common ion effect
Le Chatelier's principle
If you put solid Ca(OH)2 into water, both processes run at once. Eventually, a balance is struck between the two, and solid forms at exactly the same rate that it dissolves. There will be no apparent change in the amount of solid or in the concentrations of the calcium and hydroxide ions in the tank at that point. Equilibrium has been established.
Dumping more hydroxide ions into the solution will upset that equilibrium. More hydroxide ions mean more encounters between between calcium and hydroxide ions in solution, so more solid calcium hydroxide will form in the first process. The extra hydroxide won't directly affect the second process; it runs at the same rate as before. The net result is that some of the calcium hydroxide precipitates.
All of this assumes that the solution in the tank was saturated with calcium hydroxide before the sodium hydroxide was added. What if it wasn't? You'll need to do an equilibrium calculation based on the reaction
Ca(OH)2(s) Ca+2(aq) + 2OH-(aq)to answer your question. Follow these steps:
Ksp = [Ca+2][OH-]2where Ksp is the solubility product constant (about 6.5 × 10-6 for Ca(OH)2) and [Ca+2] and [OH-] represent the equilibrium concentrations of calcium and hydroxide ions. The expression on the right side of equation is called the solubility product or ion product.
|concentration of Ca+2 =||0.010 M|
|concentration of OH- =||2 x 0.010 M + 0.001 M = 0.021 M|
|ion product =||(0.010)(0.021)2 = 4.4 × 10-6|
Your experiment illustrates the common ion effect: forcing an ionic compound to precipitate from solution by adding one of its ions to the solution. It's also called "salting out" a precipitate. It has a number of important applications. For example, you can use this trick to remove heavy metal ions from wastewater. You can also use it to understand why the body can't absorb minerals present in some foods; if an ion such as iron or calcium is "salted out" in the digestive system, the body won't be able to absorb it very efficiently.
Author: Fred Senese firstname.lastname@example.org
Copyright © 1997-2010 by Fred Senese
Comments & questions to email@example.com
Last Revised 08/17/15.URL: http://antoine.frostburg.edu/chem/senese/101/solutions/faq/print-common-ion-effect.shtml