With this learning objective, we are going to be learning a whole new world of heterogeneous equilibria. We are leaving behind the acid/base equilibria we have done all the way through the previous unit with Dr. Solt. And the first several learning objectives of this unit. We are now ready to enter a world of where there will be a solid in an aqueous solution, both combined. And we are going to be studying the world of insoluble salts. We are going to learn how to write the reaction for the dissociation of these insoluble salts. We are going to learn how to write the reaction for the dissociation of these insoluble salts. We call it a solubility equilibria. We are going to write an equilibria constant expression for these reaction, which we call the law of mass action. So lets begin by talking about insoluble salts. Earlier, now when I say earlier I would mean before you talking about insoluble salts. Earlier, now when I say earlier I would mean before you ever even began working through the lessons with us you would have learned about insoluble salts it is how some salts are insoluble. You learned law or rules to help govern which salts are soluble and which salts are insoluble. But when you have these insoluble salts it does not mean, that nothing dissolves But when you have these insoluble salts it does not mean, that nothing dissolves in solution. It means that just a very very tiny amount will dissolve. So we are going to have an equilibrium exist between the undissolved salt and the small amount of salt that does dissolve, and that is what we are going to be considering. In this learning objective and the next several through the rest of this unit. So here is an example of an insoluble salt. It is silver chloride. When it dissolves in water we are going to be able to write this reaction. When it dissolves in water we are going to be able to write this reaction. We have, and this is the way we always will write it, we will put the solid on the left as a reactant, and we will put the ions it breaks apart into as a reactant, and we will put the ions it breaks apart into on the right, as products. Now this is a reversible reaction so it could be written the other direction. Now this is a reversible reaction so it could be written the other direction. But we never will, because when we start studying this process, we are going to be using equilibrium constants that we are going to look up in tables and they always apply to the reaction written in this fashion. So if we have this reaction written up at the top you should be able to write the equilibrium constant expression or the law of mass action, for this reaction. So choose the right answer. Well if you selected answer B, then you are correct. We would not include the silver chloride in this because it is a solid and you never include the solids when you write these expressions. This equilibrium constant we are going to give a subscript of SP. So we have seen K_a's K_b's, K_w's we have seen K_c's and K_p's this is K_sp and _sp stands for solubility product. Because it is a product of the ions that have dissolved into the solution. There is a table, a solubility folder of solubility product, provided for you in your resource folder for this MOOC, and you will be able to look up values when they are needed. So once you have one of these equilibrium constant expressions we can do all the same types of calculations we have done with any other equilibria problem that we have seen. Throughout the units we have been covering on equilibria. So lets try one. No, no lets not try one, lets work on this guy practice writing out expressions. Here is a reaction is Magnesium Chloride. The first thing you need to be able to write is the reaction itself. So remember you put the solid on the left and the ions it breaks apart into on the right, you give it a try. Did you write this? Did you remember that there are two Chlorides and not write it as Cl_2? It is It is not the element, it is the they are the ions. The mono-atomic ion. So you have 2 of the Cl-'s. So once you have that, you would be able to write So you have 2 of the Cl-'s. So once you have that, you would be able to write this expression. The law of mass action for this reaction. Hopefully, you tried it out and wrote this. It has the _sp for the solubility product. And we have the magnesium to the first power and because there is a 2 here we would have the chloride to the second power. So this is the ends of our learning objective number 9, it was a very short lesson. But I would like you to be able to take any insoluble salt and write the reaction, as we did here in this problem and also write the equilibrium constant expression or the law of mass action for that.
