classroom
activity
4
Acid Base Equilibrium Reactions
Show You Know!
1. Using the data in Appendix 1 (in Activity 3), determine the amount of energy in pKa units released
or required for each of the following reactions.
bond broken = energy required
bond formed = energy released
a. Forming water from hydroxide 16 units released (so -16)
b. Forming ammonia from ammonium
c. Forming ethoxide from ethanol
9 units required (so +9)
16 units required (so +16)
2. For each the following reactions:
a. Add electron pushing arrows to indicate the forward reaction of the equilibrium.
b. Add electron pushing arrows to indicate the reverse reaction of the equilibrium.
c. Using the pKa data in Appendix 1, determine if the reaction is product favored, reactant favored
or thermoneutral.
d. Draw a net potential energy diagram for the reaction including
● Labels for the x and y axes
● Labels for the net ∆Grxn in pKa energy units
● Labels for the activation energy (Ea) for the forward reaction
● Labels for the activation energy (Ea) for the reverse reaction
Rxn 2
R
O
+
R
C
H
R
O
R
+
C
pKa = -50
Rxn 2
O
R
C
R
R
+
H
H
C
O
R
R
C
O
R
R
pKa = +5
net pKa = -50 + 5 = -45 pKa units (product favored)
Potential Energy
Ea
∆GRxn = -45 pka units
Reaction Coordinate
+
C
O
R
4 Acid Base Equilibrium Reactions
3. When sulfuric acid is added to water, the solution gets very warm. Use pKa equilibrium profile
analysis to explain why this happens.
Sulfuric acid releases 9 pKa energy units when deprotonating to form its conjugate base.
Water requires 2.5 pKa energy units to protonate and form Hydronium.
H2SO4  H+ + HSO4- = – 9
H2O + H+  H3O+ = + 2.5
Net = – 9 + 2.5 = – 6.5 pKa energy units released. This energy is released as heat and warms the
solution.