by Steven S. Zumdahl & Donald J. DeCoste University of Illinois Introductory Chemistry: A Foundation, 6 th Ed. Introductory Chemistry, 6 th Ed. Basic Chemistry, 6 th Ed.

Chapter 7 Reactions in Aqueous Solutions

Copyright © Houghton Mifflin Company. All rights reserved. 7 | 3 Predicting Whether a Reaction Will Occur “Forces” that drive a reaction Formation of a solid Formation of water Transfer of electrons Formation of a gas When chemicals (dissolved in water) are mixed and one of these 4 things can occur, the reaction will generally happen.

Copyright © Houghton Mifflin Company. All rights reserved. 7 | 4 Precipitation Reactions In all precipitation reactions, the ions of one substance are exchanged with the ions of another substance when their aqueous solutions are mixed. At least one of the products formed is insoluble in water. KI(aq) + AgNO 3 (aq)  KNO 3 (aq) + AgI  s  K+K+ I-I- Ag + NO 3 - K+K+ Ag I

Copyright © Houghton Mifflin Company. All rights reserved. 7 | 5 Dissociation Ionic compounds –Metal + nonmetal (Type I & II) –Metal + polyatomic anion –Polyatomic cation + anion Dissociation: when ionic compounds dissolve in water the anions and cations are separated from each other We know that ionic compounds dissociate when they dissolve in water because the solution conducts electricity.

Copyright © Houghton Mifflin Company. All rights reserved. 7 | 6 Dissociation (cont.) Potassium chloride dissociates in water into potassium cations and chloride anions. KCl(aq) = K + (aq) + Cl - (aq) Copper(II) sulfate dissociates in water into copper(II) cations and sulfate anions. CuSO 4 (aq) = Cu +2 (aq) + SO 4 2- (aq) K+K+ Cl - K Cl Cu +2 SO 4 2- Cu SO 4

Copyright © Houghton Mifflin Company. All rights reserved. 7 | 7 Dissociation (cont.) Potassium sulfate dissociates in water into potassium cations and sulfate anions. K 2 SO 4 (aq) = 2 K + (aq) + SO 4 2- (aq) K+K+ SO 4 2- K+K+ KK SO 4

Copyright © Houghton Mifflin Company. All rights reserved. 7 | 8 Process for Predicting the Products of a Precipitation Reaction Determine what ions each aqueous reactant has Exchange ions –(+) ion from one reactant with (-) ion from other Balance charges of combined ions to get formula of each product Balance the equation –Count atoms Determine solubility of each product in water –Solubility rules –If product is insoluble or slightly soluble, it will precipitate.

Copyright © Houghton Mifflin Company. All rights reserved. 7 | 9 Solubility Rules Most compounds that contain NO 3 - ions are soluble. Most compounds that contain Na +, K +, or NH 4 + ions are soluble Most compounds that contain Cl - ions are soluble, except AgCl, PbCl 2, and Hg 2 Cl 2 Most compounds that contain SO 4 2- ions are soluble, except BaSO 4, PbSO 4, CaSO 4

Copyright © Houghton Mifflin Company. All rights reserved. 7 | 10 Solubility Rules (cont.) Most compounds that contain OH - ions are slightly soluble (will precipitate), except NaOH and KOH, which are soluble, and Ba(OH) 2, Ca(OH) 2, which are moderately soluble. Most compounds that contain S 2-, CO 3 2-, or PO 4 3- ions are slightly soluble (will precipitate).

Copyright © Houghton Mifflin Company. All rights reserved. 7 | 11 Ionic Equations Molecular equations: equations that describe the chemicals put into the water and the product molecules KCl(aq) + AgNO 3 (aq)  KNO 3 (aq) + AgCl(s) Ionic equations: equations that describe the actual ions and molecules in the solutions, as well as the molecules of solid, liquid, and gas not dissolved K + (aq) + Cl - (aq) + Ag + (aq) + NO 3 - (aq)  K + (aq) + NO 3 - (aq) + AgCl (s)

Copyright © Houghton Mifflin Company. All rights reserved. 7 | 12 Ionic Equations (cont.) Spectator ions: ions that are both reactants and products K + (aq) + Cl - (aq) + Ag + (aq) + NO 3 - (aq)  K + (aq) + NO 3 - (aq) + AgCl (s) Net ionic equation: an ionic equation in which the spectator ions are dropped Cl - (aq) + Ag + (aq)  AgCl (s)

Copyright © Houghton Mifflin Company. All rights reserved. 7 | 13 Electrolytes Electrolytes: substances whose aqueous solutions conducts electricity All electrolytes have ions dissolved in water There are strong electrolytes, weak electrolytes, and nonelectrolytes.

Copyright © Houghton Mifflin Company. All rights reserved. 7 | 14 Electrolytes (cont.)

Copyright © Houghton Mifflin Company. All rights reserved. 7 | 15 Reactions that Form Water: Acids + Bases Acids all contain H + cations and an anion. Bases all contain OH - anions and a cation, except for weak bases such as NH 3 When acids dissociate in water they release H + ions and their anions. When bases dissociate in water they release or form OH - ions and their cations.

Copyright © Houghton Mifflin Company. All rights reserved. 7 | 16 Acid-Base Reactions In the reaction of an acid with a base, the H + from the acid combines with the OH - from the base to make water The cation from the base combines with the anion from the acid to make the salt acid + base  salt + water H 2 SO 4 (aq) + Ca(OH) 2 (aq)  CaSO 4 (aq) + 2 H 2 O(l) The net ionic equation for a strong acid-strong base reaction is always H + (aq) + OH - (aq)  H 2 O(l)

Copyright © Houghton Mifflin Company. All rights reserved. 7 | 17 Reactions of Metals with Nonmetals (Oxidation-Reduction) The metal loses electrons and becomes a cation (oxidation) The nonmetal gains electrons and becomes an anion (reduction) In the reaction, electrons are transferred from the metal to the nonmetal.

Copyright © Houghton Mifflin Company. All rights reserved. 7 | 18 Oxidation-Reduction Reactions Oxidation-reduction reactions: reactions that involve a transfer of one or more electrons The substance that loses electrons in the reaction is oxidized. The substance that gains electrons in the reaction is reduced.

Copyright © Houghton Mifflin Company. All rights reserved. 7 | 19 Predicting Products of Metal + Nonmetal Reactions Metal + nonmetal  ionic compound –Ionic compounds always solids unless dissolved in water In the ionic compound the metal is now a cation. In the ionic compound the nonmetal is now an anion.

Copyright © Houghton Mifflin Company. All rights reserved. 7 | 20 Predicting Products of Metal + Nonmetal Reactions (cont.) To predict direct synthesis of metal + nonmetal: –Determine the charges on the cation and anion from their position on the periodic table. –Determine numbers of cations and anions needed to have charges cancel. –Balance the equation.

Copyright © Houghton Mifflin Company. All rights reserved. 7 | 21 Another Kind of Oxidation-Reduction Reaction Some reactions between two non-metals are also oxidation-reduction reaction. Any reaction in which O 2 is a reactant or a product will be an oxidation-reduction reaction. CH 4 (g) + 2 O 2 (g)  CO 2 (g) + 2 H 2 O(g) 2 SO 3 (g)  2 SO 2 (g) + O 2 (g)

Copyright © Houghton Mifflin Company. All rights reserved. 7 | 22 Ways to Classify Reactions Precipitation reactions: reactions that involve solid formation Acid-base reactions: reactions that involve water formation Double displacement reactions: ion exchange reactions Both precipitation reactions and acid-base reactions involve compounds exchanging ions.

Copyright © Houghton Mifflin Company. All rights reserved. 7 | 23 Double Displacement Reactions Two ionic compounds exchange ions X  Y  (aq) + A  B  (aq)  XB + AY Reaction will not occur unless one of the products either (1) precipitates, or (2) is water

Copyright © Houghton Mifflin Company. All rights reserved. 7 | 24 Ways to Classify Reactions (cont.) Oxidation-reduction reactions: reactions that involve electron transfer –Metals + Nonmetal –O 2 as a reactant or product Gas forming reactions: reactions that occur in aqueous solution because one of the products is a gas NaHCO 3 (aq) + HCl(aq)  NaCl(aq) + CO 2 (g) + H 2 O(l)

Copyright © Houghton Mifflin Company. All rights reserved. 7 | 25 Ways to Classify Reactions (cont.) Single replacement reaction: reactions that involve an ion being transferred from one cation to another X  Y  + A   X + A  Y  Zn(s) + 2 HCl(aq)  ZnCl 2 (aq) + H 2 (g)

Copyright © Houghton Mifflin Company. All rights reserved. 7 | 26 Other Ways to Classify Reactions Combustion reactions: reactions in which O 2 (g) is reacted with a carbon compound –Release a lot of energy –Subclass of oxidation-reduction reactions Combustion of carbon compounds produces CO 2 (g) Combustion of compounds that contain hydrogen produces H 2 O(g) C 3 H 8 (g) + 5 O 2 (g)  3 CO 2 (g) + 4 H 2 O(g)

Copyright © Houghton Mifflin Company. All rights reserved. 7 | 27 Other Ways to Classify Reactions Synthesis reactions: reactions in which chemicals combine to make one product –Metal + Nonmetal reactions can be classified as synthesis reactions. 2 Na(s) + Cl 2 (g)  2 NaCl(s) –Reactions of Metals or Nonmetals with O 2 can be classified as synthesis reactions. N 2 (g) + O 2 (g)  2 NO(g) These two types of synthesis reactions are also subclasses of oxidation-reduction reactions.

Copyright © Houghton Mifflin Company. All rights reserved. 7 | 28 Other Ways to Classify Reactions Decomposition reactions: reactions in which one reactant breaks down into smaller molecules Generally initiated by addition of energy –Addition of electric current or heat Opposite of a synthesis reaction 2 NaCl(l)  2 Na(l) + Cl 2 (g) electric current