1. Acid-Base Reactions - Definitions
Assume the acids and bases are in water.
acid base
Arrhenius: donates H+ donates OH–
Bronsted-Lowry donates H+ accepts H+ (classic: NH3)
Lewis: electron pair acceptor electron pair donor

2. Acid Dissociation + 1- HCl Cl Conjugate base Acid Conjugate pair H
When a strong acid dissolves in water, the proton that is released is transferred to a water molecule that acts as a proton acceptor or base.
Resulting H3O+ ion is called the hydronium ion.
Bases that do not contain the hydroxide ion accept a proton from water, so small amounts of OH– are produced—water acts like an acid by donating a proton.
Substances that can behave as both an acid and a base are said to be amphoteric.
Products of an acid-base reaction are also an acid and a base. The acid product is the conjugate acid of the base, and the base product is the conjugate base of the acid.
All acid-base reactions involve two conjugate acid-base pairs.
Conjugate base
Acid
Conjugate pair
Kelter, Carr, Scott, Chemistry A World of Choices 1999, page 280

3. Conjugate Acid-Base Pairs Use Brønsted–Lowry Definitions of Acid & Base
conjugates
base
conj. acid
HCl H2O H3O Cl-
acid
conj. base
conjugates
Two species that differ by only a proton constitute a conjugate acid-base pair.
1. Conjugate base has one less proton than its acid; A– is the conjugate base of HA
2. Conjugate acid has one more proton than its base; BH+ is the conjugate acid of B
• In the reaction of HCl with water, HCl, the parent acid, donates a proton to a water molecule, the parent base, forming Cl–; HCl and Cl– constitute a conjugate acid-base pair.
• In the reverse reaction, the Cl– ion in solution acts as a base to accept a proton from H3O+, forming H2O and HCl; H3O+ and H2O constitute a second conjugate acid-base pair. Any acid-base reaction must contain two conjugate acid-base pairs, which in this example are HCl/Cl– and H3O+/H2O
• HCl (aq) H2O (l)  H3O+ (aq) Cl– (aq)
parent acid parent base conjugate acid conjugate base
HCl H2O H3O Cl-
acid base CA CB

4. Conjugate Acid-Base Pairs
conjugates
acid
base
NH H2O NH OH-
base
acid
conjugates
base acid CA CB
NH H2O NH OH-

5. Conjugate Acid-Base Pairs: Direction of Reaction The reaction proceeds in the direction such that the stronger acid donates its proton to the stronger base.
acid base CA CB
HCl H2O H3O Cl-

6. Amphoteric: A substance that can act as either an acid or a base.
1. Water can act as an acid:
2. Water can act as a base: H+ H+
S Water is amphoteric.

7. Zwitterion: A neutral molecule with both positive and negative electrical charges.
acid
cong. base
base
conj. acid
Water is amphiprotic: it can act as an acid by donating a proton to a base to form the hydroxide ion, or as a base by accepting a proton from an acid to form the hydronium ion, H3O+
Structure of the water molecule
1. Polar O–H bonds and two lone pairs of electrons on the oxygen atom
2. Liquid water has a highly polar structure
example: amino acid (glycine)

8. Strong Acids & Bases Strong Acids Strong Bases HCl HBr Group 1 HI LiOH
HNO3
HClO3
H2SO4
Group 1
LiOH
NaOH
KOH
(Rb & Cs)
Heavy Group 2
Ca(OH)2
Sr(OH)2
Ba(OH)2
Strong acids & bases completely dissociate (e.g., HCl -> H+ + Cl–)
Weak acids & bases don’t “ “ (e.g., HF H+ + F – )

9. Types of Acid-Base Reactions
Types: (1) strong acid + strong base
(2) weak acid + strong base
(3) strong acid + weak base
[(4) weak acid + weak base] {treated separately with titration}
(N.B., general formulas for acid = HB dissociates into H+ and B–)

10. Strong Acid + Strong Base
strong acid + strong base (using HCl and NaOH as examples)
Both completely dissociate in water before the reaction occurs.
HCl + NaOH  H+(aq) + Cl–(aq) + Na+(aq) + OH–(aq)
net ionic equation: H+(aq) + OH–(aq)  H2O(l)
(Na+ and Cl– are spectator ions) (neutralization reaction)

11. acetic acid, HC2H3O2, and NaOH as an example. HC2H3O2 + NaOH  ?
Weak Acid + Strong Base
acetic acid, HC2H3O2, and NaOH as an example.
HC2H3O2 + NaOH  ?
(A) Two reactions occur:
(1) weak acid: HC2H3O H+ + C2H3O2– (reversible reaction)
(2) strong base: (NaOH is completely dissociated so it is Na+ + OH–)
NaOH  Na+ + OH–
(S) overall reaction: Na+ + OH– + H+ + C2H3O2–  Na+ + H2O + C2H3O2–
(B) net ionic equation: HC2H3O2 + OH–  C2H3O2– + H2O(l)
(Na+ is a spectator ion.)

12. Practice
Write the net ionic equation for sulfuric acid and barium hydroxide:
H2SO4 + Ba(OH)2
strong acid strong base
2H OH– -> 2H2O

13. Practice
Ex. 4.3: Write the net ionic equation for the following reaction:
a) Hypochlorous acid + barium hydroxide
weak acid strong base
HClO2 + OH– -> ClO2– + H2O

14. Strong Acid + Weak Base HCl and NH4 as examples HCl + NH3 
Two reactions occur:
(1) weak base: NH3 + H2O NH4+ + OH– (reversible reaction)
(2) strong acid: HCl  H+ + Cl–
Overall Reaction: NH3 + H2O + H+ + Cl– NH4+ + H2O + Cl– (H2O from H+ + OH–)
(Cl– is a spectator ion)
net ionic equation: H+ + NH3  NH4+

15. Practice
Ex. 4.3: Write the net ionic equation for each of the following:
a) Hypochlorous acid + barium hydroxide
weak acid strong base
2H OH– -> 2H2O
H OH– -> H2O

16. Acid-Base Titration
Commonly used to determine the concentration of a solution.

17. Acid-Base Titration
Titration: Measuring the concentration of an unknown using the volume and concentration of a standard solution of known concentration.
Objective: To determine the point at which the reaction is complete. Equivalence point.

18. Acid-Base Titration
What is the molar mass of H2X of mL of M NaOH are required to neutralize a solution prepared by adding g of H2X to enough water to make mL of solution?
Plan: Determine number of moles of H2X
Then you can calculate the mass, molar mass, and even volume or molarity.
number of moles NaOH moles OH– mole H2X MM mole H2X
(Then mL volume is irrelevant.)

19. Acid-Base Titration
What is the molar mass of H2X of mL of M NaOH are required to neutralize a solution prepared by adding g of H2X to enough water to make mL of solution?
mol H2X:
MM H2X: