Fall 2015
Chapter 2
Acids and bases
AN INTRODUCTION TO ACIDS AND
BASES
Brønsted–Lowry defined an acid as a species that donates
a proton, and a base as a species that accepts a proton
When a compound loses a proton (HCl), the resulting
species is called its conjugate base (Cl-).
When a compound accepts a proton (H2O), the resulting
species is called its conjugate acid (H3O+).
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Water (H2O) can behave as either an acid or a base
Acidity is a measure of the tendency of a compound to
give up a proton
Basicity is a measure of a compound’s affinity for a
proton
A strong acid has a strong tendency to give up a proton
• The weaker the base, the stronger is its conjugate acid
• The stronger the acid, the weaker is it conjugate base
pKa AND pH
• When a strong acid such as hydrogen chloride is
dissolved in water, almost all the molecules dissociate.
• When a much weaker acid, such as acetic acid, is
dissolved in water, very few molecules dissociate.
Ka = 107
Ka = 1.74 x 10-5
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An Acid/Base Equilibrium
[H3O+] [A-]
Ka =
[HA]
pKa = -log Ka
Very strong
moderately strong
Weak
Very weak
Extremely weak
pKa < 1
pKa = 1-3
pKa = 3-5
pKa = 5-15
pKa > 15
• Ka: The acid dissociation constant.
• The stronger the acid, the larger its Ka value and the
smaller its pKa value.
• pH indicates the
concentration of positively
charged hydrogen ion [H+]
in the solution
• pH = -log [H+]
• The lower the pH, the more
acidic is the solution
• The pH scale is used to
describe the acidity of a
solution; the pKa is
characteristic of a particular
compound
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ORGANIC ACIDS AND BASES
• The most common organic acids are carboxylic acids
– Compounds have a COOH group
– Have pKa values range from about 3-5
• Alcohols
– Compounds have an OH group
– Are much weaker acids than carboxylic acids
– With pKa values close to 16
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• Water can behave both as an acid and as a base
• An alcohol behaves similarly
• A protonated compound has gained an additional
proton
– Very strong acids
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HOW TO DETERMINE THE POSITION OF
EQUILIBRIUM
• Strong Acids / Bases React to Form Weak Acids / Bases
• Compare the pKa values of the acids
HOW THE STRUCTURE OF AN ACID
AFFECTS ITS pKa
• Two factors that affect the stability of a base are its size
and its electronegativity
• When atoms are similar in size, the strongest acid will
have its hydrogen attached to the most electronegative
atom.
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• When atoms are very different in size, the strongest acid
will have its hydrogen attached to the largest atom.
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Fall 2015
HOW pH AFFECTS THE STRUCTURE OF
AN ORGANIC COMPOUND
• Whether an acid will lose a proton in an aqueous solution
depends on both the pKa of the acid and the pH of the
solution.
• pKa > pH; acidic form (with its proton)
• pKa = pH; acidic form = basic form
• pKa < pH; basic form (without its proton)
• Physiological pH (pH=7.3)
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Fall 2015
BUFFER SOLUTIONS
• A solution containing a weak acid (HA) and its
conjugated base (A-) is called a buffer solution.
• A buffer solution will maintain nearly constant pH when
small amounts of acid or base are added to it
LEWIS ACIDS AND BASES
• Lewis acid:
– Accepts a share in an electron pair
– non-proton-donating acid (AlCl3)
• Lewis base:
– donates a share in an electron pair
– all bases are Lewis bases
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Quiz 2
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Which of the following is a conjugate acid/base pair?
1. HCl/H2O
2. H2O/Cl–
3. HCl/Cl–
4. All of the above
5. None of the above
Which of the following hydrogen halides is the
strongest acid?
1. HCl
2. HF
3. HBr
4. HI
5. None of these
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Fall 2015
Which form predominates for a molecule of the
ethyl ammonium ion (CH3CH2NH3+, pKa = 11.0)
at a pH of 5.0?
1. CH3CH2NH3+
2. CH3CH2NH2
3. CH3CH2NH–
4. Equal amounts of RNH3+ and RNH2
5. None of these
Which acid has the strongest conjugate base?
1. HNO2 (Ka = 4.0 × 10–4)
2. HNO3 (Ka = 22)
3. HCO3– (Ka = 6.3 × 10–11)
4. HCN (Ka = 7.9 × 10–10)
5. HCOOH (Ka = 2.0 × 10–4)
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Fall 2015
Which form will predominate for a molecule of
acetic acid (CH3COOH, pKa = 4.76) at a pH of 7.0?
1. CH3COOH
2. CH3COO–
3. CH3COOH2+
4. All of the above
5. None of the above
Quiz 2 - Answers
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Fall 2015
Which of the following is a conjugate acid/base pair?
1. HCl/H2O
2. H2O/Cl–
3. HCl/Cl– – this is a strong acid and its conjugate base
4. All of the above
5. None of the above
Which of the following hydrogen halides is the
strongest acid?
1. HCl
2. HF
3. HBr
4. HI – when atoms differ in size, the strongest acid is the
one whose hydrogen is attached to the largest atom
5. None of these
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Which form predominates for a molecule of the
ethyl ammonium ion (CH3CH2NH3+, pKa = 11.0)
at a pH of 5.0?
1. CH3CH2NH3+ – the pH is below the pKa, so the
molecule stays protonated
2. CH3CH2NH2
3. CH3CH2NH–
4. Equal amounts of RNH3+ and RNH2
5. None of these
Which acid has the strongest conjugate base?
1.
HNO2 (Ka = 4.0 × 10–4)
2. HNO3 (Ka = 22)
3. HCO3– (Ka = 6.3 × 10–11) – the weaker the
acid, the stronger its conjugate base
4. HCN (Ka = 7.9 × 10–10)
5. HCOOH (Ka = 2.0 × 10–4)
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Fall 2015
Which form will predominate for a molecule of
acetic acid (CH3COOH, pKa = 4.76) at a pH of 7.0?
1. CH3COOH
2. CH3COO– – the pH is above the pKa, so the
molecule will lose its proton
3. CH3COOH2+
4. All of the above
5. None of the above
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