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

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Presentation on theme: "Equilibrium in Acid- Base Systems 16.4b: pH curve shape vs. acid-base strength."— Presentation transcript:

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 TypesThere are four types of acid-base titration; each has a characteristic curve. strong acid (HCl) v. strong base (NaOH) weak acid (CH 3 COOH) v. strong alkali (NaOH) strong acid (HCl) v. weak base (NH 3 ) weak acid (CH 3 COOH) v. weak base (NH 3 ) In the following examples, alkali (0.1M) is added to 25cm 3 of acid (0.1M)

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

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

6 Very little pH change during the initial 20cm 3 pH 1 at the start due to 0.1M HCl (strong monoprotic acid) strong acid (HCl) v. strong base (NaOH)

7 Very little pH change during the initial 20cm 3 Very sharp change in pH over the addition of less than half a drop of NaOH pH 1 at the start due to 0.1M HCl (strong monoprotic acid) strong acid (HCl) v. strong base (NaOH)

8 Very little pH change during the initial 20cm 3 Very sharp change in pH over the addition of less than half a drop of NaOH Curve levels off at pH 13 due to excess 0.1M NaOH (a strong alkali) pH 1 at the start due to 0.1M HCl (strong monoprotic acid) strong acid (HCl) v. strong base (NaOH)

9 Any of the indicators listed will be suitable - they all change in the ‘vertical’ portion PHENOLPHTHALEIN LITMUS METHYL ORANGE

10 Very little pH change during the initial 20cm 3 Sharp change in pH over the addition of less than half a drop of NH 3 Curve levels off at pH 10 due to excess 0.1M NH 3 (a weak alkali) pH 1 at the start due to 0.1M HCl strong acid (HCl) v. weak base (NH 3 )

11 PHENOLPHTHALEIN LITMUS METHYL ORANGE strong acid (HCl) v. weak base (NH 3 ) Only methyl orange is suitable - it is the only one to change in the ‘vertical’ portion

12 Steady pH change Sharp change in pH over the addition of less than half a drop of NaOH Curve levels off at pH 13 due to excess 0.1M NaOH (a strong alkali) pH 4 due to 0.1M CH 3 COOH (weak monoprotic acid) weak acid (CH 3 COOH) v. strong base (NaOH)

13 PHENOLPHTHALEIN LITMUS METHYL ORANGE Only phenolphthalein is suitable - it is the only one to change in the ‘vertical’ portion weak acid (CH 3 COOH) v. strong base (NaOH)

14 weak acid (CH 3 COOH) v. weak base (NH 3 ) Types Steady pH change pH 4 due to 0.1M CH 3 COOH (weak monoprotic acid) NO SHARP CHANGE IN pH Curve levels off at pH 10 due to excess 0.1M NH 3 (a weak alkali)

15 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. weak acid (CH 3 COOH) v. weak base (NH 3 )

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