Chemistry 12
Acids & Bases Unit Notes
4.4. Bronsted-Lowry Theory of A&B
acid : a substance which ___________________________
base : a substance which ___________________________
H+
Typical Bronsted-Lowry A-B rxn eqn:
eg1) NH3 +
base
H2O ⇔
acid
eg2) CH3COOH
______
+
NH4+
+
CH3COO-
H2O ⇔
_____
OHH3O+
+
H2O an acid and a base????? H2O is said to be amphiprotic
Amphiprotic substance: a substance which can _______________________________________
• If a substance acts as an acid around a stronger base, or a base around a stronger acid, it is an
AMPHIPROTIC substance. (eg. H2O
Note: every Bronsted-Lowry rxn there is an acid and a base on both sides of the rxn eqn
N2H5+
A
+
S2B
⇔
N2H4
B*
HSA*
+
• Acids classified as:
releases one proton.
Monoprotic
releases two protons.
Diprotic
releases three protons.
Triprotic
releases many protons
Polyprotic
B* & A* are acids and bases
for the reverse reaction
• Bases classified as:
accepts one proton.
Monobasic
accepts two protons.
Dibasic
accepts three protons.
Tribasic
accepts many protons
Polybasic
Polyprotic acids release H+ ions ____ ___ __ ______:
step 1
H3PO4
step 2
H2PO4-
step 3
HPO42-
PO43-
If a substance has a ______________ charge and still has an easily removable ______________ then the
substance is ________________
Also, _______________ acids which have lost one proton are amphiprotic
Hebden #11-14
-
2-
H2S ↔ HS ↔ S
1
Chemistry 12
Acids & Bases Unit Notes
4.5 Conjugate Acids and Bases
Conjugate acid-base pair: the pair of molecules in a rxn eqn that _________________________.
Conjugate acid: _________ the extra proton
Conjugate base: _________ the extra proton
Bronsted-Lowry eqns have two conjugate A-B pairs:
eg1)
base2
acid2
____________
|
|
HF + H2O ⇔ F + H3O+
|____________|
acid1
base1
Conjugate Pair
eg1) HF and FH3O+ and H2O
Conjugate A
Conjugate B
What is the conjugate B for the following compounds?
NH4+
HCO3-
H2PO4-
What is the conjugate A for the following compounds?
CH3COO-
HC2O4-
OH-
Connect and label the conjugate pairs:
a)
HCl(aq) + H2O(l) → H3O+ (aq) + Cl- (aq)
b)
H2SO4(aq) + H2O(l) → H3O+ (aq) + HSO4- (aq)
Hebden #16, 19
Summary for a Bronsted-Lowry A-B rxn:
Conjugate Acid
+Form of A
+
Conjugate Base
form of B
Conjugate Base
form+of A
+
Conjugate Acid
form of B
2
Chemistry 12
Acids & Bases Unit Notes
4.6. “Strong and Weak” Acids and Bases
Recall:
Concentrated = _____ molarity (M) (mol/L) and
Dilute = _____ molarity
BUT in chemistry…
And…
Strong acid ≠ _______________ acid
Weak acid ≠ ____________ acid
Strong Acid (SA) or Strong Base (SB): _____________ 100%:
Eg:
NaOH(s) Na+(aq) + OH-(aq)
HCl(g) H+(aq) + Cl-(aq)
Weak Acid (WA) or Weak Base (WB): ______ than 100% dissociated:
Eg:
NH3(aq) + H2O(l) ⇔ NH4+(aq) + OH-(aq)
HF(aq) + H2O(l) ⇔ H3O+(aq) + F-(aq)
not complete dissociation
∴ eqb arrows
Characteristics of Acids and Bases:
are STRONG if they:
dissociate __________________ in water
have an undefined, or very _______ Ka value
have ______________ dissociation rxn (→)
are WEAK if they:
do _______ dissociate completely in water
have a ____________ Ka value
have an ___________ dissociation rxn (⇔)
Examples of STRONG Acids (p334): _____________________________________________________
Examples of STRONG Bases (p. 122): ____________________________________________________
PRACTICE: Describe the acid below as:
a)monoprotic, b) diprotic, c) triprotic
H2SO4(aq) → H+ (aq) + HSO4- (aq)
d) strong,
_________
e) weak
metal
hydroxides
HSO4- (aq) ⇔ H+ (aq) + SO42-(aq)
_________
Which is a stronger acid?
HNO2
_
HPO42-
Which is a stronger base?
HC2O4- _
CN3
Chemistry 12
Acids & Bases Unit Notes
6. The Levelling Effect
Strong acids are 100% dissociated in soln.
are all equiv. to H3O+ solns of same conc.
∴
eg)
1M HI produces 1M H3O+ and 1M I1M HClO4 produces 1M H3O+ and 1M ClO4-
Strengths are
“leveled”
Similarly, the strong bases are 100% dissociated in soln.
Therefore, are all equiv. to OH- solns of that conc
eg)
1M KOH produces 1M OH- and 1M K+
1M NaOH produces 1M OH- and 1M Na+
Strengths are
“leveled”
Weaker A&B dissociate to different extents, so have unique strengths.
Hebden # 21-27
4
Chemistry 12
Acids & Bases Unit Notes
4.4. Bronsted-Lowry Theory of A&B
acid : a substance which donates a proton
base : a substance which accepts a proton
H+
Typical Bronsted-Lowry A-B rxn eqn:
eg1) NH3 +
base
H2O ⇔
acid
eg2) CH3COOH
acid
+
NH4+
+
CH3COO-
H2O ⇔
base
OHH3O+
+
H2O an acid and a base????? H2O is said to be amphiprotic
Amphiprotic substance: a substance which can act as either an acid or a base
• If a substance acts as an acid around a stronger base, or a base around a stronger acid, it is an
AMPHIPROTIC substance. (eg. H2O
Note: every Bronsted-Lowry rxn there is an acid and a base on both sides of the rxn eqn
N2H5+
A
+
S2B
⇔
N2H4
B*
HSA*
+
• Acids classified as:
releases one proton.
Monoprotic
releases two protons.
Diprotic
releases three protons.
Triprotic
releases many protons
Polyprotic
B* & A* are acids and bases
for the reverse reaction
• Bases classified as:
accepts one proton.
Monobasic
accepts two protons.
Dibasic
accepts three protons.
Tribasic
accepts many protons
Polybasic
Polyprotic acids release H+ ions one at a time:
step 1
H3PO4
step 2
H2PO4-
step 3
HPO42-
PO43-
If a substance has a negative charge and still has an easily removable hydrogen then the substance is
amphiprotic
Also, polyprotic acids which have lost one proton are amphiprotic
Hebden #11-14
-
2-
H2S ↔ HS ↔ S
5
Chemistry 12
Acids & Bases Unit Notes
4.5 Conjugate Acids and Bases
Conjugate acid-base pair: the pair of molecules in a rxn eqn that differ by one proton.
Conjugate acid: has the extra proton
Conjugate base: lacks the extra proton
Bronsted-Lowry eqns have two conjugate A-B pairs:
eg1)
base2
acid2
____________
|
|
HF + H2O ⇔ F + H3O+
|____________|
acid1
base1
Conjugate Pair
eg1) HF and FH3O+ and H2O
Conjugate A
HF
H3O+
Conjugate B
FH2O
What is the conjugate B for the following compounds?
NH4+
HCO3-
NH3
CO32-
H2PO4-
HPO42-
HC2O4-
H2C2O4
What is the conjugate A for the following compounds?
CH3COO-
OH-
CH3COOH
H2O
Connect and label the conjugate pairs:
a)
HCl(aq) + H2O(l) → H3O+ (aq) + Cl- (aq)
c)
H2SO4(aq) + H2O(l) → H3O+ (aq) + HSO4- (aq)
Hebden #16, 19
Summary for a Bronsted-Lowry A-B rxn:
Conjugate Acid
+Form of A
+
Conjugate Base
form of B
Conjugate Base
form+of A
+
Conjugate Acid
form of B
6
Chemistry 12
Acids & Bases Unit Notes
4.6. “Strong and Weak” Acids and Bases
Recall:
concentrated = high molarity (M) (mol/L) and
Dilute = low molarity
BUT in chemistry…
And…
Strong acid ≠ concentrated acid
Weak acid ≠ dilute acid
Strong Acid (SA) or Strong Base (SB): dissociate 100%:
Eg:
NaOH(s) Na+(aq) + OH-(aq)
HCl(g) H+(aq) + Cl-(aq)
Weak Acid (WA) or Weak Base (WB): less than 100% dissociated:
Eg:
NH3(aq) + H2O(l) ⇔ NH4+(aq) + OH-(aq)
HF(aq) + H2O(l) ⇔ H3O+(aq) + F-(aq)
not complete dissociation
∴ eqb arrows
Characteristics of Acids and Bases:
are STRONG if they:
dissociate completely in water
have an undefined, or very large Ka value
have complete dissociation rxn (→)
are WEAK if they:
do NOT dissociate completely in water
have a defined Ka value
have an equilib dissociation rxn (⇔)
Examples of STRONG Acids (p334): HClO4, HI, HBr, HCl, HNO3, H2SO4
Examples of STRONG Bases (p. 122): NaOH, KOH, Mg(OH) 2,Ca(OH) 2,Fe(OH)3, Zn(OH)2
PRACTICE: Describe the acid below as:
a)monoprotic, b) diprotic, c) triprotic
H2SO4(aq) → H+ (aq) + HSO4- (aq)
b&d
HSO4- (aq) ⇔ H+ (aq) + SO42-(aq)
a&e
metal
d) strong,
e) weak
hydroxides
Which is a stronger acid?
HNO2
>
HPO42-
Which is a stronger base?
HC2O4- <
CN7
Chemistry 12
Acids & Bases Unit Notes
6. The Levelling Effect
Strong acids are 100% dissociated in soln.
are all equiv. to H3O+ solns of same conc.
∴
eg)
1M HI produces 1M H3O+ and 1M I1M HClO4 produces 1M H3O+ and 1M ClO4-
Strengths are
“leveled”
Similarly, the strong bases are 100% dissociated in soln.
Therefore, are all equiv. to OH- solns of that conc
eg)
1M KOH produces 1M OH- and 1M K+
1M NaOH produces 1M OH- and 1M Na+
Strengths are
“leveled”
Weaker A&B dissociate to different extents, so have unique strengths.
Hebden # 21-27
8