Topic 8.1

General Characteristics
• Acids
– normally C, O, F, Cl and especially H covalently bonded
• H
2
SO
4
, HCl, H
2
NO
3
– they dissociate/ionize (break apart) to form at least one cation (H + ) and an anion
• Bases
– many contain a metal and a hydroxide ion (OH )
• NaOH, Mg(OH)
2
, NH
3
– many produce OH when dissolved in water

Background (none of these are good definitions now)
• concepts acids and bases were loosely defined as substances that change some properties of water
• criteria that was often used was taste
– substances were classified
• salty-tasting, sour-tasting, sweet-tasting, bitter-tasting
• sour-tasting substances would give rise to the word
'acid', which is derived from the Greek word oxein since it was once thought that oxygen was responsible for a compounds acid properties

Nobel Prize in Chemistry 1903
– too narrow (but works for most acids and bases) because it had to include water
• left out some acids and bases
– acids ionize in water to form (H+) (or H
3
0 + called hydronium ions)
– bases form hydroxide ions (OH ) in water

– involves the transfer of a proton (H + )
• a “proton” is really just a hydrogen atom that has lost its electron
– acids : molecule or ion that acts as proton
(H + ) donor
– bases : molecule or ion that acts as proton
(H + ) acceptor

acid base acid base
HCl
H
2
O
⇌ H
3
O +
Cl -
– in the forward reaction,
• HCl is the acid (will donate H + )
• H
2
O is the base (will accept H + )
– in the reverse reaction,
• H
3
O + is the acid (will donate H + )
• Cl is the base (will accept H + )

(joined together)
• an acid-base reaction always involves (at least) two conjugate pairs that differ by an H +
• conjugate acids and conjugate bases are compounds formed when a H+ ion is gained and a H+ is lost
• a conjugate pair is:
– an acid and its conjugate base
– a base and its conjugate acid

Brønsted-Lowry conjugate pairs base acid acid base

Acid
H
2
SO
4
HCl
HNO
3
H
3
O +
HSO
4
-
CH
3
COOH
H
2
CO
3
NH
4
+
HCO
3
-
H
2
O
• Strong acid  Weak conjugate base
– ALL of the acid donates H +
– almost NO H+ is accepted back
• Weak acid  Strong conjugate base
• Strong base  Weak conjugate acid
• Weak base  Strong conjugate acid
Strength
Very Strong
Fairly Strong
Weak
Very Weak
Base
HSO
4
-
Cl -
NO
3
-
H
2
O
SO
4
2-
CH
3
COO -
HCO
3
-
NH
3
CO
3
2-
OH -
Strength
Very Weak
Weak
Less Weak
Fairly Strong

Amphiprotic substances can act as an acid or a base (amphoteric substances are slightly different when considering the Lewis definition in topic 17)
Acid Base Conjugate Acid Conjugate Base
HCl
H
2
PO
4
-
NH
4
+
+
+
+
H
2
O 

H
2
O 

H
2
O 

H
3
O + + Cl -
H
3
O + + HPO
4
2-
H
3
O + + :NH
3
Base
:NH
3
PO
4
3-
+
+
Acid Conjugate Acid Conjugate Base
H
H
2
2
O 
O 


NH
4
+
HPO
4
2-
+
+
OH
OH
-
-
17

• mono protic acids contain a single proton that can be donated ( H Cl, H NO
3
, H , CH
3
COO H )
• di protic acids contain two protons that can be
CO
3
, H
2
SO
4
, H
NO
2
) donated (
• tri protic acids contain three protons that can be donated (
H
H
2
3
PO
4
)
2
SO
3
• for a substance to be an acid, the hydrogen usually has to be attached to oxygen or a halogen
– for example, in CH
3
COOH, only the H on “OH” is able to be donated, the three hydrogens on carbon are non-acidic (do not write this C
2
H
4
O
2
)

Lewis Acid and Base Definitions (this is now
HL (topic 18) and not required for topic 8 )
– most general/encompassing definition
• must have lone pairs ( ligands ) available to donate
– Lewis acids accept a pair of electrons to form a coordinate covalent bond
– Lewis bases donate a pair of electrons to form a dative covalent bond acid

LONE PAIR
DONOR = Base
LONE PAIR
ACCEPTOR = Acid

LONE PAIR
DONOR = Base
LONE PAIR
ACCEPTOR = Acid

• when ammonia donates a pair of electrons to hydrogen, its a Lewis base
H H
H
+
+ N H H N
+
H
H
H
• when boron trifluoride accepts a pair of electrons from nitrogen (in NH
3
) its a Lewis acid
F H H
F
F B
+ N H H N
+
B
-
F
F
H
H F