What Really Determines the Strength of an Acid?

This computational project uses the Gaussian software through a WebMO interface to illuminate the relationship between the strength of an acid, molecular structure, and a change in thermodynamic properties for the dissociation of the acid. In undergraduate chemistry courses it is common to use structural properties such as bond length and partial positive charge on the acidic hydrogen to determine relative acid strength. However, these properties do not explain the true physics of acid strength. In order to understand contributing factors the change in Gibbs energy for the dissociation of halogenic acids and derivatives of acetic acids was calculated. The change in the standard state Gibbs energy was shown to become more negative as the strength of an acid increased. The main contribution to the higher negative change in Gibbs energy was the increase in the change in entropy for the dissociation of the acid. For all the acids compared, the change in entropy for the dissociation of an acid increased as the acid strength increased. Moreover, the trend in this change of entropy value was like the trends observed when comparing acids using structural properties. This result led the students to conclude that the increase in the change of entropy for the dissociation of an acid is the true factor which makes one acid stronger than another.