Predicting Reactions Presented by Mr. Mark Langella AP Chemistry Instructor College Board Consultant Why do the reactions occur? GibbsFree Energy drives the Spontaneous reactions Lower PE energy Formation of Stronger Bonds Greater Entropy ( Formation of Gases) Solubility Formation Constant Ways of Expressing CHEMICAL EQUATIONS Word equation In word equations, the names of the reactants and products are written out. The following example is a word equation: carbon + oxygen carbon dioxide Formula equations Formula equations consist of formulas substituted for the names in the word equation. The reaction above becomes C + O2 CO2 Ionic equations In ionic equations, all water soluble compounds in an aqueous solution are separated into ions. 2NaI(aq) + Pb(NO3)2(aq) 2NaNO3(aq) + PbI2(s) 2Na+(aq) + 2I-(aq) + Pb2+(aq) + 2NO3-(aq) 2Na+(aq) + 2NO3-(aq) + PbI2(s) Spectator ions - Spectator ions do not participate in the chemical reaction. That is, they are identical on both sides of the equation. Net ionic equations The net ionic equation contains all of the particles in the ionic equation less any spectator ions. 2Na+(aq) + 2I-(aq) + Pb2+(aq) + 2NO3-(aq) 2Na+(aq) + 2NO3-(aq) + PbI2(s) Pb2+(aq) + 2I-(aq) PbI2(s) Balancing We were reminded (especially by the combustion reaction above) that the coefficients used for balancing should be the lowest possible WHOLE NUMBER coefficients. All other rules for the balanced equations are the same - omit spectator ions, assume a reaction occurs, write the formulas for molecular compounds (e.g. HF) as undissociated species in solution, etc. Synthesis or Combination Reactions In synthesis or combination reactions, two or more substances combine together to form a single product. The general form is A + B C The products must contain only those elements found in the reactants. Metal + Nonmetal Salt Magnesium ribbon is burned in oxygen A strip of magnesium metal is heated strongly in pure nitrogen gas Online Demos Reaction of Magnesium and Oxygen http://boyles.sdsmt.edu/magburn/magnesium_burning.htm Reaction of Iron and Sulfur http://www.pc.chemie.uni-siegen.de/pci/versuche/english/v21-1.html Reaction of Potassium and Oxygen http://neon.chem.ox.ac.uk/vrchemistry/FilmStudio/alkalimetals/HTML/pag e08.htm Reaction of Lithium and Oxygen http://neon.chem.ox.ac.uk/vrchemistry/FilmStudio/alkalimetals/HTML/pag e02.htm Reaction of Lithium and Chlorine http://neon.chem.ox.ac.uk/vrchemistry/FilmStudio/alkalimetals/HTML/pag e04.htm Reaction of Sodium and Oxygen http://neon.chem.ox.ac.uk/vrchemistry/FilmStudio/alkalimetals/HTML/pag e05.htm Reaction of Zinc and Sulfur http://boyles.sdsmt.edu/znsulf/zincsul.htm Nonmetal + Nonmetal Molecular compounds PureSolid Phosphorus (White Form) is burned in air Reaction of Phosphorus and Chlorine Website: http://boyles.sdsmt.edu/pwithcl/reaction_of _white_phosphorus_and.htm Reaction of Hydrogen and Oxygen Hydrides Preparation of the Group 1 hydrides These are made by passing hydrogen gas over the heated metal. For example, for lithium hydride: 2Li (s) + H2(g) 2LiH Nonmetal Oxide + Water Oxyacid Oxy Acid= Contains H+ ions attached to common Polyatomic ion of Nonmetal Oxide plus one more oxygen Solid dinitrogen pentoxide is added to water Sulfur trioxide gas is bubbled into water Nonmetal Oxide + Water Oxyacid Phosphorus (V) oxide powder is sprinkled over distilled water Sulfur dioxide gas is bubbled into distilled water Metal oxide + water metal hydroxide Solid Cesium Oxide is added to water Solid sodium oxide is added to distilled water Powdered strontium oxide is added to distilled water Calcium oxide powder is added to distilled water Solid barium oxide is added to distilled water Demo Cmd Na2O MgO Al3O2 SiO2 P4O10 SO3 Cl2O7 %Ionic 79 68 56 45 32 17 2 Solid Ionic Ionic Ionic Network Molecular Molecular Molecular Hydroxide NaOH Mg(OH)2 Al(OH)3 Si(OH)4 OP(OH)3 O2S(OH)2 O3Cl(OH) Formula Formula SiO2+2H2O H3PO4 H2SO4 HClO3 Very Basic Ampho. Slightly Acidic Stronger Very Acidic Basic acidic Acid Hydrogen-Nonmetal + Water Acidic Solution Hydrogen Chloride gas bubbled into water Metal oxide + nonmetal oxide metal ion attached to common polyatomic ion Metal oxide + carbon dioxide metal carbonate Carbon dioxide gas is passed over hot, solid sodium oxide Metal oxide + sulfur dioxide Metal sulfite Sulfur Dioxide is passed over solid calcium oxide DECOMPOSITION REACTIONS Substances break down by means of decomposition reactions The general form of a decomposition reaction is C A+B Decomposition reactions are the opposite of combination or synthesis reactions Decomposition of Metal Carbonate Heating a metal carbonate always yields the metal oxide and carbon dioxide. MCO3 MO + CO2 Powdered magnesium carbonate is heated strongly Solid calcium carbonate is strongly heated Metal Hydrogen Carbonate Decomposition Heating a metal bicarbonate gives the metal oxide, carbon dioxide, and water. MHCO3 MO + H2O + CO2 http://www.chemguide.co.uk/inorganic/gro up1/compounds.html Sodium Hydrogen Carbonate is Solid strongly heated Heating the nitrates Most nitrates tend to decompose on heating to give the metal oxide, brown fumes of nitrogen dioxide, and oxygen. For example, a typical Group 2 nitrate like magnesium nitrate decomposes like this: 2 Mg(NO3)2 = 2 MgO + 2 NO2 + 3 O2 In Group 1, lithium nitrate behaves in the same way - producing lithium oxide, nitrogen dioxide and oxygen. The rest of the Groups, however, don't decompose so completely (at least not at Bunsen temperatures) - producing the metal nitrite and oxygen, but no nitrogen dioxide. Decomposition of Metal Hydroxides Heating a metal hydroxide gives the metal oxide and water MOH MO + H2O Metal Sulfite Decomposition Heating a metal sulfite produces a metal oxide and sulfur dioxide MSO3 MO + SO2 Solid calcium sulfite is heated in a vacuum Metal Chlorate Decomposition Heating a metal chlorate gives the metal chloride plus oxygen. MClO3 MCl + O2 Electrolysis of Binary Compound Electrolysisof a molten salt (ionic compound) separates the substance into its elements. MN M+N Ammonium Compound Decomposition Ammonium Carbonates Solid ammonium carbonate is heated (NH4)2CO3 = 2NH3 + 2CO2 + H2O Ammonium Hydroxide NH4OH NH3+ H2O 2007 Questions (NH4)2CO3 = 2NH3 + 2CO2 + H2O Peroxide Decomposition Elephant’s Toothpaste Website: http://boyles.sdsmt.edu/tp aste/elephants.htm Genie in a Bottle Demo Website: http://boyles.sdsmt.edu/g eniebot/genie.htm Reactions Based on Reduction Potentials EMF Potential Reduction and Oxidation Single replacement Cation Replacement There are two types of single replacement reactions, in one, a metal or hydrogen replaces a positive ion M0 + A+B- M+B- + A0 Replacement of Hydrogen Reaction of Magnesium and Different Concentrations of Acids Website: http://boyles.sdsmt.edu/kinetic/mercadokinetics.htm Hydrochloric Acid(g) + Magnesium(s) Magnesium(II)Chloride(aq) + Hydrogen(g) 2HCl(g) + Mg(s) MgCl2(aq) + H2(g) Small piece of sodium metal is added to distilled water Website: http://boyles.sdsmt.edu/sodwat/reaction_of_sodium_and_water.htm Sodium(s) + Water(l) Sodium Hydroxide(aq) + Hydrogen(g) 2Na(s) + 2H2O(l) 2NaOH(aq) + H2(g) ) Reaction of Potassium and Water Website: http://www.chem.shef.ac.uk/webelements- moov/K_H2O.mov Potassium(s) + Water(l) Potassium Hydroxide(aq) + Hydrogen(g) 2K(s) + 2H2O 2KOH + H2(g) A strip of zinc is added to a solution of 6.0-molar hydrobromic acid Group I with water video http://video.google.com/videoplay?docid=- 2134266654801392897&q=rubidium+water 2007 Question Cation Replacement Reaction of Zinc and Tin (II) Chloride Website: http://www.chemtopics.com/lectures/unit02 /lecture1/displace.htm Zinc(s) + Tin (II) Chloride(aq) Tin(s) + Zinc (II) Chloride(aq) Zn(s) + SnCl2(aq) Sn(s) + ZnCl2(aq) Thermite Reaction 2Al(s) + Fe2O3 (s) Al2O3 (s) + 2Fe(l) http://boyles.sdsmt.edu/thermite/therm.htm http://www2.chemie.uni- erlangen.de/education/medprak/videos/the rmit_v.mpg A solution of copper (II) sulfate is spilled onto a sheet of freshly polished aluminum metal. Bar of strontium metal is immersed in a 1.0 M copper (II) nitrate solution. A piece of copper wire is placed in a solution of silver nitrate A small piece of calcium metal is added to hot distilled water A solution of tin (II) nitrate is added to a solution of silver nitrate A mixture of powdered iron (III) oxide and powdered aluminum metal is heat strongly. A bar of zinc metal is immersed in a solution of copper (II) sulfate A piece of nickel metal is immersed in a solution of copper (II) sulfate Solutions of tin (II) chloride and iron (III) chloride are mixed Cu+2 (aq) + Fe(s) Cu(s) + Fe+2 Cu+2 (aq) + Zn(s) Cu(s) + Zn+2 Growing Crystals in Gels are a great activity you can use to demonstrate single replacement reactions. Solutions: 1. Saturated Sodium metasilicate- Flinn Scientific (CAT No. S0102) – Needs to be diluted to a density of 1.06 g/ml. About 158 ml of the commercial solution diluted to one liter of solution will yield a proper density. 2. Commercial White Vinegar 3. 1.0 M CuCl2 (13.5g/100ml H2O) 4. 1.0 M Pb(NO3)2 (33.1g/100ml H2O) Pb+2(aq) + Zn(s) Zn+2(aq) + Pb(s) Anion Replacement In the second, a halogen replaces another halogen as the negative ion. N20 + A+B- A+N- + B20 Chlorine gas is bubbled through a solution of potassium bromide Liquid bromine is shaken with 0.5M sodium iodide solution Chlorine gas is bubbled into a solution of sodium bromide Halogen Replacement NaClO(aq) 2HCl(aq) Cl (aq) NaCl(aq) H O( ) 2 2 Cl (g) 2NaBr(aq) 2NaCl(aq) Br (aq) 2 2 To prepare a saturated solution of aqueous bromine, mix equal volumes of 1.28 M NaBr, 1.28 M HCl, and household bleach. Prepare only as much solution as you need to the activity in working fume hood to avoid unnecessary exposure to bromine vapor. There is no need to store the bromine water since it is easily prepared. It may also be helpful to mix the solutions in a clear glass bottle allowing your students to view the color of elemental bromine. Discussion: Since the solubility of bromine in water at 25EC is 0.214 moles/liter , a saturated aqueous bromine solution can easily be prepared as needed by mixing equal volumes of a 1.28 M sodium or potassium bromide solution, a 1.28 M hydrochloric acid solution, and common household bleach. The three reagents react according to the equation: The reaction between bleach and hydrochloric acid produces elemental chlorine which then will replace the bromine in sodium bromide producing bromine and sodium chloride. A saturated solution of aqueous chlorine can be prepared in the same fashion. To prepare 100 ml a saturated aqueous solution of chlorine, mix 1.5 ml of concentrated HCl with 13 ml of bleach in 84 ml of water. The solubility of chlorine at 25EC is 0.092 M (2) 2007 Question Special Red-ox Reactions Hydrogen reacts with hot metallic oxide to produce the elemental metal and water Hydrogen gas is passed over hot iron(II) oxide powder Oxygen react with Metal Sulfides to produce Metallic Oxides and Sulfur Dioxide Copper reacts with Concentrated Sulfuric Acid Cu + 2 H2SO4 CuSO4+SO2+2 H2O Copper Reacts with Concentrated Nitric Acid 3Cu + 8HNO3 Cu(NO3)2+ NO + 4H2O Typical Reactions Oxidation States of Manganese Procedure Place 10 ml of a Water into Four Graduated Cylinders Place one tablet #1 into each Cylinder .01 M KMnO4 solution is in four small cylinders labeled A , B, N ( Place Tablet 1/10 ml water) To Flask A, Add 10 ml of 3M H2SO4 MnO4- + H+ To Flask B, add 10 ml of 5 M NaOH. MnO4- + OH- To Flask N add nothing. MnO4- Special Redox To Flask A add .01M NaHSO3 ( Tablet 2) slowly till you get a colorless Mn2+ ion. MnO4- + 5H++ HSO3- 3H2O + 2Mn2+ + 5SO42- To Flask N add .01M NaHSO3 ( Tablet 2)until a brown precipitate forms. 2MnO4- + 3HSO3- 3SO42- + H++ H2O +MnO2 To Flask B slowly add .01M NaHSO3 ( Tablet 2) until a green solution forms. 2MnO4- + OH-+ HSO3- 2MnO42- + 2H2O + SO42- Special Redox Sulfite ion is readily oxidized to sulfate. On prolonged exposure to air, this oxidation occurs with atmospheric oxygen: 2SO32-(aq) + O2(g) --> 2SO42-(aq) Sulfite or sulfur dioxide will decolorize permanganate. This de-colorization serves as a convenient test for sulfur dioxide: 2MnO4-(aq) + 5SO2(g) + 2H2O(l) --> 5SO42-(aq) + 2Mn2+(aq) + 4H+(aq) 2007 Question Special Redox Hydrogen chloride gas is oxidized by oxygen gas. (i) Balanced equation: (i) 2 HCl + O2 H2O + OCl2 or 4 HCl + O2 2 H2O + 2 Cl2 (ii) If three moles of hydrogen chloride gas and three moles of oxygen gas react as completely as possible, which reactant, if any, is present in excess? Justify your answer. (ii) O2 in excess; reacting on a 2:1 mole ratio, 3 mol HCl requires only 1.5 mol of O2 ) Formation of a Silver Mirror Website: http://boyles.sdsmt.edu/agmirror/agmir.ht m : Preparing Oxygen from Bleach Website: http://boyles.sdsmt.edu/prepoxyg/preparati on_of_oxygen_gas_from_b.htm Special Redox An acidic solution of potassium dichromate is added to a solution of iron (II) nitrate. Acidified solutions of potassium permanganate and iron (II) nitrate are mixed together Solution of iron (II) chloride is added to an acidified solution of sodium dichromate A concentrated solution of hydrochloric acid is added to solid potassium permanganate. Special Redox A solution of potassium dichromate is added to an acidified solution of iron(II) chloride. A concentrated solution of hydrochloric acid is added to solid potassium permanganate. Solutions of potassium permanganate and sodium oxalate are mixed. A solution of sodium bromide is added to an acidified solution of potassium bromate Reactions Driven by and Precipitation Solubility Formation of Gases ( Increase in entropy) Formation of Water Coordinate Covalent Bond Formation ( Lewis Acid-Base) Formation Constants Formation of Water Metal Oxide + an Acid Salt + Water Metal Hydroxide + an Acid Salt + Water (a special type of reaction called neutralization) A 0.1 M nitrous acid solution is added to the same volume of a 0.1 M sodium hydroxide solution A 0.02 M hydrochloric acid solution is mixed with an equal volume of a 0.01 M calcium hydroxide solution. Acid Base Solid strontium hydroxide is added to a solution of nitric acid. (i) Balanced equation: (i) Sr(OH)2 + 2 H+ 2 H2O + Sr2+ (ii) How many moles of strontium hydroxide would react completely with 500. mL of 0.40 M nitric acid? (ii) 0.10 mol strontium hydroxide Lewis Acid-Base Reactions (Coordinate Covalent Bond Formation) Ammonia gas is mixed with hydrogen chloride gas. Methylamine gas is bubbled into distilled water Lewis Acid/ Base Phosphine (phosphorus trihydride) gas is added to boron trichloride gas. Question: Which species acts as a Lewis acid in the reaction? Explain. Formation of Gas and Water Metal Carbonate + an Acid Salt + Carbon Dioxide + Water Hydrogen iodide gas is bubbled into a solution of lithium carbonate Solid zinc carbonate is added to 1.0 M sulfuric acid. Carbon dioxide and Water- Carbon Dioxide is easily produced by the reaction of sodium bicarbonate and vinegar. Nonmetal Oxide and Metal Hydroxide ) Reaction of Carbon Dioxide and Limewater Website: http://boyles.sdsmt.edu/respira/respir.htm CO2(g) + Ca(OH)2(aq) CaCO3(s) + H2O(l) 2007 Question Metal hydrides + Water Metal Hydroxide + Hydrogen Gas Solid calcium hydride is added to distilled water Solid sodium hydride is added to water Formation of Gas and Water Metal Bicarbonate (Hydrogen Carbonate) + an Acid Salt + Carbon Dioxide + Water Excess hydrobromic acid solution is added to a solution of potassium hydrogen carbonate. PREDICTIONS BASED ON SOLUBILITY Ifone or both of the products in the double replacement reaction is insoluble in water, the reaction will occur. SOLUBILITY RULES FOR COMMON IONIC COMPOUNDS IN WATER 1. All nitrates, chlorates, and acetates are soluble in water. Silver acetate is sparingly soluble. 2. Most common acids are soluble in water. 3. All common IA, and ammonium compounds are soluble in water. 4. All chlorides, bromides, and iodides are soluble in water except silver, mercury (I), and lead. HgI2 and HgBr2 are insoluble in water. 5. All sulfates are soluble in water except CaSO4, SrSO4, BaSO4, PbSO4, Hg2SO4. Ag2SO4 is sparingly soluble in water. 6. All carbonates, phosphates, oxides, and sulfites are insoluble in water but soluble in dilute acids except the IA and ammonium compounds. 7. The sulfides of all metals are insoluble in water except the IA, IIA, and ammonium sulfides. 8. All hydroxides are insoluble in water except the IA, Ca(OH)2, Sr(OH)2, and Ba(OH)2 hydroxides. Solubility A solution of sodium phosphate is added to a solution of aluminum nitrate A solution of potassium phosphate is mixed with a solution of calcium acetate A solution of sodium Iodide is added to a solution of lead (II) acetate A solution of lead(II) nitrate is added to a solution of potassium sulfate. 2007 Question Complex Ion Formation Often an ion in water solution is in a more complex species, in which the ion is bound to several water molecules A complex ion consists of a central ion to which are bonded two, four, or six neutral or ionic species called ligands AMPHOTERIC Lead and Zinc form complexes with hydroxide The hydroxides of lead and zinc are soluble in both acidic and basic solutions Hydroxides that have this property are called amphoteric Halide complexes Al3+ + 6 F- <----------> [AlF6]3- 2.5 x 104 Al3+ + 4 F- <----------> [AlF4]-1 2.0 x 108 Be2+ + 4 F- <----------> [BeF4]2- 1.3 x 1013 Sn4+ + 6 F- <----------> [SnF6]2- 1.0 x 1025 Cu+ + 2 Cl- <----------> [CuCl2]-1 3.0 x 105 Ag+ + 2 Cl- <----------> [AgCl2]-1 1.8 x 105 Pb2+ + 4 Cl- <----------> [PbCl4]2- 2.5 x 1015 Zn2+ + 4 Cl- <----------> [ZnCl4]2- 1.6 Hg2+ + 4 Cl- <----------> [HgCl4]2- 5.0 x 1015 Cu+ + 2 Br- <----------> [CuBr2]-1 8.0 x 105 Ag+ + 2 Br- <----------> [AgBr2]-1 1.0 x 1011 Ammonia complexes Ag+ + 2 NH3 <----------> [Ag(NH3)2]+ 1.6 x 107 Zn2+ + 4 NH3 <----------> [Zn(NH3)4]2+ 7.8 x 108 Cu2+ + 4 NH3 <----------> [Cu(NH3)4]2+ 1.1 x 1013 Hg2+ + 4 NH3 <----------> [Hg(NH3)4]2+ 1.8 x 1019 Co2+ + 6 NH3 <----------> [Co(NH3)6]2+ 5.0 x 104 Co3+ + 6 NH3 <----------> [Co(NH3)6]3+ 4.6 x 1033 Cd2+ + 6 NH3 <----------> [Cd(NH3)6]2+ 2.6 x 105 Ni2+ + 6 NH3 <----------> [Ni(NH3)6]2+ 2.0 x 108 Cyanide complexes Fe2+ + 6 CN- <----------> [Fe(CN)6]4- 1.0 x 1024 Fe3+ + 6 CN- <----------> [Fe(CN)6]3- 1.0 x 1031 Ag+ + 2 CN- <----------> [Ag(CN)2]-1 5.3 x 1018 Cu+ + 2 CN- <----------> [Cu(CN)2]-1 1.0 x 1016 Cd2+ + 4 CN- <----------> [Cd(CN)4]2- 7.7 x 1016 Au+ + 2 CN- <----------> [Au(CN)2]-1 2.0 x 1038 Complexes with other monodentate ligands Ag+ + 2 CH3NH2 <---------> [Ag(CH3NH2)2]+1 7.8 x 106 Cd2+ + 4 SCN- <----------> [Cd(SCN)4]2- 1.0 x 103 Cu2+ 2 SCN- <----------> [Cu(SCN)2] 5.6 x 103 Fe3+ 3 SCN- <----------> [Fe(SCN)3] 2.0 x 106 Hg2+ 4 SCN- <----------> [Hg(SCN)4]2- 5.0 x 1021 Cu2+ 4 OH- <----------> [Cu(OH)4]2- 1.3 x 1016 Zn2+ 4 OH- <----------> [Zn(OH)4]2- 2.0 x 1020 Complex Ion Formation Aqueous sodium hydroxide is added to a saturated solution of aluminum hydroxide, forming a complex ion. (i) Balanced equation: (ii) If the resulting mixture is acidified, would the concentration of the complex ion increase, decrease, or remain the same? Explain. (a) (i) Al(OH)3 + OH- [Al(OH)4]- (ii) decrease; H+ will neutralize OH- and destroy the complex Formation Constants Excess concentrated aqueous ammonia is added to a solution of nickel (II) bromide Excess concentrated hydrochloric acid is added to a 1.0 M solution of cobalt (II) chloride A drop of potassium thiocyanate solution is added to a solution of iron (III) nitrate. Excess concentrated ammonia solution is added to a solution of nickel (II) sulfate Excess sodium cyanide solution is added to a solution of silver nitrate Balancing Red-Ox Reactions Red-Ox Continued Red-Ox Under Basic Conditions Red-Ox continued Organic Where in The Curriculum? Thermochemistry Stoichiometry Bonding and Molecular Structure Metal Oxide and Water Solid potassium oxide is added to water. (i) Balanced equation: (i) K2O + H2O 2 K+ + 2 OH- (ii) If a few drops of phenolphthalein are added to the resulting solution, what would be observed? Explain. (ii) pink color; phenolphthalein is pink in an alkaline solution Anion Replacement (a) Chlorine gas, an oxidizing agent, is bubbled into a solution of potassium bromide at 25°C. (i) Balanced equation: (i) Cl2 + 2 Br– 2 Cl– + Br2 (ii) Predict the algebraic sign of ∆S˚ for the reaction. Explain your reasoning. (ii) negative, a decrease in entropy as a gas converts into aqueous and a ion converts into a liquid Precipitate formation A solution of barium chloride is added drop by drop to a solution of sodium carbonate, causing a precipitate to form. (i) Balanced equation: (i) 3 Ba2+ + H2CO3 + HCO3- + CO32- 3 BaCO3 + 3 H+ (ii) What happens to the pH of the sodium carbonate solution as the barium chloride is added to it? (ii) pH will decrease Cation Replacement 1. A strip of magnesium metal is added to an aqueous solution of silver nitrate. Question: Which substance is oxidized in the reaction? Decomposition Solid potassium chlorate is strongly heated. Question: What is the oxidation number of chlorine before and after the reaction occurs? Complex Ion Formation 3. Solid silver chloride is added to a solution of concentrated hydrochloric acid to form a complex ion. Question: Which species acts as a Lewis base in the reaction? Explain. Acid Base 4.A solution of Ethanoic (acetic) acid is added to a solution of barium hydroxide. Question: Explain why a mixture of equal volumes of equimolar solutions of acetic acid and barium hydroxide is basic. Acid Base 5. Ammonia gas is bubbled into a solution of hydrofluoric acid. Question: Identify a conjugate acid- base pair in the reaction. Cation Replacement Zinc metal is placed into a solution of copper (II) sulfate. Question: Describe the change in color that the original solution undergoes as the reaction proceeds. Precipitation A solution of nickel (II) bromide is added to a solution of potassium hydroxide. Question: Identify the spectator ions in the reaction mixture. Combustion Hexane is combusted in air. Question: When one molecule of hexane is completely combusted, how many molecules of products are formed?