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Precipitates and Solubility

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1 Precipitates and Solubility
Unit 8 – Solutions Chapter 10, section 3

2 Precipitates & Solubility
Precipitates are insoluble products that occur as the result of an ionic reaction. Happens when two soluble ionic compounds undergo a double replacement reaction. —This means that the cations switch partners. NaCl + KBr  NaBr + KCl If one of the new products formed is insoluble in water, it precipitates out. AgNO3 + NaCl  NaNO3 + AgCl(s) Precipitate

1. All nitrate (Things bonded with NO3-) salts are soluble. 2. All salts containing the alkali metal ions (Li+, Na+, and K+) and the ammonium ion (NH4+) are soluble. Most chloride, bromide, and iodide salts are soluble; **Silver (Ag+), Lead (Pb2+), and Mercury (Hg22+) are exceptions. 4. Most sulfates (Things bonded with SO42-) are soluble *** Calcium (Ca+2), Barium (Ba+2), Lead (Pb+2), and Mercury (Hg2+2) and Strontium (Sr+2) are exceptions.

4 5. All Acetate (C2H3O2-) salts are soluble,
5. All Acetate (C2H3O2-) salts are soluble, **except when bonded with iron (Fe+3) 6. Most hydroxide salts (Things bonded with OH -) are insoluble. 7. All sulfide (S2-), carbonate (CO32-), chromate (CrO42-), and phosphate (PO43-) salts are insoluble. Rules 1-5 override rules 6-7. For example, this means that anything bonded to an alkali metal like sodium, or bonded to ammonium ion (NH4+) is soluble

5 These rules help you before and after a reaction to figure out what will happen.
If something will not dissolve in water, then it won’t react with anything in the water. If a solution forms a precipitate, then you can determine what that precipitate is just by using these rules.

6 Example: AgNO3 + NaCl  Forms what?
First check if these two things dissolve at all. Rules 1 and 3 say they do, so they do. You can use (aq) , which means aqueous, in the reaction equation to show that it is dissolved in water. This means that the ionic compound has separated into its ionic parts and is solvated by water molecules. To find the products, just switch the partners of the reactants around. AgNO3 (aq) + NaCl (aq)  AgCl + NaNO3

7 Now check if any of the products form a precipitate
Now check if any of the products form a precipitate. According to Rule 3, AgCl is insoluble and will form a precipitate. You can then use (s) for solid next to AgCl to indicate it is a precipitate. AgNO3 (aq) + NaCl (aq)  AgCl (s) + NaNO3 (aq)

8 Writing Full Ionic equations
Full Ionic Equations show all soluble ionic compounds as ions. Ag+(aq) + NO3- (aq) + Na+(aq) + Cl-(aq)  AgCl(s) + Na+(aq) + NO3-(aq) Ions that appear on both sides of the equation cancel out to leave the Net Ionic Equation.

9 Writing Net Ionic Equations
The equation only includes the ions and compounds that form a precipitate in the reaction Ag+ (aq) + Cl-(aq)  AgCl (s) Spectator Ions – Those ions that remain in solution after the reaction. Na+ and NO3- are the spectator ions for the above reaction.


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