Equilibria involving ions: acids and bases

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Equilibria involving ions acids and bases

• AH Unit 2(b)(iii)

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Key question

• What is are acids and bases?

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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)

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Key question

• What are the limitations of these definitions?

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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

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Hydronium ion
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Conjugate acids and bases
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Ionisation of water

• Water is amphoteric.
• Can you write an expression for the equilibrium
  constant?

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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.

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pH scale
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Dissociation of acids
This is a measure of the strength of an acid -
the larger the value of Ka, the stronger the acid.
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Calculating pH of weak acids
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Dissociation of bases
This is a measure of the strength of a base - the
larger the value of Ka, the weaker the base.
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Indicators
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Universal indicator
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Methyl orange
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Phenolphthalein
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Indicators

• Are weak acids

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• 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

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• The theoretical point at which the colour change
  occurs is when HIn In-
• Therefore the colour change occurs when
  KIn H3O
• pKIn pH

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• 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

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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?

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Methyl red
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Phenolphthalein
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• 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.

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Strong acid / strong base
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Phenolphthalein
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Examples
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Practice
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Buffers
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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

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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.

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• 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)
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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.
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Practice
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Basic buffers

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