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Titrations and Indicators

IB Chemistry Power Points Topic 18 Acids and Bases. www.pedagogics.ca. Titrations and Indicators. Acid Base Titrations. If a strong acid is titrated using a strong base the pH will start off as 0-1 and gradually increase.

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Titrations and Indicators

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  1. IB Chemistry Power Points Topic 18 Acids and Bases www.pedagogics.ca Titrations and Indicators

  2. Acid Base Titrations • If a strong acid is titrated using a strong base the pH will start off as 0-1 and gradually increase. • Once enough base is added that it is now in excess the pH will change very suddenly to about 12. • The point at which this change is seen is when the amount of acid = amount of base. • This is called the equivalence point. • With this combination it occurs at pH 7 as the acid and base combine to make a neutral solution.

  3. Although use of phenolphthalein is common, most indicators will work for this combination. • the sharp change in pH (vertical line on plot) is called the inflection point.

  4. Weak Acid + Strong Base • A weak acid will have a pH of 3-5. When a strong base is added the pH will increase gradually as HA is converted to A-. • This is called the buffering region as the solution is acting like a buffer. • When half the amount of base required for neutralization has been added it is called the half-neutralization point and [HA] = [A-] • at this point, Ka= [H+] and pKa = pH

  5. Ka= [H+] and pKa = pH • This is the best way to determine the Ka for a weak acid. • The equivalence point occurs when pH >7. Why? • Most suitable indicator is phenolphthalein

  6. Consider a titration of 50 cm3 of 0.05 mol dm-3 of acetic acid with 0.10 mol dm-3NaOH • Use your graph to determine the pH of the half equivalence point and the Ka for a acetic acid.

  7. Weak Base + Strong Acid • A suitable indicator here is methyl orange.

  8. When a strong acid is added to a weak base B(aq) + H+(aq) ∏ BH+(aq) • At half equivalence point [B] = [BH+] so Kb = [OH-] • And pKb = pOH = 14 – pH • At the equivalence point pH is <7 and exact pH can be found from pKb of base (or pKa of the conjugate acid). Why is this?

  9. When equivalence point is reached moles of B = moles of H+ B(aq) + H+(aq) ∏ BH+(aq) • BH+ is the conjugate acid and will react with water to form the equilibrium BH+(aq) + H2O ∏B(aq) + H3O+(aq) • pKb of the base + pKa of conjugate acid = 14 • Knowing pKa and [BH+] (or pKb and [A-]) means you can calculate equivalence point pH – see next two slides if you are brave

  10. 50.0 cm3 of 0.1 M NH3 are titrated with 0.20 M HCl. What is the pH at the equivalence point? NH3(aq) + H+(aq) ∏NH4+(aq) At equivalence point, 0.1 x 0.050 = 0.0050 mol of NH4+ is produced by the neutralization (moles NH3 = moles HCl) NH4+ is the conjugate acid and will react with water to form the equilibrium NH4+(aq) + H2O ∏ NH3 (aq) + H3O+(aq) pKb of the NH3 + pKa of NH4+ = 14 4.75 + 9.25 = 14

  11. NH4+(aq) + H2O ∏ NH3 (aq) + H3O+(aq) pKb of the NH3 + pKa of NH4+ = 14 4.75 + 9.25 = 14

  12. Weak Acid + Weak Base • When a weak acid is added to a weak base the change in pH is gradual from acidic to basic so it is hard to detect the equivalence point. • It is hard to find a suitable indicator.

  13. How Indicators Work • An indicator is a substance (often an organic dye) that has a different color in acidic and alkaline solutions. • The color change is seen because the indicator is a weak acid/base in which the two forms have different colors. HIn(aq)  H+(aq) + In-(aq) litmus: red blue • HIn stands for the indicator and In- is the other form when it dissociates.

  14. HIn(aq)  H+(aq) + In-(aq) What happens with the addition of acid? • the eqm is driven to the left to form HIn as the H+ ions combine with In- to reduce [H+]. What happens with the addition of base? • the equilibrium shifts to the right to form more In- as the OH- of the base combines with H+ reducing [H+] so more HIn will dissociate.

  15. The color present depends on the pH and [H+] and also on Ka so different indicators change color over different pH ranges. • When pH = pKa the two colored forms will have equal concentrations and the indicator will be in the middle of its color change. • Rule of thumb: If one form is in excess by 10 fold then the color will be of that form.

  16. Consider Bromophenol blue pKa = 4.0yellow in acid blue in alkali • at pH = 4.0, pH = pKa and the [H+] = 10-4

  17. Consider Bromophenol blue pKa = 4.0yellow in acid blue in alkali • at pH = 5.0, pH > pKa and the [H+] = 10-5

  18. Consider Bromophenol blue pKa = 4.0yellow in acid blue in alkali • at pH = 3.0, pH < pKa and the [H+] = 10-3

  19. Choosing an Indicator – data book • Phenolphthalein • pKa = 9.6 • pH range: 8.3 -10.0 • Color in acid: colorless • Color in alkali: pink • Good for titrations with strong bases

  20. Choosing an Indicator – data book • Methyl Orange • pKa = 3.7 • pH range: 3.1 - 4.4 • Color in acid: red • Color in alkali: yellow • Good for titrations with strong acids

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