A chemist is tasked with reducing the fluoride concentration in groundwater. She considers adding solid aluminum chloride, AlCl₃(s), to water that is already saturated with calcium fluoride, CaF₂(s), and in equilibrium with excess undissolved CaF₂.
In aqueous solution, Al³⁺ ions can react with fluoride ions to form a series of soluble complex ions (for example, AlF₆³⁻). Assume the complexation reaction proceeds to completion under the conditions used.
(1) Predict qualitatively how the addition of AlCl₃ will affect (i) the concentration of free F⁻(aq) ions and (ii) the mass of CaF₂(s) present once a new equilibrium is reached. Provide reasoning for each prediction.
(2) Describe, at the particulate level, the sequence of events that occurs in the solution immediately after AlCl₃ dissolves and before the system reaches its final equilibrium state.
(3) Explain how the chemist’s strategy takes advantage of equilibrium principles to lower dissolved fluoride levels, and discuss one practical limitation or consideration that should be addressed in applying this method on a large scale.