Solutions, Solubility, and Reaction Types

1. Solutions, Solubility,and Reaction Types Brown, LeMay Ch 4 AP Chemistry

2. 4.1: Solutions & Expressing Concentration Solution: homogeneous mixture of 2 or more substances • solute:component present in lesser quantity • solvent:component present in greater quantity • Molarity (M)= moles solute = molvolume of solution L • Molality (m)= moles solute = molkg of solvent kg • *Normality (N):associated with acid & base strength = (moles solute)(# of acid/base equivalents) = molvolume of solution L

3. Dilution:(Minitial)(Vinitial) = (Mfinal)(Vfinal) Ex: How much water must be added to a solution that contains 100.0 g NaOH in 500. mL of solution to make the final concentration 2.00 M? Minitial = (100.0 g NaOH)/(40.00 g/mol NaOH) = 5.00 M 0.500 L (Mi)(Vi) = (Mf)(Vf) = (5.00 M)(0.500 L) = (2.00 M)(Vf) Vf = 1.25 L, so 0.75 L water must be added.

4. 4.2: Properties of Solutes • Ionic compounds (salts):dissociate in water by solvation, in which polar H2O molecules surround and separate ions from ionic crystal lattice • Molecular compounds:do not ionize because intramolecular bonds are stronger than bonds that form between H2O and molecule • Acids and bases, sometimes considered molecular (depending on strength), may also ionize.

5. Electrolyte:substance which, in aqueous solution, ionizes and thus conducts electricity • Non-electrolyte:substance which, in aqueous solution, does not dissociate and thus does not conduct electricity • Strong & weak electrolytes:conductivity depends on degree of dissociation and equilibrium position: HA (aq) ↔ H+ (aq) + A- (aq)Strong = nearly completely dissociated Weak = partially dissociated

6. 4.3: Acids & Bases • Bronsted-Lowry definitions: • Acids: H+ donor • Bases: H+ acceptor Group 1A’s: LiOH, NaOH, KOH, RbOH, CsOH Heavy 2A’s: Ca(OH)2, Sr(OH)2, Ba(OH)2 HBr, HCl, HI HNO3, H2SO4 HClO4, HClO3

7. 4.4: Ionic Equations • Molecular equation: shows complete chemical equation with states of matter, undissociated BaCl2 (aq) + Na2SO4 (aq) → 2 NaCl (aq) + BaSO4 (s) • Complete ionic equation: shows complete chemical equation with states of matter, dissociated if appropriate Ba2+(aq) + 2 Cl-(aq) + 2 Na+(aq) + SO42-(aq) → 2 Cl-(aq) + 2 Na+(aq) + BaSO4 (s) • Spectator ions:present in reaction but do not “participate”; depend on solubility rulesCl- (aq) and Na+ (aq) 3. Net ionic equation:shows chemical equation without spectator ionsBa2+ (aq) + SO42- (aq) → BaSO4 (s)

8. Solubility Rules (memorize them!) • Also, compounds with Group 1A and NH4+ cations

9. Scene from The G Block Featuring 2003-04 AP Chemistry students Daniel Fong, Ashwin Nirmalkumar, Veselin Kolev, and Shintaro Funasaki. Cameo here by Rob Palmiotto.

10. Other anions (e.g., BrO3- or Cr2O72-) are generally insoluble except with the soluble cations (Group 1A and NH4+)

11. AP Exam: Predicting Reaction Products In general, when writing the reactants: • Write all soluble ionic compounds in aqueous solution as their constituent ions. Never write NaCl, Ca(NO3)2, etc. • Write all strong acids in water as H+ or H3O+ and the anion. Never write HCl, HNO3, etc. • Write all strong bases as OH- and the cation. Never write NaOH, Ba(OH)2, etc. • Don’t omit any ions unless absolutely sure that they are be spectator ions. For example, although the Cl- in HCl is usually a spectator ion, it is sometimes a reactant.

12. In general, after writing the products: • Inspect all of the ions in the products to see if there are any precipitates. • Cross out any spectator ions. • Balance the reaction. Practice Predicting Precipitation Reactions Observations from previous AP exams: • These types of reactions generally involve mixing two solutions, but may include the addition of hydrogen sulfide gas. • Many involve formation of a single insoluble precipitate, but watch for the possibility of two insoluble products.

13. Examples • Dilute hydrochloric acid is added to a dilute solution of mercury (I) nitrate. • Dilute sulfuric acid is added to a solution of barium chloride. H+ + Cl- + Hg22+ + NO3-→ Hg2Cl2+ NO3- + H+ 2 Cl- + Hg22+ → Hg2Cl2 H+ + SO42- + Ba2+ + Cl- → BaSO4+ H+ + Cl- SO42- + Ba2+ → BaSO4

14. Hydrogen sulfide gas is added to a solution of cadmium nitrate. • Solutions of silver nitrate and sodium chromate are mixed. H2S + Cd2+ + NO3- → CdS + H+ + NO3- H2S + Cd2+ → CdS + 2 H+ Ag+ + NO3- + Na+ + CrO42- → Ag2CrO4 + Na+ + NO3- 2 Ag+ + CrO42- → Ag2CrO4

15. Other reactions to know: • Acid + Base → Salt + Water Ex: A solution of cesium hydroxide is added to a solution of nitric acid. • Nonmetal oxide + Water → Oxyacid Ex: Sulfur trioxide gas is added to excess water. • Metal oxide + Water → Base Ex: Solid sodium oxide is added to water. Cs+ + OH- + H+ + NO3-→ Cs+ + NO3- + H2O H+ + OH-→ H2O SO3 + H2O → H2SO4 SO3 + H2O → 2 H+ + SO42- Na2O + H2O → NaOH Na2O + H2O → 2 Na+ + 2 OH-

16. Nonmetal oxide + Metal oxide → Salt Ex: Solid calcium oxide is heated in the presence of sulfur trioxide gas. CaO + SO3 → CaSO4 If this were in water: CaO + H2O → Ca2+ + OH- and SO3 + H2O → H2SO4→ H+ + SO42- So, our overall reaction would be: Ca2+ + H2O + SO42- → CaSO4 + H2O But this can happen without water: CaO + SO3 → CaSO4

17. Metal oxide + Acid → Salt + Water Ex: Potassium oxide solid is added a solution of hydrochloric acid. • Nonmetal oxide + Base → Salt + Water Ex: Carbon dioxide is bubbled through a solution of barium hydroxide. K2O + H+ + Cl- →K+ + Cl- + H2O (K+ + O2-) + H+ + Cl- →K+ + Cl- + H2O K2O + 2 H+ →2 K+ + H2O CO2 + Ba2+ + OH-→ BaCO3+ H2O CO2 + Ba2+ + 2 OH- →BaCO3+ H2O

19. Any suggestions for others you’ve seen on old/practice exams, let me know.