Acid Strength Exercise
James E. Finholt
This discussion was written for instructors. It was written to provide a starting point for the development of a fully detailed set of student instructions to fit a particular course, available hardware and available software. The exercise has been tested on a computer using a Windows 95 operating system with CAChe 3.0 software.
Computer calculations can provide considerable insight into the factors influencing acid strength. In this exercise nitric acid, HONO2, will be compared to nitrous acid, HONO.
Draw Lewis structures for all resonance forms of the following species: HONO2, HONO, NO3-, and NO2-. Calculate the formal charge of each atom in each resonance structure. Pick out the most stable resonance structure for each molecule and ion and explain.
Use the computer to build HONO2 and beautify it. Optimize the geometry using AM1 parameters. Note the heat of formation. Record all bond distances and angles. Record the partial charge of each atom. Repeat for HONO, NO3- and NO2-.
Use the heat of formation data to predict the relative strength of HONO2 and HONO. By convention the heat of formation of H+ is taken to be 0.
A number of parameters have been suggested that might be used to predict relative acid strength. Discuss the relative value of each of the following: the partial charge on the nitrogen atoms in HONO2 and HONO, the partial charge on the hydrogen atoms in HONO2 and HONO, the partial charge on the oxygen atoms attached to the hydrogen atoms in HONO2 and HONO, and the partial charge on the oxygen atoms in NO2- and NO3-.
Optional; test your conclusions by conducting calculations on other acids.
Compare the three NO distances in HONO2 to test your conclusion about the most stable resonance form of HONO2 and explain.
The AM1 calculation for HONO2 yields a planar structure with the H atom closer to one terminal oxygen than to the other terminal oxygen rather than a nonplanar structure with the H atom equidistant from the two terminal oxygen atoms. Speculate on what might be causing this situation. It has been suggested that this might be due to restricted rotation about the bond between the N and the O to which the H is attached. Others have suggested H bonding is the cause. There is no "right answer" to this part of the exercise.
Certain factors have been neglected in this analysis. Acids and bases are usually used in aqueous solution but the calculations described in this exercise were all for gas phase conditions. Discuss whether this was a fatal flaw. Some calculation packages allow aqueous phase calculations to be conducted. It might be interesting to compare the results of such calculations with the gas phase calculations. Entropy was ignored. Speculate on whether or not this was a fatal flaw.