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StarterIn pairs discuss everything you remember about
titrations:1)How they are carried out.2) What are indicators for?3)You carry out a titration of a 25 cm3 acid
solution (c= 0.5 moldm-3) with a 1 mol dm-3 solution of NaOH. Calculate the number of moles of acid after the addition of 10 cm3 of NaOH
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Neutralisation
Interpret and sketch acid-base pH curves for strong and weak acid and bases.
Explain the choice of suitable indicator for acid-base titration
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Simulation: http://users.skynet.be/eddy/titratie.swfStrong acid/ strong base
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pH curvespH curves
Types There are four types of acid-base titration; each has a characteristic curve.
strong acid (HCl) v. strong base (NaOH) weak acid (CH3COOH) v. strong alkali (NaOH)
strong acid (HCl) v. weak base (NH3)
weak acid (CH3COOH) v. weak base (NH3)
In the following examples, alkali (0.1M) is added to 25cm3 of acid (0.1M)
End points need not be “neutral‘ due to the phenomenon of salt hydrolysis
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Equivalence point: the point in a titration at which the volume of one solution has reacted exactly with the volume of the second solution
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strong acidstrong acid (HC (HCll) ) v. strong base v. strong base (NaOH)(NaOH)
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Equivalence point is the centre of the vertical section of the titration curve.
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Titration curve for a strong acid–strong base titration
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pH 1 at the start due to 0.1M HCl
(strong monoprotic acid)
strong acidstrong acid (HC (HCll) ) v. strong base v. strong base (NaOH)(NaOH)
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Very little pH change during the initial 20cm3
pH 1 at the start due to 0.1M HCl
(strong monoprotic acid)
strong acidstrong acid (HC (HCll) ) v. strong base v. strong base (NaOH)(NaOH)
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Very little pH change during the initial 20cm3
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 acidstrong acid (HC (HCll) ) v. strong base v. strong base (NaOH)(NaOH)
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Very little pH change during the initial 20cm3
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)
strong acidstrong acid (HC (HCll) ) v. strong base v. strong base (NaOH)(NaOH)
pH 1 at the start due to 0.1M HCl
(strong monoprotic acid)
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Very little pH change during the initial 20cm3
Sharp change in pH over the addition of less than
half a drop of NH3
Curve levels off at pH 10 due to excess 0.1M NH3
(a weak alkali)
pH 1 at the start due to 0.1M HCl
strong acidstrong acid (HC (HCll) ) v. weak basev. weak base (NH (NH33))
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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 CH3COOH (weak monoprotic acid)
weak acid weak acid (CH(CH33COOH) COOH) v. strong basev. strong base (NaOH) (NaOH)
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weak acidweak acid (CH (CH33COOH) COOH) v. weak basev. weak base (NH (NH33))
Types
Steady pH change
pH 4 due to 0.1M CH3COOH (weak monoprotic acid)
NO SHARPCHANGE IN pH
Curve levels off at pH 10 due to excess 0.1M NH3
(a weak alkali)
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4.3 exercise 4
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Indicator
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Hin = H+ + In-
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pH colour ranges for some common indicators
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Watch demo?Why do you get a different result with a different indicator?
phenolthalein
Methyl orange
COLOUR CHANGES OF SOME COMMON INDICATORS
PHENOLPHTHALEIN
LITMUS
METHYL ORANGE
1 2 3 4 5 6 7 8 9 10 11 12 13 14
CHANGE
CHANGE
CHANGE
pH
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Indicator change colours around +- 1 unit of pH from their end point.
An indicator has to be chosen so that their end point is as close as the pH value of the titration’s equivalence point.
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So the colour of the indicator changes over the pH range pKIn 1
HIn(aq) H+(aq) + In¯(aq)
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Which indicator is the best?
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Simulation
What happens when you titrate Na2CO3 with HCl?
What happens when you titrate H3PO4 with NaOH?
4.3 exercise 4
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Choice of indicators for titrations
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To be useful, an indicator mustchange over the “vertical” section of the curve where there is a large change in pH for the addition of a very small volume of alkali.
The indicator used depends on the pH changes around the end point - the indicator must change during the ‘vertical’ portion of the curve.
In the example, the only suitable indicator is PHENOLPHTHALEIN.
Must have an easily observed colour change.
Must change immediately in the required pH rangeover the addition of ‘half’ a drop of reagent.
Acid-base indicatorsAcid-base indicators
PHENOLPHTHALEIN
LITMUS
METHYL ORANGE
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COLOUR CHANGES OF SOME COMMON INDICATORS
Must have an easily observed colour change.
Must change immediately in the required pH rangeover the addition of ‘half’ a drop of reagent.
Acid-base indicatorsAcid-base indicators
PHENOLPHTHALEIN
LITMUS
METHYL ORANGE
1 2 3 4 5 6 7 8 9 10 11 12 13 14
CHANGE
CHANGE
CHANGE
pH
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pH curvespH curves
Types There are four types of acid-base titration; each has a characteristic curve.
strong acid (HCl) v. strong base (NaOH) weak acid (CH3COOH) v. strong alkali (NaOH)
strong acid (HCl) v. weak base (NH3)
weak acid (CH3COOH) v. weak base (NH3)
In the following examples, alkali (0.1M) is added to 25cm3 of acid (0.1M)
End points need not be “neutral‘ due to the phenomenon of salt hydrolysis
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strong acidstrong acid (HC (HCll) ) v. strong base v. strong base (NaOH)(NaOH)
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pH 1 at the start due to 0.1M HCl
(strong monoprotic acid)
strong acidstrong acid (HC (HCll) ) v. strong base v. strong base (NaOH)(NaOH)
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Very little pH change during the initial 20cm3
pH 1 at the start due to 0.1M HCl
(strong monoprotic acid)
strong acidstrong acid (HC (HCll) ) v. strong base v. strong base (NaOH)(NaOH)
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Very little pH change during the initial 20cm3
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 acidstrong acid (HC (HCll) ) v. strong base v. strong base (NaOH)(NaOH)
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Very little pH change during the initial 20cm3
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)
strong acidstrong acid (HC (HCll) ) v. strong base v. strong base (NaOH)(NaOH)
pH 1 at the start due to 0.1M HCl
(strong monoprotic acid)
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strong acidstrong acid (HC (HCll) ) v. strong base v. strong base (NaOH)(NaOH)
Any of the indicators listed will be suitable - they all change in the ‘vertical’ portion
PHENOLPHTHALEIN
LITMUS
METHYL ORANGE
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Very little pH change during the initial 20cm3
Sharp change in pH over the addition of less than
half a drop of NH3
Curve levels off at pH 10 due to excess 0.1M NH3
(a weak alkali)
pH 1 at the start due to 0.1M HCl
strong acidstrong acid (HC (HCll) ) v. weak basev. weak base (NH (NH33))
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PHENOLPHTHALEIN
LITMUS
METHYL ORANGE
strong acid strong acid (HC(HCll) ) v. weak basev. weak base (NH (NH33))
Only methyl orange is suitable - it is the only one to change in the ‘vertical’ portion
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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 CH3COOH (weak monoprotic acid)
weak acid weak acid (CH(CH33COOH) COOH) v. strong basev. strong base (NaOH) (NaOH)
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PHENOLPHTHALEIN
LITMUS
METHYL ORANGE
Only phenolphthalein is suitable - it is the only one to change in the ‘vertical’ portion
weak acidweak acid (CH (CH33COOH) COOH) v. strong basev. strong base (NaOH) (NaOH)
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weak acidweak acid (CH (CH33COOH) COOH) v. weak basev. weak base (NH (NH33))
Types
Steady pH change
pH 4 due to 0.1M CH3COOH (weak monoprotic acid)
NO SHARPCHANGE IN pH
Curve levels off at pH 10 due to excess 0.1M NH3
(a weak alkali)
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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 acidweak acid (CH (CH33COOH) COOH) v. weak base v. weak base (NH(NH33))