How does the addation of BF3 to a solution of NH3 affect the PH of the solution and Why?

Question

This question is related to Humo and Lumo interactions

Answer 1

The quick answer is that addition of BF3 will lower the pH because some of the NH3 will bind to the BF3 and remove NH3 from the acid/base equilibrium. Removal of a reactant by Le Chatelier's principal will push towards reactants and lower pH.

NH3 + H2O --> NH4+ + OH-

If your teacher specifically wants you to use HOMO and LUMO interactions to explain this, we can look at the question a little differently. In both cases NH3 acts as the base so we will use the HOMO of NH3 and the LUMO of either BF3 or H3O+. I'm using H3O+ as a model for acid. The LUMO for BF3 will be the nonbonding p orbital of the B and the LUMO of H2O would be an O-H antibonding orbital.

In general we would expect a nonbonding orbital to be lower in energy than an antibonding orbital. A lower LUMO energy will result in a stronger HOMO-LUMO interaction due to a smaller difference in energy between the HOMO and LUMO. Therefore BF3 would bind stronger to NH3 than the H+ of H2O. This interaction will also explain less NH4+ and OH- formation resulting in a decrease in pH.