Chapter 4 Solutions and Chemical Reactions Water Importance Life (as we know it) depends on water Human civilization requires water for many purposes Many important chemical reactions occur in Aqueous Solutions, where other compounds are dissolved in water The nature of water Bent shape and unequal sharing of electrons makes water polar This aids water in dissolving ionic compounds (cations and anions) Water hydrates the ions by interacting with its oppositely charged ends

NaCl(s) -----> Na+(aq) + Cl-(aq) The ionic substance breaks up into independent cations and anions Nonionic compounds can also dissolve in water if they are polar Nonpolar substances generally don’t dissolve in water: grease, oils, skin Electrolytes Solutions A solution is a homogeneous mixture the same throughout We can vary the composition by adding more or less of the components Solvent = usually a liquid; the most abundant component of a solution Solute = the lesser abundant component(s) of a solution Ethanol NaCl(s) -----> Na+(aq) + Cl-(aq)

that the more ions present, the better the conductivity B. Solutions and Electrical Conductance A substance allowing current to flow through it is electrically conductive Pure water does not conduct electricity Different solutes dissolved in water help it to be conductive Strong electrolyte = completely ionized; strongly conductive solution Weak electrolyte = partially ionized; somewhat conductive solution Nonelectrolyte = not ionized; nonconductive solution Arrhenius (1859-1927) found that the more ions present, the better the conductivity

C. Strong Electrolytes Completely ionized when dissolved in water Many salts (ionic compounds) are strong electrolytes Strong Acids are strong electrolytes Acid = substance that produces H+ when dissolved in water Strong Acids completely ionize in solution Hydrochloric Acid HCl(g) -------> H+(aq) + Cl-(aq) Nitric Acid HNO3(g) -------> H+(aq) + NO3-(aq) Sulfuric Acid H2SO4(l) -------> H+(aq) + HSO4-(aq) Strong Bases are strong electrolytes Base = substance that produces OH- when dissolved in water Strong bases completely ionize in solution NaOH(s) -------> Na+(aq) + OH-(aq) KOH(s) -------> K+(aq) + OH-(aq)

D. Weak Electrolytes Only partially ionized when dissolved in water Weak Acids are weak electrolytes Weak acid only produces a few H+ ions Acetic acid is a weak acid HC2H3O2(aq) -------> H+(aq) + -C2H3O2(aq) Only 1 molecule in a 100 dissociates Weak Bases are weak electrolytes Weak base produces only a few OH- ions Ammonia is a weak base NH3(aq) + H2O(l) -----> NH4+(aq) + OH-(aq) Only 1 molecule in 100 reacts Nonelectrolytes Does not ionize when dissolve in water Sugar is a nonelectrolyte C12H22O11(s) -------> C12H22O11(aq)

III. Solution Concentration The Stoichiometry of Chemical Reactions We must know what the reactants and products are We must know the amounts of the reactants and products How do we describe the amounts in a solution? Molarity Unit for the concentration of a solute in a solution M = moles solute/liters of solution 1.0 M NaCl = 1 mole of NaCl dissolved in 1 L of solution Any volume having the same concentration is also 1.0 M NaCl 500 ml (0.500 L) of 1.0 M NaCl would contain 0.5 mol NaCl Example: Calculate M of 11.5 g NaOH in 1.5 L of total solution.

5. Example: M = ? for 1.56 g HCl in a total of 26.8 ml of solution? 6. Molarity descriptions of a solution reflect composition before dissolution 1.0 M NaCl actually contains no NaCl 1.0 M NaCl is 1.0 M in Na+ and 1.0 M in Cl- 1.0 M CaCl2 is 1.0 M in Ca2+ and 2.0 M in Cl- CaCl2(s) -------> Ca2+(aq) + 2Cl-(aq) 7. Example: Give the concentration of each ion 0.5 M Co(NO3)2 = 0.5M in Co2+ and 1.0 M in NO3- Co(NO3)2 (s) -------> Co2+(aq) + 2NO3-(aq) 1M Fe(ClO4)3 = 1M Fe3+ and 3M ClO4- 8. Example: ??? moles of Cl- in 1.75L of 0.001 M ZnCl2

9. Example: What volume of 0.14 M NaCl contains 1.0mg NaCl? 10. Standard Solution = concentration is accurately known Accurate masses come from an analytical balance (0.4563g) Accurate volumes are obtained using a Volumetric Flask Example: How much K2Cr2O7 needed for 1.00 L of 0.200 M?

Dilution Chemicals are often purchased or prepared as concentrated stock solutions Dilution = adding water to stock solution to make a less concentrated one M1V1 = M2V2 is a useful equation to calculate dilutions Example: ??? volume of 16 M H2SO4 is needed for 1.5 L 0.10M H2SO4

IV. Precipitation Reactions Definitions When two solutions are mixed and a solid forms Precipitate = solid that forms from a precipitation reaction K2CrO4(aq) + Ba(NO3)2(aq) = 2K+(aq) + CrO42-(aq) + Ba2+(aq) +2NO3-(aq) K2CrO4 and Ba(NO3)2 are both soluble (all dissolve in water) A yellow precipitate forms when these solutions are mixed K2CrO4(aq) + Ba(NO3)2(aq) -----> BaCrO4(s) + 2KNO3(aq) AgNO3(aq) + KCl(aq) ------> AgCl(s) + KNO3(aq) + = Precipitate Spectator Ions

B. Solubility Rules Example: predict what will happen when you mix: KNO3(aq) + BaCl2(aq) ------> Na2SO4(aq) + Pb(NO3)2(aq) ------> 3KOH(aq) + Fe(NO3)2(aq) ------>

Describing Reactions in Solution Molecular Equation shows what compounds the ions came from Does not give clear picture of what happens in solution K2CrO4(aq) + Ba(NO3)2(aq) -----> BaCrO4(s) + 2KNO3(aq) Complete Ionic Equation represents the form of the ions in solution All strong electrolytes are represented as their ions 2K+(aq) + CrO42-(aq) + Ba2+(aq) +2NO3-(aq) ----> BaCrO4(s) + 2K+(aq) + 2NO3-(aq) Net Ionic Equation shows only the ions participating in the reaction The K+ and NO3- ions occur on both sides of the complete ionic eqn. These spectator ions can be cancelled out of each side (algebra) Ba2+(aq) + CrO42-(aq) -------> BaCrO4(s) Example 3KOH(aq) + Fe(NO3)3(aq) -----> Fe(OH)3(s) + 3KNO3(aq) b. 3K+(aq) + 3OH-(aq) + Fe3+(aq) + 3NO3-(aq) ----> Fe(OH)3(s) + 3K+(aq) + 3NO3-(aq) c. Fe3+(aq) + 3OH-(aq) -------> Fe(OH)3(s)