Chemistry question

Question

•What is the common-ion effect and how does it related to Le Chatelier's Principle

Answer 1

First, remind yourself what Le Chatelier's Principle is! For a given chemical reaction (or we could say for a given system), there is a given chemical equilibrium that the reaction will want to shift towards. Disturbing this will cause a shift in this dynamic equilibrium. Let's say we have this given reaction:

CaCO3 <---> Ca(2+) + CO3(2-)

What will happen if we add a bunch of CaCO3 to an aqueous solution? It will push the equilibrium of the equation towards the products, because we added a bunch of reactants! This is in essence Le Chatelier's Principle!

Ok, what if now we added a new salt and dissolved it in the same solution, let's say we added CaCl2. If this salt dissolves into Ca(2+) and 2Cl(-), then we just added a bunch of Ca(2+) to the solution. Why does this matter? Take a look at our original equation-- there is Ca(2+) on the products side!

In other words, by adding a new salt, CaCl2, to our aqueous solution of CaCO3, we added more Ca(2+) to the solution, which is adding more products. According to Le Chatelier's Principle, if I add more products, the equilibrium will shift towards the reactants.

The common ion in this case is Ca(2+), since it is present in both CaCO3 and CaCl2. Adding a common ion to a chemical reaction will shift the equilibrium AWAY from that common ion. In the case of aqueous solutions, adding a common ion will DECREASE solubility our salt.