Common Solution Equilibria

Greetings,

This post covers common equilibria of aqueous solutions; namely, slightly soluble ionic compounds plus weak acids and bases.

Slightly Soluble Ionic Compounds

Many ionic compounds are often considered insoluble to the beginning chemistry student, but a chemist understands that they are actually slightly soluble.  The tiny amounts of ions which dissociate from a crystal of a slightly soluble ionic compound generally do not produce a visually-apparent change in a solution, however these ions can be easily detected using analytical testing techniques.  Slightly soluble compounds have been studied extensively and there is much information available on the extent of their dissociation in chemistry reference books.  A slightly soluble compound crystal in water is actually in a state of dynamic equilibrium in which some ions dissociate from the solid into solution while other ions (previously dissociated) are deposited back onto the crystal.  The dissociation and deposition processes occur simultaneously and at the same rate, therefore we have a state of dynamic equilibrium.  The following diagram shows this equilibrium process for a crystal lattice of the compound, silver chloride.

Referring to the diagram, free silver and chloride ions further from the lattice move about in random directions (known as Brownian Motion) while some ions closer to the crystal are drawn in and deposited.  While the deposition is happening bound ions from the crystal surface are released into solution.  The dissociation/deposition process commonly occurs for "insoluble" ionic compounds but even happens for saturated solutions of ionic compounds considered to be "soluble".

Weak Acids and Bases

Weak acids and bases are also in a state of dynamic equilibrium, but the process involves molecules ionizing instead of ionic crystals dissociating.  Note that molecules which break apart to produce ions are said to "ionize" while ionic compound crystals produce ions (in solution) by simply releasing them from the solid surface.  Another important distinction is that all parts (reactants and products) of weak acid and base equilibrium systems are in a dissolved state.  The equilibrium, then, is between intact molecular species and proton/anion pairs in a hydrated state.  The next diagram shows equations for a strong acid vs the equilibria for a weak acid and a weak base.

Note that the hydrochloric acid ionization is not an equilibrium equation because every single molecule breaks apart to produce chloride and hydronium ions.

My next post will cover stoichiometric calculations.

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