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+ (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
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 basesLewis acids and bases
Bronsted-Lowry Acids & Bases
Bronsted-Lowry Acidany 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
Bronsted-Lowry Acids & Bases
Bronsted-Lowry Baseany 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
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.
H+ + OH
H H OH
H
hydronium ion
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 amphotericCapable 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
Bronsted-Lowry Acids & Bases
Examples of other amphoteric substances include:NaHCO3NaH2PO4NaHSO4
Alcohols such as ethanol:
H CH
HCH
HO H
In any acid-base equilibrium, both forward and reverse reactions involve proton transfers.
HNO2 (aq) + H2O (l) NO2 – (aq) + H3O+
(aq)
Bronsted-Lowry Acids & Bases
Forward Reaction:B-L acid =B-L base =
Reverse Reaction:acid =base =
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.
Bronsted-Lowry Acids & Bases
BL acid conjugate base
BL base conjugate acid
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.
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 NO2
-
H3O+ and H2O HCO3
- and CO32-
NH4+ and NH3
Bronsted-Lowry Acids & Bases
Example: Identify the acid, base, conjugate acid and conjugate base for the following reaction.
HSO4– (aq) + CO3
2- (aq) SO42- (aq) + HCO3
- (aq)
Bronsted-Lowry Acids & BasesExample: Identify the BL acid, BL base, conjugate acid and conjugate base in the following reactions.
NaH + CH3CH2OH CH3CH2O- Na+ + H2
CH3C CH + NaNH2 NH3 + CH3C C - Na+
In any acid-base equilibrium, both forward and reverse reactions involve proton transfers.
HNO2 (aq) + H2O (l) NO2 – (aq) + H3O+
(aq)
Bronsted-Lowry Acids & Bases
How can we predict the position of the chemical equilibrium?
baseacid Conjugateacid
Conjugatebase
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 completelyweak acid
ionizes partially solutions contain mixture of acid molecules, hydronium ion, and conjugate base
negligible acidity no tendency to form H+ in solution
The seven most common strong acids:HCl hydrochloric acidHBr hydrobromic acidHI hydroiodic acidHNO3 nitric acidHClO3 chloric acidHClO4 perchloric acidH2SO4 sulfuric acid
Bronsted-Lowry Acids & Bases
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:CH4H2OH –
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
Example: Does the following acid/base equilibrium favor the reactants or products?
Bronsted-Lowry Acids & Bases
HF (aq) + HSO4- (aq) H2SO4 (aq) + F- (aq)
