronium ion
+
H
+
O H
H
H O H
H
hydronium ion
 The hydronium ion is responsible for the
characteristic properties of aqueous solutions of
acids.
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)
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)
base
NH3 (g) + H2O (l)  NH4+ (aq) + OH- (aq)
acid
Bronsted-Lowry Acids & Bases
 Examples of other amphoteric substances
include:
 NaHCO3
 NaH2PO4
 NaHSO4
 Alcohols such as ethanol:
H H
H C C O H
H H
Bronsted-Lowry Acids & Bases
 In any acid-base equilibrium, both forward and
reverse reactions involve proton transfers.
HNO2 (aq) + H2O (l)
 Forward Reaction:
 B-L acid =
 B-L base =
 Reverse Reaction:
 acid =
 base =
NO2 – (aq) + H3O+ (aq)
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)
BL acid
BL base
NO2 – (aq) + H3O+ (aq)
conjugate
base
conjugate
acid
 HNO2 and NO2- are called a conjugate acid-
base pair.
 H2O and H3O+ are also a conjugate acid-base
pair.
Bronsted-Lowry Acids & Bases
 Conjugate acid:
 The acid formed when a base gains a proton
 Conjugate acid of H2O
H O +
3
 Conjugate acid of SO4 2-
 HSO4
-
 NOTE: The conjugate acid is always shown on
the product side.
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.
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 NO2H3O+ and H2O
HCO3 - and CO32NH4+ and NH3
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)
Bronsted-Lowry Acids & Bases
Example: Identify the BL acid, BL base,
conjugate acid and conjugate base in the
following reactions.
NaH
+
CH3CH2OH
CH3C
CH +
NaNH2
CH3CH2O- Na+
NH3
+
CH3C
+
H2
C - Na+
Bronsted-Lowry Acids & Bases
 In any acid-base equilibrium, both forward
and reverse reactions involve proton transfers.
HNO2 (aq) + H2O (l)
acid
base
NO2 – (aq) + H3O+ (aq)
Conjugate
base
Conjugate
acid
 How can we predict the position of the
chemical equilibrium?
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
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
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.
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
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.
Bronsted-Lowry Acids and Bases
The stronger the acid,
the weaker the
conjugate base
The stronger the base,
the weaker the
conjugate acid
Bronsted-Lowry Acids & Bases
Example: Does the following acid/base
equilibrium favor the reactants or products?
-
HF (aq) + HSO4 (aq)
-
H2SO4 (aq) + F (aq)