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. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Outline Reversible reactions Chemical Equilibrium Le Chatelier’s Principle Equilibrium constants

3. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Reversible Reactions BiCl 3 (aq) + H 2 O(l) ↔ BiOCl(s) + 2HCl(aq) CH 3 CO 2 H + CH 3 CH 2 OH ↔ CH 3 CO 2 CH 2 CH 3 + H 2 O Cr 2 O 7 2- (aq) + 2OH - (aq) ↔ 2CrO 4 2- (aq) + H 2 O(l) CH 3 CO 2 H(aq) + H 2 O(l) ↔ CH 3 CO 2 - (aq) + H 3 O + (aq)

4. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Chemical Equilibrium Reactions not 100% complete –Products and Reactants exist together A dynamic equilibrium Position of equilibrium ??? Can the position of equilibrium be changed?

5. 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 External Changes Concentration Pressure (gases) Temperature Link to external video

6. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Concentration 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?

7. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Pressure 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

8. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Temperature 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

9. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Equilibrium constants a measure of equilibrium position aA + bB ↔ cC + dD 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. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Calculating Equilibrium Constants HNO 2 (aq) ↔ H + (aq) + NO 2 - (aq) Solution [HNO 2(aq) ] mol litre -1 [H + (aq) ] mol litre -1 [NO 2 - (aq)] mol litre -1 A 0.0906.2 x 10 -3 B 0.209.3 x 10 -3 C 0.3011.4 x 10 -3 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

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. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Acids and Bases

13. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Outline Definitions Weak Acids Dissociation Constants Weak Bases Drugs pH Buffers

14. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Acids and Bases Several definitions available - most common is Bronsted and Lowry Acid is a proton donor –HCl is able to transfer H + Base is a proton acceptor –NH 3 is able to accept H + and become NH 4 + Aqueous solutions Proton species is H 3 O + (hydroxonium ion) –HCl(aq) + H 2 O(l)  H 3 O + (aq) + Cl - (aq) –HCl(aq)  H + (aq) + Cl - (aq)

15. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Strong Acids Strong acids are fully dissociated HCl (aq) + H 2 O (l)  H 3 O + (aq) + Cl - (aq) all dissolved HCl molecules are ionised 1 mol dm -3 HCl (aq) there are: –Approx 1 mol dm -3 H 3 O + (aq) –Approx 1 mol dm -3 Cl - (aq) DO NOT confuse ‘strong’ and ‘concentrated’ 1 x 10 -4 mol dm -3 HCl (aq) is a dilute solution of a strong acid

16. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Other strong acids HNO 3 (nitric) H 2 SO 4 (sulfuric) HClO 4 (perchloric) Write equations showing the dissociation of the above acids Which are monoprotic? Are any diprotic? Chemical equilibrium – K very large

17. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Weak Acids Acids that dissociate in a reversible reaction (e.g. CH 3 COOH; ethanoic (acetic) acid) CH 3 COOH (aq) + H 2 O (l) ↔ H 3 O + (aq) + CH 3 COO - (aq) Solution of CH 3 COOH (aq) contains: –CH 3 COOH (aq) –H 3 O + (aq) –CH 3 COO - (aq) CH 3 COOH is partially dissociated

18. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License How weak is a weak acid? 0.1 mol dm -3 HCl is dissociated 91.4% [H 3 O+] = 0.091 mol dm -3 pH=1.04 0.1 mol dm -3 CH 3 COOH is dissociated 1.34% [H 3 O + ] = 0.0013 mol dm -3 pH=2.87 Extent given by K

19. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Weak Acids HA (aq) + H 2 O (l) ↔ H 3 O + (aq) + A - (aq) HA Bronsted acid H 2 O Bronsted base H 3 O + Bronsted acid A - Bronsted base

20. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Acid dissociation constant (K a ) The higher the K a value: –greater degree of ionisation –stronger the acid –Data tables

21. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License K a Values HCO 2 H1.8 x 10 -4 mol dm -3 CH 3 CO 2 H1.7 x 10 -5 mol dm -3 Are these weak or strong acids? Which is the stronger acid? HCO 2 H3.75 CH 3 CO 2 H4.77

22. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License pK a values (data tables) AcidpK a Conjugate base H 3 PO 4 2.12H 2 PO 4 - HNO 2 3.34NO 2 - H 2 CO 3 6.37HCO 3 - HCN9.31CN - HCO 3 - 10.25CO 3 2-

23. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License pK a Values Controlling the ionisation of weak acids pH = pKa then [HA] = [A-] pH > pKa then [A-] > [HA] pH [A-] CH 3 COOH (aq) + H 2 O (l) ↔ H 3 O + (aq) + CH 3 COO - (aq) CH 3 COOH: CH 3 COO - at pH = 4.77 ? CH 3 COOH: CH 3 COO - at pH = 3 ? CH 3 COOH: CH 3 COO - at pH = 7 ?

24. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Henderson-Hasselbach 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. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Weak Bases B (aq) + H 2 O (l)  BH + (aq) + OH - (aq) CH 3 NH 2(aq) +H 2 O (l) ↔ CH 3 NH 3 + (aq) + OH - (aq) pK a = 10.66 (of conjugate acid) [B]=[BH + ] pH = 10.66 pH =8 what happens to CH 3 NH 3 + (aq) : CH 3 NH 2(aq) pH =13 ?

26. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Henderson-Hasselbach 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. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Acidic drugs 2-[4-(2-methylpropyl)phenyl]propanoic acid How does this molecule ionise? pK a =4.5 pH =3 (stomach pH)? pH=6 (intestine)? ibuprofen

28. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Basic drugs 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?

29. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Water Can dissociate: H 2 O (l) ↔ H + (aq) + OH - (aq) 2H 2 O (l) ↔ H 3 O + (aq) + OH - (aq) H 2 O is amphoteric

30. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Water K w = [H 3 O + ][OH - ]= 1 x 10 -14 mol 2 dm -6 K w the ionic product of water In pure water what is [H 3 O + ] and [OH - ] ? K w is a very small constant –water is only very partially ionised

31. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License pH pH is defined as: pH = -log 10 [H 3 O + ] pH is a measure of the H 3 O + concentration in solution and can vary from 1 to 14 pH=7 – neutral [H 3 O + ] = [OH - ] = 1 x 10 -7 mol dm -3 at 25 o C pH [OH - ] pH>7 - alkaline/basic [H 3 O + ] <[OH - ]

32. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License pH-examples 0.1M HNO 3 0.1M CH 3 COOH What is the pH? pH is dependent on the ionisation of the acid

33. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License pH-examples What about alkaline solutions? E.g. 0.1M NaOH solution Will also depend on degree of ionisation use equation: [H + ] x [OH - ] = 10 -14

34. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Buffers

35. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Buffers A buffer solution resists pH changes on addition of small amounts of acid or base (alkali) to a system. Very important –e.g. blood has a pH of 7.4. If it varies by ± 0.4, death can occur Buffer solutions rely upon the effects of a weak acid or base and the salt of that acid or base

36. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Buffers Ethanoic acid (a weak acid) and sodium ethanoate (salt) CH 3 COOH  CH 3 COO - + H + (1) CH 3 COONa  CH 3 COO - + Na + (2) (1)-partially ionised (2)-fully ionised

37. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Buffers Henderson-Hasselbach equation Acidic buffers

38. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Making a buffer solution Choose a weak acid with a pK a close to the required pH of the buffer. Choose an appropriate salt of the weak acid Determine [salt]/[acid] ratio needed to give correct pH

39. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License An acidic buffer: Ethanoic acid and sodium ethanoate What is the [salt] if the acid is 0.1 mol dm -3 to give buffer solutions of pH = 5 pH = 4 What would be the pH of an ethanoate buffer with equal acid and sodium ethanoate concentrations?

40. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License An alkaline buffer: ammonia solution and ammonium chloride 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. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Acknowledgements JISC HEA Centre for Educational Research and Development School of natural and applied sciences School of Journalism SirenFM http://tango.freedesktop.org