CMY 385: Theme 5 Acids and Bases
What is a Bronsted acid? base? - correct answer 1. Acid: proton donor
2. base: proton acceptor.
Water example of amphiprotic substance, why? - correct answer can act as either
Brønsted acid or Brønsted base
What is a conjugate base/ acid? - correct answer 1. Conjugate base: species that is
formed when proton is lost
2. Conjugate acid: species that is formed when proton is gained
When is a Bronsted acid strong or weak? - correct answer 1. Strong acid: fully
deprotonated in water pKa < 0; Ka >> 1
2. Weak acid: pKa > 0; Ka < 1
When is a Bronsted base strong or weak? - correct answer 1. Strong base: fully
protonated in water pKb < 0; Kb >> 1
2. Weak base: pKb > 0; Kb < 1
What are polyprotic acids? - correct answer Acids that can donate more than one
proton and they have more than one acidity constants (Ka's)
For polyprotic acids, why is Ka2 almost always smaller than Ka1 - correct answer
because deprotonation is less favourable since a proton has to be removed from a negative ion
Characteristics of Brønsted acids - correct answer - Largest class of acids in water
contains an -OH group (acidic proton) attached to central atom
1. Aqua acid: acidic proton is on water molecule coordinated to central metal ion
2. Hydroxoacid: acidic proton is on hydroxyl group without a neighbouring oxo (=O) group e.g. Te(OH)6
, 3. Oxoacid: acidic proton is on hydroxyl group with a neighbouring oxo (=O) group e.g. H2SO4
Successive stages in deprotonation of aqua acid - correct answer slide 6
Periodic trends in aqua acid strength - correct answer Increases with increasing
positive charge of central metal ion and decreasing ionic radius of central metal ion
Why do some D block metal ions deviate from the ionic model/ have high pka/ are strong acids? -
correct answer Metal ions repel departing proton more strongly than predicted by
ionic model because cation's positive charge is delocalized over ligands and closer to departing proton,
thus there is greater repulsion
Types of oxoacids - correct answer 1. Substituted oxoacids: One or more -OH groups
are replaced by other groups e.g. fluorosulfuric acid, O2SF(OH); F high electronegativity, withdraws
electrons from central S, making it more positive and a stronger acid than sulfuric acid
2. Aminosulfuric acid contains NH2 group which are electron donating and places electron density on S
and weakens acid compared to sulfuric acid
3. Phosphonic acid, H3PO3
- exception to oxoacid pattern
- contains P-H bond and is only a diprotic acid
- For Pauling's rules: OPH(OH)2
Describe Pauling's rules in determining acid strength - correct answer 1. Acid
strengths increase with increasing number of oxygen atoms; Oxygen is electron-withdrawing, weakens
O-H bond and proton is more readily released
2. More resonance structures for a conjugate base of the acid means the acid is a stronger acid.
3. Rules:
- For oxoacid OpE(OH)q pKa = 8 - 5p
- Successive pKa values of polyprotic acids increase by 5 units for each proton transfer
What is a Bronsted acid? base? - correct answer 1. Acid: proton donor
2. base: proton acceptor.
Water example of amphiprotic substance, why? - correct answer can act as either
Brønsted acid or Brønsted base
What is a conjugate base/ acid? - correct answer 1. Conjugate base: species that is
formed when proton is lost
2. Conjugate acid: species that is formed when proton is gained
When is a Bronsted acid strong or weak? - correct answer 1. Strong acid: fully
deprotonated in water pKa < 0; Ka >> 1
2. Weak acid: pKa > 0; Ka < 1
When is a Bronsted base strong or weak? - correct answer 1. Strong base: fully
protonated in water pKb < 0; Kb >> 1
2. Weak base: pKb > 0; Kb < 1
What are polyprotic acids? - correct answer Acids that can donate more than one
proton and they have more than one acidity constants (Ka's)
For polyprotic acids, why is Ka2 almost always smaller than Ka1 - correct answer
because deprotonation is less favourable since a proton has to be removed from a negative ion
Characteristics of Brønsted acids - correct answer - Largest class of acids in water
contains an -OH group (acidic proton) attached to central atom
1. Aqua acid: acidic proton is on water molecule coordinated to central metal ion
2. Hydroxoacid: acidic proton is on hydroxyl group without a neighbouring oxo (=O) group e.g. Te(OH)6
, 3. Oxoacid: acidic proton is on hydroxyl group with a neighbouring oxo (=O) group e.g. H2SO4
Successive stages in deprotonation of aqua acid - correct answer slide 6
Periodic trends in aqua acid strength - correct answer Increases with increasing
positive charge of central metal ion and decreasing ionic radius of central metal ion
Why do some D block metal ions deviate from the ionic model/ have high pka/ are strong acids? -
correct answer Metal ions repel departing proton more strongly than predicted by
ionic model because cation's positive charge is delocalized over ligands and closer to departing proton,
thus there is greater repulsion
Types of oxoacids - correct answer 1. Substituted oxoacids: One or more -OH groups
are replaced by other groups e.g. fluorosulfuric acid, O2SF(OH); F high electronegativity, withdraws
electrons from central S, making it more positive and a stronger acid than sulfuric acid
2. Aminosulfuric acid contains NH2 group which are electron donating and places electron density on S
and weakens acid compared to sulfuric acid
3. Phosphonic acid, H3PO3
- exception to oxoacid pattern
- contains P-H bond and is only a diprotic acid
- For Pauling's rules: OPH(OH)2
Describe Pauling's rules in determining acid strength - correct answer 1. Acid
strengths increase with increasing number of oxygen atoms; Oxygen is electron-withdrawing, weakens
O-H bond and proton is more readily released
2. More resonance structures for a conjugate base of the acid means the acid is a stronger acid.
3. Rules:
- For oxoacid OpE(OH)q pKa = 8 - 5p
- Successive pKa values of polyprotic acids increase by 5 units for each proton transfer
