Chapter16AcidsandBases
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a) H2SO4 b) HSO4c) H3O+ d) NH4+
19
Problem
What is the conjugate acid of each of the following Brønsted- Lowry bases? a) HCO3b) CO32c) OHd) H2PO4-
20
Problem
Ka
CH 3COO H3O CH 3COOH
1.8 x 10-5 at 25 C
12
BL acid strength
We give the equilibrium constant a special name for reactions like these – the acid dissociation (or ionization) constant Ka. Since the value of the constant is less than one, the acetic acid does not dissociate to a great extent – it is a weak acid!
Ka
Cl H3O
HCl
1 x106 at 25 C
14
Hydrated protons and hydronium ions
What is the strongest Brønsted-Lowry acid there is? The strongest Brønsted-
Lowry acid is H+. The ultimate proton-donor is a proton itself!
Chapter 16
Acids and Bases
Arrhenius theory of acids and bases
The Arrhenius concept: acids dissociate in water (aqueous solution) to produce hydrogen ions H+, and bases dissociate in water to give hydroxide ions, OH-.
Of the following species, one is acidic, one
is basic, and one is amphiprotic in their reactions with water: HNO2, PO43-, HCO3-. Write the four equations needed to
8
BL base strength
The strength of a Brønsted-Lowry base can be quantified by the equilibrium constant as it relates to completeness of reaction with water. For the reaction of ammonia with water
11
BL acid strength
The strength of a Brønsted-Lowry acid can be quantified by the equilibrium constant as it relates to completeness of reaction with water. For the reaction of acetic acid with water
17
Amphiprotic substances
Some substances, like water, have protons that can be donated (BL acid),
and lone pairs of electrons that can accept protons (BL base). This is why it
Many substances contain OH- act
(like NH3) like bases
that do not in water!
The Brønsted-Lowry Theory: an acid is any substance that donates protons (H+) while a base is any substance that can accept protons.
Ka
CH3COO H3O CH3COOH
1.8 x10-5 at 25 C
13
Strong BL acids and bases
Strong BL acids and bases have the exact same reaction with water as do weak acids and bases, they just are much more complete. This is reflected in the acid or base dissociation constant – it is much larger than one! Hydrochloric acid is a strong acid.
This means that Brønsted-Lowry acidbase reactions are proton transfer reactions.
6
Proton transfer reactions
Pairs of compounds are related to each other through Brønsted-Lowry acid-base reactions. These are conjugate acid-base pairs. Generally, an acid HA has a conjugate base A- (a proton has transferred away from the acid). Conversely, a base B has a conjugate acid BH+ (a proton has transferred toward the base).
with the formula [H3O(H2O)n]+ where n is 1 to 4.
16
Requirements of Brønsted-Lowry bases
For a molecule or ion to accept a proton (to act as a base) requires it to have an unshared pair of
can act like an acid AND a base.
Such substances are said to be
amphiprotic.
18
Problem
Write a balanced equation for the dissociation of each of the following Brønsted-Lowry acids in water:
H+ (aq) + A- (aq) + M+ (aq) + OH- (aq) H2O (l) + M+ (aq) + A- (aq)
The net ionic equation is
H+ (aq) + OH- (aq) H2O (l)
3
Aqueous salts
The aqueous salt in the reaction comes from the spectator ions in the reaction. These ions are present to balance the proton positive charge or the hydroxide ion negative charge in the acid or base.
In water there is no such thing as H+.
H H2O H3O K very large!
15
Hydrated protons and hydronium ions
Often more than one water molecule will crowd around the hydronium ion (H3O+) to give hydrates
The key to the Arrhenius description is that we need water to act as a solvent to
promote the dissociation of the acid or base.
5
Brønsted-Lowry theory of acids and bases
If we evaporate all the water, we are left with an ionic solid called a salt e.g. NaCl
4
Arrhenius theory of acids and bases
Many like bases!
Arrhenius acid: HA (aq) H+ (aq) + A- (aq) Arrhenius base: MOH (aq) M+ (aq) + OH- (aq)
2
Neutralization reactions
Arrhenius acids and bases react with each other to form water and aqueous salts in neutralization reactions.
Kb
NH4 OHNH3
1.8 x10-5 at 25 C
10
Water as a base in BL reactions
When a Brønsted-Lowry acid is placed in water, it reacts with the water (which acts as a Brønsted-Lowry base) and establishes an acid-base equilibrium.
electrons which can then be used to create a bond to the H+.
All Brønsted-Lowry bases have at least one lone pair of electrons.
In the previous reactions we’ve seen NH3 has a lone pair and can act as a base. Also, water has two lone pairs, and can act as a base.
represent these facts.
7
Water as an acid in BL reactions
When a Brønsted-Lowry base is placed in water, it reacts with the water (which acts as a Brønsted-Lowry acid) and establishes an acid-base equilibrium.
Kb
NH4 OHNH3
1.8 x10-5 at 25 C
9
BL base strength
We give the equilibrium constant a special name for reactions like these – the base dissociation (or ionization) constant Kb. Since the value of the constant is less than one, the ammonia does not dissociate to a great extent – it is a weak base!
19
Problem
What is the conjugate acid of each of the following Brønsted- Lowry bases? a) HCO3b) CO32c) OHd) H2PO4-
20
Problem
Ka
CH 3COO H3O CH 3COOH
1.8 x 10-5 at 25 C
12
BL acid strength
We give the equilibrium constant a special name for reactions like these – the acid dissociation (or ionization) constant Ka. Since the value of the constant is less than one, the acetic acid does not dissociate to a great extent – it is a weak acid!
Ka
Cl H3O
HCl
1 x106 at 25 C
14
Hydrated protons and hydronium ions
What is the strongest Brønsted-Lowry acid there is? The strongest Brønsted-
Lowry acid is H+. The ultimate proton-donor is a proton itself!
Chapter 16
Acids and Bases
Arrhenius theory of acids and bases
The Arrhenius concept: acids dissociate in water (aqueous solution) to produce hydrogen ions H+, and bases dissociate in water to give hydroxide ions, OH-.
Of the following species, one is acidic, one
is basic, and one is amphiprotic in their reactions with water: HNO2, PO43-, HCO3-. Write the four equations needed to
8
BL base strength
The strength of a Brønsted-Lowry base can be quantified by the equilibrium constant as it relates to completeness of reaction with water. For the reaction of ammonia with water
11
BL acid strength
The strength of a Brønsted-Lowry acid can be quantified by the equilibrium constant as it relates to completeness of reaction with water. For the reaction of acetic acid with water
17
Amphiprotic substances
Some substances, like water, have protons that can be donated (BL acid),
and lone pairs of electrons that can accept protons (BL base). This is why it
Many substances contain OH- act
(like NH3) like bases
that do not in water!
The Brønsted-Lowry Theory: an acid is any substance that donates protons (H+) while a base is any substance that can accept protons.
Ka
CH3COO H3O CH3COOH
1.8 x10-5 at 25 C
13
Strong BL acids and bases
Strong BL acids and bases have the exact same reaction with water as do weak acids and bases, they just are much more complete. This is reflected in the acid or base dissociation constant – it is much larger than one! Hydrochloric acid is a strong acid.
This means that Brønsted-Lowry acidbase reactions are proton transfer reactions.
6
Proton transfer reactions
Pairs of compounds are related to each other through Brønsted-Lowry acid-base reactions. These are conjugate acid-base pairs. Generally, an acid HA has a conjugate base A- (a proton has transferred away from the acid). Conversely, a base B has a conjugate acid BH+ (a proton has transferred toward the base).
with the formula [H3O(H2O)n]+ where n is 1 to 4.
16
Requirements of Brønsted-Lowry bases
For a molecule or ion to accept a proton (to act as a base) requires it to have an unshared pair of
can act like an acid AND a base.
Such substances are said to be
amphiprotic.
18
Problem
Write a balanced equation for the dissociation of each of the following Brønsted-Lowry acids in water:
H+ (aq) + A- (aq) + M+ (aq) + OH- (aq) H2O (l) + M+ (aq) + A- (aq)
The net ionic equation is
H+ (aq) + OH- (aq) H2O (l)
3
Aqueous salts
The aqueous salt in the reaction comes from the spectator ions in the reaction. These ions are present to balance the proton positive charge or the hydroxide ion negative charge in the acid or base.
In water there is no such thing as H+.
H H2O H3O K very large!
15
Hydrated protons and hydronium ions
Often more than one water molecule will crowd around the hydronium ion (H3O+) to give hydrates
The key to the Arrhenius description is that we need water to act as a solvent to
promote the dissociation of the acid or base.
5
Brønsted-Lowry theory of acids and bases
If we evaporate all the water, we are left with an ionic solid called a salt e.g. NaCl
4
Arrhenius theory of acids and bases
Many like bases!
Arrhenius acid: HA (aq) H+ (aq) + A- (aq) Arrhenius base: MOH (aq) M+ (aq) + OH- (aq)
2
Neutralization reactions
Arrhenius acids and bases react with each other to form water and aqueous salts in neutralization reactions.
Kb
NH4 OHNH3
1.8 x10-5 at 25 C
10
Water as a base in BL reactions
When a Brønsted-Lowry acid is placed in water, it reacts with the water (which acts as a Brønsted-Lowry base) and establishes an acid-base equilibrium.
electrons which can then be used to create a bond to the H+.
All Brønsted-Lowry bases have at least one lone pair of electrons.
In the previous reactions we’ve seen NH3 has a lone pair and can act as a base. Also, water has two lone pairs, and can act as a base.
represent these facts.
7
Water as an acid in BL reactions
When a Brønsted-Lowry base is placed in water, it reacts with the water (which acts as a Brønsted-Lowry acid) and establishes an acid-base equilibrium.
Kb
NH4 OHNH3
1.8 x10-5 at 25 C
9
BL base strength
We give the equilibrium constant a special name for reactions like these – the base dissociation (or ionization) constant Kb. Since the value of the constant is less than one, the ammonia does not dissociate to a great extent – it is a weak base!