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Autoionization of Water

Autoionization of Water. Although water is a molecular substance, very low concentrations of hydronium ions and hydroxides ions are formed by autoionization. Autoinization. 2 H 2 O (l) H 3 O + (aq) + OH - (aq). Autoionization of Water. Autoionization:

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Autoionization of Water

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  1. Autoionization of Water • Although water is a molecular substance, very low concentrations of hydronium ions and hydroxides ions are formed by autoionization. Autoinization

  2. 2 H2O (l) H3O+ (aq) + OH- (aq) Autoionization of Water • Autoionization: • the process in which water spontaneously forms low concentrations of H+ and OH - ions by proton transfer from one water molecule to another • At any given time only a very small number of water molecules are ionized. • If every letter in our text represented a water molecule, you would have to look through about 50 texts to find one H3O+!

  3. Autoionization of Water • An equilibrium constant expression can be written for the autoionization of water: 2 H2O (l) H3O+ (aq) + OH - (aq) Kw = [H3O+] [OH -] = [H+ ] [OH- ] where Kw = ionization constant for water = ion-product constant = 1.00 x 10-14 at 25oC

  4. Autoionization of Water • The value of Kw (and all other equilibrium constants) varies with temperature: • Kw = 1.14 x 10-15at 0oC • Kw = 1.00 x 10-14at 25oC • Kw = 9.61 x 10-14 at 60oC • You should assume 25oC unless otherwise stated in a problem.

  5. Autoionization of Water By definition: • Neutral solution: [H+ ] = [OH- ] • Acidic solution: [H+ ] > [OH- ] • Basic solution: [H+ ] < [OH- ]

  6. Autoionization of Water Example:Calculate the [H+ ] and [OH- ]in a neutral solution at 25oC.

  7. Autoionization of Water Example:What is the [H+ ] at 25oC for a solution in which [OH- ] = 0.010 M.

  8. Autoionization of Water Example:What is the [OH- ] at 25oC in a solution in which [H+ ] = 2.5 x 10-6 M.

  9. pH • Since the [H+] is usually very small in aqueous solutions, we normally express the [H+] in terms of pH. pH = - log10 [H+] • Sig Figs and logs: only the digits after the decimal point are significant.

  10. pH Example:Calculate the pH of a solution with [H+] = 2.52 x 10-5.

  11. pH Example:Calculate the pH of a solution with [OH-] = 6.5 x 10-5.

  12. pH • The negative log is also used to express the magnitude of other small quantities: • pOH = - log [OH- ] • pH and pOH are related by the following equation that is derived by taking the negative log of the expression for Kw pH + pOH = 14.00 at 25oC

  13. pH Example: Calculate the pOH of a solution with [OH - ] = 2.5 x 10-3 M.

  14. pH Example: Calculate the pH of a solution with [OH - ] = 2.5 x 10-3 M. Approach 1:

  15. pH Approach # 2:

  16. pH • Given the pH of a solution, you can also find the [H+] and the [OH-]. • Since pH = - log [H+], [H+] = 10 –pH • Since pOH = - log [OH-], [OH-] = 10 -pOH

  17. pH Example: What are the [H+] and [OH-] for a solution with a pH of 2.50 at 25oC?

  18. Strong Acids • Strong acid: • Strong electrolyte • Ionizes completely in aqueous solution HNO3(aq) H +(aq) + NO3–(aq) • The only significant source of H+ ion in an aqueous solution of a strong acid is usually the strong acid.

  19. HNO3(aq) H +(aq) + NO3–(aq) • In a 0.05 M HNO3 (aq) solution, [H+] = 0.05 mol HNO3 x 1 mol H+= 0.05 M L 1 mol HNO3 Strong Acids • Consequently, the [H+] in a solution of a strong monoprotic acid can be determined easily using the concentration of the strong acid itself.

  20. Strong Acids Example: What is the pH of a 0.25 M HCl (aq) solution?

  21. Strong Bases • Strong Base • strong electrolyte • ionizes completely in aqueous solution NaOH (aq) Na+(aq) + OH-(aq) • Common strong bases • alkali metal hydroxides • hydroxides of Ca, Sr, and Ba

  22. Strong Bases • The pH of an aqueous solution of a strong base can be determined using the concentration of the strong base NaOH (aq) Na+(aq) + OH-(aq) • A 0.25 M solution of NaOH has an [OH-] of 0.25 M: 0.25 mol NaOH x 1 mol OH- = 0.25 M L 1 mol NaOH

  23. Strong Bases • The pH of the base solution can then be found in two ways: • Calculate pOH • use pH + pOH = 14.00 to determine pH • Calculate [H+] • use [H+] [OH-] = 1.00 x 10-14 • Then calculate pH

  24. Strong Bases Example: Calculate the pH of a 0.25 M Ca(OH)2 (aq) solution. Step 1: Determine [OH-] Step 1: Determine [OH-]

  25. Strong Bases Step 2: Calculate pOH Step 3: Calculate pH

  26. Strong Bases Example:What is the pH of a solution prepared by mixing 10.0 mL of 0.015 M Ba(OH)2 and 30.0 mL of 7.5 x 10-3 M NaOH?

  27. Strong Bases Step 1: Find the total [OH-]

  28. Strong Bases Step 2: Find pOH Step 3: Find pH

  29. Weak Acids • Most acidic substances are weak acids: • partially ionize in solution • the solution contains an equilibrium mixture of acid molecules and its component ions CH3CO2HH+ (aq) + CH3CO2- (aq)

  30. Weak Acids • The extent to which a weak acid ionizes can be expressed using an equilibrium constant known as the acid-dissociation constant (Ka). • For a general reaction: HX (aq) H+(aq) + X- (aq) Ka = [H+][X-] [HX] Note: The rules for writing an expression for Ka are the same as those for Kc, Kp and Ksp.

  31. Weak Acids • The magnitude of Ka indicates the tendency of the hydrogen ion in an acid to ionize. • The larger the value of Ka, the stronger the acid is. • The pH of a weak acid solution can be calculated using the initial concentration of the weak acid and its Ka.

  32. Weak Acids • To calculate the pH of a weak acid solution: • Write the ionization equilibrium for the acid. • Write the equilibrium constant expression and its numerical value. • Set up a table showing initial concentration, change, equilibrium concentration. • Substitute equilibrium concentrations into the equilibrium constant expression.

  33. Weak Acids • To calculate the pH of a weak acid solution (cont): • Solve for the change in concentration. • Assume that the change in concentration is small (i.e. < 5%) compared to the initial concentration of the weak acid. • Check the validity of previous assumptions. • If x/initial concentration x 100% >5.0%, you must use the quadratic equation to solve for x. • Calculate the final concentrations and pH.

  34. Weak Acids Example:Calculate the pH of a 0.20 M solution of HCN. Ka = 4.9 x 10-10 Step 1: Write the equation for the ionization. Step 2: Write the expression for Ka.

  35. Weak Acids Step 3: Set up a table.

  36. Weak Acids Step 4: Substitute equilibrium concentrations into the Ka expression. Step 5: Assume that x << 0.20 M and solve for x.

  37. Weak Acids Step 6: Check the validity of our assumption.

  38. Weak Acids • Step 7: Substitute value for x into the table to find the [H+].

  39. Weak Acids Step 8: Calculate the pH using the [H+]

  40. Weak Bases • Many substances behave as weak bases in water. Weak base + H2O conjugate acid + OH- • The extent to which a weak base reacts with water to form its conjugate acid and OH- ion can be expressed using an equilibrium constant known as the base-dissociation constant (Kb).

  41. Weak Bases • Kb always refers to the equilibrium in which a base reacts with water to form its conjugate acid and OH- ion. • For the reaction: NH3(aq) + H2O (l) NH4+(aq) + OH-(aq) Kb = [NH4+] [OH-] [NH3] Note: The rules for writing an expression for Kb are the same as those for Kc, Kp and Ksp.

  42. Weak Base • To calculate the pH of a weak base solution: • Write the ionization equilibrium for the base. • Write the equilibrium constant expression and its numerical value. • Set up a table showing initial concentration, change, equilibrium concentration. • Substitute equilibrium concentrations into the equilibrium constant expression.

  43. Weak Bases • To calculate the pH of a weak base solution (cont): • Solve for the change in concentration. • Assume that the change in concentration is small (i.e. < 5%) compared to the initial concentration of the weak base. • Check the validity of previous assumption. • Calculate the [OH-] concentration and pOH • Use pOH to calculate pH.

  44. Weak Bases Example: Calculate the pH of a 0.20 M solution of methylamine, CH3NH2. Kb = 3.6 x 10-4. Step 1: Write the equation for the ionization. Step 2: Write the expression for Kb.

  45. Weak Bases Step 3: Set up a table.

  46. Weak Bases Step 4: Substitute equilibrium concentrations into the Kb expression. Step 5: Assume that x << 0.20 M and solve for x.

  47. Weak Bases Step 6: Check the validity of our assumption.

  48. Weak Bases • Step 7: Substitute value for x into the table to find the [OH-].

  49. Weak Acids Step 8: Calculate the pOH Step 9: Calculate the pH

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