Download Acid-Base Reactions

Acid-Base Reactions - Definitions
Assume the acids and bases are in water.
acid
Arrhenius:
donates H+
base
donates OH–
Bronsted-Lowry
donates H+
accepts H+
Lewis:
electron pair acceptor
electron pair donor
(classic: NH3)
Acid Dissociation
+
H
1-
HCl
Cl
Acid
Conjugate base
Conjugate pair
Kelter, Carr, Scott, Chemistry A World of Choices 1999, page 280
Conjugate Acid-Base Pairs
Use Brønsted–Lowry Definitions of Acid & Base
conjugates
base
HCl
+
H2O
conj. acid
H3O+
+
acid
Clconj. base
conjugates
HCl
acid
+
H2O
H3O+
base
CA
+
ClCB
Conjugate Acid-Base Pairs
conjugates
acid
NH3
+ H2O
base
base
NH41+
+ OH-
acid
conjugates
NH3
base
+
H2O
acid
NH41+
CA
+ OHCB
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.
HCl + H2O
acid
base
H3O1+
CA
+ ClCB
Amphoteric: A substance that can act as either an acid or a base.
1. Water can act as an acid:
H+
2. Water can act as a base:
+
H
S Water is amphoteric.
Zwitterion:
A neutral molecule with both positive and negative electrical
charges.
acid
conj. acid
example: amino acid (glycine)
base
cong. base
Strong Acids & Bases
Strong Acids
HCl
HBr
HI
HNO3
HClO3
H2SO4
Strong Bases
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 – )
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–)
Strong Acid + Strong Base
1. 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)
2.
net ionic equation:
H+(aq) + OH–(aq)  H2O(l)
(Na+ and Cl– are spectator ions) (neutralization reaction)
Weak Acid + Strong Base
acetic acid, HC2H3O2, and NaOH as an example.
HC2H3O2 + NaOH
(A) Two reactions occur:
(1) weak acid:
(2) strong base:
 ?
HC2H3O2
H+ + C2H3O2–
(reversible reaction)
(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.)
Practice
Write the net ionic equation for sulfuric acid and barium hydroxide:
H2SO4 + Ba(OH)2
strong acid strong base
2H+ + 2OH– -> 2H2O
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
Strong Acid + Weak Base
(2)HCl and NH4 as examples
HCl + NH3 
Two reactions occur:
(1) weak base:
(2) strong acid:
Overall Reaction:
net ionic equation:
NH3 + H2O
NH4+ + OH–
(reversible reaction)
HCl  H+ + Cl–
NH3 + H2O + H+ + Cl–
NH4+ + H2O + Cl–
H+ + NH3  NH4+
(H2O from H+ + OH–)
(Cl– is a spectator ion)
Practice
Ex. 4.3: Write the net ionic equation for each of the following:
a) Hypochlorous acid + barium hydroxide
weak acid
strong base
2H+ + 2OH– -> 2H2O
H+ + OH– -> H2O
Acid-Base Titration
Commonly used to determine the concentration of a solution.
https://youtu.be/wQ98RfLJ5Uk?list=PLnEQfEiSza7A2yiSG8sun5YGxJ1sg0Rwe
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.
Acid-Base Titration
What is the molar mass of H2X of 29.45 mL of 0.187 M NaOH are required to neutralize a
solution prepared by adding 0.242 g of H2X to enough water to make 25.00 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–
(Then 25.00 mL volume is irrelevant.)
mole H2X
MM mole H2X
Acid-Base Titration
What is the molar mass of H2X of 29.45 mL of 0.187 M NaOH are required to neutralize a
solution prepared by adding 0.242 g of H2X to enough water to make 25.00 mL of solution?
mol H2X:
MM H2X: