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21.3 Neutralisation

Monday, 26 September 2016. 21.3 Neutralisation. Acid-base titrations Acid-base titrations are used to monitor neutralisation reactions closely. The results can then provide unknown information about the acid or base involved.

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21.3 Neutralisation

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  1. Monday, 26 September 2016 21.3 Neutralisation

  2. Acid-base titrations Acid-base titrations are used to monitor neutralisation reactions closely. The results can then provide unknown information about the acid or base involved. pH meters are typically used with the titration to monitor the pH changes

  3. pH titration curve

  4. Acid-base titrations The equivalence point of the titration is where the volume of one solution exactly reacts with the volume of the other solution. The solutions have reacted exactly and the amounts used match the stoichiometry of the reaction. The equivalence point is the exact centre of the vertical section of the titration graph

  5. Titration of strong acids with strong bases 1.00 mol dm-3 sodium hydroxide vs. 1.00 mol dm-3 hydrochloric acid The first 22cm3 has little effect. But what if 25.1cm3 or 24.9cm3 of HCl

  6. Titration of weak acids with strong bases 1.00 mol dm-3 sodium hydroxide vs. 1.00 mol dm-3ethanoic acid Again the first 22cm3 has little effect. What is the equivalence point here?

  7. Titration of strong acids with weak bases 1.00 mol dm-3 ammonia vs. 1.00 mol dm-3 hydrochloric acid The first 22cm3 is a little different here. What is the equivalence point here?

  8. Titration of weak acids with weak bases 1.00 mol dm-3 ammonia vs. 1.00 mol dm-3ethanoic acid What is the equivalence point here? What issues do you have here?

  9. Acid-base indicators An acid-base indicator is a weak acid, HA (or HIn), that has a distinctively different colour between the acid and the conjugate base. For methyl orange the weak acid, HA, is red and the conjugate base, A-, is yellow. At the end point of the titration, the indicator contains equal concentrations of the weak acid and the conjugate base of the indicator and the colour will be between the two

  10. Acid-base indicators Can you explain why an indicator changes colour in terms of equilibrium? HA(aq) H+(aq) + A-(aq) Red Yellow What happens when an acid is added? What happens when a base/alkali is added?

  11. Acid-base indicators Different indicators have different Ka values and therefore change colour over different pH ranges. Because the end point is the point where [HA] and [A-] are equal the pH of the end point is the same as the pKa value of the indicator.

  12. Selecting the right indicator for the titration The colour change of the indicator must coincide with the vertical section of the pH titration curve. Ideally the end point and the equivalence point would coincide. This isn’t always possible and the volumes between the two may be slightly different but the difference will be a matter of one or two drops (i.e. 0.05-0.10 cm3)

  13. Choosing an indicator: A good indicator needs to show a dramatic colour change. The indicator needs to change during the equivalence point (which can be over several pH values) What indicators would you choose for the four types of titration and why?

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