1. Solubility Rules Predicting whether a chemical reaction will occur in an aqueous solution and what its products will be

2. Remember, just because we put two substances together, they will not necessarily chemically react. We can think of ‘tendencies’ in reactants that drive them toward making products (i.e., chemically reacting together). The most common driving forces that pull reactants toward making products are: 1)Formation of a solid (precipitate) 2)Formation of water 3)Formation of electrons 4)Formation of a gas

3. Predicting Products in Aqueous Solutions In this unit, we will be discussing chemical reactions among reactants in aqueous solutions. Why? The most important chemical reactions to us occur in water— breathing (oxygen dissolves in blood) and production of fuel for our cells (food digestion). When ionic compounds dissolve in water, almost always the ions separate (into cations and anions) and move around independently. Ba(NO 3 ) 2 → Ba 2+ + 2NO 3 -

4. We say that the ions of the solid dissociate (come apart) when the solid dissolves in water. That is, Ba(NO 3 ) 2 (aq) does not contain units of Ba(NO 3 ) 2 but contains separated Ba 2+ and NO 3 - ions in the aqueous solution. Chemists know that the separated ions are present because the solution conducts electricity. (Pure water does not conduct electricity because ions must be present for current to flow.)

5. When each unit of a substance that dissolves in water produces cations and anions, the substance is called a strong electrolyte. Barium nitrate and sodium chloride (table salt) are strong electrolytes. A weak electrolyte is an electrolyte that does not completely dissociate in solution. The solution will contain both ions and molecules of the electrolyte. HC 2 H 3 O 2 (acetic acid), H 2 CO 3 (carbonic acid), and NH 3 (ammonia) are all weak electrolytes.

6. Predicting Products Let’s look at a reaction between two ionic compounds, barium nitrate and potassium chromate, which are dissolved in water. Ba(NO 3 ) 2 (aq) + K 2 CrO 4 (aq) → products Because each compound dissociates, a more accurate equation is: Ba 2+ (aq) + 2NO 3 - (aq) + 2K + (aq) + CrO 4 2- (aq) → products ions in Ba(NO 3 ) 2 (aq) ions in K 2 CrO 4 (aq) DEMO

7. Making an Educated Guess The only sure way to know what the products are is to test them experimentally, but we can look at the likelihood of the various possible outcomes and make an educated guess. Remember that compounds have zero net electrical charge, so the products of any reaction will have both cations and anions, not two cations put together or two anions (couldn’t be K 2 Ba or CrO 4 (NO 3 ) 2, for instance).

8. Making an Educated Guess Most ionic materials only contain one cation and one anion. When two of these ionic compounds react together, they form two new ionic compounds. You will recognize this category of reaction as a DOUBLE DISPLACEMENT REACTION. AB + CD → AD + CB The outside ions react together to form a new compound, as do the inside ions.

9. So what’re the products? Our options for the products in this reaction have to be some combination of the cations and anions in the solution: NO 3 - CrO 4 2- Ba 2+ Ba(NO 3 ) 2 BaCrO 4 K+K+ KNO 3 K 2 CrO 4

10. So what’re the products? They’re not going to be either of the reactants (Ba(NO 3 ) 2 or K 2 CrO 4 ) because they are already present in the solution. So the precipitate must be either KNO 3 or BaCrO 4, but which one? Here we need more information. What if I told you that KNO 3 is a white solid? What if I told you that the chromate ion (CrO 4 2- ) is bright yellow?

11. So what’re the products? Yep, the solid that precipitated is barium chromate (BaCrO 4 ). So the chemical equation for this reaction is Ba(NO 3 ) 2 (aq) + K 2 CrO 4 (aq) → BaCrO 4 (s) + 2KNO 3 (aq) So what happened to the K + and NO 3 - ions? They stayed dissolved in the solution, as aqueous ions. If we filtered out the precipitate, which we have identified as BaCrO 4, and evaporated all the water, we would have solid KNO 3.

12. In this example, we used our knowledge of facts and of concepts to identify the products. As you continue to study chemistry, you will apply BOTH types of knowledge. You will need to memorize important facts and understand crucial concepts to be successful.

13. Solubilities of Ionic Compounds To predict whether a given pair of dissolved ions (one cation and one anion) will form a solid (precipitate) when mixed, it is useful to know some facts about the solubilities of various types of ionic compounds:  Soluble solid —readily dissolves in water (disappears from view)  Insoluble (or slightly soluble) solid —does NOT appear to dissolve or only such a tiny amount dissolves in water that is undetectable with the naked eye

14. Organizing the Rules SolubleInsoluble You may prefer to put your solubility rules in a table like this, or a list (next slide). You don’t need to have them numbered, but keep the order. Insoluble ions/compounds will be put in bold italics.

15. Solubility Rules 1.Salts containing Group I elements are soluble (Li +, Na +, K +, Cs +, Rb + ). Exceptions to this rule are rare. 2.Salts containing the ammonium ion (NH 4 + ) are also soluble. 3.Salts containing nitrate ion (NO 3 - ) are generally soluble, as are those containing perchlorate (ClO 4 - ), chlorate (ClO 3 - ), and acetate (C 2 H 3 O 2 - ) anions. 4.Salts containing Cl -, Br -, I - are generally soluble. Important exceptions to this rule are halide salts of Ag +, Pb 2+, and (Hg 2 ) 2+. Thus, AgCl, PbBr 2, and Hg 2 Cl 2 are all insoluble.

16. Solubility Rules (cont.) 5. Most silver salts are insoluble. AgNO 3 and Ag(C 2 H 3 O 2 ) are common soluble salts of silver; virtually anything else is insoluble. 6. Most sulfate (SO 4 2-) salts are soluble. Important exceptions to this rule are those containing Ba 2+, Pb 2+, Ca 2+, Hg 2 2+, or Sr 2+. 7. Most hydroxide (OH - ) salts are only slightly soluble. Hydroxide salts of Group 1A elements are soluble. Hydroxide salts of Group 2A elements (Ca, Sr, and Ba) are slightly soluble. Hydroxide salts of transition metals and Al 3+ are insoluble. Thus, Fe(OH) 3, Al(OH) 3, Co(OH) 2 are not soluble.

17. Solubility Rules (cont.) 8. All compounds containing PO 4 3- (phosphate), CO 3 2- (carbonate), chromates (CrO 4 2- ), and SO 3 2- (sulfite) ions are insoluble except those that also contain alkali metals (Group 1A) or NH 4 +. 9. All compounds containing S 2- (sulfide) ions are insoluble except those that also contain alkali metals (Group 1A), alkaline earth metals (Group 2A), or NH 4 +. 10. Fluorides are frequently insoluble. Examples: BaF 2, MgF 2 PbF 2.

18. Solubility Lab You are going to predict precipitates when solutions of two ionic compounds are mixed. Follow these easy steps for success: 1.Write the reactions as they actually exist before any reaction occurs. Remember that when a salt dissolves, its ions separate. 2.Consider the various solids that could form by exchanging the anions of the added salts. 3.Use the solubility rules to decide whether a solid forms and, if so, to predict the identity of the solid.

19. Describing Reactions in Aqueous Solutions Remember our demonstration: Ba(NO 3 ) 2 (aq) + K 2 CrO 4 (aq) → BaCrO 4 (s) + 2KNO 3 (aq) This is called the molecular equation for the reaction; it shows the complete formulas of all reactants and products. However, it is not give a very clear picture of what actually occurs in solution. What could we use instead?

20. Ionic Equations Ba 2+ (aq) + 2NO 3 - (aq) + 2K + (aq) + CrO 4 2- (aq) → ions in Ba(NO 3 ) 2 (aq) ions in K 2 CrO 4 (aq) BaCrO 4 (s) + 2K + (aq) + 2NO 3 - (aq) precipitate ions in solution This is the complete ionic equation.

21. In complete ionic equations, all substances that are strong electrolytes are represented as ions. Solids, such as barium chromate, are not dissolved and so are written as compounds. Ba 2+ (aq) + 2NO 3 - (aq) + 2K + (aq) + CrO 4 2- (aq) → BaCrO 4 (s) + 2K + (aq) + 2NO 3 - (aq) Remember the K + and NO 3 - ions? They are present in the solution both before and after the reaction. Since they did not participate directly in the reaction in solution, we call them spectator ions. The ions that participate directly in a reaction are part of the net ionic equation. Ba 2+ (aq) + CrO 4 2- (aq) → BaCrO 4 (s)

22. Practice Let’s consider another reaction. Here is the molecular equation. Copy this in your notes: Pb(NO 3 ) 2 (aq) + Na 2 SO 4 (aq) → PbSO 4 (s) + 2NaNO 3 (aq) Now write the complete ionic equation. Pb + (aq) + 2(NO 3 ) - (aq) + 2Na + (aq) + SO 4 2- (aq) → PbSO 4 (s) + 2Na + (aq) + 2NO 3 - (aq) Now write the net ionic equation Pb + (aq) + SO 4 2- (aq) → PbSO 4 (s) What are the spectator ions? Na + and NO 3 -