Solubility! What it is how it works Ionic solids in water have specific and important characteristics An ionic substance has a lattice structure of ions, +/ - that equal out. In solution, the water attracts and pulls apart the + and -. This is called dissolution or dissociation NOT THE SAME THING as DECOMPOSITION

Dissociation vs decomposition NaCl Na (s) + Cl2 g This is decomposition What we want to do is Dissociation NaCl Na+ aq + Cl- aq

How do you get precipitates? You can mix two soluble salts to create an insoluble product, creating a precipitate How do you know if a salt is insoluble? Memorize the chart on page 144

Solubility chart After you memorize it you can tell if salts are insoluble. 2 soluble reactants can create an insoluble product Na2 S ( aq) + ZnCl ( aq) NaCl + ZnSs

Are these soluble or not? NaF Na2S BCl Ca3 (PO4)2 BaCl2 AuNO3 MgS Now find 2 you can combine to make a nonsoluble product!

Net ionic equations When writing equations, if a element exists as an ion on both sides of the equation, can leave it out. Complete equation NaCl(aq) + AgNO3 ( aq)  NaNO3 + AgCl s Net ionic equation Ag+ + Cl-  AgCl (s)

Some insolubles are though… If you have very little of an insouble compound, if it is way below it’s Ksp, That little bit will dissolve

Ions have certain levels that they will dissolve in water Ions have certain levels that they will dissolve in water. The more soluble they are, the more ions you have in solution. The equation that represents this is called a solubility equation or solubility equilibrium equation. It is equal to the concentratation of the 2 products, raised to their coefficients

Precipitation is the opposite of dissolution As ions will go into solution, so to they will come out This creates a reversible reaction that continues until equilibrium is reached!

How to set up the math Write out the dissolution equation. Remember that precipitation is the opposite of dissolution, so this is in fact a reversible reaction and can therefore be written just like Keq statements, but we are going to call them Ksp statements. Since we always start with a solid, there is never a reactant for the statement

AgCl dissolution AgCls Ag+ + Cl- make sure the product side is electrically neutral The Ksp shows the concentration of the 2 ions, makes it equal the solubility constant for that compound Ksp = {Ag} {Cl}

Try the Ksp for MgCl2 First the statement MgCl2 Mg+2 +2 Cl- Ksp = {Mg} {2Cl}2 Try one more Al (OH)3

Precipitation reactions! Even if a salt is soluble, there is a point at which it will precipitate! How do you find it? You just need to know if the amount you have in solution is larger than the amount that the solution will hold! Sounds easy huh?

The ion product is the amount that is in your solution, and is represented by a Q. ( yep, this is the reaction quotient like last chapter) The solubility product is the Ksp. Compare the 2 and see. If Q is larger than Ksp, you will have a precipitate If Q is smaller than Ksp, no precipitate!

How to do the math! You will be given a gram or molar amount, for the salt and a volume amount for the solvent. Make sure your salt is in moles ( if it is in grams divide by molar mass) then divide by Liters to get molarity. If your equation is unequal, take the 2 or 3 into account, but no sqares or cubes at this time.

Math continued Now plug your molar values into the equation and do the math. Now is the time for the squared values if needed, the answer is Q Compare to Ksp and see if you will get a precipitate! See pg 572 for example!

Common Ion effect Ok so you are at Ksp and all your compound is dissolved and you are at equilibrium… and you pour in something else that has an ion in common! Remember LeChatlier! It will force the equation to shift back towards the reactants and some will precipitate!

Check it out A saturated solution of KCl add NaCl and get precipitate, Add KF and get a precipitate Add MgF and get nothing! All done! Enjoy studying