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AP Chem Acids/Bases

AP Chem Acids/Bases. Friday April 13, 2012. Write the net ionic equation for the reaction between hydrofluoric acid, HF, and the sulfite ion, SO 3 2-, and predict which side will be favored at equilibrium. HF(aq) + SO 3 2- (aq)  F - (aq) + HSO 3 1- (aq).

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AP Chem Acids/Bases

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  1. AP Chem Acids/Bases Friday April 13, 2012

  2. Write the net ionic equation for the reaction between hydrofluoric acid, HF, and the sulfite ion, SO32-, and predict which side will be favored at equilibrium.

  3. HF(aq) + SO32- (aq)  F- (aq) + HSO31- (aq) • A B CB CA • Refer to the acid-base strength table and identify strength. • HF is stronger acid than HSO3- • SO32- is stronger base than F- • Equilibrium favors the side with the weaker acid and base. Forward reaction favored. • “The weak shall inherit the test tube.”

  4. Write the net ionic equation for the acid-base reaction between HCO3- and ClO- and predict which side will be favored at equilibrium.

  5. HCO3- (aq) + ClO- (aq)  CO32-(aq) + HClO (aq) • A B CB CA • Check strengths on table • HCO3- is weaker acid than HClO • ClO- is weaker base than CO32- • Equilibrium favors reverse reaction.

  6. Interesting twist – no product given:HC2H3O2(aq) + HS-(aq)  • Reactions could be either: • HC2H3O2(aq) + HS-(aq) H2C2H3O2+ (aq) + S2- (aq) • B A CA CB • or • HC2H3O2(aq) + HS-(aq) C2H3O2- (aq) + H2S (aq) • A B CB CA • Check the strengths of acid actions • HC2H3O2 is a stronger acid than HS- • HC2H3O2 will serve as acid over HS-

  7. Weak Acids • Most acids are weak and only partially ionize. • HA (aq) + H2O (l)  H3O+ (aq) + A- (aq) • or time to simply our equations: • HA (aq)  H+ (aq) + A- (aq) • You can eliminate the H2O • cuz of the (aq) behind HA, and • H3O+ is more easily shown as just H+

  8. Equilibrium-constant expression • Keq = [H+] [A-] [H2O] is solvent (omitted) [HA] We will use the subscript a on Ka to denote that it is an equilibrium constant for the ionization of an acid; called acid-dissociation constant. Table 16.2 on page 628 in textbook gives Ka values for several weak acids, another list is in Appendix D. The larger the Ka value, the stronger the acid.. Notice that Ka is typically less than 10-3.

  9. pKa is found by the equation: pKa = -log(Ka) In fact, a lower-case p in all this chapter means: -log

  10. Polyprotic Acids • Acids with more than one ionizable H atom. • Sulfurous acid, H2SO3 ionizes in two steps: • H2SO3(aq) H+(aq) + HSO3-(aq) Ka1 = 1.7 x 10-2 • HSO3- (aq)  H+(aq) + SO32- (aq) Ka2 = 6.4 x 10-8 • The Ka1 and Ka2 labels refers to the removal of the first H+ and the second H+.

  11. In previous example, the Ka2 is much smaller than Ka1 . • It is always easier to remove the first proton from a polyprotic acid than the second. • Based on electrostatic attractions. Expect a H+ to be lost more easily from the neutral H2SO3 than from the negatively-charged HSO3- ion. • Also, for an acid with three ionizable H+, the second proton is easier to remove than the third. • Ka values get successively smaller as protons are removed.

  12. Page 635 or Appendix D (Page 1044)

  13. Because Ka1 is so much larger than subsequent K values, almost all the H+ in the solution comes from the first ionization reaction. • As long as the successive Ka values differ by a factor of 103 or more, it is possible to obtain a satisfactory estimate of the pH of polyprotic acid solutions by considering only Ka1.

  14. What is “negligible acidity”? • Substances, such as CH4 and H2, that contain H but do NOT demonstrate any acidic behavior in water are considered “negligible”. • Their conjugate bases (CH3- and H-), while forming in very small concentrations, are strong bases – gaining H+ from water to form OH- ions. • CH3- + H2O  CH4 + OH- • H- + H2OH2 + OH-

  15. Amphoteric: capable of acting as either an acid or base • Acts as a base when combined with something more strongly acidic than itself, • and as an acid when combined with something more strongly basic than itself.

  16. HSO3-1 is amphoteric. • Write an equation for a reaction with water, where HSO3- acts as an acid; • HSO3- (aq) + H2O (l)  SO32- (aq) + H3O+ (aq) • A B CB CA • Write an equation for a reaction with water where HSO3- acts as an base: • HSO3- (aq) + H2O (l)  H2SO3(aq) + OH- (aq) • B A CA CB

  17. HCO3-1 is amphoteric. • Write an equation for a reaction with water, where HCO3- acts as an acid; • HCO3- (aq) + H2O (l)  CO32- (aq) + H3O+ (aq) • A B CB CA • Write an equation for a reaction with water where HCO3- acts as an base: • HCO3- (aq) + H2O (l)  H2CO3(aq) + OH- (aq) • B A CA CB

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