Reactions in Aqueous Solution - Chapter 4 - PDF

Reactions in Aqueous Solution Chapter 4 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Solutions A solution is a homogenous mixture of 2 or more substances The solute is(are) the substanc...

Reactions in Aqueous Solution Chapter 4 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Solutions A solution is a homogenous mixture of 2 or more substances The solute is(are) the substance(s) present in the smaller amount(s) The solvent is the substance present in the larger amount Solution Solvent Solute Liquid solution Soft drink (l) H2O Sugar, CO2 Gaseous solution Air (g) N2 O2, Ar, CH4 Solid solution Soft Solder (s) Pb Sn Aqueous solutions: the solvent is water  Electrolytic Properties An electrolyte is a substance that, when dissolved in water, results in a solution that can conduct electricity. A nonelectrolyte is a substance that, when dissolved, results in a solution that does not conduct electricity. nonelectrolyte weak electrolyte strong electrolyte  Which solutes conduct electricity in solution? Cations (+) and Anions (-) Strong Electrolyte – 100% dissociation H2O NaCl (s) Na+ (aq) + Cl- (aq) Ionization of acetic acid CH3COOH CH3COO- (aq) + H+ (aq) A reversible reaction. The reaction can occur in both directions. Weak Electrolyte – not completely dissociated Acetic acid is a weak electrolyte because its ionization in water is incomplete. Water is a very effective solvent for ionic compounds Water is a polar solvent; although water is electrically neutral , it has a positive region (the H atoms) and a negative region (the O atom) δ− Hydration is the process in which an ion is surrounded by water molecules arranged δ+ in a specific manner. H2O Nonelectrolyte does not conduct electricity? No cations (+) and anions (-) in solution H2O C6H12O6 (s) C6H12O6 (aq)  Precipitation Reactions A Precipitation reaction is a reaction that occurs in aqueous solution and results in the formation of an insoluble product Precipitate – insoluble solid that separates from solution precipitate molecular equation Pb(NO3)2 (aq) + 2NaI (aq) PbI2 (s) + 2NaNO3 (aq) ionic equation Pb2+ + 2NO3- + 2Na+ + 2I- PbI2 (s) + 2Na+ + 2NO3- net ionic equation Pb2+ + 2I- PbI2 (s) PbI2 Na+ and NO3- are spectator ions Solubility is the maximum amount of solute that will dissolve in a given quantity of solvent at a specific temperature Classify the following ionic compounds as soluble or insoluble Ag2SO4 CaCO3 Na3PO4 Mn(OH)2 AgClO3 K2S Two aqueous solutions of NaCl and AgNO3 are mixed together. Which of the following best represents the mixture?  Writing Net Ionic Equations 1. Write the balanced molecular equation. 2. Write the ionic equation showing the strong electrolytes completely dissociated into cations and anions. 3. Cancel the spectator ions on both sides of the ionic equation Write the net ionic equation for the reaction of silver nitrate with sodium chloride. AgNO3 (aq) + NaCl (aq) AgCl (s) + NaNO3 (aq) Ag+ + NO3- + Na+ + Cl- AgCl (s) + Na+ + NO3- Ag+ + Cl- AgCl (s) Two aqueous solutions of KOH and MgCl2 are mixed together. Which of the following best represents the mixture?  Rules for Writing Ionic Equations 1. Strong electrolytes in solution are written in their ionic form. 2. Weak electrolytes are written in their molecular (un-ionized) form. 3. Nonelectrolytes are written in their molecular form. 4. Insoluble substances, precipitates and gases are written in their molecular forms. 5. The net ionic equation should include only substances that have undergone a chemical change. Spectator ions are omitted from the net ionic equation. 6. Equations must be balanced both in atoms and in electrical charge.  Chemistry In Action: An Undesirable Precipitation Reaction Ca2+ (aq) + 2HCO3- (aq) CaCO3 (s) + CO2 (aq) + H2O (l) CO2 (aq) CO2 (g) Solid calcium carbonate is the main component of the scale that accumulates in boilers, water heaters, pipes and tea kettles  Predict what happens when the following solutions are mixed together. Write the ionic and net ionic equation for the reaction 1. calcium nitrate with potassium phosphate. 2. Barium chloride and zinc sulfate 3. Ammonium carbonate and calcium chloride  Acids Have a sour taste. Vinegar owes its taste to acetic acid. Citrus fruits contain citric acid. Cause color changes in plant dyes. React with certain metals to produce hydrogen gas. 2HCl (aq) + Mg (s) MgCl2 (aq) + H2 (g) React with carbonates and bicarbonates to produce carbon dioxide gas 2HCl (aq) + CaCO3 (s) CaCl2 (aq) + CO2 (g) + H2O (l) Aqueous acid solutions conduct electricity.  Bases Have a bitter taste. Feel slippery. Many soaps contain bases. Cause color changes in plant dyes. Aqueous base solutions conduct electricity.  Arrhenius Definition of Acids and Bases Arrhenius acid is a substance that produces H+ (H3O+) in water Arrhenius base is a substance that produces OH- in water  Bronsted Definition of Acids and Bases A Brønsted acid is a proton donor A Brønsted base is a proton acceptor acid base conjugate conjugate acid base Hydronium ion, hydrated proton, H3O+ A Brønsted acid is a proton donor A Brønsted base is a proton acceptor base acid conjugate conjugate acid base A Brønsted acid must contain at least one ionizable proton! Conjugate acid-base pairs differ by a proton. Monoprotic acids Yield one hydrogen ion upon ionization HCl H+ + Cl- Strong electrolyte, strong acid HNO3 H+ + NO3- Strong electrolyte, strong acid CH3COOH H+ + CH3COO- Weak electrolyte, weak acid Diprotic acids Yield two hydrogen ion upon ionization H2SO4 H+ + HSO4- Strong electrolyte, strong acid HSO4- H+ + SO42- Weak electrolyte, weak acid Triprotic acids Yield three hydrogen ion upon ionization H3PO4 H+ + H2PO4- Weak electrolyte, weak acid H2PO4- H+ + HPO42- Weak electrolyte, weak acid HPO42- H+ + PO43- Weak electrolyte, weak acid  Identify each of the following species as a Brønsted acid, base, or both. (a) HI, (b) CH3COO-, (c) H2PO4- HI (aq) H+ (aq) + I- (aq) Brønsted acid CH3COO- (aq) + H+ (aq) CH3COOH (aq) Brønsted base H2PO4- (aq) H+ (aq) + HPO42- (aq) Brønsted acid H2PO4- (aq) + H+ (aq) H3PO4 (aq) Brønsted base H2PO4- is called amphoteric because it possesses both acidic and basic properties  Acid-Base Neutralization A Neutralization Reaction is a reaction between an acid and a base Aqueous acid-base reactions produce salt and water A salt is an ionic compound made up of a cation other than H+ and an anion other than OH - or O2- acid + base salt + water HCl (aq) + NaOH (aq) NaCl (aq) + H2O(l) H+ + Cl- + Na+ + OH- Na+ + Cl- + H2O H+ + OH- H2O Na+ and Cl- are spectator ions  Acid-Base reactions can be characterized as proton transfer reactions acid + base salt + water HF (aq) + KOH (aq) KF (aq) + H2O (l) H2SO4 (aq) + 2NaOH (aq) Na2SO4 (aq) + 2H2O (l) HNO3(aq) + NH4+(aq)+ OH-(aq) NH4NO3(aq) + H2O(l) The above reaction can also be written as: HNO3(aq) + NH3 (aq) NH4NO3 (aq) Acid base reactions leading to formation of a Gas: Salts like: carbonates CO32-, bicarbonates HCO3-, sulfites SO32- and sulfides S2- react with acids to form gaseous products K2S(aq) + 2HCl (aq) → 2KCl (aq) + H2S(g) Na2CO3(aq) + 2HCl(aq) → 2NaCl(aq) + H2CO3(aq) Carbonic acid is unstable and if present in solutions in sufficient concentrations decomposes as follows H2CO3(aq) → CO2(g) + H2O(l) Similarly H2SO3(aq) → SO2(g) + H2O(l)  Oxidation-Reduction Reactions Oxidation-Reduction reactions are considered electron transfer reactions Oxidation is loss of electrons Reduction is gain of electrons 2Mg (s) + O2 (g) 2MgO (s) 2Mg 2Mg2+ + 4e- Oxidation half-reaction (lose e-) O2 + 4e- 2O2- Reduction half-reaction (gain e-) 2Mg + O2 + 4e- 2Mg2+ + 2O2- + 4e- 2Mg + O2 2MgO  Oxidizing and Reducing Agents Zn (s) + CuSO4 (aq) ZnSO4 (aq) + Cu (s) Zn Zn2+ + 2e- Zn is oxidized Zn is the reducing agent Cu2+ + 2e- Cu Cu2+ is reduced Cu2+ is the oxidizing agent Note: The number of electrons lost by the reducing agent should be equal to the number of electrons gained by the oxidizing agent  Oxidation Number The charge the atom would have in a molecule (or an ionic compound) if electrons were completely transferred. 1. Free elements (uncombined state) have an oxidation number of zero. Na, Be, K, Pb, H2, O2, P4 = 0 2. In monatomic ions, the oxidation number is equal to the charge on the ion. Li+, Li = +1; Fe3+, Fe = +3; O2-, O = -2 3. The oxidation number of oxygen is usually –2. Exceptions: in peroxides, H2O2 and O22- it is –1. in OF2 it is +2 4. The oxidation number of hydrogen is +1 except when it is bonded to metals in binary compounds. In these cases, its oxidation number is –1. 5. Group IA metals are +1, IIA metals are +2 and fluorine is always –1. 6. The sum of the oxidation numbers of all the atoms in a molecule or ion is equal to the charge on the molecule or ion. HCO3- Oxidation numbers of all O = -2 H = +1 the elements in HCO3- ? 3x(-2) + 1 + C = -1 C = +4  IF7 Oxidation numbers of all the elements in the F = -1 following ? 7x(-1) + I = 0 I = +7 K2Cr2O7 NaIO3 Na = +1 O = -2 O = -2 K = +1 3x(-2) + 1 + I = 0 7x(-2) + 2x(+1) + 2x(Cr) = 0 I = +5 Cr = +6 The oxidation numbers of elements in their compounds  Summary of Oxidation Numbers Metallic elements have only positive oxidation numbers Nonmetallic elements may have either positive or negative oxidation numbers The highest oxidation number an element in Groups (1A-7A) can have is its group number. e.g. the halogens are in Group 7A, so their highest possible oxidation number is +7 The transition metal (Groups 1B, 3B-8B) usually have several oxidation numbers  1. Copper wire reacts with silver nitrate to form silver metal. What is the oxidizing agent in the reaction? Cu (s) + 2AgNO3 (aq) Cu(NO3)2 (aq) + 2Ag (s) Cu Cu2+ + 2e- Cu is oxidized Ag+ + 1e- Ag Ag+ is reduced Ag+ is the oxidizing agent 2. Arrange the following species in order of increasing oxidation number of the sulfur atom: a) H2S, b) S8, c) H2SO4, d) S2-, e) HS-, f)SO2, g) SO3 H2S (−2), S2− (−2), HS− (−2) < S8 (0) < SO2 (+4) < SO3 (+6), H2SO4 (+6)  Types of Oxidation-Reduction Reactions 1. Combination Reactions are reactions in which two or more substances combine to form a single product A+B C 0 0 +3 -1 2Al + 3Br2 2AlBr3 0 0 +2 -3 3Mg (s) + N2 (g) Mg3N2 (s) 2. Decomposition reactions are the opposite of combination , a compound breaks down into two or more components C A+B If either A or B is an element , then the reaction is redox reaction. +2 -2 0 0 2HgO (s) 2Hg (l) + O2 (g) +1 +5 -2 +1 -1 0 +1 2KClO3 2KCl + 3O2 3. Combustion reactions in which a substance reacts with oxygen, usually with the release of heat and light to produce a flame A + O2 B 0 0 +4 -2 S + O2 SO2 0 0 +2 -2 2Mg + O2 2MgO 4. Displacement reaction, an ion or atom in a compound is replaced by an ion or atom of another element A + BC AC + B There are three subcategories: a) Hydrogen Displacement: A metal displaces hydrogen from water, or from an acid 0 +1 +2 0 Ca + 2H2O Ca(OH)2 + H2 Zn (s) + 2HCl (aq) ZnCl2 (aq) + H2 (g) Net ionic equation 0 +1 +2 0 Zn (s) + 2H+ (aq) Zn2+ (aq) + H2 (g) Note: Some metals such as copper Cu, silver Ag and gold Au do not displace hydrogen when placed in hydrochloric acid b) Metal Displacement: a metal in a compound can be displaced by another metal in the elemental state Zn (s) + CuSO4 (aq) ZnSO4 (aq) + Cu (s) Cu (s) + ZnSO4 (aq) No Reaction +4 0 0 +2 TiCl4 + 2Mg Ti + 2MgCl2 How to predict whether a metal or hydrogen displacement will take place? Refer to the activity series also called electrochemical series  The Activity Series for Metals Any species listed in the series will react with any species (in a compound) below it Hydrogen Displacement Reaction Ba + 2H2O Ba(OH)2 + H2 Zn (s) + 2HCl (aq) ZnCl2 (aq) + H2 (g) Cu (s)+HCl (aq) No Reaction Metal Displacement Reaction Mg (s)+ZnSO4 (aq) MgSO4 (aq) +Zn (s) Pb (s)+ NaNO3 (aq) No Reaction c) Halogen Displacement a halogen in a compound can be displaced by another halogen in the elemental state A + BC BA + C 0 -1 -1 0 Cl2 + 2KBr 2KCl + Br2 Halogen activity series F2 > Cl2 > Br2 > I2 Br2(l) + 2NaI (aq) NaBr (aq) + I2(s) I2(s) + NaCl (aq) No Reaction 5. Disproportionation Reaction An element in one oxidation state is simultaneously oxidized and reduced. -1 -2 0 2H2O2 (aq) 2H2O (l) + O2 (g) 0 +1 -1 Cl2 + 2OH- ClO- + Cl- + H2O Chlorine Chemistry  Classify the following reactions. Ca2+ + CO32- CaCO3 Precipitation NH3 + H+ NH4+ Acid-Base Zn + 2HCl ZnCl2 + H2 Redox (H2 Displacement) Ca + F2 CaF2 Redox (Combination) Sr + 2H2O Sr(OH)2 + H2 Redox (H2 Displacement) Chemistry in Action: Breath Analyzer +6 3CH3CH2OH + 2K2Cr2O7 + 8H2SO4 Ethanol +3 3CH3COOH + 2Cr2(SO4)3 + 2K2SO4 + 11H2O Acetic acid  Solution Stoichiometry The concentration of a solution is the amount of solute present in a given quantity of solvent or solution. moles of solute M = molarity = Volume of solution (L) 1.46 M C6H12O6 solution contains 1.46 moles of C6H12O6 in 1L of the solution. Mass # of moles of solute = Molar Mass 1.46 M C6H12O6 solution contains (1.46 mol x 120.16 g/mol) = 175.4 g of C6H12O6 in 1L of the solution. Strong electrolytes undergo complete dissociation in water H2O KCl (s) K+ (aq) + Cl- (aq) 1mol 1mol ion 1mol ion 1M KCl solution contains 1 mole of K + ions and 1 mole of Cl- ions The concentration of the ions: [K+] = 1M; [Cl-] = 1M [ ] indicate the molar concentration or Molarity H2O Ba(NO3)2(s) Ba2+ (aq) + 2NO3- (aq) 1M Ba(NO3)2 solution [Ba2+] = 1M; [NO3-] = 2M  What mass of KI is required to make 5.00 x 102 mL of a 2.80 M KI solution? 1L Convert volume to liters 5.00 x 102 mL x = 0.500 L 1000 mL n=MXV = 2.80 mol/L x 0.500 L = 1.40 mol Mass = n x Molar mass = 1.40 mol x 166 g/mol = 232 g KI  Dilution of Solutions Dilution is the procedure for preparing a less concentrated solution from a more concentrated solution. If a solution is diluted by adding pure solvent: – the volume of the solution increases. – the number of moles of solute remain the same. Dilution Add Solvent Moles of solute Moles of solute before dilution (i) = after dilution (f) MiVi = MfVf  How would you prepare 60.0 mL of 0.200 M HNO3 from a stock solution of 4.00 M HNO3? MiVi = MfVf Mi = 4.00 M Mf = 0.200 M Vf = 60.0 mL Vi = ? mL MfVf 0.200 x 60.0 Vi = = = 3.00 mL Mi 4.00 3.00 mL of acid + 57.0 mL of water = 60.0 mL of solution  Acid-Base Titration In a titration a solution of accurately known concentration is added gradually to another solution of unknown concentration until the chemical reaction between the two solutions is complete. Equivalence point – the point at which the reaction is complete Indicator – substance that changes color at (or near) the equivalence point Slowly add base to unknown acid UNTIL The indicator changes color (pink)  Acid- Base Titration Acid- base titration involves the complete reaction between an acid and a base. Neutralization reaction Acid + base Salt + water HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l) Net ionic equation of the reaction H+(aq) + OH (aq) H2O(l)  42.00 mL of 0.150 M NaOH solution is required to neutralize 50.00 mL of hydrochloric acid solution. What is the molarity of the acid solution? HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l) acid base salt Complete neutralization: # of moles H+ = # of moles OH- Mole ratio of HCl:NaOH is 1:1 # of moles HCl = # of moles NaOH M acid x V acid = M base x V base M acid = 0.150 M × 42.00 mL = 0.126 M 50.00 mL  42.00 mL of 0.150 M NaOH solution is required to neutralize 50.00 mL of sulfuric acid solution. What is the molarity of the acid solution? H2SO4 + 2NaOH 2H2O + Na2SO4 Complete neutralization: # of moles H+ = # of moles OH- Mole ratio of H2SO4:NaOH is 1:2 # of moles H2SO4 x 2 = # of moles NaOH M acid x V acid x 2 = M base x V base 0.150 M × 42.00 mL M acid = = 0.0630 M 50.00 mL × 2