Practice Problems -­‐ Answers 1. Draw the structure of the conjugate acid of each of the following ions or compounds. H 3C
C
CH
H 3C
C
C
HNO3
NO3 O
O
OH
O
O
OH
quite unstable
O
OH
H 3C
NH 2
H 3C
NH 3
2. Draw the structure of the conjugate base of each of the following ions or compounds. NH 3
NH 2
H 2C
C
N
H 3C
C
N
OH
OH
O
C
C
N
N
O
O
H 3C
SH
H 3C
S
OH
O
3. Classify each compound as a Lewis acid or Lewis base in the reactants. H
H
C
C
N
+
N
Lewis base
Lewis acid
Cl
BF3Cl +
+
BF3
Lewis base
Lewis acid
NH 3
NH 2
+ H 3C
NH
+ H 3C
NH 2
Lewis acid
Lewis base
O
4. Assign the pKas to the structures; pKa = 3.98 and 2.83. O
O
Cl
OH
OH
Cl
B
A
2.83
3.98
pKa = _____
pKa = _____
The inductive (polarity) effect is responsible for the pKa difference.
5. When sodium metal reacts with methanol hydrogen gas evolves from the solution and sodium methoxide is formed according to the following reaction: 2 CH3OH(l) + 2 Na(s) à 2 CH3O-­‐ Na+(sol) + H2(g) Write a similar chemical reaction with NH3. 2 NH3(l) + 2 Na(s) à 2 NH!! Na!
(!"#) + H!(!) 6. The pKas of two acids are shown below. HCOOH pKa = 3.75 CH3(CH2)14COOH pKa = 6.76 Will a chemical reaction occur if CH3(CH2)14COO-­‐Na+ is added to a solution of HCOOH? Explain. CH3(CH2)14COO-­‐Na+ + HCOOH CH3(CH2)14COOH + HCOO-­‐Na+ pKa = 3.75 pKa = 6.76 The stronger acid is on the reactant side of the chemical reaction. All systems favor
the formation of the weaker acid. The Gibbs free energy (ΔG°) value for the written
reaction is negative, that is, product favored.
Prepared by José Laboy