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Indicators

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Indicators

  1. 1. Indicators CONTENTS • Acid-base indicators - theory • Titration curves - introduction • Titration curve; strong acid - strong base • Titration curve; strong acid - weak base • Titration curve; weak acid - strong base • Titration curve; weak acid - weak base • Titration curve; acid - carbonate • Titration curve; phosphoric acid • Check list
  2. 2. Acid-base indicators General Many indicators are weak acids and partially dissociate in aqueous solution HIn(aq) H+(aq) + In¯(aq) The un-ionised form (HIn) is a different colour to the anionic form (In¯).
  3. 3. Acid-base indicators General Many indicators are weak acids and partially dissociate in aqueous solution HIn(aq) H+(aq) + In¯(aq) The un-ionised form (HIn) is a different colour to the anionic form (In¯). Apply Le Chatelier’s Principle to predict any colour change In acid - increase of [H+] - equilibrium moves to the left to give red undissociated form In alkali - increase of [OH¯] - OH¯ ions remove H+ ions to form water; H+(aq) + OH¯(aq) - equilibrium will move to the right to produce a blue colour H2O(l)
  4. 4. Acid-base indicators General Many indicators are weak acids and partially dissociate in aqueous solution HIn(aq) H+(aq) + In¯(aq) The un-ionised form (HIn) is a different colour to the anionic form (In¯). Apply Le Chatelier’s Principle to predict any colour change In acid - increase of [H+] - equilibrium moves to the left to give red undissociated form In alkali - increase of [OH¯] - OH¯ ions remove H+ ions to form water; H+(aq) + OH¯(aq) - equilibrium will move to the right to produce a blue colour Summary In acidic solution HIn(aq) H+(aq) + In¯(aq) In alkaline solution H2O(l)
  5. 5. Acid-base indicators Must have an easily observed colour change. Must change immediately in the required pH range over the addition of ‘half’ a drop of reagent. COLOUR CHANGES OF SOME COMMON INDICATORS pH METHYL ORANGE LITMUS PHENOLPHTHALEIN 1 2 3 4 5 6 7 8 9 10 CHANGE CHANGE CHANGE 11 12 13 14
  6. 6. Acid-base indicators Must have an easily observed colour change. Must change immediately in the required pH range over the addition of ‘half’ a drop of reagent. To be useful, an indicator must change over the “vertical” section of the curve where there is a large change in pH for the addition of a very small volume of alkali. The indicator used depends on the pH changes around the end point - the indicator must change during the ‘vertical’ portion of the curve. In the example, the only suitable indicator is PHENOLPHTHALEIN. PHENOLPHTHALEIN LITMUS METHYL ORANGE
  7. 7. pH curves Types There are four types of acid-base titration; each has a characteristic curve. strong acid (HCl) v. strong base (NaOH) weak acid (CH3COOH) v. strong alkali (NaOH) strong acid (HCl) v. weak base (NH3) weak acid (CH3COOH) v. weak base (NH3) In the following examples, alkali (0.1M) is added to 25cm3 of acid (0.1M) End points need not be “neutral‘ due to the phenomenon of salt hydrolysis
  8. 8. strong acid (HCl) v. strong base (NaOH)
  9. 9. strong acid (HCl) v. strong base (NaOH) pH 1 at the start due to 0.1M HCl (strong monoprotic acid)
  10. 10. strong acid (HCl) v. strong base (NaOH) Very little pH change during the initial 20cm3 pH 1 at the start due to 0.1M HCl (strong monoprotic acid)
  11. 11. strong acid (HCl) v. strong base (NaOH) Very sharp change in pH over the addition of less than half a drop of NaOH Very little pH change during the initial 20cm3 pH 1 at the start due to 0.1M HCl (strong monoprotic acid)
  12. 12. strong acid (HCl) v. strong base (NaOH) Curve levels off at pH 13 due to excess 0.1M NaOH (a strong alkali) Very sharp change in pH over the addition of less than half a drop of NaOH Very little pH change during the initial 20cm3 pH 1 at the start due to 0.1M HCl (strong monoprotic acid)
  13. 13. strong acid (HCl) v. strong base (NaOH) PHENOLPHTHALEIN LITMUS METHYL ORANGE Any of the indicators listed will be suitable - they all change in the ‘vertical’ portion
  14. 14. strong acid (HCl) v. weak base (NH3) Curve levels off at pH 10 due to excess 0.1M NH3 (a weak alkali) Sharp change in pH over the addition of less than half a drop of NH3 pH 1 at the start due to 0.1M HCl Very little pH change during the initial 20cm3
  15. 15. strong acid (HCl) v. weak base (NH3) PHENOLPHTHALEIN LITMUS METHYL ORANGE Only methyl orange is suitable - it is the only one to change in the ‘vertical’ portion
  16. 16. weak acid (CH3COOH) v. strong base (NaOH) Curve levels off at pH 13 due to excess 0.1M NaOH (a strong alkali) Sharp change in pH over the addition of less than half a drop of NaOH Steady pH change pH 4 due to 0.1M CH3COOH (weak monoprotic acid)
  17. 17. weak acid (CH3COOH) v. strong base (NaOH) PHENOLPHTHALEIN LITMUS METHYL ORANGE Only phenolphthalein is suitable - it is the only one to change in the ‘vertical’ portion
  18. 18. weak acid (CH3COOH) v. weak base (NH3) Curve levels off at pH 10 due to excess 0.1M NH3 (a weak alkali) Steady pH change pH 4 due to 0.1M CH3COOH (weak monoprotic acid) Types NO SHARP CHANGE IN pH
  19. 19. weak acid (CH3COOH) v. weak base (NH3) PHENOLPHTHALEIN LITMUS METHYL ORANGE NOTHING SUITABLE There is no suitable indicator- none change in the ‘vertical’ portion. The end point can be detected by plotting a curve using a pH meter.
  20. 20. Other pH curves - acid v. carbonate Sodium carbonate reacts with hydrochloric acid in two steps... Step 1 Na2CO3 + HCl ——> NaHCO3 + NaCl Step 2 NaHCO3 + HCl ——> NaCl + H2O + CO2 Overall Na2CO3 + 2HCl ——> 2NaCl + H2O + CO2
  21. 21. Other pH curves - acid v. carbonate Sodium carbonate reacts with hydrochloric acid in two steps... Step 1 Na2CO3 + HCl ——> NaHCO3 + NaCl Step 2 NaHCO3 + HCl ——> NaCl + H2O + CO2 Overall Na2CO3 + 2HCl ——> 2NaCl + H2O + CO2 There are two sharp pH changes The second addition of HCl is exactly the same as the first because the number of moles of HCl which react with the NaHCO3 is the same as that reacting with the Na2CO3. 17.50cm3 35.00cm3
  22. 22. Other pH curves - acid v. carbonate Sodium carbonate reacts with hydrochloric acid in two steps... Step 1 Na2CO3 + HCl ——> NaHCO3 + NaCl Step 2 NaHCO3 + HCl ——> NaCl + H2O + CO2 Overall Na2CO3 + 2HCl ——> 2NaCl + H2O + CO2 There are two sharp pH changes First rapid pH change around pH = 8.5 due to the formation of NaHCO3 . Can be detected using phenolphthalein
  23. 23. Other pH curves - acid v. carbonate Sodium carbonate reacts with hydrochloric acid in two steps... Step 1 Na2CO3 + HCl ——> NaHCO3 + NaCl Step 2 NaHCO3 + HCl ——> NaCl + H2O + CO2 Overall Na2CO3 + 2HCl ——> 2NaCl + H2O + CO2 There are two sharp pH changes First rapid pH change around pH = 8.5 due to the formation of NaHCO3 . Can be detected using phenolphthalein Second rapid pH change around pH = 4 due to the formation of acidic CO2 . Can be detected using methyl orange.
  24. 24. Other pH curves - polyprotic acids (H3PO4) Phosphoric acid is triprotic; it reacts with sodium hydroxide in three steps... Step 1 H3PO4 Step 2 Step 3 + NaOH ——> NaH2PO4 + H2O NaH2PO4 + NaOH ——> Na2HPO4 + H2O Na2HPO4 + NaOH ——> Na3PO4 + H2O
  25. 25. Other pH curves - polyprotic acids (H3PO4) Phosphoric acid is triprotic; it reacts with sodium hydroxide in three steps... Step 1 H3PO4 Step 2 Step 3 + NaOH ——> NaH2PO4 + H2O NaH2PO4 + NaOH ——> Na2HPO4 + H2O Na2HPO4 + NaOH ——> Na3PO4 + H2O There are three sharp pH changes Each successive addition of NaOH is the same as equal number of moles are involved.
  26. 26. Other pH curves - polyprotic acids (H3PO4) Phosphoric acid is triprotic; it reacts with sodium hydroxide in three steps... Step 1 H3PO4 Step 2 Step 3 + NaOH ——> NaH2PO4 + H2O NaH2PO4 + NaOH ——> Na2HPO4 + H2O Na2HPO4 + NaOH ——> Na3PO4 + H2O pH of H3PO4 = 1.5
  27. 27. Other pH curves - polyprotic acids (H3PO4) Phosphoric acid is triprotic; it reacts with sodium hydroxide in three steps... Step 1 H3PO4 Step 2 Step 3 + NaOH ——> NaH2PO4 + H2O NaH2PO4 + NaOH ——> Na2HPO4 + H2O Na2HPO4 + NaOH ——> Na3PO4 + H2O pH of NaH2PO4 = 4.4 pH of H3PO4 = 1.5
  28. 28. Other pH curves - polyprotic acids (H3PO4) Phosphoric acid is triprotic; it reacts with sodium hydroxide in three steps... Step 1 H3PO4 Step 2 Step 3 + NaOH ——> NaH2PO4 + H2O NaH2PO4 + NaOH ——> Na2HPO4 + H2O Na2HPO4 + NaOH ——> Na3PO4 + H2O pH of Na2HPO4 = 9.6 pH of NaH2PO4 = 4.4 pH of H3PO4 = 1.5
  29. 29. Other pH curves - polyprotic acids (H3PO4) Phosphoric acid is triprotic; it reacts with sodium hydroxide in three steps... Step 1 H3PO4 Step 2 Step 3 + NaOH ——> NaH2PO4 + H2O NaH2PO4 + NaOH ——> Na2HPO4 + H2O Na2HPO4 + NaOH ——> Na3PO4 + H2O pH of Na3PO4 = 12 pH of Na2HPO4 = 9.6 pH of NaH2PO4 = 4.4 pH of H3PO4 = 1.5
  30. 30. REVISION CHECK What should you be able to do? Recall the definition of a weak acid Understand why indicators can be made from weak acids Understand why indicators must change colour quickly over a small pH range Recall and explain the shape of titration curves involving acids and bases Explain why particular indicators are used for certain titrations Explain the shape of the titration curve for phosphoric acid Explain the shape of the titration curve for acid -sodium carbonate reactions CAN YOU DO ALL OF THESE? YES NO
  31. 31. You need to go over the relevant topic(s) again Click on the button to return to the menu
  32. 32. WELL DONE! Try some past paper questions
  33. 33. INDICATORS & pH CURVES THE END © 2004 JONATHAN HOPTON & KNOCKHARDY PUBLISHING

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