What physical evidence do we have that sodium hydroxide dissociates into ions when it dissolves in water? NaOH(s) Na + + OH -

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1 Chemistry Date Period Name CW double replacement reactions doc Double Replacement Reactions Double replacement reactions are some of the most common reactions that are performed in the laboratory. They almost always occur in an aqueous (water) solution. They frequently result in the formation of one or more insoluble compounds which precipitate as a solid. Before going very far into double replacement reactions, it will be necessary to discuss aqueous solutions. Compounds can be broadly classified as being either soluble or insoluble in water. Those compounds which are soluble in water form aqueous solutions. Suppose that solid sodium hydroxide (NaOH) pellets are placed in water: What physical evidence do we have that sodium hydroxide dissociates into ions when it dissolves in water? NaOH(s) Na + + OH This chemical equations represents solid NaOH dissolving in water to form sodium ions and hydroxide ions. The solid NaOH breaks apart (dissociates) into two ions. (Ions are electrically charged atoms formed by either gaining or losing the electrons involved in bonding. Since ions and atoms are extremely small they cannot be seen with the eye.) IMPORTANT When a soluble salt dissolves in water it will dissociate (break apart) into a positively charged ion and a negatively charged ion. Here are some examples of the dissociation of salts producing two ions in solution: FeSO 4 (s) Fe 2+ + SO 4 2 MgCl 2 (s) Mg Cl Al(NO 3 ) 3 (s) Al NO 3 Na 2 CO 3 (s) 2 Na CO 3 There are many solids which are not soluble in water. AgCl, PbSO 4, FeS, Cu(OH) 2, CaCO 3, and CdS are some insoluble salts. These don t dissolve in water and don t dissociate to form ions. Insoluble salts can also be formed by combining two soluble salts. When the two ions of an insoluble salt come together, the insoluble salt will form and come out of solution as a solid precipitate. Such is the case when solutions of sodium sulfide and cadmium nitrate are combined. Yellow cadmium sulfide, CdS, is the insoluble product. Na 2 S(aq) + Cd(NO 3 ) 2 CdS(s) + 2NaNO 3 (aq)

2 Double Replacement Reactions Precipitation Reactions Consider the case where solutions of silver nitrate and sodium chloride are mixed. Both are soluble salts. But they form an insoluble product, silver chloride. AgNO 3 (aq) + NaCl(aq) AgCl(s) + NaNO 3 (aq) Notice how the positive ion of one compound will combine with the negative ion of the other compound: AgNO 3 (aq) + NaCl(aq) AgCl(s) + NaNO 3 (aq) Since silver nitrate and sodium chloride are both soluble in water, silver nitrate actually exists as silver ions (Ag + ) and nitrate ions (NO 3 ), and sodium chloride actually exists as sodium ions (Na + ) and chloride ions (Cl ). When the two solutions are placed together in a beaker, all four ions have a chance to "mingle". When the silver ions (Ag + ) collide with chloride ions (Cl ), solid silver chloride precipitates. Silver chloride is NOT soluble in water. This reaction is called a "molecular equation" since all the compounds are written as molecules. AgNO 3 (aq) + NaCl(aq) AgCl(s) + NaNO 3 (aq) But the molecular equation is not very accurate since the two salts exists as ions in solution. Therefore, a better way to represent the reaction the ionic equation. In the ionic equation all of the compounds which are dissociated into ions are written as ions, and the insoluble compounds are written as molecules. The ionic equation is more accurate because it represents what is actually in solution. The ionic equation for the silver nitratesodium chloride reaction is: Ag + + NO 3 + Na + + Cl AgCl(s) + Na + + NO 3 + If we look at the reaction that occurs, we see there are only two ions which actually react. Ag + + NO 3 + Na + + Cl AgCl(s) + Na + + NO 3 + The reaction produces solid AgCl which is insoluble. The sodium nitrate (NaNO 3 ) doesn't form an insoluble precipitate since it is soluble in water. Therefore, the sodium ions and nitrate ions stay just as they were. They are referred to as spectator ions. Before reaction After reaction Spectator ions Insoluble precipitate

3 We can represent the ions that actually react with the "net ionic equation": Ag + + Cl AgCl(s) The net ionic equation says that "in any reaction in which there is a silver ion and a chloride ion, the two ions will react and precipitate as solid silver chloride." In the net ionic equation the spectator ions are omitted. Consider another example where solutions of iron(ii) chloride and sodium phosphate are mixed together. Balanced molecular equation: 3FeCl 2 (aq) + 2Na 3 PO 4 (aq) Fe 3 (PO 4 ) 2 (s) + 6NaCl(aq) Balanced ionic equation: 3Fe Cl + 6Na + + 2PO 4 3 Fe 3 (PO 4 ) 2 (s) + 6Na + + 6Cl Balanced net ionic equation: 3Fe PO 4 3 Fe 3 (PO 4 ) 2 (s) Double Replacement Reactions AcidBase Reactions A acid is a solution which dissolves in water to produce an excess of hydrogen ions. A base is a compound which dissolves in water to produce an excess of hydroxide ions. Usually acids contain hydrogen and produce hydrogen ions by dissociating. Bases can dissociate in water to produce hydroxide ions, OH. HCl H + + Cl HNO 3 H + + NO 3 H 2 SO 4 H + + HSO 4 NaOH Na + + OH KOH K + + OH Ca(OH) 2 Ca OH Some substances produce basic solutions by reacting with water. Na(s) + H 2 O Na + + OH + H 2 (g) MgO(s) + H 2 O Mg OH K 2 CO 3 (s) + H 2 O 2K + + 2OH + CO 2 (aq) When an acid reacts with a base (a compound that contains a hydroxide ion), water is produced. Water is a molecular liquid and does not dissociate into ions*. Water does NOT split apart into H + and OH. In an acidbase reaction water is the "insoluble product." Molecular equation: HCl(aq) + NaOH(aq) NaCl(aq) + H 2 O(l) Ionic equation: H + + Cl + Na + + OH Na + + Cl + H 2 O(l) Net ionic equation: H + + OH H 2 O(l) The formation of water is the main reaction in an acid/base reaction. Consider the reaction between sulfuric acid and aluminum hydroxide: Molecular equation: Ionic equation: Net ionic equation 3H 2 SO 4 (aq) + 2Al(OH) 3 (s) Al 2 (SO 4 ) 3 (aq) + 6H 2 O(l) 3H + + HSO 4 + 2Al(OH) 3 (s) 2Al SO H 2 O(l) 3H + + HSO 4 + 2Al(OH) 3 (s) 2Al SO H 2 O(l) Sulfuric acid dissociates in water to make H + and HSO 4 ions. Aluminum hydroxide is insoluble in water and does not dissociate into ions. The net ionic equation is the same as the ionic equation since there are no spectator ions.

4 Exercise 1 Write balanced ionic and net ionic equations for the following reactions. 1. BaCl 2 (aq) + (NH 4 ) 2 CO 3 (aq) BaCO 3 (s) + 2NH 4 Cl(aq) 2. 2AgNO 3 (aq) + CuCl 2 (aq) 2AgCl(s) + Cu(NO 3 ) 2 (aq) 3. 3FeCl 2 (aq) + 2Na 3 PO 4 (aq) Fe 3 (PO 4 ) 2 (s) + 6NaCl(aq) 4. 2Fe(OH) 3 (s) + 3H 2 SO 4 (aq) Fe 2 (SO 4 ) 3 (aq) + 6HOH(l) Exercise 2. Predict the possible products of these double replacement reactions and write them to the right of the arrow. Use the solubility rules to predict whether the possible products are soluble in water. If there is no reaction, write NR and continue to the next problem. (You don t need to balance an equation if no reaction actually occurs.) Where there is an insoluble product and therefore, a reaction, balance the molecular equation and write the balanced net ionic equation in the space below. All of the reactants are soluble in water. Include the state symbols of the products: (s) or (l) for an insoluble solid or liquid; (aq) if it is in aqueous solution. 1. NaCl + AgNO 3 2. BaCl 2 + Na 2 CO 3 3. KNO 3 + LiCl 4. Pb(C 2 H 3 O 2 ) 2 + Cu(NO 3 ) 2

5 5. FeI 2 + NaCl 6. CaCl 2 + K 3 PO 4 7. CaCl 2 + Pb(NO 3 ) 2 8. Na 2 S + NH 4 Br 9. FeSO 4 + KOH 10. NaF + CaCl Cs 3 PO 4 + Pb(NO 3 ) HCl + Sr(OH) ZnCl 2 + Al(C 2 H 3 O 2 ) Cu(NO 3 ) 2 + NH 4 OH 15. Bi(NO 3 ) 3 + Na 2 SO 4

6 Chemistry Date Period Name Answers to Exercise 1: Double Replacement Reactions CW double replacement reactions doc 1. BaCl 2 (aq) + (NH 4 ) 2 CO 3 (aq) BaCO 3 (s) + 2 NH 4 Cl(aq) Ba Cl + 2 NH CO 3 BaCO 3 (s) + 2 NH Cl Ba CO 3 BaCO 3 (s) 2. 2 Fe(OH) 3 (s) + 3 H 2 SO 4 (aq) Fe 2 (SO 4 ) 3 (aq) + 6 HOH(l) 2Fe OH + 3 H HSO 4 2 Fe SO HOH(l) H + + OH HOH(l) HSO 4 + OH 2 HOH(l) + SO AgNO 3 (aq) + CuCl 2 (aq) 2 AgCl(s) + Cu(NO 3 ) 2 (aq) 2 Ag + 2 NO 3 + Cu Cl 2 AgCl(s) + Cu NO 3 Ag + + Cl AgCl(s) Sulfuric acid, H 2 SO 4 is a strong acid, which means that it dissociates completely, but it does not dissociate into 2H + and SO 4 2. It dissociates to form H + and HSO 4, the bisulfate ion, which is a weak acids. Weak acids do not completely dissociate in water. H 2 SO 4 H + + HSO 4 HSO 4 H + + SO FeCl 2 (aq) + 2 Na 3 PO 4 (aq) Fe 3 (PO 4 ) 2 (s) + 6 NaCl(aq) 3 Fe Cl + 6 Na PO 4 Fe 3 (PO 4 ) 2 (s) + 6 Na Cl 3 Fe PO 4 Fe 3 (PO 4 ) 2 (s) Answers to Exercise 2: 1. NaCl(aq) + AgNO 3 (aq) NaNO 3 (aq) + AgCl(s). (BAW) 2. BaCl 2 (aq) + Na 2 CO 3 (aq) BaCO 3 (s) + 2 NaCl (aq) 3. KNO 3 (aq) + LiCl(aq) NR.. KCl(aq) + LiNO 3 (aq) both soluble in water 4. Pb(C 2 H 3 O 2 ) 2 (aq) + Cu(NO 3 ) 2 (aq) NR. Pb(NO 3 ) 2 (aq) + Cu(C 2 H 3 O 2 ) 2 (aq) 5. FeI 2 (aq) + 2 NaCl(aq) NR. FeCl 2 aq) + NaI(aq) both soluble in water 6. 3 CaCl 2 (aq) + 2 K 3 PO 4 (aq) Ca 3 (PO 4 ) 2 (s) + 6 KCl(aq) 7. CaCl 2 (aq) + Pb(NO 3 ) 2 (aq) Ca(NO 3 ) 2 (aq) + PbCl 2 (s).. (BAW) 8. Na 2 S(aq) + NH 4 Br(aq) NR.. NaBr(aq) + (NH 4 ) 2 S(aq) both soluble in water 9. FeSO 4 (aq) + 2 KOH(aq) Fe(OH) 2 (s) + K 2 SO 4 (aq) NaF(aq) + CaCl 2 (aq) 2 NaCl(aq) + CaF 2 (s) Cs 3 PO 4 (aq) + 3 Pb(NO 3 ) 2 (aq) 6 CsNO 3 (aq) + Pb 3 (PO 4 ) 2 (s) HCl(aq) + Sr(OH) 2 (aq) 2 HOH(l) + SrCl 2 (aq) 13. ZnCl 2 (aq) + Al(C 2 H 3 O 2 ) 3 (aq) NR.. Zn(C 2 H 3 O 2 ) 2 (aq) + AlCl 3 (aq) both soluble in water 14. Cu(NO 3 ) 2 (aq) + 2 NH 4 OH(aq) Cu(OH) 2 (s) + 2 NH 4 NO 3 (aq) 15. Bi(NO 3 ) 3 (aq) + Na 2 SO 4 (aq) NR. Bi 2 (SO 4 ) 3 (aq) + NaNO 3 (aq) both soluble in water