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Equilibria involving ions: acids and bases

Equilibria involving ions: acids and bases. AH Unit 2(b)(iii). Key question. What is are acids and bases?. Arrhenius definition. An acid is a substance that when added to water increases the concentration of H + (aq) ions . HA + (aq)  H + (aq) + A - (aq)

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Equilibria involving ions: acids and bases

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  1. Equilibria involving ions: acids and bases AH Unit 2(b)(iii)

  2. Key question • What is are acids and bases?

  3. Arrhenius definition • An acid is a substance that when added to water increases the concentration of H+(aq) ions. HA + (aq)  H+(aq) + A-(aq) • A base is a substance that when added to water increases the concentration of OH-(aq) ions. BOH + (aq)  B+(aq) + OH-(aq)

  4. Key question • What are the limitations of these definitions?

  5. Bronsted-Lowry definitions • An acid is a proton donor. HA  H++ A- HA + H2O  H3O++ A- • A base is a proton acceptor. B + H+ BH+ B + H3O+  BH+ + H2O

  6. Hydronium ion

  7. Conjugate acids and bases

  8. Ionisation of water • Water is amphoteric. • Can you write an expression for the equilibrium constant?

  9. Ionic product of water • Kw has a value of 1 x 10-14 at 25ºC. • Known as the ionic product of water. • Value varies with temperature.

  10. pH scale

  11. Dissociation of acids This is a measure of the strength of an acid - the larger the value of Ka, the stronger the acid.

  12. Calculating pH of weak acids

  13. Dissociation of bases This is a measure of the strength of a base - the larger the value of Ka, the weaker the base.

  14. Indicators

  15. Universal indicator

  16. Methyl orange

  17. Phenolphthalein

  18. Indicators • Are weak acids

  19. HIn and In- have different colours • Their ratio is dependant on [H3O+] • The colour of an indicator in any given solution therefore depends on the ratio, which in turn is determined by pH

  20. The theoretical point at which the colour change occurs is when [HIn] = [In-] • Therefore the colour change occurs when KIn = [H3O+] • pKIn = pH

  21. In practice, the colour change is not visible when [HIn] = [In-] • Instead, they must differ by a factor of 10 • i.e. when [H+] = KIn± 10 • OR when pH = pKIn ± 1

  22. Choice of indicator • Colour change must occur as close to the equivalence point as possible. • Equivalence point – the point at which all of the acid has been exactly “neutralised” by all of the alkali. • Does this always occur at pH 7?

  23. Methyl red

  24. Phenolphthalein

  25. The colour chance must occur in the region of rapid pH change. • This means that the addition of half a drop of acid/base will cause a colour change. • The choice of indicator must therefore be made with reference to titration curves.

  26. Strong acid / strong base

  27. Phenolphthalein

  28. Examples

  29. Practice

  30. Buffers

  31. Buffer solutions • Is a solution where the pH remains approximately constant when small amounts of acid or bases are added. • Common examples: • blood • sea water

  32. Acid buffers • Consists of a weak acid with one of its salts (of a strong alkali) • e.g. ethanoic acid + sodium ethanoate • The acid is partially dissociated and equilibrium with its ions. • The salt is fully ionised.

  33. Addition of alkali: Supplies H3O+(aq) ions if any removed in reacting with an added base. • Addition of acid: CH3COONa(s) → Na+(aq) + CH3OO-(aq) The conjugate base removes any added H+(aq)

  34. pH of buffer solutions Because the by diluting a buffer the concentration of acid and salt will decrease in proportion, dilution will not affect the pH of a buffer solution.

  35. Practice

  36. Basic buffers • Consist of a weak base with one of its salts (of a strong acid).

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