Ionic Product versus Solubility Product

In this video we want to discuss the concept of Ionic Product versus Solubility Product.

Let's use the dissociation of a sparingly soluble salt calcium hydroxide as an example.

Solubility Product

IP versus Ksp solubility product

Based on the dissociation of calcium hydroxide, solubility product is given as:

Ksp = [Ca2+][OH-]2

Equilibrium constant Ksp describes the concentration of ions for a saturated solution, hence the concentration of Ca2+ and OH- are at their maximum and equilibrium value.

This means Ksp measures the maximum amount of ions that the solution can handle or dissolve.

Since Ksp is an equilibrium constant, it is only affected by changes in temperature, so will be a fixed value at constant temperature.

Ionic Product

IP versus Ksp ionic product

Interestingly ionic product looks exactly the same as solubility product:

IP = [Ca2+][OH-]2

However the concentration of ions are for any solution so the solution can be diluted, saturated or supersaturated.

Since ionic product measures the actual amount of ions in solution, it will vary depending on the concentration of ions dissolved.

Ionic Product versus Solubility Product

To visualise the relationship between IP and Ksp, we can see IP as the number line, which increases as concentration of ion increases, and Ksp as a specific point along this number line.

IP versus Ksp ionic product versus solubility product

1. IP = Ksp

When actual amount of ions equal to maximum amount of ions, the solution is saturated but there is no precipitation.

Any addition of ions will result in precipitation since the saturated solution can no longer dissolve any more ions.

2. IP < Ksp

When actual amount of ions is less than maximum amount of ions, the solution is diluted and there is no precipitation.

We will expect more salt to dissolve when added since there are still available slots in the diluted solution to accommodate these additional ions.

3. IP > Ksp

When actual amount of ions is more than maximum amount of ions, the solution is supersaturated and is hanging on to more ions than it can keep in solution.

This usually happens when 2 solutions containing separate sources of cation and anion are mixed together.

A supersaturated solution doesn't stay supersaturated and it has to precipitate out the excess ions that it cannot handle in solution.

The resultant outcome will be a saturated solution and precipitate.

For the detailed comparison between ionic product and solubility product, check out this video!

Topic: Solubility Product, Physical Chemistry, A Level Chemistry, Singapore

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