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Types of Chemical Reactions

Types of Chemical Reactions

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Types of Chemical Reactions

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  1. Types of Chemical Reactions • Dissolution reactions • Precipitation reactions • Acid-base reactions • Redox reactions • Complexation reactions Read Chapter 5 (Introduction to Reactions in Aqueous Solutions)

  2. Dissolution Reactions Solute Dissolves in solvent to produce a solution Usually driven by solvent-solute attractive interactions e.g. Dissociation (ionization): NaCl (s)  Na+ (aq) + Cl- (aq) The lattice energy is NaCl (s)  Na+ (g) + Cl- (g), 797 kJ mol-1 Compare to dissolution energy Why does MgSO4 lower the temperature of water? MgSO4 Mg2+ + SO42- Depends on lattice energy (requires energy, endothermic) (H2O)n  nH2O Solvent-solvent interactions (overcoming H-bonds, slightly endothermic) Mg2+ + H2O  Mg2+ (aq) Formation of aquo complex and solvation sphere (exothermic) SO42- + H2O  SO42- (aq) Solvation (exothermic) E (solvation) < E (lattice)  dissolution is overall endothermic More energy is required to break up MgSO4 than is released by solvation. Why does this happen?

  3. Ether (1.36g of water per 100g of ether) 25oC Water (6.41g of ether per 100g of water) Solubility limit – largest amount that can dissolve in a given amount of solvent at a particular temperature Dissolution Reactions • Ionic compounds dissolve in water to give electrolytes • Resulting electrolyte has higher conductivity than pure water • Incomplete production of ions during dissolution yields “weak” electrolytes • Oppositely charged aquated ions can form ion pairs (reduces conductivity) Dissolution of non-electrolytes substances – driven by dipolar interactions e.g. Fructose C6H12O6 (s)  C6H12O6 (aq) Solubility – usually two substances are only partially miscible

  4. Precipitation Reactions When solubility limit is exceeded solids usually precipitate. Can occur if (i) Solvent evaporates (ii) Changing solvent composition e.g. NaCl in water, 36g/100g NaCl in ethanol, 0.12/100g  adding ethanol to NaCl (aq) can precipitate NaCl (s) (iii) Changing temperature For slightly soluble substance BaCl2 mixed with very soluble K2SO4 BaCl2 (aq) + K2SO4 (aq)  BaSO4 (s) + 2KCl (aq) Determining the “net ionic equation” 1. Write down all the ions involved: Ba2+ (aq) + 2Cl- (aq) + 2K+ (aq) + SO42- (aq) 2. Identify the “insoluble product/s: BaSO4 (s)

  5. Ba2+ (aq) + 2Cl- (aq) + 2K+ (aq) + SO42- (aq)  BaSO4 (s) + 2K+ (aq) + 2Cl- (aq) 3. Write down the complete ionic equation: Ba2+ (aq) + 2Cl- (aq) + 2K+ (aq) + SO42- (aq)  BaSO4 (s) + 2K+ (aq) + 2Cl- (aq) 4. Kill the “spectator ions”: 5. Obtain the net ionic equation: Ba2+ (aq) + SO42- (aq)  BaSO4 (s) For weak electrolytes HgCl2 (aq) and KI (aq), insoluble HgI2 forms. The net ionic equation is: Hg2+ (aq) + 2I- (aq)  HgI2 (s)

  6. Acid-Base Reactions Autoionization is an intrinsic property of water: H2O (l)  H+ (aq) + OH- (aq)  water always contains very small amounts of H+ and OH- ions (a weak electrolyte) • Predicting precipitations; some general rules: • Check the solubilities of the compounds formed by switching partners • (b) If either is insoluble (or slightly soluble)  precipitation occurs • (c) If both insoluble  two precipitation reactions • (d) If both solution  no precipitation An aqueous solution ofNaNO3is mixed with a solution ofKCl.The reaction, NaNO3 + KCl  NaCl + KNO3,is anticipated, but nothing happens. Explain. All compounds: NaCl, KNO3, NaNO3, KCl Na+ (aq) + NO3- (aq) + K+ (aq) + Cl- (aq)  Na+ (aq) + Cl- (aq) + K+ (aq) + NO3- (aq) NR (no reaction)

  7. Arrhenius acid: A substance, when dissolved in water, delivers H+ ions to the solution Arrhenius base: A substance, when dissolved in water, delivers OH- ions to the solution Water is amphoteric, i.e. both an Arrhenius acid and base. Strong acids (e.g. HCl, HNO3, H2SO4) and bases (NaOH, KOH) dissociate almost completely in water. e.g. NaOH (s)  Na+ (aq) + OH- (aq) Neutralization reactions: Acid + Base  Salt + Water e.g. HCl (aq) + NaOH (aq)  NaCl (aq) + H2O (l) The complete ionic equation is: H+ (aq) + Cl- (aq) + Na+ (aq) + OH- (aq)  Na+ (aq) + Cl- (aq) + H2O (l) The net ionic equation is: H+ (aq) + OH- (aq)  H2O (l)

  8. The others donate OH- poorly, i.e. weak bases. However, weak bases will neutralize acids (will dissolve in acidic solution) e.g. 2HCl (aq) + Mg(OH)2 (s)  2H2O (l) + MgCl2 (aq) Neutralization reactions: Acid + Base  Salt + Water There are exceptions: NH3 (aq) + HCl (aq)  NH4Cl (aq), no water Bases The only soluble hydroxides are Ba(OH)2and alkali metal hydroxides (e.g. NaOH, KOH) Note: NH3, methylamine (CH3NH2), pyridine (C5H5N) are bases, but do not contain OH- groups. e.g. NH3 (aq) + H2O(l)  NH4+ (aq) + OH- (aq) Arrhenius acid: A substance, when dissolved in water, increases [H+] Arrhenius base: A substance, when dissolved in water, increases [OH-]

  9. Strong oxoacid Acid anhydride e.g. SO3 (aq) + H2O (l)  H2SO4 (aq) Base anhydride e.g. Na2O (s) + H2O (l)  2NaOH (aq) e.g. SO3 (aq) + H2O (l)  H2SO4 (aq) and H2SO4  2H+ (aq) + SO42- (aq)  Sulfur trioxide is a strong acid Acid and base anhydrides

  10. Give the anhydride corresponding to the following acid or base. (a) H3AsO4 H2MoO4 MoO3 + H2O The anhydride of this acid is molybdenum (VI) oxide 2H3AsO4 As2O5 + 3H2O The anhydride of this acid is diarsenic pentoxide (b) H2MoO4 (c) RbOH 2RbOH  Rb2O + H2O The anhydride of this base is rubidium oxide (d) H2SO3 H2SO3 SO2 + H2O The anhydride of this acid is sulfur dioxide

  11. Reactions of Acid and Bases • acid + carbonate/hydrogen carbonate  CO2 + salt + water • e.g. NaHCO3 (s) + HCl (aq)  NaCl (aq) + H2O (l) + CO2 (g) 2. acid + metal oxide  salt + water e.g. CuO (s) + H2SO4 (aq)  CuSO4 (aq) + H2O (l) 3. acid + many metals  salt + H2 e.g. Zn (s) + 2HCl (aq)  ZnCl2 (aq) + H2 (g) 4. base + ammonium salts  salt + water + NH3 e.g. NH4Cl (s) + NaOH (aq)  NaCl (aq) + NH3 (g) + H2O (g) 5. base + non metal oxide  salt + water e.g. SO3 (g) + 2NaOH (aq)  Na2SO4 (aq) + H2O (l)