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IV. Percent Ionization

IV. Percent Ionization. Instead of characterizing a weak acid by its K a , we can calculate how much it ionizes. The concentration of ionized acid is simply equal to the [H + ] at equilibrium. IV. Sample Problem. Find the % ionization of a 0.200 M acetic acid solution if its pK a = 4.74.

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IV. Percent Ionization

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  1. IV. Percent Ionization • Instead of characterizing a weak acid by its Ka, we can calculate how much it ionizes. • The concentration of ionized acid is simply equal to the [H+] at equilibrium.

  2. IV. Sample Problem • Find the % ionization of a 0.200 M acetic acid solution if its pKa = 4.74.

  3. IV. Sample Problem • In a 0.0100 M solution of butyric acid at 20 °C, the acid is 4.0% ionized. Calculate the Ka and pKa of butyric acid at these conditions.

  4. IV. Mixtures of Acids • If there are multiple acids in solution, then there are multiple sources of H3O+. • If one of the acids is strong, it will be the major contributor, so much so that we can ignore contributions from others. HCl(aq) + H2O(l) H3O+(aq) + Cl-(aq) HCOOH(aq) + H2O(l)  H3O+(aq) + HCOO-(aq)

  5. IV. Mixtures of Weak Acids • If we have a mixture of weak acids, we need to examine the Ka’s. • If the Ka’s differ by more than a factor of several hundred, then we can just calculate based on the strongest acid. • If the Ka’s are too close together, we must solve two weak acid equilibria! • Start with the strongest weak acid and use that result in the second weak acid equilibrium.

  6. IV. Sample Problem • Calculate the pH of a solution that is 0.100 M in acetic acid, CH3COOH (Ka = 1.8 x 10-5), and 0.200 M in formic acid, HCOOH (Ka = 1.8 x 10-4).

  7. V. Basic Solutions • Just like acidic solutions, there are strong bases and weak bases. • Everything we learned about weak acids applies to weak bases. • The two systems are analogous to each other.

  8. V. Strong Bases • Strong bases ionize completely. • Strong bases are typically ionic compounds containing the hydroxide anion.

  9. V. Weak Bases • Weak bases typically do not produce OH- by partially ionizing. • Weak bases produce OH- by pulling a proton off water. B(aq) + H2O(l) BH+(aq) + OH-(aq)

  10. V. Weak Bases • The strength of a weak base depends on its base ionization constant, Kb. • For the generic weak base B(aq) + H2O(l) BH+(aq) + OH-(aq):

  11. V. Common Weak Bases

  12. V. Weak Base Structures • Weak bases tend to have lone pair e-’s that can accept a proton.

  13. V. Weak Base Problems • The method of solving weak base problems is no different than the method of solving weak acid problems! • Instead of Ka, you use Kb. • To get to pH, remember that you can go through pOH.

  14. V. Sample Problem • The pain reliever morphine is a weak base. In a 0.010 M morphine solution, the pH is 10.10. Calculate the Kb and pKb or morphine.

  15. V. Sample Problem • The pKb for pyridine is 8.77. What’s the pH of a 0.010 M aqueous solution of pyridine?

  16. VI. Ions as Weak Acids/Bases • Some ions can act as either weak acids or weak bases. • e.g. NH4+ and CH3COO- • These ions must be introduced into a solution as a salt. • e.g. NH4Cl and CH3COONa • These ionic salts ionize, and then the weak acid/base sets up its equilibrium.

  17. VI. Anions as Weak Bases • Any anion can be thought of as the conjugate base of an acid. • Anions that are conjugate bases of weak acids are themselves weak bases. • Anions that are conjugate bases of strong acids are pH neutral.

  18. VI. Conjugate Ka/Kb Pairs

  19. VI. Sample Problem • What’s the pH of a 0.10 M NaNO2 solution? Note that Ka for nitrous acid is 7.1 x 10-4.

  20. VI. Cations as Weak Acids • When cations go into aqueous solutions,we need to examine whether or not they will set up an equilibrium. • Cations of strong bases do nothing and are thus pH neutral. • Cations that are conjugate acids of weak bases will establish an acid equilibrium. • Small, highly-charged metal cations form weakly acidic solutions.

  21. VI. The Case of Al3+ • Al3+(aq) will form Al(H2O)63+ which will establish an acid equilibrium.

  22. VI. pH of Salt Solutions • To determine whether a salt solution will be acidic, basic, or neutral, we need to consider the nature of the cation and anion. • There are 4 possibilities: • Neither cation nor anion acts as acid or base. • Cation acts as acid, anion is neutral. • Anion acts as base, cation is neutral. • Cation acts as acid, and anion acts as base.

  23. VI. pH Neutral Salt Solutions • Cation comes from a strong base. • Anion comes from a strong acid.

  24. VI. Acidic Salt Solutions • Cation is conjugate acid of weak base or is a small, highly-charged cation. • Anion comes from a strong acid.

  25. VI. Basic Salt Solutions • Cation comes from a strong base. • Anion is a conjugate base of a weak acid.

  26. VI. “It Depends” • Sometimes, cation will be a conjugate acid and the anion will be a conjugate base. • In this case, the pH of the salt solution depends on relative values of Ka and Kb. • If Ka > Kb, solution will be acidic; if Kb > Ka, solution will be basic.

  27. VI. Analyzing Salt Solutions • Use the following steps to determine whether an aqueous solution of a salt will be acidic, basic, or neutral. • Break up salt into its cation and anion. • Ask yourself whether the cation can donate a proton or whether it is small and highly charged. If so, it is a weak acid. • Ask yourself whether the anion can accept a proton. If so, it is a weak base. • Consider the combined effect of having the cation and anion in solution.

  28. VI. Sample Problem • For each compound, predict whether its 0.1 M solution in water will be acidic, basic, or neutral. • NaNO2 • KCl • NH4Br • Fe(NO3)3 • NH4CN

  29. VII. Acids w/ More than one H+ • Some acids have more than one acidic proton; these are called polyprotic acids. • Generally, the Ka of the 2nd proton is much smaller than the 1st, so we generally just solve for the 1st ionization. • Exceptions: H2SO4 and when Ka’s are within a few hundred of each other. • For exceptions, it’s a double equil. problem!

  30. VII. Some Polyprotic Acids

  31. VII. Sample Problem • What is the pH and [SO42-] of a 0.0075 M sulfuric acid solution if Ka2 = 0.012 for sulfuric acid?

  32. VIII. Acid Strength & Structure • Why are some acids strong and some acids weak? • Depends on structure and composition of the acid. • We examine the factors that contribute to acid strength in binary acids and oxyacids.

  33. VIII. Binary Acids • The strength of a binary acid depends on bond polarity and bond strength. • The H must have the partial positive charge. • Weaker bond leads to greater acidity.

  34. VIII. Binary Group 16/17 Acids • The combined influence of polarity and bond strength can be seen in the Group 16 and Group 17 binary acids.

  35. VIII. Oxyacids • An oxyacid (a.k.a. oxoacid) has the general form H-O-Y- in which Y is some atom which may or may not have additional atoms bonded to it. • Oxyacid strength depends on the electronegativity of Y and the number of O atoms attached to Y.

  36. VIII. Electronegativity of Y • The more electronegative Y is, the more polar and weaker the O-H bond becomes. • If the O kicks off the H as H+, it can claim both electrons in the bond.

  37. VIII. # of O Atoms on Y • More O atoms draw electron density away from Y, which draws electron density from the O-H bond, leading to greater acidity.

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