Chapter 4 (semester 2/2010) Reactions in Aqueous Solution prepared by A. Kyi Kyi Tin Ref: Raymong Chang. Chemistry Ninth Edition, McGraw – Hill International Edition 4.1 General Properties of Aqueous Solutions 4.2 Precipitation Reactions 4.3 Acid- Base Reactions 4.4 Oxidation – Reduction Reactions 4.5 Concentration of Solutions 4.1 General Properties of Aqueous Solutions Solution = [ solute + [smaller amount [CLEAR, solvent] + larger amount] HOMOGENEOUS MIXTURE] Aqueous solution = solute (liquid “or” solid) + solvent (water) Solution Solvent Solute Sea water H2O Salt (NaCl) Air (g) N2 O2, Ar, CH4 Alloy Cu Ni 4.1 Two types of Solutes Electrolyte Non-electrolyte When dissolved in water can conduct electricity When dissolved in water does not conduct electricity nonelectrolyte weak electrolyte Incomplete dissociation/reversible strong electrolyte 100%dissociation/ reversible reaction Ref: Raymond Chang Chemistry, Ninth Edition Figure 4.1, Page 120 4.2 Conduct electricity in solution? Dissociation means breaking up into..Cations (+) and Anions (-) Strong Electrolyte – 100% dissociation NaCl (s) H 2O Na+ (aq) + Cl- (aq) Weak Electrolyte – not completely dissociated CH3COOH CH3COO- (aq) + H+ (aq) Note: Pure water contains very few ions, cannot conduct electricity (extremely weak electrolyte) 4.3 Hydration: is the process in which an ion is surrounded by water molecules arranged in a specific manner. Water, electrically neutral molecule has a positive poles and negative poles, it is a polar solvent. Ex: when NaCl dissolves in water Na+ ions and Clions are separated from each other and undergo “hydration”. Hydration helps to stabilize ions in solution and prevents cations from combining with anions. 4.4 Ionization of acetic acid CH3COOH CH3COO- (aq) + H+ (aq) A reversible reaction. The reaction can occur in both directions. Acetic acid is a weak electrolyte because its ionization in water is incomplete. 4.5 Nonelectrolyte does not conduct electricity? No cations (+) and anions (-) in solution C6H12O6 (s) H 2O C6H12O6 (aq) 4.6 4.2 Precipitation Reactions (Metathesis reaction) or (Double Displacement reaction) One product is insoluble solid precipitate Pb(NO3)2 (aq) + 2NaI (aq) PbI2 (s) + 2NaNO3 (aq) molecular equation Pb2+ + 2NO3- + 2Na+ + 2I- PbI2 (s) + 2Na+ + 2NO3- ionic equation Pb2+ + 2I- PbI2 (s) net ionic equation Na+ and NO3- are spectator ions 4.7 Solubility: Maximum amount of solute that will dissolve in a given quantity of solvent in a specific temperature. soluble Substances: Slightly soluble insoluble Soluble : fair amount is visibly dissolves when added to water -All ionic compounds are strong electrolytes, but they are not equally soluble. -Even insoluble compounds dissolve to a certain extent Examples are: (NH4)2CO3, (NH4)3PO4,(NH4)2S,(NH4)2CrO4 4.8 4.3 Acid-Base Reactions ACID: Arrhenius acid is a substance that produces H+ (H3O+) in water BASE: Arrhenius base is a substance that produces OH- in water 4.9 Hydronium ion, hydrated proton, H3O+ 4.10 A Brønsted acid is a proton donor A Brønsted base is a proton acceptor base acid acid base A Brønsted acid must contain at least one ionizable proton! 4.11 Monoprotic acids HCl H+ + Cl- HNO3 H+ + NO3H+ + CH3COO- CH3COOH Strong electrolyte, strong acid Strong electrolyte, strong acid Weak electrolyte, weak acid Diprotic acids H2SO4 H+ + HSO4- Strong electrolyte, strong acid HSO4- H+ + SO42- Weak electrolyte, weak acid Triprotic acids H3PO4 H2PO4HPO42- H+ + H2PO4H+ + HPO42H+ + PO43- Weak electrolyte, weak acid Weak electrolyte, weak acid Weak electrolyte, weak acid 4.12 Practice question: Identify each of the following species as a Brønsted acid, base, or both. (a) HI, (b) CH3COO-, (c) H2PO4HI (aq) H+ (aq) + Br- (aq) CH3COO- (aq) + H+ (aq) H2PO4- (aq) Brønsted acid CH3COOH (aq) H+ (aq) + HPO42- (aq) H2PO4- (aq) + H+ (aq) H3PO4 (aq) Brønsted base Brønsted acid Brønsted base 4.13 Neutralization Reaction acid + base HCl (aq) + NaOH (aq) H+ + Cl- + Na+ + OH- H+ + OH- salt + water NaCl (aq) + H2O Na+ + Cl- + H2O H2O 4.14 4.4 Oxidation-Reduction Reactions REDOX REACTIONS(electron transfer reactions) 2Mg (s) + O2 (g) 2Mg O2 + 4e- 2Mg + Oxidized Reducing Agent (donates electrons to oxygen and causes oxygen to be reduced) 2Mg2+ + 4e2O2- O2 Reduced Oxidizing Agent (accepts electrons from Magnesium and causes Magnesium to be oxidized) 2MgO (s) Oxidation half-reaction (loss of e-) Reduction half-reaction (gain e-) 2MgO OIL RIG Oxidation Is Loss Reduction Is Gain 4.15 Zn (s) + CuSO4 (aq) Zn ZnSO4 (aq) + Cu (s) Zn2+ + 2e- Zn is oxidized Cu2+ + 2e- Zn is the reducing agent Cu Cu2+ is reduced Cu2+ is the oxidizing agent Copper wire reacts with silver nitrate to form silver metal. What is the oxidizing agent in the reaction? Cu (s) + 2AgNO3 (aq) Cu Ag+ + 1e- Cu(NO3)2 (aq) + 2Ag (s) Cu2+ + 2eAg Ag+ is reduced Ag+ is the oxidizing agent 4.16 IF7 Oxidation numbers of all the elements in the following ? F = -1 7x(-1) + ? = 0 I = +7 NaIO3 Na = +1 O = -2 3x(-2) + 1 + ? = 0 I = +5 K2Cr2O7 O = -2 K = +1 7x(-2) + 2x(+1) + 2x(?) = 0 Cr = +6 4.17 Types of Redox Reactions (i) Combination Reaction Two or more substances combine to form a single product. 0 0 S(s) + O2 (g) +4 -2 SO2(g) (ii) Decomposition Reaction Breakdown of a compound into two or more components. +2 -2 2 HgO(s) 0 2Hg(l) 0 + O2(g) 4.18 (iii) Displacement Reaction Halogen displacement According to Activity Series F2 > Cl2 > Br2 > I2 i.e Molecular fluorine can replace chloride, bromide and iodide ions in solution. On the other hand, Molecular chlorine can replace bromide and iodide ions in solution 0 -1 -1 Cl2 (g) + 2 KBr(aq) 0 0 2KCl(aq) + Br2(l) -1 Cl2 (g) + 2 NaI(aq) -1 0 2NaCl(aq) + I2(l) 4.19 Classify the following reactions. Ca2+ + CO32NH3 + H+ Zn + 2HCl Ca + F2 CaCO3 NH4+ ZnCl2 + H2 CaF2 Precipitation Acid-Base Redox (H2 Displacement) Redox (Combination) 4.20 4.5 Concentration of Solutions concentration : amount of solute present in a given quantity of solvent or solution. Most commonly used unit is “Molarity” M = molarity = moles of solute liters of solution n M V 4.21 Dilution is the procedure for preparing a less concentrated solution from a more concentrated solution. Dilution Add Solvent Moles of solute before dilution (i) = Moles of solute after dilution (f) MiVi = MfVf 4.22 How would you prepare 60.0 mL of 0.2 M HNO3 from a stock solution of 4.00 M HNO3? MiVi = MfVf Mi = 4.00 Vi = Mf = 0.200 MfVf Mi Vf = 0.06 L Vi = ? L 0.200 x 0.06 = = 0.003 L = 3 mL 4.00 3 mL of acid + 57 mL of water = 60 mL of solution Ref: power point presentation for instructors (Mc- Graw hill series,www.mhhe.com/chemistry) 4.23