Hydrochloric Acid in Gas Phase and in Solution

Case Study

Hydrochloric acid, HCl, is known as a "strong" acid, one which completely dissociates in water. However, we know that HCl is not ionic in the gas phase. In this study, you will use semi-empirical calculations to show how dissociation is favored in solution and gather thermochemical information about hydrochloric acid.

Construct HCl and constrain the bond length to several different lengths between 1.0 and 5.0 Angstroms. Do several points so that you can see the overall behavior. Optimize the geometry (this won't take long!) with both the AM1 and AM1-SM2 basis sets. Plot energy over bond length for gaseous and solvated molecules (use a spreadsheet to make life easy here). What happens in each case as the distance approaches infinity? How many minima do each curve have? Can you explain why HCl dissociates in solution and not in gas phase? Why do you think this happens?

Figure out what the energy would be for HCl in solvent at infinite separation according to your earlier graph. Find the difference between this energy and the undissociated energy (at 1.3 Angstroms distance). Divide this number by 1.36 to find pKa for HCl in water. This constant, pKa, expresses the acidity of the substance. A high pKa corresponds to a weak acidity and vice versa. Does your calculation of pKa confirm the idea that hydrochloric acid is a "strong" acid?

Extension

Plot the solvation energy (difference between AM1 and AM1-SM2 energies) over distance. Describe how bond length and solvation energy are related.

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