Question

What direction do you predict the addition of an acid to the solution containing bromophenol blue will drive equilibrium? What about the addition of a base? Explain your prediction in terms of Le Chatelier's principle.

Answer 1

Le Chatlier's principle states that when a system in equilibrium is perturbed by external changes, the system responds by adjusting the equilibrium concentrations of its components such that the perturbation is nullified and attains an equilibrium state at this perturbed condition, with equilibrium concentrations different than before.

Bromothymol blue is a phenolic suflone that acts a weak acid in aqueous media. Being a weak acid, it does not ionize completely and establishes an equilibrium with its associated and dissociated form. The general equilibrium expression for this weak diprotic indicator can be written as:

where In is the dianionic form of bromothymol blue.

From Le Chatlier's principle, it can be ascertained that when a strong acid is added, the latter's complete dissociation, without establishment of equilibrium causes a spike in the proton concentration in the medium. This increase in proton concentration is the external perturbation and the system will respond by shifting to the left, thus protonating more dianions of bromothymol in an effort to nullify the proton concentration increase.

Similarly, when a base is added, it consumes the protons in the medium to form water. So, the concentration of protons in the medium decreases. To compensate this change, the equilibrium responds by shifting to the right i.e., it causes more of the neutral indicator to dissociate into the dianion and protons, thus attempts to nullify the perturbation.