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Reversible Reactions and Chemical Equilibrium. University of Lincoln presentation. 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

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  1. Reversible Reactions and Chemical Equilibrium University of Lincoln presentation This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

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

  3. Reversible Reactions BiCl3(aq) + H2O(l) ↔ BiOCl(s) + 2HCl(aq) CH3CO2H + CH3CH2OH ↔ CH3CO2CH2CH3 + H2O Cr2O72-(aq) + 2OH-(aq) ↔ 2CrO42-(aq) + H2O(l) CH3CO2H(aq) + H2O(l) ↔ CH3CO2-(aq) + H3O+(aq) This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

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

  5. 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 Link to external video This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

  6. Concentration 1. BiCl3(aq) + H2O(l) ↔ BiOCl(s) + 2HCl(aq) 2. Cr2O72-(aq) + 2OH-(aq) ↔ 2CrO42-(aq) + H2O(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? This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

  7. Pressure N2(g) + 3H2(g) ↔ 2NH3(g) CO(g) + 2H2(g) ↔ CH3OH(g) 2NO2(g) ↔ 2NO(g) + O2(g) PCl5(g) ↔ PCl3(g) + Cl2(g) H2(g) + I2(g) ↔ 2HI(g) CO(g) + H2O(g) ↔ CO2(g) + H2(g) How do the above equilibria respond to: An increase in pressure A decrease in pressure This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

  8. Temperature N2(g) + 3H2(g) ↔ 2NH3(g) rH = -92.2 kJ mol-1 H2(g) + I2(g) ↔ 2HI(g) rH = -9.4 kJ mol-1 CO(g) + H2O(g) ↔ CO2(g) + H2(g) rH = -41.2 kJ mol-1 PCl5(g) ↔ PCl3(g) + Cl2(g) rH = 87.9 kJ mol-1 How do the above respond to an Increase in temperature Decrease in temperature This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

  9. Equilibrium constantsa measure of equilibrium position aA + bB ↔cC + dD BiCl3(aq) + H2O(l) ↔ BiOCl(s) + 2HCl(aq) Write the expressions for Kc for the reactions given in previous slides This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

  10. Calculating Equilibrium Constants HNO2(aq) ↔ H+(aq) + NO2-(aq) The table shows the equilibrium molar concentrations for three solutions of nitrous acid in water at 25 oC Calculate the equilibrium constant for this reaction at 25oC This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

  11. Solution A Units of Kc Now try for solutions B and C This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

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

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

  14. 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 NH3 is able to accept H+ and become NH4+ Aqueous solutions Proton species is H3O+ (hydroxonium ion) HCl(aq) + H2O(l)  H3O+(aq) + Cl-(aq) HCl(aq)  H+(aq) + Cl-(aq) This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

  15. Strong Acids Strong acids are fully dissociated HCl (aq) + H2O(l) H3O+ (aq) + Cl- (aq) all dissolved HCl molecules are ionised 1 mol dm-3 HCl(aq) there are: Approx 1 mol dm-3 H3O+ (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 This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

  16. Other strong acids HNO3 (nitric) H2SO4 (sulfuric) HClO4 (perchloric) Write equations showing the dissociation of the above acids Which are monoprotic? Are any diprotic? Chemical equilibrium – K very large This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

  17. Weak Acids Acids that dissociate in a reversible reaction (e.g. CH3COOH; ethanoic (acetic) acid) CH3COOH (aq) + H2O(l) ↔ H3O+ (aq) + CH3COO- (aq) Solution of CH3COOH (aq) contains: CH3COOH (aq) H3O+ (aq) CH3COO- (aq) CH3COOH is partially dissociated This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

  18. How weak is a weak acid? 0.1 mol dm-3 HCl is dissociated 91.4% [H3O+] = 0.091 mol dm-3 pH=1.04 0.1 mol dm-3 CH3COOH is dissociated 1.34% [H3O+] = 0.0013 mol dm-3 pH=2.87 Extent given by K This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

  19. Weak Acids HA(aq) + H2O(l)↔ H3O+(aq) + A-(aq) HA Bronsted acid H2O Bronsted base H3O+ Bronsted acid A- Bronsted base This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

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

  21. Ka Values HCO2H 1.8 x 10-4 mol dm-3 CH3CO2H 1.7 x 10-5 mol dm-3 Are these weak or strong acids? Which is the stronger acid? HCO2H 3.75 CH3CO2H 4.77 This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

  22. pKa values (data tables) This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

  23. pKa Values Controlling the ionisation of weak acids pH = pKa then [HA] = [A-] pH > pKa then [A-] > [HA] pH < pKa then [HA] > [A-] CH3COOH (aq) + H2O(l) ↔ H3O+ (aq) + CH3COO- (aq) CH3COOH: CH3COO- at pH = 4.77 ? CH3COOH: CH3COO- at pH = 3 ? CH3COOH: CH3COO- at pH = 7 ? This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

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

  25. Weak Bases B(aq) + H2O(l) BH+(aq) + OH-(aq) CH3NH2(aq) +H2O(l) ↔ CH3NH3+(aq) + OH-(aq) pKa = 10.66 (of conjugate acid) [B]=[BH+] pH = 10.66 pH =8 what happens to CH3NH3+(aq): CH3NH2(aq) pH =13 ? This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

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

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

  28. Basic drugs amphetamine (C6H5CH2CH(NH2)CH3) Write an equation for the reaction of amphetamine with water. The pKa 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? This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

  29. Water Can dissociate: H2O(l) ↔ H+(aq) + OH-(aq) 2H2O(l) ↔ H3O+(aq) + OH-(aq) H2O is amphoteric This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

  30. Water Kw = [H3O+][OH-]= 1 x 10-14 mol2 dm-6 Kw the ionic product of water In pure water what is [H3O+] and [OH-] ? Kw is a very smallconstant water is only very partially ionised This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

  31. pH pH is defined as: pH = -log10[H3O+] pH is a measure of the H3O+ concentration in solution and can vary from 1 to 14 pH=7 – neutral [H3O+] = [OH-] = 1 x 10-7 mol dm-3 at 25 oC pH<7 – acidic [H3O+] >[OH-] pH>7 - alkaline/basic [H3O+] <[OH-] This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

  32. pH-examples 0.1M HNO3 0.1M CH3COOH What is the pH? pH is dependent on the ionisation of the acid This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

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

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

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

  36. Buffers Ethanoic acid (a weak acid) and sodium ethanoate (salt) CH3COOH  CH3COO- + H+(1) CH3COONa  CH3COO- + Na+ (2) (1)-partially ionised (2)-fully ionised This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

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

  38. Making a buffer solution Choose a weak acid with a pKa 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 This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License

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

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

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

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