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Chapter 16

Chapter 16. Ionic Equilibria: Acids and Bases. Chapter Goals. A Review of Strong Electrolytes The Autoionization of Water The pH and pOH Scales Ionization Constants for Weak Monoprotic Acids and Bases Polyprotic Acids Solvolysis Salts of Strong Bases and Strong Acids. Chapter Goals.

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Chapter 16

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  1. Chapter 16 • Ionic Equilibria: Acids and Bases

  2. Chapter Goals • A Review of Strong Electrolytes • The Autoionization of Water • The pH and pOH Scales • Ionization Constants for Weak Monoprotic Acids and Bases • Polyprotic Acids • Solvolysis • Salts of Strong Bases and Strong Acids

  3. Chapter Goals • Salts of Strong Bases and Weak Acids • Salts of Weak Bases and Strong Acids • Salts of Weak Bases and Weak Acids • Salts That Contain Small, Highly Charged Cations

  4. A Review of Strong Electrolytes • This chapter details the equilibria of weak acids and bases. • We must distinguish weak acids and bases from strong electrolytes. • Weak acids and bases ionize or dissociate partially, much less than 100%. • In this chapter we will see that it is often less than 10%! • Strong electrolytes ionize or dissociate completely. • Strong electrolytes approach 100% dissociation in aqueous solutions.

  5. A Review of Strong Electrolytes • There are three classes of strong electrolytes. • Strong Water Soluble Acids Remember the list of strong acids from Chapter 4.

  6. A Review of Strong Electrolytes

  7. A Review of Strong Electrolytes • Strong Water Soluble Bases The entire list of these bases was also introduced in Chapter 4.

  8. A Review of Strong Electrolytes • Most Water Soluble Salts The solubility guidelines from Chapter 4 will help you remember these salts.

  9. A Review of Strong Electrolytes • The calculation of ion concentrations in solutions of strong electrolytes is easy. • Example 18-1: Calculate the concentrations of ions in 0.050 M nitric acid, HNO3.

  10. A Review of Strong Electrolytes • Example 18-2: Calculate the concentrations of ions in 0.020 M strontium hydroxide, Sr(OH)2, solution. You do it!

  11. The Autoionization of Water • Pure water ionizes very slightly. • The concentration of the ionized water is less than one-millionth molar at room temperature.

  12. We can write the autoionization of water as a dissociation reaction similar to those previously done in this chapter. Because the activity of pure water is 1, the equilibrium constant for this reaction is: The Autoionization of Water

  13. The Autoionization of Water • Experimental measurements have determined that the concentration of each ion is 1.0 x 10-7 M at 25oC. • Note that this is at 25oC, not every temperature! • We can determine the value of Kc from this information.

  14. The Autoionization of Water • This particular equilibrium constant is called the ion-product for water and given the symbol Kw. • Kw is one of the recurring expressions for the remainder of this chapter and Chapters 19 and 20.

  15. The Autoionization of Water • Example 18-3: Calculate the concentrations of H3O+ and OH- in 0.050 M HCl.

  16. The Autoionization of Water • Use the [H3O+] and Kw to determine the [OH-]. You do it!

  17. The Autoionization of Water • The increase in [H3O+] from HCl shifts the equilibrium and decreases the [OH-]. • Remember from Chapter 17, increasing the product concentration, [H3O+], causes the equilibrium to shift to the reactant side. • This will decrease the [OH-] because it is a product!

  18. The Autoionization of Water • Now that we know the [H3O+] we can calculate the [OH-]. You do it!

  19. The pH and pOH scales • A convenient way to express the acidity and basicity of a solution is the pH and pOH scales. • The pH of an aqueous solution is defined as:

  20. The pH and pOH scales • In general, a lower case p before a symbol is read as the ‘negative logarithm of’ the symbol. • Thus we can write the following notations.

  21. The pH and pOH scales • If either the [H3O+] or [OH-] is known, the pH and pOH can be calculated. • Example 18-4: Calculate the pH of a solution in which the [H3O+] =0.030 M.

  22. The pH and pOH scales • Example 18-5: The pH of a solution is 4.597. What is the concentration of H3O+? You do it!

  23. A convenient relationship between pH and pOH may be derived for all dilute aqueous solutions at 250C. Taking the logarithm of both sides of this equation gives: The pH and pOH scales

  24. Multiplying both sides of this equation by -1 gives: Which can be rearranged to this form: The pH and pOH scales

  25. The pH and pOH scales • Remember these two expressions!! • They are key to the next three chapters!

  26. The usual range for the pH scale is 0 to 14. And for pOH the scale is also 0 to 14 but inverted from pH. pH = 0 has a pOH = 14 and pH = 14 has a pOH = 0. The pH and pOH scales

  27. The pH and pOH scales

  28. The pH and pOH scales • Example 18-6: Calculate the [H3O+], pH, [OH-], and pOH for a 0.020 M HNO3 solution. • Is HNO3 a weak or strong acid? • What is the [H3O+] ?

  29. The pH and pOH scales • Example 18-6: Calculate the [H3O+], pH, [OH-], and pOH for a 0.020 M HNO3 solution.

  30. The pH and pOH scales • To help develop familiarity with the pH and pOH scale we can look at a series of solutions in which [H3O+] varies between 1.0 M and 1.0 x 10-14M.

  31. Let’s look at the dissolution of acetic acid, a weak acid, in water as an example. The equation for the ionization of acetic acid is: The equilibrium constant for this ionization is expressed as: Ionization Constants for Weak Monoprotic Acids and Bases

  32. Ionization Constants for Weak Monoprotic Acids and Bases • The water concentration in dilute aqueous solutions is very high. • 1 L of water contains 55.5 moles of water. • Thus in dilute aqueous solutions:

  33. Ionization Constants for Weak Monoprotic Acids and Bases • The water concentration is many orders of magnitude greater than the ion concentrations. • Thus the water concentration is essentially that of pure water. • Recall that the activity of pure water is 1.

  34. Ionization Constants for Weak Monoprotic Acids and Bases • We can define a new equilibrium constant for weak acid equilibria that uses the previous definition. • This equilibrium constant is called the acid ionization constant. • The symbol for the ionization constant is Ka.

  35. Ionization Constants for Weak Monoprotic Acids and Bases • In simplified form the dissociation equation and acid ionization expression are written as:

  36. Ionization Constants for Weak Monoprotic Acids and Bases • The ionization constant values for several acids are given below. • Which acid is the strongest?

  37. From the above table we see that the order of increasing acid strength for these weak acids is: The order of increasing base strength of the anions (conjugate bases) of these acids is: Ionization Constants for Weak Monoprotic Acids and Bases

  38. Ionization Constants for Weak Monoprotic Acids and Bases • Example 18-8: Write the equation for the ionization of the weak acid HCN and the expression for its ionization constant.

  39. Ionization Constants for Weak Monoprotic Acids and Bases • Example 18-9: In a 0.12 M solution of a weak monoprotic acid, HY, the acid is 5.0% ionized. Calculate the ionization constant for the weak acid. You do it!

  40. Ionization Constants for Weak Monoprotic Acids and Bases • Since the weak acid is 5.0% ionized, it is also 95% unionized. • Calculate the concentration of all species in solution.

  41. Ionization Constants for Weak Monoprotic Acids and Bases • Use the concentrations that were just determined in the ionization constant expression to get the value of Ka.

  42. Ionization Constants for Weak Monoprotic Acids and Bases • Example 18-10: The pH of a 0.10 M solution of a weak monoprotic acid, HA, is found to be 2.97. What is the value for its ionization constant? pH = 2.97 so [H+]= 10-pH

  43. Ionization Constants for Weak Monoprotic Acids and Bases • Use the [H3O+] and the ionization reaction to determine concentrations of all species.

  44. Ionization Constants for Weak Monoprotic Acids and Bases • Calculate the ionization constant from this information.

  45. Ionization Constants for Weak Monoprotic Acids and Bases • Example 18-11: Calculate the concentrations of the various species in 0.15 M acetic acid, CH3COOH, solution. • It is always a good idea to write down the ionization reaction and the ionization constant expression.

  46. Ionization Constants for Weak Monoprotic Acids and Bases • Next, combine the basic chemical concepts with some algebra to solve the problem.

  47. Ionization Constants for Weak Monoprotic Acids and Bases • Next we combine the basic chemical concepts with some algebra to solve the problem

  48. Ionization Constants for Weak Monoprotic Acids and Bases • Next we combine the basic chemical concepts with some algebra to solve the problem

  49. Ionization Constants for Weak Monoprotic Acids and Bases • Substitute these algebraic quantities into the ionization expression.

  50. Ionization Constants for Weak Monoprotic Acids and Bases • Solve the algebraic equation, using a simplifying assumption that is appropriate for all weak acid and base ionizations.

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