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The Danger of Antifreeze

ethylene glycol (aka 1,2–ethandiol). The Danger of Antifreeze. each year, thousands of pets and wildlife die from consuming antifreeze most brands of antifreeze contain ethylene glycol sweet taste initial effect drunkenness metabolized in the liver to glycolic acid HOCH 2 COOH

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The Danger of Antifreeze

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  1. ethylene glycol (aka 1,2–ethandiol) The Danger of Antifreeze • each year, thousands of pets and wildlife die from consuming antifreeze • most brands of antifreeze contain ethylene glycol • sweet taste • initial effect drunkenness • metabolized in the liver to glycolic acid • HOCH2COOH • if present in high enough concentration in the bloodstream, it overwhelms the buffering ability of HCO3−, causing the blood pH to drop • when the blood pH is low, it ability to carry O2 is compromised • acidosis • the treatment is to give the patient ethyl alcohol, which has a higher affinity for the enzyme that catalyzes the metabolism of ethylene glycol

  2. Buffers • buffers are solutions that resist changes in pH when an acid or base is added • they act by neutralizing the added acid or base • but just like everything else, there is a limit to what they can do, eventually the pH changes • many buffers are made by mixing a solution of a weak acid with a solution of soluble salt containing its conjugate base anion Tro, Chemistry: A Molecular Approach

  3. Making an Acid Buffer Tro, Chemistry: A Molecular Approach

  4. How Acid Buffers WorkHA(aq) + H2O(l) A−(aq) + H3O+(aq) • buffers work by applying Le Châtelier’s Principle to weak acid equilibrium • buffer solutions contain significant amounts of the weak acid molecules, HA – these molecules react with added base to neutralize it • you can also think of the H3O+ combining with the OH− to make H2O; the H3O+ is then replaced by the shifting equilibrium • the buffer solutions also contain significant amounts of the conjugate base anion, A− - these ions combine with added acid to make more HA and keep the H3O+ constant Tro, Chemistry: A Molecular Approach

  5. H2O How Buffers Work new HA HA HA A− A−  H3O+ + Added H3O+ Tro, Chemistry: A Molecular Approach

  6. H2O How Buffers Work new A− A− HA A− HA  H3O+ + Added HO− Tro, Chemistry: A Molecular Approach

  7. Common Ion Effect HA(aq) + H2O(l) A−(aq) + H3O+(aq) • adding a salt containing the anion, NaA, that is the conjugate base of the acid (the common ion) shifts the position of equilibrium to the left • this causes the pH to be higher than the pH of the acid solution • lowering the H3O+ ion concentration Tro, Chemistry: A Molecular Approach

  8. Common Ion Effect Tro, Chemistry: A Molecular Approach

  9. Ex 16.1 - What is the pH of a buffer that is 0.100 M HC2H3O2 and 0.100 M NaC2H3O2? HC2H3O2 + H2O  C2H3O2 + H3O+ Tro, Chemistry: A Molecular Approach

  10. Ex 16.1 - What is the pH of a buffer that is 0.100 M HC2H3O2 and 0.100 M NaC2H3O2? HC2H3O2 + H2O  C2H3O2 + H3O+ x +x +x x 0.100 x 0.100 + x Tro, Chemistry: A Molecular Approach

  11. Ex 16.1 - What is the pH of a buffer that is 0.100 M HC2H3O2 and 0.100 M NaC2H3O2? Ka for HC2H3O2 = 1.8 x 10-5 0.100 x 0.100 +x Tro, Chemistry: A Molecular Approach

  12. Ex 16.1 - What is the pH of a buffer that is 0.100 M HC2H3O2 and 0.100 M NaC2H3O2? Ka for HC2H3O2 = 1.8 x 10-5 x = 1.8 x 10-5 the approximation is valid Tro, Chemistry: A Molecular Approach

  13. Ex 16.1 - What is the pH of a buffer that is 0.100 M HC2H3O2 and 0.100 M NaC2H3O2? 0.100 x 0.100 + x x x = 1.8 x 10-5 Tro, Chemistry: A Molecular Approach

  14. Ex 16.1 - What is the pH of a buffer that is 0.100 M HC2H3O2 and 0.100 M NaC2H3O2? Tro, Chemistry: A Molecular Approach

  15. Ex 16.1 - What is the pH of a buffer that is 0.100 M HC2H3O2 and 0.100 M NaC2H3O2? Ka for HC2H3O2 = 1.8 x 10-5 the values match Tro, Chemistry: A Molecular Approach

  16. Practice - What is the pH of a buffer that is 0.14 M HF (pKa = 3.15) and 0.071 M KF? Tro, Chemistry: A Molecular Approach

  17. Practice - What is the pH of a buffer that is 0.14 M HF (pKa = 3.15) and 0.071 M KF? HF + H2O  F + H3O+ Tro, Chemistry: A Molecular Approach

  18. Practice - What is the pH of a buffer that is 0.14 M HF (pKa = 3.15) and 0.071 M KF? HF + H2O  F + H3O+ x +x +x x 0.14 x 0.071 + x Tro, Chemistry: A Molecular Approach

  19. Practice - What is the pH of a buffer that is 0.14 M HF (pKa = 3.15) and 0.071 M KF? Ka for HF = 7.0 x 10-4 0.14 x 0.071 +x Tro, Chemistry: A Molecular Approach

  20. Practice - What is the pH of a buffer that is 0.14 M HF (pKa = 3.15) and 0.071 M KF? Ka for HF = 7.0 x 10-4 x = 1.4 x 10-3 the approximation is valid Tro, Chemistry: A Molecular Approach

  21. Practice - What is the pH of a buffer that is 0.14 M HF (pKa = 3.15) and 0.071 M KF? 0.14 x 0.071 + x x x = 1.4 x 10-3 Tro, Chemistry: A Molecular Approach

  22. Practice - What is the pH of a buffer that is 0.14 M HF (pKa = 3.15) and 0.071 M KF? Tro, Chemistry: A Molecular Approach

  23. Practice - What is the pH of a buffer that is 0.14 M HF (pKa = 3.15) and 0.071 M KF? Ka for HF = 7.0 x 10-4 the values are close enough Tro, Chemistry: A Molecular Approach

  24. Henderson-Hasselbalch Equation • calculating the pH of a buffer solution can be simplified by using an equation derived from the Ka expression called the Henderson-Hasselbalch Equation • the equation calculates the pH of a buffer from the Ka and initial concentrations of the weak acid and salt of the conjugate base • as long as the “x is small” approximation is valid Tro, Chemistry: A Molecular Approach

  25. Deriving the Henderson-Hasselbalch Equation Tro, Chemistry: A Molecular Approach

  26. Ex 16.2 - What is the pH of a buffer that is 0.050 M HC7H5O2 and 0.150 M NaC7H5O2? HC7H5O2 + H2O  C7H5O2 + H3O+ Ka for HC7H5O2 = 6.5 x 10-5 Tro, Chemistry: A Molecular Approach

  27. Practice - What is the pH of a buffer that is 0.14 M HF (pKa = 3.15) and 0.071 M KF? Tro, Chemistry: A Molecular Approach

  28. Practice - What is the pH of a buffer that is 0.14 M HF (pKa = 3.15) and 0.071 M KF? HF + H2O  F + H3O+ Tro, Chemistry: A Molecular Approach

  29. Do I Use the Full Equilibrium Analysis or the Henderson-Hasselbalch Equation? • the Henderson-Hasselbalch equation is generally good enough when the “x is small” approximation is applicable • generally, the “x is small” approximation will work when both of the following are true: • the initial concentrations of acid and salt are not very dilute • the Ka is fairly small • for most problems, this means that the initial acid and salt concentrations should be over 1000x larger than the value of Ka Tro, Chemistry: A Molecular Approach

  30. How Much Does the pH of a Buffer Change When an Acid or Base Is Added? • though buffers do resist change in pH when acid or base are added to them, their pH does change • calculating the new pH after adding acid or base requires breaking the problem into 2 parts • a stoichiometry calculation for the reaction of the added chemical with one of the ingredients of the buffer to reduce its initial concentration and increase the concentration of the other • added acid reacts with the A− to make more HA • added base reacts with the HA to make more A− • an equilibrium calculation of [H3O+] using the new initial values of [HA] and [A−] Tro, Chemistry: A Molecular Approach

  31. Ex 16.3 - What is the pH of a buffer that has 0.100 mol HC2H3O2 and 0.100 mol NaC2H3O2 in 1.00 L that has 0.010 mol NaOH added to it? HC2H3O2 + OH− C2H3O2 + H2O Tro, Chemistry: A Molecular Approach

  32. Ex 16.3 - What is the pH of a buffer that has 0.100 mol HC2H3O2 and 0.100 mol NaC2H3O2 in 1.00 L that has 0.010 mol NaOH added to it? HC2H3O2 + OH− C2H3O2 + H2O Tro, Chemistry: A Molecular Approach

  33. Ex 16.3 - What is the pH of a buffer that has 0.100 mol HC2H3O2 and 0.100 mol NaC2H3O2 in 1.00 L that has 0.010 mol NaOH added to it? HC2H3O2 + H2O  C2H3O2 + H3O+ Tro, Chemistry: A Molecular Approach

  34. Ex 16.3 - What is the pH of a buffer that has 0.100 mol HC2H3O2 and 0.100 mol NaC2H3O2 in 1.00 L that has 0.010 mol NaOH added to it? HC2H3O2 + H2O  C2H3O2 + H3O+ x +x +x x 0.090 x 0.110 + x Tro, Chemistry: A Molecular Approach

  35. Ex 16.3 - What is the pH of a buffer that has 0.100 mol HC2H3O2 and 0.100 mol NaC2H3O2 in 1.00 L that has 0.010 mol NaOH added to it? Ka for HC2H3O2 = 1.8 x 10-5 0.110 +x 0.090 x Tro, Chemistry: A Molecular Approach

  36. Ex 16.3 - What is the pH of a buffer that has 0.100 mol HC2H3O2 and 0.100 mol NaC2H3O2 in 1.00 L that has 0.010 mol NaOH added to it? Ka for HC2H3O2 = 1.8 x 10-5 x = 1.47 x 10-5 the approximation is valid Tro, Chemistry: A Molecular Approach

  37. Ex 16.3 - What is the pH of a buffer that has 0.100 mol HC2H3O2 and 0.100 mol NaC2H3O2 in 1.00 L that has 0.010 mol NaOH added to it? 0.090 x x 0.110 + x x = 1.47 x 10-5 Tro, Chemistry: A Molecular Approach

  38. Ex 16.3 - What is the pH of a buffer that has 0.100 mol HC2H3O2 and 0.100 mol NaC2H3O2 in 1.00 L that has 0.010 mol NaOH added to it? Tro, Chemistry: A Molecular Approach

  39. Ex 16.3 - What is the pH of a buffer that has 0.100 mol HC2H3O2 and 0.100 mol NaC2H3O2 in 1.00 L that has 0.010 mol NaOH added to it? Ka for HC2H3O2 = 1.8 x 10-5 the values match Tro, Chemistry: A Molecular Approach

  40. Ex 16.3 - What is the pH of a buffer that has 0.100 mol HC2H3O2 and 0.100 mol NaC2H3O2 in 1.00 L that has 0.010 mol NaOH added to it? HC2H3O2 + H2O  C2H3O2 + H3O+ Ka for HC2H3O2 = 1.8 x 10-5 Tro, Chemistry: A Molecular Approach

  41. Ex 16.3 - What is the pH of a buffer that has 0.100 mol HC2H3O2 and 0.100 mol NaC2H3O2 in 1.00 L that has 0.010 mol NaOH added to it? HC2H3O2 + H2O  C2H3O2 + H3O+ pKa for HC2H3O2 = 4.745 Tro, Chemistry: A Molecular Approach

  42. Ex 16.3 – Compare the effect on pH of adding 0.010 mol NaOH to a buffer that has 0.100 mol HC2H3O2 and 0.100 mol NaC2H3O2 in 1.00 L to adding 0.010 mol NaOH to 1.00 L of pure water? HC2H3O2 + H2O  C2H3O2 + H3O+ pKa for HC2H3O2 = 4.745 Tro, Chemistry: A Molecular Approach

  43. H2O(l) + NH3(aq) NH4+(aq) + OH−(aq) Basic BuffersB:(aq) + H2O(l)  H:B+(aq) + OH−(aq) • buffers can also be made by mixing a weak base, (B:), with a soluble salt of its conjugate acid, H:B+Cl− Tro, Chemistry: A Molecular Approach

  44. Ex 16.4 - What is the pH of a buffer that is 0.50 M NH3 (pKb = 4.75) and 0.20 M NH4Cl? NH3 + H2O  NH4+ + OH− Tro, Chemistry: A Molecular Approach

  45. Buffering Effectiveness • a good buffer should be able to neutralize moderate amounts of added acid or base • however, there is a limit to how much can be added before the pH changes significantly • the buffering capacity is the amount of acid or base a buffer can neutralize • the buffering range is the pH range the buffer can be effective • the effectiveness of a buffer depends on two factors (1) the relative amounts of acid and base, and (2) the absolute concentrations of acid and base Tro, Chemistry: A Molecular Approach

  46. Effect of Relative Amounts of Acid and Conjugate Base a buffer is most effective with equal concentrations of acid and base Buffer 1 0.100 mol HA & 0.100 mol A- Initial pH = 5.00 Buffer 12 0.18 mol HA & 0.020 mol A- Initial pH = 4.05 pKa (HA) = 5.00 HA + OH− A + H2O after adding 0.010 mol NaOH pH = 5.09 after adding 0.010 mol NaOH pH = 4.25

  47. Effect of Absolute Concentrations of Acid and Conjugate Base a buffer is most effective when the concentrations of acid and base are largest Buffer 1 0.50 mol HA & 0.50 mol A- Initial pH = 5.00 Buffer 12 0.050 mol HA & 0.050 mol A- Initial pH = 5.00 pKa (HA) = 5.00 HA + OH− A + H2O after adding 0.010 mol NaOH pH = 5.02 after adding 0.010 mol NaOH pH = 5.18

  48. Effectiveness of Buffers • a buffer will be most effective when the [base]:[acid] = 1 • equal concentrations of acid and base • effective when 0.1 < [base]:[acid] < 10 • a buffer will be most effective when the [acid] and the [base] are large Tro, Chemistry: A Molecular Approach

  49. Buffering Range • we have said that a buffer will be effective when 0.1 < [base]:[acid] < 10 • substituting into the Henderson-Hasselbalch we can calculate the maximum and minimum pH at which the buffer will be effective Lowest pH Highest pH therefore, the effective pH range of a buffer is pKa± 1 when choosing an acid to make a buffer, choose one whose is pKa is closest to the pH of the buffer

  50. Ex. 16.5a – Which of the following acids would be the best choice to combine with its sodium salt to make a buffer with pH 4.25? Chlorous Acid, HClO2 pKa = 1.95 Nitrous Acid, HNO2 pKa = 3.34 Formic Acid, HCHO2 pKa = 3.74 Hypochlorous Acid, HClO pKa = 7.54 Tro, Chemistry: A Molecular Approach

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