Chemical Reactions: pH Equilibria Seminar

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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 Seminar

  1. 1. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Equilibria seminar University of Lincoln presentation
  2. 2. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Le Chatelier’s Principle When an external change is made to a system in equilibrium, the system will respond to oppose the change 1. BiCl 3 (aq) + H 2 O(l) ↔ BiOCl(s) + 2HCl(aq) 2. Cr 2 O 7 2- (aq) + 2OH - (aq) ↔ 2CrO 4 2- (aq) + H 2 O(l) How does reaction 1 respond to addition of hydrochloric acid? How does reaction 2 respond to addition of alkali? How does reaction 2 respond to addition of acid?
  3. 3. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License N 2 (g) + 3H 2 (g) ↔ 2NH 3 (g) CO(g) + 2H 2 (g) ↔ CH 3 OH(g) 2NO 2 (g) ↔ 2NO(g) + O 2 (g) PCl 5 (g) ↔ PCl 3 (g) + Cl 2 (g) H 2 (g) + I 2 (g) ↔ 2HI(g) CO(g) + H 2 O(g) ↔ CO 2 (g) + H 2 (g) How do the above equilibria respond to: An increase in pressure A decrease in pressure
  4. 4. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License N 2 (g) + 3H 2 (g) ↔ 2NH 3 (g)  r H = -92.2 kJ mol -1 H 2 (g) + I 2 (g) ↔ 2HI(g)  r H = -9.4 kJ mol -1 CO(g) + H 2 O(g) ↔ CO 2 (g) + H 2 (g)  r H = -41.2 kJ mol -1 PCl 5 (g) ↔ PCl 3 (g) + Cl 2 (g)  r H = 87.9 kJ mol -1 How do the above respond to an Increase in temperature Decrease in temperature
  5. 5. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License <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>pKa= 4.77
  6. 6. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License CH 3 NH 2(aq) +H 2 O (l) ↔ CH 3 NH 3 + (aq) + OH - (aq) pK a = 10.66 (of conjugate acid) pH = 10.66 [B]=[BH + ] pH =8 what happens to CH 3 NH 3 + (aq) : CH 3 NH 2(aq) pH =13 ?
  7. 7. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Acidic drugs How does this molecule ionise? pK a =4.5 pH =3 (stomach pH)? pH=6 (intestine)? ibuprofen
  8. 8. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License amphetamine ( C 6 H 5 CH 2 CH (NH 2 )CH 3 ) Write an equation for the reaction of amphetamine with water. The pK a of the conjugate acid is 9.8. What will happen to the ratio of ionised to unionised amphetamine at: pH 7 pH 12 Why might this be important? Basic drugs
  9. 9. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License 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><ul><li>What would be the pH of an ethanoate buffer with equal acid and sodium ethanoate concentrations? </li></ul>
  10. 10. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License pK a values (data tables) 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-
  11. 11. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License pH = 5 pH = 4 Ethanoate buffers
  12. 12. 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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