Unit Eight, Chapter Twenty – Acids and Bases
Notes – N – C – U8C20S3-4
I.
Acid-Base Theories
A.
Arrhenius Acids and Bases
1.
Acids contain hydronium ions (H3O+) commonly referred to as hydrogen ions
(H+) that dissociate in water
a.
Different acids release different numbers of H+, known as protons since
the hydrogen loses its electron, resulting in only one proton (positive
charge)
Common Acids and Types
Acid
HNO3 (nitric acid)
HC2H3O2 (acetic acid)
HCl (hydrochloric acid)
HBr (hydrobromic acid)
HF (hydrofluoric acid)
HI (hydroiodic acid)
H2SO4 (sulfuric acid)
H2CO3 (carbonic acid)
H3PO4 (phosphoric acid)
H3PO3 (phosphorous acid)
Number of H+ ions per mole
Type of acid
1
Monoprotic
2
3
Diprotic
Triprotic
b.
c.
d.
2.
Not all compounds containing hydrogen are acids
Not all hydrogens in an acid will necessarily dissociate in water
Dissociation only occurs when very polar bonds are present because
the hydrogen ions are stabilized by dissolving in solution (i.e., forming
hydronium ions in solution)
Bases contain hydroxide ions (OH-) that dissociate in water
a.
Differences in solubility in water (page 227, “Solubility Rules for Ionic
Compounds”)
1.
High solubility: KOH, NaOH, hydroxides with Group 1 elements
2.
Low solubility: Ca(OH)2, Mg(OH)2, hydroxides with Group 2
elements
b.
React with acids to produce salt and water via double-replacement
reaction
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
B.
Bronsted-Lowry Acids and Bases
1.
Some bases do not give off hydroxide ions but are still basic (i.e., NH3, Na2CO3)
2.
Acid: hydrogen-ion (H+) donor; Base: hydrogen-ion (H+) acceptor
3.
Conjugate acid: what makes the solution acid; Conjugate base: what makes
the solution basic
4.
Conjugate acid-base pair related by the loss or gain of a single hydrogen ion
page 1 – N – C – U8C20S3-4
5.
Examples
NH3 (aq ) + H2O(aq ) ⇔ NH+4 (aq ) + OH- (aq )
Ammonia
Wate r
hydrogen ion
acceptor; BronstedLowry base
Am monium ion
hydrogen ion
donor; BronstedLowry acid
Conjugate acid
Hydroxide ion
Conjugate base
since this makes
the solution basic
HCl(aq ) + H2O(l ) ⇔ H3O+ (aq ) + Cl- (aq )
Hydrochloric acid
hydrogen ion
donor; BronstedLowry acid
Water
Hydronium ion
hydrogen ion
acceptor; BronstedLowry base
Conjugate acid
since this makes the
solution acidic
Chloride ion
Conjugate base
6.
Amphoteric substance
a.
Substance that can act as either an acid or a base
b.
Example: water
Lewis Acids and Bases
1.
Lewis Acid: can accept a pair of electrons to form a covalent bond
2.
Lewis Base: can donate a pair of electrons to form a covalent bond
Summary of acids and bases
C.
D.
Acid-Base Definitions
Type
Arrhenius
Bronsted-Lowry
Lewis
II.
Acid
H producer
H+ donor
Electron-pair acceptor
+
Base
OH producer
H+ acceptor
Electron-pair donor
-
Strengths of Acids and Bases
A.
Strong and Weak Acids and Bases
1.
Strong acids completely ionize in water
2.
Weak acids only slightly ionize in water
B.
Acid dissociation constant
HCl(aq ) + H2O(l ) ⇔ H3O+ (aq ) + Cl- (aq )
+
[H ]
[acid]
Ka =
1.
2.
[conjugate base]
[H ][conjugate base] since the concentration of water is a constant
+
[acid]
Calculation done at equilibrium
The smaller the constant, the less likely the acid will ionize in water
page 2 – N – C – U8C20S3-4
C.
3.
The smaller the constant, the weaker the acid
4.
Each hydrogen ionizing in water has a different ionization constant
Base dissociation constant
NH3 (aq ) + H2O(aq ) ⇔ NH+4(aq ) + OH- (aq )
[base]
[conjugate acid]
[OH ]
Kb =
[OH ][conjugate acid] since the concentration of water is a constant
1.
2.
3.
Calculation done at equilibrium
The smaller the constant, the less likely the base will ionize in water
The smaller the constant, the weaker the base
-
[base]
Relative Strengths of Common Acids and Bases
Substance
Hydrochloric acid
Nitric acid
Sulfuric acid
Phosphoric acid
Ethanoic acid
Carbonic acid
Hydrosulfuric acid
Hypochlorous acid
Boric acid
Formula
HCl
HNO3
H2SO4
H3PO4
CH3COOH
H2CO3
H2S
HclO
H3BO3
Relative Strength
Strong acids
↑
|
Increasing strength of acid
Neutral solution
Sodium cyanide
Ammonia
Methylamine
Sodium silicate
Calcium hydroxide
Sodium hydroxide
Potassium hydroxide
NaCN
NH3
CH3NH2
Na2SiO3
Ca(OH)2
NaOH
KOH
page 3 – N – C – U8C20S3-4
Increasing strength of base
|
↓
Strong bases