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Chemical Reactions: pH Equilibria

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Lecture materials for the Introductory Chemistry course for Forensic Scientists, University of Lincoln, UK. See http://forensicchemistry.lincoln.ac.uk/ for more details.

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Chemical Reactions: pH Equilibria

  1. 1. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Reversible Reactions and Chemical Equilibrium University of Lincoln presentation
  2. 2. Outline <ul><li>Reversible reactions </li></ul><ul><li>Chemical Equilibrium </li></ul><ul><li>Le Chatelier’s Principle </li></ul><ul><li>Equilibrium constants </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  3. 3. Reversible Reactions <ul><li>BiCl 3 (aq) + H 2 O(l) ↔ BiOCl(s) + 2HCl(aq) </li></ul><ul><li>CH 3 CO 2 H + CH 3 CH 2 OH ↔ CH 3 CO 2 CH 2 CH 3 + H 2 O </li></ul><ul><li>Cr 2 O 7 2- (aq) + 2OH - (aq) ↔ 2CrO 4 2- (aq) + H 2 O(l) </li></ul><ul><li>CH 3 CO 2 H(aq) + H 2 O(l) ↔ CH 3 CO 2 - (aq) + H 3 O + (aq) </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  4. 4. Chemical Equilibrium <ul><li>Reactions not 100% complete </li></ul><ul><ul><li>Products and Reactants exist together </li></ul></ul><ul><li>A dynamic equilibrium </li></ul><ul><li>Position of equilibrium ??? </li></ul><ul><li>Can the position of equilibrium be changed? </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  5. 5. Le Chatelier’s Principle <ul><li>When an external change is made to a system in equilibrium, the system will respond to oppose the change </li></ul><ul><li>External Changes </li></ul><ul><li>Concentration </li></ul><ul><li>Pressure (gases) </li></ul><ul><li>Temperature </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Link to external video Link to external video
  6. 6. Concentration <ul><li>1. BiCl 3 (aq) + H 2 O(l) ↔ BiOCl(s) + 2HCl(aq) </li></ul><ul><li>2. Cr 2 O 7 2- (aq) + 2OH - (aq) ↔ 2CrO 4 2- (aq) + H 2 O(l) </li></ul><ul><li>How does reaction 1 respond to addition of hydrochloric acid? </li></ul><ul><li>How does reaction 2 respond to addition of alkali? </li></ul><ul><li>How does reaction 2 respond to addition of acid? </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  7. 7. Pressure <ul><li>N 2 (g) + 3H 2 (g) ↔ 2NH 3 (g) </li></ul><ul><li>CO(g) + 2H 2 (g) ↔ CH 3 OH(g) </li></ul><ul><li>2NO 2 (g) ↔ 2NO(g) + O 2 (g) </li></ul><ul><li>PCl 5 (g) ↔ PCl 3 (g) + Cl 2 (g) </li></ul><ul><li>H 2 (g) + I 2 (g) ↔ 2HI(g) </li></ul><ul><li>CO(g) + H 2 O(g) ↔ CO 2 (g) + H 2 (g) </li></ul><ul><li>How do the above equilibria respond to: </li></ul><ul><ul><li>An increase in pressure </li></ul></ul><ul><ul><li>A decrease in pressure </li></ul></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  8. 8. Temperature <ul><li>N 2 (g) + 3H 2 (g) ↔ 2NH 3 (g)  r H = -92.2 kJ mol -1 </li></ul><ul><li>H 2 (g) + I 2 (g) ↔ 2HI(g)  r H = -9.4 kJ mol -1 </li></ul><ul><li>CO(g) + H 2 O(g) ↔ CO 2 (g) + H 2 (g)  r H = -41.2 kJ mol -1 </li></ul><ul><li>PCl 5 (g) ↔ PCl 3 (g) + Cl 2 (g)  r H = 87.9 kJ mol -1 </li></ul><ul><li>How do the above respond to an </li></ul><ul><li>Increase in temperature </li></ul><ul><li>Decrease in temperature </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  9. 9. Equilibrium constants a measure of equilibrium position <ul><li>aA + bB ↔cC + dD </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License BiCl 3 (aq) + H 2 O(l) ↔ BiOCl(s) + 2HCl(aq) Write the expressions for K c for the reactions given in previous slides
  10. 10. Calculating Equilibrium Constants <ul><li>HNO 2 (aq) ↔ H + (aq) + NO 2 - (aq) </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License The table shows the equilibrium molar concentrations for three solutions of nitrous acid in water at 25 o C Calculate the equilibrium constant for this reaction at 25 o C Solution [HNO 2(aq) ] mol litre -1 [H + (aq) ] mol litre -1 [NO 2 - (aq)] mol litre -1 A 0.090 6.2 x 10 -3 6.2 x 10 -3 B 0.20 9.3 x 10 -3 9.3 x 10 -3 C 0.30 11.4 x 10 -3 11.4 x 10 -3
  11. 11. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Units of K c Solution A Now try for solutions B and C
  12. 12. Acids and Bases This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  13. 13. Outline <ul><li>Definitions </li></ul><ul><li>Weak Acids </li></ul><ul><li>Dissociation Constants </li></ul><ul><li>Weak Bases </li></ul><ul><li>Drugs </li></ul><ul><li>pH </li></ul><ul><li>Buffers </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  14. 14. Acids and Bases <ul><li>Several definitions available - most common is Bronsted and Lowry </li></ul><ul><li>Acid is a proton donor </li></ul><ul><ul><li>HCl is able to transfer H + </li></ul></ul><ul><li>Base is a proton acceptor </li></ul><ul><ul><li>NH 3 is able to accept H + and become NH 4 + </li></ul></ul><ul><li>Aqueous solutions </li></ul><ul><li>Proton species is H 3 O + (hydroxonium ion) </li></ul><ul><ul><li>HCl(aq) + H 2 O(l)  H 3 O + (aq) + Cl - (aq) </li></ul></ul><ul><ul><li>HCl(aq)  H + (aq) + Cl - (aq) </li></ul></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  15. 15. Strong Acids <ul><li>Strong acids are fully dissociated </li></ul><ul><li>HCl (aq) + H 2 O (l)  H 3 O + (aq) + Cl - (aq) </li></ul><ul><li>all dissolved HCl molecules are ionised </li></ul><ul><li>1 mol dm -3 HCl (aq) there are: </li></ul><ul><ul><li>Approx 1 mol dm -3 H 3 O + (aq) </li></ul></ul><ul><ul><li>Approx 1 mol dm -3 Cl - (aq) </li></ul></ul><ul><li>DO NOT confuse ‘strong’ and ‘concentrated’ </li></ul><ul><li>1 x 10 -4 mol dm -3 HCl (aq) is a dilute solution of a strong acid </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  16. 16. Other strong acids <ul><li>HNO 3 (nitric) </li></ul><ul><li>H 2 SO 4 (sulfuric) </li></ul><ul><li>HClO 4 (perchloric) </li></ul><ul><li>Write equations showing the dissociation of the above acids </li></ul><ul><li>Which are monoprotic? </li></ul><ul><li>Are any diprotic? </li></ul><ul><li>Chemical equilibrium – K very large </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  17. 17. Weak Acids <ul><li>Acids that dissociate in a reversible reaction (e.g. CH 3 COOH; ethanoic (acetic) acid) </li></ul><ul><li>CH 3 COOH (aq) + H 2 O (l) ↔ H 3 O + (aq) + CH 3 COO - (aq) </li></ul><ul><li>Solution of CH 3 COOH (aq) contains: </li></ul><ul><ul><li>CH 3 COOH (aq) </li></ul></ul><ul><ul><li>H 3 O + (aq) </li></ul></ul><ul><ul><li>CH 3 COO - (aq) </li></ul></ul><ul><li>CH 3 COOH is partially dissociated </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  18. 18. How weak is a weak acid? <ul><li>0.1 mol dm -3 HCl is dissociated 91.4% </li></ul><ul><li>[H 3 O+] = 0.091 mol dm -3 pH=1.04 </li></ul><ul><li>0.1 mol dm -3 CH 3 COOH is dissociated 1.34% </li></ul><ul><li>[H 3 O + ] = 0.0013 mol dm -3 pH=2.87 </li></ul><ul><li>Extent given by K </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  19. 19. Weak Acids <ul><li>HA (aq) + H 2 O (l) ↔ H 3 O + (aq) + A - (aq) </li></ul><ul><li>HA Bronsted acid </li></ul><ul><li>H 2 O Bronsted base </li></ul><ul><li>H 3 O + Bronsted acid </li></ul><ul><li>A - Bronsted base </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  20. 20. Acid dissociation constant (K a ) <ul><li>The higher the K a value: </li></ul><ul><ul><li>greater degree of ionisation </li></ul></ul><ul><ul><li>stronger the acid </li></ul></ul><ul><ul><li>Data tables </li></ul></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  21. 21. K a Values <ul><li>HCO 2 H 1.8 x 10 -4 mol dm -3 </li></ul><ul><li>CH 3 CO 2 H 1.7 x 10 -5 mol dm -3 </li></ul><ul><li>Are these weak or strong acids? </li></ul><ul><li>Which is the stronger acid? </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License HCO 2 H 3.75 CH 3 CO 2 H 4.77
  22. 22. pK a values (data tables) This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Acid pK a Conjugate base H 3 PO 4 2.12 H 2 PO 4 - HNO 2 3.34 NO 2 - H 2 CO 3 6.37 HCO 3 - HCN 9.31 CN - HCO 3 - 10.25 CO 3 2-
  23. 23. pK a Values <ul><li>Controlling the ionisation of weak acids </li></ul><ul><li>pH = pKa then [HA] = [A-] </li></ul><ul><li>pH > pKa then [A-] > [HA] </li></ul><ul><li>pH < pKa then [HA] > [A-] </li></ul><ul><li>CH 3 COOH (aq) + H 2 O (l) ↔ H 3 O + (aq) + CH 3 COO - (aq) </li></ul><ul><li>CH 3 COOH: CH 3 COO - at pH = 4.77 ? </li></ul><ul><li>CH 3 COOH: CH 3 COO - at pH = 3 ? </li></ul><ul><li>CH 3 COOH: CH 3 COO - at pH = 7 ? </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  24. 24. Henderson-Hasselbach This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License For weak acids Use the equation with the example in the previous slide. Do you come to the same conclusion regarding the ratio of un-ionised to ionised acid molecules?
  25. 25. Weak Bases <ul><li>B (aq) + H 2 O (l)  BH + (aq) + OH - (aq) </li></ul><ul><li>CH 3 NH 2(aq) +H 2 O (l) ↔ CH 3 NH 3 + (aq) + OH - (aq) </li></ul><ul><li>pK a = 10.66 (of conjugate acid) [B]=[BH + ] </li></ul><ul><li>pH = 10.66 </li></ul><ul><li>pH =8 what happens to CH 3 NH 3 + (aq) : CH 3 NH 2(aq) </li></ul><ul><li>pH =13 ? </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  26. 26. Henderson-Hasselbach This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License For weak bases Use the equation with the example in the previous slide. Do you come to the same conclusion regarding the ratio of un-ionised to ionised acid molecules?
  27. 27. Acidic drugs <ul><li>2-[4-(2-methylpropyl)phenyl]propanoic acid </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License How does this molecule ionise? pK a =4.5 pH =3 (stomach pH)? pH=6 (intestine)? ibuprofen
  28. 28. Basic drugs <ul><li>amphetamine ( C 6 H 5 CH 2 CH(NH 2 )CH 3 ) </li></ul><ul><li>Write an equation for the reaction of amphetamine with water. </li></ul><ul><li>The pK a of the conjugate acid is 9.8. What will happen to the ratio of ionised to unionised amphetamine at: </li></ul><ul><ul><li>pH 7 </li></ul></ul><ul><ul><li>pH 12 </li></ul></ul><ul><li>Why might this be important? </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  29. 29. Water <ul><li>Can dissociate: </li></ul><ul><li>H 2 O (l) ↔ H + (aq) + OH - (aq) </li></ul><ul><li> 2H 2 O (l) ↔ H 3 O + (aq) + OH - (aq) </li></ul><ul><li>H 2 O is amphoteric </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  30. 30. Water <ul><li>K w = [H 3 O + ][OH - ]= 1 x 10 -14 mol 2 dm -6 </li></ul><ul><li>K w the ionic product of water </li></ul><ul><li>In pure water what is [H 3 O + ] and [OH - ] ? </li></ul><ul><li>K w is a very small constant </li></ul><ul><ul><li>water is only very partially ionised </li></ul></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  31. 31. pH <ul><li>pH is defined as: </li></ul><ul><li>pH = -log 10 [H 3 O + ] </li></ul><ul><li>pH is a measure of the H 3 O + concentration in solution and can vary from 1 to 14 </li></ul><ul><li>pH=7 – neutral [H 3 O + ] = [OH - ] </li></ul><ul><ul><ul><ul><li>= 1 x 10 -7 mol dm -3 at 25 o C </li></ul></ul></ul></ul><ul><li>pH<7 – acidic [H 3 O + ] >[OH - ] </li></ul><ul><li>pH>7 - alkaline/basic [H 3 O + ] <[OH - ] </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  32. 32. pH-examples <ul><li>0.1M HNO 3 </li></ul><ul><li>0.1M CH 3 COOH </li></ul><ul><li>What is the pH? </li></ul><ul><li>pH is dependent on the ionisation of the acid </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  33. 33. pH-examples <ul><li>What about alkaline solutions? </li></ul><ul><li>E.g. 0.1M NaOH solution </li></ul><ul><li>Will also depend on degree of ionisation </li></ul><ul><li>use equation: [H + ] x [OH - ] = 10 -14 </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  34. 34. Buffers This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  35. 35. Buffers <ul><li>A buffer solution resists pH changes on addition of small amounts of acid or base (alkali) to a system . </li></ul><ul><li>Very important </li></ul><ul><ul><li>e.g. blood has a pH of 7.4. If it varies by ± 0.4, death can occur </li></ul></ul><ul><li>Buffer solutions rely upon the effects of a weak acid or base and the salt of that acid or base </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  36. 36. Buffers <ul><li>Ethanoic acid (a weak acid) and sodium ethanoate (salt) </li></ul><ul><ul><li>CH 3 COOH  CH 3 COO - + H + (1) </li></ul></ul><ul><ul><li>CH 3 COONa  CH 3 COO - + Na + (2) </li></ul></ul><ul><li>(1)-partially ionised </li></ul><ul><li>(2)-fully ionised </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  37. 37. Buffers This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Henderson-Hasselbach equation Acidic buffers
  38. 38. Making a buffer solution <ul><li>Choose a weak acid with a pK a close to the required pH of the buffer. </li></ul><ul><li>Choose an appropriate salt of the weak acid </li></ul><ul><li>Determine [salt]/[acid] ratio needed to give correct pH </li></ul>This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  39. 39. An acidic buffer: Ethanoic acid and sodium ethanoate This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License What is the [salt] if the acid is 0.1 mol dm -3 to give buffer solutions of pH = 5 pH = 4 <ul><ul><li>What would be the pH of an ethanoate buffer with equal acid and sodium ethanoate concentrations? </li></ul></ul>
  40. 40. An alkaline buffer: ammonia solution and ammonium chloride This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Note the base/salt ratio What is the pH of a buffer with base:salt ratio = 1? Calculate the base:salt ratios for pH 8.5 and pH 10.5
  41. 41. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Acknowledgements <ul><li>JISC </li></ul><ul><li>HEA </li></ul><ul><li>Centre for Educational Research and Development </li></ul><ul><li>School of natural and applied sciences </li></ul><ul><li>School of Journalism </li></ul><ul><li>SirenFM </li></ul><ul><li>http:// tango.freedesktop.org </li></ul>

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