For the dissociation of acetic acid in water, CH3COOH CH3COO- + H+, the entropy
ID: 980413 • Letter: F
Question
For the dissociation of acetic acid in water, CH3COOH CH3COO- + H+, the entropy change is negative (indicating greater order in the system of acetic acid and water) even though some of the acetic acid molecules have dissociated and are less ordered (a positive entropic effect). Why is the entropy change negative?
Water molecules become ordered around the ions.
The ionic product of water, Kw, ensures that H+ and OH- associate to form H2O and become more ordered.
The partial dissociation of a weak acid means that most of the molecules are undissociated and more ordered.
The heat absorbed by the dissociation (+H) makes a positive contribution to -TS so the entropy becomes negative to maintain equilibrium and G = 0.
Explanation / Answer
Though a few acetic acid molecules get dissociated which contributes to increase in entropy, at the same time to stabilise the ions obtained from acetic acid many of the water molecules orderily arrange around the ions which contributes to decrease in entropy. Thus the net entopy change of the system is decreased, hence negative sign.