if you have a reaction with an acid dissociation, where one HA goes to H + A how can the entropy be negative? I thought that one mole going to two moles would have positive entropy because of the greater disorder.

Think about the possibility of hydration of both + and - ions and ion-dipole type "clusters" forming so those "two" particles really aren't two particles.

hydration just means H2O would attach to the ions, right? would the H2O attach to the -F to form larger molecules? how does the entropy change from that?

responded above.

The change in entropy (∆S) during a chemical reaction is determined by the total number of microstates available to the system before and after the reaction. Microstates refer to the different ways in which the particles of a system can be arranged without changing the macroscopic properties like temperature, pressure, and volume.

In the case of an acid dissociation reaction, where one molecule HA breaks down into H+ and A-, it may seem counterintuitive that the entropy change could be negative. To understand why this can happen, we need to consider other factors influencing entropy.

While it's true that the number of particles increases from one HA molecule to two ions (H+ and A-), resulting in increased disorder, entropy is not solely determined by the number of particles. It also depends on the system's energy distribution, molecular freedom, and other factors.

In an acid dissociation reaction, there is a decrease in the degree of molecular freedom. Initially, the HA molecule is a single entity, while after dissociation, its constituent ions become more restricted in their movement due to electrostatic attraction between oppositely charged ions. This decrease in molecular freedom can outweigh the increase in disorder caused by the formation of two separate particles.

Furthermore, the energy distribution can also play a role. The energy associated with the bonds within HA might be higher than the energy of H+ and A- ions. During the dissociation, some of this energy is redistributed among the ions, resulting in lower overall energy and decreased entropy.

Overall, the entropy change occurring in an acid dissociation reaction depends on a variety of factors, including the number of particles, molecular freedom, energy distribution, and any other influencing factors. Hence, it is possible for the entropy to be negative, despite the increase in the number of particles.