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Equilibrium in Acid-Base Systems

Equilibrium in Acid-Base Systems. 16.4b: pH curve shape vs. acid-base strength. Acid-Base Strength. quantitative reactions are not limited to SA-SB they can occur between SA-WB and WA-SB, but not between WA-WB equivalence pt. pH for a SA-SB is 7

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Equilibrium in Acid-Base Systems

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  1. Equilibrium in Acid-Base Systems 16.4b: pH curve shape vs. acid-base strength

  2. Acid-Base Strength • quantitative reactions are not limited to SA-SB • they can occur between SA-WB and WA-SB, but not between WA-WB • equivalence pt. pH for a SA-SB is 7 • equivalence pt. pH for a quantitative SA-WB is <7 • equivalence pt. pH for a quantitative WA-SB is >7 • WA-WB reactions do not have a detectable equivalence pt. since they are usually not quantitative

  3. 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)

  4. strong acid (HCl) v. strong base (NaOH)

  5. strong acid (HCl) v. strong base (NaOH) pH 1 at the start due to 0.1M HCl (strong monoprotic acid)

  6. 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)

  7. 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)

  8. 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)

  9. 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

  10. 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 Very little pH change during the initial 20cm3 pH 1 at the start due to 0.1M HCl

  11. 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

  12. 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)

  13. 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

  14. weak acid (CH3COOH) v. weak base (NH3) Curve levels off at pH 10 due to excess 0.1M NH3 (a weak alkali) NO SHARP CHANGE IN pH Steady pH change pH 4 due to 0.1M CH3COOH (weak monoprotic acid) Types

  15. 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.

  16. Buffer Capacity amount of H+ or OH- the solution can absorb without a significant change in pH determined by the size of [HA] and [A-] if the amounts are equal and have large concentrations, it will have the largest buffer capacity

  17. Homeostasis (Increase breathing rate)

  18. Homework • Textbook p762 #11-13(a,c), 14 • Textbook p767 #1,3,5,7 • LSM D,F,H • LSM K,L summary Finished Chem 30. Congrats and study for your Diploma!

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