Selected thermodynamic values for the ionization of ethanoic acid, H3CCOOH, and trichloroethanoic ac — Thermodynamics / Acid-Base Equilibrium Chemistry Question
Problem Context
Selected thermodynamic values for the ionization of ethanoic acid, H3CCOOH, and trichloroethanoic acid, Cl3CCOOH, at 25 ˚C are given in the table below.
Use the given values to provide answers to the following questions.
What is the ∆S˚ value for the ionization of H3CCOOH at 25 ˚C?
Model Answer
Go = Ho – T So
27120 = –2510 – 298.15 K × So
So = –99.4 K
What is the Ka value for Cl3CCOOH at 25 ˚C?
Model Answer
Go = –RT ln K
ln (K) = –1.58
K = 2.0 × 10–1
What is the Ka value for H3CCOOH at 35 ˚C?
Model Answer
ln ( ) = –301.9 × (0.003354–0.003245)
ln ( ) = –0.0329
( ) = 0.9677
K2 = 1.71 × 10–5
Account for the difference in the Ka values of the two acids at 25 ˚C in terms of their molecular structures.
Model Answer
Cl3CCOOH has a greater K than H3CCOOH because the Cl atom draw electrons from C–OH, weakening the O–H bond.
State whether the ∆S˚ values suggest an increase or a decrease in the order of the system upon ionization and offer an explanation in molecular-level terms.
Model Answer
So values are less than zero, suggesting an increase in order H+ and A– are solvated more tightly than HA with CH3CO2 – being solvated better than larger Cl3CO–.