How does the presence of a common ion affect the solubility of a salt? Explain in terms of equilibrium.
I can help you through this but I don't want to write it for you. The common ion effect is just an example of Le Chatelier's Principle.
Oh that's what it is? Thanks
Right. The "common ion effect" is called that because there is a "common ion."
For example, Ag2CrO4 ==> 2Ag^+ + CrO4^2-
So if we add AgNO3 the Ag^+ is the common ion.
If we add Na2CrO4 the CrO4^2- is the common ion. Both decrease the solubility of Ag2CrO4
The presence of a common ion can impact the solubility of a salt by affecting the equilibrium between the dissolved ions and the undissolved salt. To understand this, let's first review the concept of solubility and equilibrium.
When a salt is dissolved in water, it dissociates into its constituent ions. For example, consider the salt sodium chloride (NaCl). When NaCl dissolves in water, it breaks into sodium ions (Na+) and chloride ions (Cl-).
The solubility of a salt is a measure of how much of it can dissolve in a given amount of solvent, usually water. It is often denoted by the concentration of the dissolved ions.
Now, let's discuss equilibrium. When a salt is added to water, an equilibrium is established between the dissolved ions and the undissolved salt. At this point, the rate of dissolution of the salt is equal to the rate of precipitation of the salt. This dynamic equilibrium is expressed using the solubility product constant, Ksp.
Now, how does the presence of a common ion affect solubility in terms of equilibrium? Well, if a solution already contains one of the ions that are produced when a salt dissolves, that ion is referred to as a common ion. Let's take the example of a solution with NaCl already present, and we are trying to dissolve more NaCl.
If the solution already contains sodium ions (Na+), due to the presence of NaCl, it will affect the equilibrium between the dissolved ions and the undissolved salt. According to Le Chatelier's principle, when we increase the concentration of one of the ions involved in the equilibrium, the equilibrium shifts to decrease the concentration of that ion.
In this case, the excess sodium ions (common ion) from the NaCl already present in the solution will reduce the solubility of additional NaCl salt. This happens because the equilibrium shifts towards the undissolved salt, suppressing further dissolution.
The same principle applies to other salts as well. If the solution already contains a common ion, its presence will decrease the solubility of any other salt that contains that same ion.
To summarize, the presence of a common ion reduces the solubility of a salt by shifting the equilibrium between the dissolved ions and the undissolved salt towards the undissolved salt, thereby decreasing the amount of the salt that can dissolve in the solution.