1 Reactions On the A.P. Test there will be one question (question #4) that will say: Give the formulas to show the reactants and the products for the following chemical reactions. Each occurs in aqueous solution unless otherwise indicated. Represent substances in solution as ions if the substance is extensively ionized. Omit formulas for any ions or molecules that are unchanged by the reaction. In all cases a reaction occurs. You need to balance the equation and answer the question that follows it.. Some suggestions: Identify what type of reaction it is: redox, combustion, single replacement, acid-base, precipitation, etc. Use your knowledge of these types of rxns to determine where to go from there. Remember that when things form they make common substances, not complex ones. If your answer looks complicated, it is probably wrong. Only do net ionic reactions. Identify those species that are actually going to react, the rest you can ignore. Make sure you know the solubility rules. What substances will precipitate? Make sure you know the strong acids and bases. Types of Reactions: Reactions with no changes in Oxidation Numbers I. Dissociation Reactions of Salts in Aqueous Solutions: (3,4,5,6) dissociation: When ionic compounds dissolve in water the ions which compose these compounds separate from each other and enter the solution as more or less independent particles (ions): Cations or anions NaCl ----> Na +1 (aq) + Cl - (aq) H 2 SO 4 (l) ----> Na + (aq) + HSO - 4 (aq) II. Nonmetallic oxides react with H 2 O to form acids This is called an acid anhydride SO 3 (g) + H 2 O ----> H 2 SO 4 + H 2 O ----> H 3 3O + (aq) + HSO 4 - N 2 O 5 (g) + H 2 O ----> 2 HNO 3 + H 2 O ----> 2 H 3 O + + 2 NO 3 - III. Metallic Oxides react with H 2 O to form bases. Called a basic anhydride Na 2 O(s) + H 2 O ----> 2 NaOH + H 2 O (l) ----> 2 Na + (aq) + 2 OH - (aq) IV. Some Decomposition Reactions
2 A. Base ----> metal oxide + water Ca(OH) 2 ----> CaO + H 2 O B. Salt containing oxygen ----> metal oxide + nonmetal oxide CaCO 3 ---->CaO + CO 2 C. Acid containing oxygen ----> water + nonmetal oxide H 2 CO 3 ----> H 2 O + CO 2 V. Metallic oxides + nonmetallic oxides ----> Salt CaO(s) + SO 2 (g) ----> CaSO 3 (s) VI. Reactions of Coordination compounds These are called complex ions. (Though an ion is not always made). It involves adding a ligand to a 4th row transition metal, and a few other selected metals. The most common ligands include: ammonia (NH 3 ), water, hydroxide (OH-), thiocynate (SCN - ), and cyanide (CN - ). The number of ligands that attach to the metal will almost always be double the amount of the charge on the metal. To determine the final charge, add the total charge from the metal to the sum of the charges from all of the ligands. Cu 2+ + OH - ----> Cu(OH) 4 VI. Ionization Reactions of Molecular Compounds Ionization: One example of ionization is when a molecular substance dissolves in water and produces ions. HCl(g) + H 2 O ---> H 3 O + (aq) + Cl - (aq) HF(aq) + H 2 O ---> H 3 O + (aq) + F - (aq) Molecular Equations Complete formulas of all reactants and products are written as if they were molecules. No explicit indication has been made that the soluble compounds are actually dissociated into ions, so molecular in this context means only that complete formulas are used. Cd(NO 3 ) 2 (aq) + Na 2 S(aq) ---> CdS(s) + 2 NaNO 3 (aq) Ionic Equations: All strong electrolytes are shown ionized Cd 2+ (aq) + 2 NO 3 - (aq) + 2 Na + (aq) + S (aq) ---> CdS(s) + 2 Na + (aq) + 2NO 3 - (aq) Net Ionic Equations: Cd 2+ (aq) + S (aq) ---> CdS(s) YOU MUST LEARN THE SOLUBILITY RULES: THEY MUST BE MEMORIZED!!! What to do when a nonsoluble reactant is used:
3 Molecular Equation Ionic Equation Mg(OH) 2 (s) + 2 HCl(aq) ---> MgCl 2 (aq) + 2H 2 O Mg(OH) 2 (s) + 2 H + (aq) + 2 Cl - (aq) ---> Mg 2+ (aq) + 2 Cl - (aq) + 2H 2 O Net Ionic Equation Mg(OH) 2 (s) + 2 H + (aq) Mg 2+ (aq) + 2 H 2 O When a Weak Electrolyte is the Reactant Molecular Equation NaC 2 H 3 O 2 (aq) + HCl(aq) ---> HC 2 H 3 O 2 (aq) + NaCl (aq) Ionic Equation Na + (aq) + C 2 H 3 O 2 - (aq) + H + (aq) + Cl - (aq) ---> HC 2 H 3 O 2 (aq) + Na + (aq) + Cl - (aq) Net Ionic Equation C 2 H 3 O 2 - (aq) + H + (aq) ---> HC 2 H 3 O 2 Reactions in Which a Gas Forms Molecular Equation 2 HCl(aq) + Na 2 S(aq) ---> 2NaCl(aq) + H 2 S(g) Ionic Equation 2H + (aq) + 2 Cl - (aq) + 2Na + (aq) + S (aq) --> 2H 2 S(g) + 2Na + (aq) + 2Cl - (aq) Net Ionic Equation 2H + (aq) + S (aq) ---> H 2 S(g) Reactions when Oxidation Numbers do change I. Rules for Oxidation Numbers The oxidation number of any free element (an element not combined chemically with a different element) is zero, regardless of how complex its molecules might be. The oxidation number for any simple, monoatomic ion is equal to the charge on the ion. The sum of all the oxidation numbers of the atoms in a molecule or ion must be equal the charge of the particle. In its compound, fluorine has an oxidation of -1
4 In its compounds, hydrogen has an oxidation number of +, except hen combined with metals when it is -1 In its compounds, oxygen has an oxidation number of -2, except in forming peroxides then it is -1. All Group I metals in compounds have a charge of +1 All Group II metals in compounds have a charge of +2 All Halogens have a charge of -1 when combined with metals in binary compounds II. Balancing Redox Equations A. Half-Reaction (Acidic Solution) 1. Divide the skeleton equation into half-reactions 2. Balance atoms other than H & O 3. Balance oxygen by adding H 2 O to the side that needs O 4. Balance hydrogen by adding H + to the side that needs H 5. Balance the charge by adding electrons 6. Make the number of electrons gained equal to the number lost and then add the two half-reactions 7. Cancel anything that is the same on both sides. B. Half-Reactions Method (Basic Solution) 1-7 Same as A above 8. Add to both sides of the equation the same number of OH - as there are H + 9. Combine H + and OH - to form H 2 O 10. Cancel any H 2 O that you can III. Types of Redox Reactions A. Simple Redox 1. Hydrogen Displacement Ca(s) + 2H 2 O(l) --> Ca(OH) 2 (s) + H 2 2. Metal Displacement Zn(s) + CuSO 4 (aq) ---> ZnSO 4 (aq) + Cu(s) 3. Halogen Displacement Cl 2 (g) + KBr(aq) ----> 2KCl(aq) + Br 2 (l) 4. Combustion CH 4 (g) + 2O 2 (g) ---> CO 2 (g) + 2H 2 O(g) B. Disportionation This is where one substance both oxidizes and reduces Cl 2 (g) + 2OH - (aq) ------> OCl - (aq) + Cl - (aq) + H 2 O(l)
5 C. Reactions involving oxoanions such as Cr 2 O 7 14H + (aq) + Cr 2 O 7 + 6 Fe 2+ ---> Cr 3+ + 7 H 2 O + 6 Fe 3+ IV. Redox Reaction Prediction Important Oxidizers Formed in reaction MnO - 4 (acid solution) Mn 2+ MnO - 4 (basic solution) MnO 2 MnO 2 (acid solution) Mn 2+ Cr 2 O 7 (acid) Cr 3+ CrO 4 Cr 3+ HNO 3, conc NO 2 HNO 3, dilute NO H 2 SO 4, hot conc SO 2 Metallic Ions Metallous Ions Free Halogens Halide ions HClO 4 Cl - Na 2 O 2 OH - H 2 O 2 O 2 Important Reducers Formed in Reaction Halide Ions Halogens Free Metals Metal Ions Metalous Ions Metallic ions Nitrite Ions Nitrate Ions Sulfite Ions SO 4 Free Halogens (dil, basic, sol) Hypohalite ions Free Halogens (conc, basic sol) Halite ions Redox reactions involve the transfer of electrons. The oxidation numbers of at least two elements must change. Single replacement, some combination and some decomposition reactions are redox reactions. To predict the products of a redox reaction, look at the reagents given to see if there is both an oxidizing agent and a reducing agent. When a problem mentions an acidic or basic solution, it is probably is redox.
6 #1: (a) (i) a solution containing iodide ions is added to a solution containing iron (III) ion. (ii) Which halogen is the best oxidizing agent? Explain your answer. (b) (i) lithium metal is burned in air (ii) What is the change in oxidization station of the Li? (c) (i) small chunks of solid sodium are added to water (ii) What might one observe in this reaction? #2: (a) (i) an excess of sodium hydroxide solution is added to a solution of magnesium nitrate (ii) What is the oxidizing agent in this reaction? (b) (i) solutions of ammonia and hydrofluoric acid are mixed. (ii) What is the ammonia acting as in this reaction? (c) (i) solutions of manganese (II) sulfate and ammonium sulfide are mixed (ii) What type of reaction is this? #3: (a) (i) a piece of aluminum metal is added to a solution of silver nitrate. (ii) Identify the oxidizing agent in this reaction. (b) (i) tetraphosphorus decoxide powder is sprinkled over distilled water (ii) Would you predict the ph of the resulting solution to be 7, above 7, or below 7 for the resulting solution? Support your answer. (c) (i) Solid ammonium carbonate is heated. (ii) What type of reaction is this? #4: (a) (i) carbon dioxide gas is bubbled through a concentrated solution of potassium hydroxide (ii) What would be your prediction for ph for the resulting solution? (b) (i) hydrogen sulfide gas is bubbled into a solution of mercury (II) chloride (ii) What would you expect to see as a result of this reaction? (c) (i) A bar of zinc metal is immersed in a solution of cupper(ii) sulfate. (ii) Identify the reducing agent in this reaction. #5: (a) (i) Solid barium oxide is added to distilled water. (ii) What type of reaction is this? (b) (i) Excess concentrated sulfuric acid is added to solid calcium phosphate (ii) What is the oxidation state of sulfur on the product side? (c) (i) hydrogen peroxide is added to an acidified solution of sodium bromide (ii) Identify the oxidizing agent.